SPring-8-II
Conceptual Design Report

RIKEN SPring-8 Center
November, 2014

General Introduction

Seventeen years have passed since the 1997 inauguration of SPring-8 as thehighest-electron-energy, large-scale third generation synchrotron radiation(SR) facility in the world. Since then, SPring-8 has been playing a leadingrole in high-energy photon science, welcoming around 170,000 users fromboth the academic and industrial sectors. It should be noted that no new SR
facilities larger than SPring-8 have been built during this time. Instead,newer SR facilities have followed the model of lower-energy, medium-size
electron storage rings with fairly low-emittance.
This trend was kicked off by the research and development at SPring-8 forin-vacuum type undulators, which are able to reduce the magnetic period
length of the undulators. SPring-8 employed in-vacuum undulators asstandard insertion devices. In the earlier stages of the medium-sizedfacilities, we observed them managing to produce hard X-rays fromundulators. But now, the performance of medium-sized facilities is about tosurpass that of the older, large-scale facilities, owing to the past decade’s
rapid technological progress. The newer technologies clearly have thepotential to convert existing large-scale facilities into super-performance SR
facilities.
At the SPring-8 site, the first adaptation of newer technology was not in thestorage ring, but in the X-ray free electron laser (XFEL). A shorter
magnetic-period, in-vacuum undulator enabled us to implement a “compact”
SASE (self-amplified spontaneous emission) XFEL. The first proposal forthe compact SASE source was submitted in 2000, followed by an R&D phasefrom 2001 to 2005, and ending up with the construction of a prototype FEL
operating in the extreme ultraviolet (EUV) region. This initiative was
highlighted in the 3rd midterm “Basic Program for Science and Technology,”
which designated the compact XFEL as one of the five “Key Technologies of
National Importance”. The SACLA (SPring-8 Angstrom Compact Laser)
project started accordingly in 2006 with a total budget of 40B
JP¥ (approximately US$400M in 2014) and a 5-year schedule. We designed
the SACLA linear accelerator (linac) to be used for a full-energy injector forthe SPring-8 storage ring. An ultra-low-emittance electron beam deliveredfrom the SACLA linac should be compatible with the future upgraded
SPring-8 facility. SACLA completed the project on time and on budget. It is
now in operation supporting user programs.
SACLA is delivering pulsed X-ray laser beams whose pulse duration is as
short as a few femtoseconds (10-15 second). The peak brilliance of the SASE
FEL is so high that samples can be destroyed after a single shot irradiationof the SASE pulse. However, the extremely short pulse width enables us to
extract sample information through scattering, absorption, photon or
photoelectron emission before the sample is destroyed. This “measure before
destroy” scheme is characteristic of FEL experiments. The scheme requires
reconstructing the full data set from many independent data results takenfrom different samples as exemplified in serial femtosecond crystallography(SFX). This technique is a marked contrast to data taking with SR, whichusually provides a full data set from measurement with a single sample.
The complementary use of storage-ring light sources and XFELs is essential
to opening new frontiers in science and technology, partly because XFELs
allow access to ultra-fast time domains, which are inaccessible with storage
ring sources.
However, there is a wide gap between what the current SPring-8 can do andwhat SACLA can do. The SPring-8 upgrade to SPring-8-II should narrow
the gap from the storage ring perspective. We expect the upgraded facility tooffer lower emittance and a higher coherent flux. For many phenomena at
the atomic scale, we know how they happen, but we do not know why they
happen. The most important role of SPring-8-II, combined with SACLA, isthe construction of the basic tool to provide the answers to the many “whys.”
A joint design team for SPring-8-II was established in collaboration between
RIKEN and JASRI. Dr. Hitoshi Tanaka, who is the Division Head of XFEL
R&D, was nominated as the head of the design team. After several monthsof rigorous design work, we are pleased to publish the “Conceptual DesignReport (CDR).” PART-I provides an overview of the light source related
design, including the accelerator, insertion devices, the control system, andradiation safety issues, while PART-II addresses scientific scope, beamline
design, and end-station considerations.
With this CDR, we would like to begin the review process with international
experts. We hope to initiate the next step of the Engineering Design Process
in 2015 FY. We welcome any feedback from machine experts and futureusers of the light source. We would like to inaugurate SPring-8-II in theearly 2020s.

Tetsuya Ishikawa
Director
RIKEN SPring-8 CenterTable of Contents

PART-I: Light Source Development

I-1

Accelerator Design………………………………..………. 1－97

I-2

Light Sources………………………………..……………... 98－133

I-3

Control System………………………………..…………...134－147

I-4 Safety Issues……………………………………..…………148－160

PART-II: Scientific Scope and Beamline Development

II-1 Overview……………………………………..…………......161－165
II-2

Beamline Design………………………………………...166－190

II-3 Detector System………………………………………….191－199

Appendix

PART-I
I-1 Accelerator DesignI-1.1.
I-1.2.
I-1.3.
I-1.4.
I-1.5.
I-1.6.
I-1.7.
I-1.8.
I-1.9.

Overview
Storage Ring Lattice
Beam Dynamics Issues
Magnet System
Vacuum System
RF System
Beam Instrumentation and Instability Feedback
Injector
Storage Ring Lattice Data2

I-1.1. Overview
The following three factors led us to adopt more conservative, lessaggressive accelerator specifications compared with other upgrade plans
under investigation. The first is the target date for restarting user
operations with the upgraded ring, currently scheduled in the early 2020’s.
This schedule allows only five more years to complete design, R&D, andmanufacturing. The second factor is the limited human resources available
for the upgrade project. We have to cover the XFEL and SPring-8 user
operations, the near-term XFEL accelerator improvements, and theSPring-8 upgrade project with our current limited resources, with noadditional headcount planned in the foreseeable future. The third factor isthat no relevant R&D activities have yet been conducted regarding theplanned upgrade. We therefore aim at the electron beam performance ashigh as possible using a conventional technology-based design.
Because this project is not a green field build, but rather an
upgrade of an operating facility, we must also consider the following four
conditions for our accelerator design: (a) maintenance of all the undulator
beamline axes, (b) reuse of the existing machine tunnel, (c) electric powersavings, and (d) a blackout period of one year or less. Taking these
conditions into account, we determined the practical target emittance to bearound 100 pmrad with undulator gaps closed and the target stored-current
to be 100 mA, neither of which is difficult to achieve.
The lattice is a nonidentical five-bend achromat composed of four
longer longitudinal gradient bends and one shorter homogeneous bend
which mainly provides BM radiation in order to reduce the emittance from
~6 to ~0.1 nmrad under the doubly achromatic condition.
For the magnet system, we adopted a separated-function
individual

magnet,primarily

to

avoid

the

risks

associatedwithmanufacture, alignment and operations. The permanent magnet-based
bending magnets and reasonably small bore-radii markedly reduce thepower consumption.
The existing RF system is reused as much as possible to reducelabor and cost for the project. In place of the current injector system, the2

linear accelerator of SACLA is introduced as the injector for the upgradedring to reduce power consumption and to provide a tiny injection beam witha low emittance of less than 1x10-9 mrad. The timing system was configured
to synchronize the beam ejection from SACLA with the specified RF bucketin the ring.
The vacuum system is generally conventional, with a design basedon discrete absorbers without NEG coating. The challenges are minimizing
the size of all the components and utilizing vacuum conditioning without
requiring in-situ baking in the machine tunnel.

3I-1.2. Storage Ring LatticeI-1.2.1. Design Basics
The lattice structure of the present SPring-8 storage ring is of the
double-bend type. The natural emittance is 6.6 nmrad for achromat optics
and 2.4 nmrad for non-achromat optics (with dispersion leakage) at thebeam energy of 8 GeV. It is assumed that the present machine tunnel isreused in the upgrade. The X-ray source points of insertion device
beamlines and beam injection point must be therefore kept unchanged.
Under these constraints, the target emittance has been set to ~100 pmrad
with undulator gaps closed. It is well known that a multi-bend latticeconfiguration is a promising way approaching to an extremely low
emittance of a 100 pmrad range. As a result of investigating several kinds ofmulti-bend lattice we have adopted a five-bend achromatic configuration for
a new lattice, which can be constructed by using conventional magnets andwith feasible strengths of quadrupole and sextupole magnets.
Another method that we introduced for reducing the emittance isto use a bending magnet with a longitudinal gradient [Nagaoka2007]. A
bending magnet in the arc was divided into three segments and thestrength of each segment was optimized to achieve a half of the emittancevalue with a conventional homogeneous dipole field.
To reduce the emittance further, we lower the operation energy
from 8 to 6 GeV since the emittance is proportional to the square of a beamenergy. Though lowering of the operation energy shifts the synchrotron
radiation spectrum to the lower energy regions, this can be compensated byshortening an undulator period-length. Lowering the operation energy also
has a merit that the energy loss by bending magnets is largely reduced andthe damping effect by insertion devices is enhanced. It is expected that thedamping effect due to undulator radiations can reduce an emittance valueof 149 pmrad by about 20 to 30%.
Figure I-1.2.1 shows a unit cell of the five-bend achromat optics. As
seen from this figure, the lattice has a moderate value of dispersion in thearc section for chromaticity correction, and the betatron phase between the4two arc sections are set to be (2n+1)π, where n is an integer, to cancel
non-linear kicks due to sextupoles. (The betatron phase is slightly detuned
from the above values for controlling the amplitude-dependent tune shift).
Though quadrupole strengths are not weak, the natural chromaticity is well
suppressed by adopting an optics design having small betatron functions.
These design considerations guarantee the stability of both on- andoff-momentum electrons. This new type of multi-bend lattice was first
proposed in the ESRF [Farvacque2013, Revol2013]. During the course ofour design work, some concepts of the ESRF design were incorporated andsome were not, and a five-bend achromat lattice was constructed to fit into
the SPring-8 boundary conditions. The maximum field strength of bending,quadrupole and sextupole magnets are 0.953T, 55.4T/m and 2620T/m2,
respectively.

Fig. I-1.2.1: A unit cell of the 5-bend achromat optics. The betatron functionsin the horizontal (βx) and vertical (βy) directions, the dispersion function (ηx)
and the betatron phase difference between the arcs (∆ψx and ∆ψy) are shown.
The blue, green and orange boxes represent bending, quadrupole andsextupole magnets, respectively. Bending magnets are of sector type, andcombined magnets are not used.
I-1.2.2. Long-Straight Section
Since the present SPring-8 storage ring has four long straightsections (LSS's) and the machine tunnel will be reused, the new ring also5has to have LSS's at the same place (by a geometrical reason). For the LSS
we have tentatively chosen a simple lattice structure without sextupole andoctupole magnets as shown in Fig. I-1.2.2. The betatron phase is matched tokeep the high symmetry of the ring for on-momentum electrons. The local
chromaticity of this section is suppressed to (ξx, ξy) = (-0.86, -1.71) not todeteriorate the off-momentum acceptance.

Fig. I-1.2.2: The lattice of LSS. This section is constructed by only
quadrupole magnets to transport the beam under a phase-matched
condition for on-momentum electrons.
I-1.2.3. Injection Section
From the viewpoint of beam injection it is favorable to have ahigh-beta section at the injection point. We then modified the structure of
two unit cells located upstream and downstream of the injection point asshown in Fig. I-1.2.3. Since local modification of the storage ring lattice
breaks its symmetry, we put a constraint on the betatron phase as shown in
the figure to recover the dynamical stability of the beam.
I-1.2.4. Ring Parameters
Twiss parameters for the first quarter of the ring are shown in Fig.
I-1.2.4 and main parameters are summarized in Table I-1.2.1. For
comparison, parameters for the present storage ring are also listed in thistable. The circumference of the new ring is slightly different from the6present one. This is because the distribution of bending angles in the unit
cell is different from the present double-bend cell and we adjusted the unit
cell length so that all of the X-ray source points of insertion device
beamlines come back to their original position within 10 µm. A full detail ofthe lattice structure of is given in Appendix.

Fig. I-1.2.3: The lattice of the injection section. Two unit cells are converted
to realize a high-beta straight at the injection point and to satisfy the
betatron phase matching condition.

Fig. I-1.2.4: Twiss parameters for the first quarter of the ring.

7

Table I-1.2.1: Comparison of machine parameters of the newly designed ringand those of the present ring.
SPring-8-II (New)SPring-8 (Present)

Energy [GeV]68Beam Current [mA]

100

100

Circumference [m]1435.4345

1435.9488

Harmonic Number

2436

2436

RF Frequency [MHz]508.762508.580

Unit Cell Structure

5-Bend Achromat

Double-Bend(w/ Long. Var.)
Natural Emittance [nmrad]

0.149

6.6 (Achromat)
2.4 (Non-Achromat)

Betatron Function (βx, βy)(5.5, 3.0)

at Straight Section [m]Betatron Function(24.4, 5.8) (Achr.)(31.2, 5.0) (NA)

(30.0, 3.0)

same as above(109.14, 42.34)(40.15, 18.35) (Achr.)

at Injection Section [m]Tune (νx, νy)(41.14, 19.35) (NA)Natural Chromaticity( -155, -142)(-117, -47) (NA)(ξx, ξy)Momentum Compaction

3.27e-5

1.46e-4 (Achr.)

FactorRelative Energy Spread [%](-90, -41) (Achr.)

1.60e-4 (NA)
0.093

0.109

Radiation Loss by Bending 2.98

9.12

Magnets [MeV/turn]8

I-1.3. Beam Dynamics Issues
I-1.3.1. Dynamic Aperture and Momentum Acceptance
It is important to enlarge the dynamic aperture and momentum
acceptance so as to achieve smooth commissioning and stable operation of
the upgraded ring. In our design, sextupole magnets are located only in thearc sections and the betatron phase between the two arc sections in the unit
cell are set to be (2n+1)π. Then, dominant effects of non-linear kicks due to
sextupoles can be suppressed under this phase-matched condition. Though
this interleaved-sextupole scheme [Brown1979, Emery1989, Oide1993,
Soutome2008] works to a certain extent, the cancellation is not enough forachieving the required stability. We therefore introduced octupole magnets
as a knob for the control of amplitude-dependent tuneshifts to increase thedegree of freedom for suppressing the nonlinearity, i.e., the first- andsecond-order chromaticities, the amplitude-dependent tune shifts and theexcitation strength of nonlinear resonances due to sextupoles and octupoles.
The tuning knobs we used presently are (a) sextuople and octupole
strengths, (b) betatron phase differences between the arcs, (c) betatrontunes per unit cell and (d) a ring operation point.
Figure I-1.3.1 shows a typical example of the dynamic aperture
after the nonlinear optimization calculated with a symplectic tracking code
CETRA [Schimizu2006]. We also show the result of frequency-map analysis
and the horizontal phase space in Fig. I-1.3.2. The amplitude-dependent
tune shifts are plotted in Fig. I-1.3.3. As discussed later in detail, ahigh-quality beam from SACLA can reduce a coherent injection amplitude
down to ~3mm. The dynamic aperture shown here is thus large enough forthe beam injection presently planned. We note that in Fig. I-1.3.2 there is
an island above the main aperture area around the origin. We evaluate thatsuch an island structure scarcely affects the injection efficiency because theinjected beam emittance is small and electrons are well within the main
aperture area.
As for the off-momentum stability we plot the energy-dependenceof the betatron tune (nonlinear chromaticity) in Fig. I-1.3.4. For the9momentum deviation of less than 2%, the fractional tunes in the horizontal
and vertical directions are below the half-integer line and they do notintersect in the displayed area. The momentum acceptance is then expected
to be around 2%. We calculated the local momentum acceptance withtracking simulations, and the result is shown in Fig. I-1.3.5. The averagevalue is 1.96% for the ring without errors. The nonlinear optimization isstill ongoing for improving the dynamic aperture and momentum
acceptance.

Fig. I-1.3.1: The dynamic aperture for on-momentum electrons calculated at
the injection point where βx = 30m and βy = 3m. The dashed line is for theideal ring without errors, and the solid lines are for the ring with sextupolealignment errors of 25µm in rms with cutoff at 50µm.

Fig. I-1.3.2: The frequency map analysis (left) and the horizontal phasespace (right).
10

Fig. I-1.3.3: The amplitude-dependence of the betatron tune. The
dependence on the horizontal amplitude (left) and on the vertical one (right)
is shown.

Fig. I-1.3.4: The energy-dependence of the betatron tune. The linear
chromaticities are set to (ξx, ξy) = (+0.9, +0.7) in this case.

11

Fig. I-1.3.5: The local momentum acceptance for a quarter of the ring.

Fig. I-1.3.6: The effect of synchrotron oscillation on the dynamic aperture.
The aperture shown by the solid line is for 4-dimensional tracking without
synchrotron oscillation, and dots represent a stable area in 6-dimensional
tracking with synchrotron oscillation.
It is known that a mixing occurs between transverse andlongitudinal oscillations in a low emittance ring with a low momentum
compaction factor of 10-5 order. The mixing source is a path length change
12caused by a transverse deviation in a strong quadrupole and sextupole
magnets. We checked this effect by tracking simulations with RF voltage
switched on. As shown in Fig. I-1.3.6 the significant reduction of the
dynamic aperture is not observed so far. Further investigation to suppress
the mixing effect has been continued.
I-1.3.2. Beam Lifetime
Since the new lattice has a very small natural emittance of 149
pmrad, the beam lifetime is dominated by the Touschek effect. We then plan
to operate the ring at a high coupling ratio of 10% or more to make the beam
lifetime longer. To relax the Touschek effect further and secure stable user
operation, we limit the maximum bunch current at around 0.5 mA. FigureI-1.3.7 shows the estimated Touschek beam lifetime. The lifetime is stillshort even in the multi-bunch filling with a low bunch current showing thatthe top-up injection is indispensable.

Fig. I-1.3.7: The Touschek beam lifetime in a multi-bunch filling mode (1920
bunches) is plotted as a function of the total beam current. The lifetime iscalculated for the two cases with the momentum acceptance of 2% and 3%.
The emittance coupling ratio is set to 10%.

13

The intra-beam scattering (IBS) effect is generally not so large
under the condition with the high coupling ratio and low bunch current. We
checked the IBS effect on the beam emittance and energy spread by using
Bane's formula [Bane2002]. The calculation results show that an incrementof the emittance is only 3.2% and that of the energy spread is 0.5% when thenatural emittance is ε0 = 149 pmrad. The emittance can potentially decrease
to ~100 pmrad when most of undulator gaps are closed. Even in such alower emittance case, evaluated increments of the emittance and the energy
spread are 6.1% and 0.7%, respectively.
I-1.3.3. Effects of Insertion Devices
Since we plan to lower the beam energy from 8 to 6 GeV, whichleads to a shorter undulator period for generating hard X-rays, the effect ofinsertion devices (ID's) on the electron beam becomes larger than thepresent ring. We then estimated the effect of ID's on the tune shift,beta-distortion and the dynamic aperture. The calculations were performed
in a symplectic manner [Forest1992] with 34 planar ID's having thefollowing parameters: period length λ = 18mm, number of period N = 200,
total length L = 3.6m and K value of 2.3. The dynamic aperture shrank toalmost zero in the vertical direction due to a large vertical tune shift of+0.077 and vertical beta-distortion of 8.4% (rms) . We then tried to recover
the dynamic aperture and found that a combination of local beta- and global
tune- corrections is effective for restoration of the dynamic aperture. The
present investigation suggests that a sophisticated scheme to correct the
distortions by ID’s is necessary for a stable operation of the new ring andthe study on developing such a scheme is under progress.
Another effect of ID's is the radiation damping. Figure I-1.3.8
shows the emittance and the relative energy spread as a function of theundulator K-value. In this calculation we assumed that the above 34 ID's
are fully in use at the same K value. The emittance goes down to 100 pmrad
when gaps of all the ID’s are closed at a K-value of 1.34.

14

Fig. I-1.3.8: A change of the emittance ε and the relative energy spread δ asa function of the undulator K value. It is assumed that 34 planar undulators(λ = 18mm, N = 200) are in use at the same K-value. For example, a K valueof 1.34 corresponds to the photon energy of 10keV.
I-1.3.4. Alignment and Field Errors
Since the quadrupole strengths required for the new ring arestronger than the present, we investigated the sensitivity factor of COD forthe misalignment of quadrupole magnets. For the case of randomly aligned
quadrupoles without common girders, we obtain the results shown in Table
I-1.3.1. We see that the sensitivity of the new ring is 1.9 times higher in thehorizontal direction and 3.3 times higher in the vertical direction whencompared to the present ring. For smooth commissioning of the new ring, it
is important to reduce the sensitivity factor as much as one can. We thenplan to introduce common girders [Tanaka1992] as shown in Fig. I-1.3.9. By
using such common girders the sensitivity factor can be reduced by a factorof 1.9 in the horizontal direction and 2.5 in the vertical direction. Based onthese calculations we set the tentative alignment tolerance as follows:•

Girder: better than 75 µm in rms with cutoff at 150 µm•

Magnet on Girder: better than 25 µm in rms with cutoff at 50 µm
15Table I-1.3.1: The sensitivity factor of COD [Tanaka1992] for the case of
randomly aligned quadrupoles without common girders. The σR is the rms
value of misalignment.
SPring-8-II (New)SPring-8 (Present)

ε = 0.149nmrad

ε = 2.4nmrad

47.3

25.0

31.4

9.63

[m-1/2][m-1/2]

Fig. I-1.3.9: Arrangement of common girders in one unit cell. The positionsof BPM's and steering magnets are also shown.
The tolerance of field errors is tentatively set to 5x10-4. This value
was determined by calculating the effects of field errors on the distortion of
optics (tune, beta, emittance and chromaticity) by changing the strength ofbending, quadrupole and sextupole magnets randomly.
I-1.3.5. Beam Injection and Commissioning
I-1.3.5.1. Injection Scheme
As mentioned before the ring has a high-beta injection section
where βx = 30m, and a high-quality beam is injected from SACLA. By
16

controlling the bunch length and peak current of an injected beam and byadopting a suitable design of optics for the beam transport line, we will
suppress the effect of coherent synchrotron radiation and keep theemittance of an injected beam to be less than 560pmrad. A transverse beam
size will then be smaller than σ = 0.12mm. Figure I-1.3.10 shows aschematic of beam injection. The coherent amplitude of the injected beam is
3mm at the injection point. In Fig. I-1.3.11 we show 3σ-contour of theinjected beam on the plot of the dynamic aperture of the ring (Fig. I-1.3.6).
Injected electrons are well within the main aperture area, and as mentioned
in section I-1.3.1, though there exists an island above the main aperture, itwill not affect the injection efficiency unless nearby resonances are strongly
excited and the dynamic aperture is largely deformed. We also note thatsuch an island will not affect the Touschek beam lifetime if nearby
resonances are well suppressed, since after collisions the electrons start tooscillate mainly in the horizontal direction.
The bump orbit is generated with conventional pulsed dipole
magnets.

The magnet arrangement around the injection point is shown in

Fig. I-1.3.12 (See also Fig. I-1.2.3.). The betatron phase difference between
the two bump magnets are set to π so that the bump orbit closes with two
kicks. To put the bump magnets to suitable positions we removed one
segment of the bending magnet located upstream and downstream of theinjection point and locally adjusted the strength of quadrupole and bending
magnets. No sextupole magnet exists inside the bump orbit (linear elementsonly), and hence the bump orbit always closes at injection [Tanaka2005].

Fig. I-1.3.10: A schematic of beam injection. A septum magnet is assumed to
be of in-vacuum type.
17

Fig. I-1.3.11: The dynamic aperture at the injection point (same as Fig.
I-1.3.6) and a typical 3σ-contour (upper half) of the injected beam with σ =0.12mm.

Fig. I-1.3.12: The arrangement of main magnets around the injection point
and an example of a pulsed bump orbit. The "BP1" and "BP2" are the pulsed
bump magnets and a kick angle of 1mrad generates a π-bump orbit shown
here with the height of 4.0mm at the injection point. At the position of bumpmagnets one segment of the bending magnet (indicated by blue boxes) wasremoved.

18

I-1.3.5.2. Commissioning Scenario
From simulations we found that the injected beam cannot be
stored without orbit correction when magnets and girders are aligned withrms errors of 25µm and 75µm, respectively. This is due to the fact that the
dynamic aperture is small and the sensitivity factor is about a few times
larger (Section I-1.3.4.) than the present ring. We hence start the beamcommissioning with on-axis injection. Our scenario of beam commissioning
is described below.
(i) First-Turn Steering with On-Axis Injection: We measure a beam
trajectory with single-pass BPM's and determine the kick angle of the firststeering magnet so that the readout of a downstream BPM becomes zero(within a tolerance). We repeat the same procedure for the next steering
magnet and fix the strength of all steering magnets on by one. Then, after
one turn we can obtain an initial set of steering strengths for the COD
correction.
(ii) Beam Storage: By computer simulations we tested the above scheme
when magnets and girders are aligned with rms errors of 25µm and 75µm,
respectively. The results were promising: by using a set of steeringstrengths obtained with the above procedure, we calculated the dynamic
aperture and found that it is large enough for proceeding to the tuning ofoff-axis injection for accumulation.
(iii) Fine Correction of COD: After the beam storage with off-axis injection,
we can experimentally measure COD, and based on it, we make fine orbit
corrections. At this stage the dynamic aperture will be recovered further.
After we succeed in accumulation, we do machine tuning such as
beta-distortion correction at a relatively low current (less than 20mA) since
the Touschek beam lifetime is expected to be short (see Fig. I-1.3.7) at thisvery early stage of the machine tuning. For improving the beam lifetime,
injection efficiency, emittance and other parameters, we continue themachine tuning step by step.

19

References of I-1.2 & I-1.3[Bane2002] K. L. F. Bane, in Proc. of EPAC2002, p.1443; K. L. F. Bane, et al.,Phys. Rev. ST Accel. Beams 5 (2002) 084403.
[Brown1979] K. L. Brown, IEEE Trans. Nucl. Sci. NS-26 (1979) 3490.
[Emery1989] L. Emery, in Proc. of 1989 IEEE PAC, p.1225.
[Farvacque2013] L. Farvacque, et al., in Proc. of IPAC2014, p.79.
[Forest1992] E. Forest and K. Ohmi, KEK Report 92-14 (1992).
[Nagaoka2007] R. Nagaoka and A. F. Wrulich, Nucl. Instr. and Meth. in

Phys. Res. A575 (2007) 292.
[Oide1993] K. Oide and H. Koiso, Phys. Rev. E47 (1993) 2010.
[Revol2013] J-L. Revol, et al., in Proc. of IPAC2014, p.1140.
[Schimizu2006] Tracking simulations were carried out by using an in-house
symplectic code CETRA; J.Schimizu, et al., in Proc. of 13th Symposium on
Acc. Science and Technology, Osaka (2001), p.80; J.Schimzu, "Workshop
SAD2006", Tsukuba. http://acc-physics.kek.jp/SAD/SAD2006/[Soutome2008] K. Soutome, et al., in Proc. of EPAC2008, p.3149.
[Tanaka1992] H. Tanaka, et al., Nucl. Instr. and Meth. in Phys. Res. A313
(1992) 529.
[Tanaka2005] H. Tanaka, et al., Nucl. Instr. and Meth. in Phys. Res. A539
(2005) 547.

20

I-1.4. Magnet System
I-1.4.1. Overview
Main requirements for the magnet system are listed in Table
I-1.4.1. The five-bend lattice of SPring-8-II is composed of a normal bending
magnet at the center of the cell and four other bending magnets withlongitudinal field gradient. Here, we shall call such a bend with the field
gradient a longitudinal gradient bend, or L.G.B. The new lattice needs not
only 2.5 times more bending magnets than the current SPring-8 storage
ring does, but also more quadrupole and sextupole magnets in between. In
total, 944 quadrupoles, including 64 for long straight sections, and 352
sextupoles are necessary in addition to other magnets such as octupoles andsteerings. On the other hand, it is a common issue in designing extremely
small emittance rings that integrated field strengths of the multi-polemagnets become higher. It is the case for us, too. Although a longer magnet
help mitigate a magnetic field of each component, it is necessary to consider
a trade-off between the peak magnetic field and the packing factor. Thus,
the lattice has been designed with the magnet settings as shown in Table
I-1.4.1 so that we can make sure that the field gradients are available byexisting technologies.
Table I-1.4.1: Main requirements for magnets.
DipoleQuadrupole

Sextupole

4

20

8

44

176

880+64352

Max. field

0.953 T

0.775 T

55.4 T/m

2,620 T/m2

Gap/Bore dia. [mm]

25

25

34

36

GFR [mm]

±12

±12

±12

±12

5 x 10-4

5 x 10-4

5 x 10-4

5 x 10-4

Norm. B.

L.G.B.

#/normal cell

1

Total # magnets

Field error
between magnets

Norm. B.: normal bending magnet.

L.G.B.: Longitudinal gradient bend.

GFR: Good field region where the integrated field deviation is within 10-3.

21

Good field regions for the integrated field deviation of within 10-3
are all assumed to be ±12 mm, and integrated field errors between magnets
are assumed to be less than 5 x 10-4. Independent requirements for themagnets may be further discussed in detail.
I-1.4.2. Dipole Magnet
Dipole magnets are designed as shown in Fig. I-1.4.1(a)(b), and the
major specifications are summarized in Table I-1.4.2. In order to reduce
power consumption, we plan to apply permanent magnet to dipole magnets[Chavanne2014]. Both the normal and the longitudinal gradient bending
magnets are C-shaped sector type with a gap of 25 mm. The C-type ischosen because we intend to install and de-install, if necessary, vacuum
chambers without moving a gap of the permanent magnet. Instead thebending magnets will be horizontally moved in and out without interferingwith vacuum chambers.
Due to a hybrid structure consisting of permanent magnet andirons, a magnetic field distribution on beam axis is smoothed out even if
there is inhomogeneous magnetization of permanent magnet pieces.
Further, since the permanent magnet pieces are placed far from the beamaxis, a degradation of the magnetization by radiation is supposed to besignificantly suppressed. Another notable feature is that "outer plates" will
be embedded with the magnet so that the magnetic field on beam axis canbe adjusted by changing the outer plate positions. This helps obtain thematching between the magnetic field and the nominal electron energy at
the initial installation, and also change the operating energy for somespecific purposes. The energy tuning range by the outer plates can be morethan 10 % [Watanabe2014].
Fringe fields in the longitudinal axis need to be suppressed as low
as possible for the magnets to fit in the designed lattice. The gap of thebending magnets, therefore, is fixed at 25 mm for both normal andlongitudinal gradient bends. The field gradient of L.G.B. required by thelattice is produced by the gradients of permanent magnet distribution and22

outer plate positions. The magnetic resistance along the longitudinal axis isenhanced by spacing between the magnets as shown in Fig. I-1.4.1.
Detailed

designsofthepermanent

magnets

are

under

development. As an example, since the temperature coefficient of remanent
induction for Neodymium permanent magnet is -0.12 %/K, a temperaturedrift of ambient air will distort the lattice functions and dynamic apertures.
Thus, the magnet temperature has to be stabilized within a fraction ofdegree, which we assume is feasible.
Table I-1.4.2: Major specifications of bending magnets.
Normal bend

L.G.B. (2 types)

Magnetic field [T]

0.953

0.166, 0.296, 0.582 / 0.221, 0.395, 0.775

Effective length [m]0.420.7, 0.7, 0.35

Gap [mm]25

25

Fig. I-1.4.1: Normal and longitudinal gradient bending magnets. (a) Cross
section of normal bending magnet. (b) Longitudinal gradient bend (birdview).
23

I-1.4.3. Quadrupole, Sextupole, and Octupole Magnets
Quadrupole magnets are designed as shown in Fig. I-1.4.2 andTable I-1.4.3. All quadrupoles have the same cross sectional design, but aregrouped by the longitudinal lengths into 7 groups (Leff = 0.20, 0.24, 0.30,
0.50, 0.55, 0.65, and 0.70 m). A bore diameter of 34 mm is defined by the
maximum field gradient, 56 T/m, and the relatively small diameter is alsopreferred from the viewpoints of power consumption and fringe field along
the longitudinal axis.
A coil distribution is one of key issues in the design. As a highpacking factor is the most challenging in the design of extremely small
emittance ring, magnets ought to be compact longitudinally, yet the peakfield should not exceed the maximum value we set. Further, power
consumption should be kept as low as possible. Considering these factors, acoil distribution is designed as indicated in Fig. I-1.4.2. In the design, coils
stick out of the effective length of magnet by just 10.5 mm in each side.
Since the quadrupole magnet will be divided into four segments inan assembly and when vacuum chambers will be installed/de-installed.
Repeatability of physical mounting of the segments has to be secured[Ohnishi1996]. For the purpose, the return yoke is made slightly thicker
than it is needed in terms of magnetic saturation.
Sextupole and octupole magnets are also designed (see Figs. I-1.4.3,
I-1.4.4, and Table I-1.4.3). Bore diameters of sextupoles and octupoles arerespectively 36 and 50 mm, which are beneficial to small fringe fields andlow power consumption. Coil distributions and yoke thicknesses are
determined as quadrupoles. For sextupoles, coils stick out of the effective
length by 14.0 mm in each side, while for octupoles, it is only 1.0 mm.
Detailed design, including shimming and coil distributions, will be
confirmed by taking interference with vacuum chambers and power
consumptions into account.

24

Fig. I-1.4.2: Cross section of quadrupole magnets. Transverse dimensions andcoil distributions are to be optimized for matching with vacuum chambers(same for sexutpoles and octupoles).

Table I-1.4.3: Major specifications of quadrupoles and sextupoles.
Quadrupole

SextupoleMax. Field gradient [T/m, T/m2]

55.4

2,620

Effective length [mm]200 - 700

180, 300

Pole length [mm]181 - 681

168, 288

Total length incl. coil [mm]221 - 721

208, 328

Bore diameter [mm]3436Max. current [A]

335

250

# turns/pole

21

9

25

Fig. I-1.4.3: Cross section of sexupole magnets.

Fig. I-1.4.4: Cross section of octupole magnets.

26

I-1.4.4. Steering Magnet
Like other magnets, a steering magnet needs to be designed so thatit can fit in the lattice. In order to spare a space, the magnet has bothhorizontal and vertical kicks in a single segment as shown in Fig.I-1.4.5.
Main parameters are listed in Table I-1.4.4. The kick angle is evaluated tobe 0.12 and 0.06 mrad in each axis. Detailed specifications of steering coils
will be further discussed from the viewpoints of commissioning scenario andspacing.

Fig. I-1.4.5: Steering magnet.
Table I-1.4.4: Main parameters for steering magnets.
Kick direction

Horizontal

VerticalGap [mm]56

90

Integrated field [T-m]

2.45 x 10-3

1.32 x 10-3

Kick angle [mrad]

0.12

0.06

Effective length [mm]

83

105

Pole length [mm]5050Total length incl. coil [mm]9595I-1.4.5. Power Supply
The power supplies and auxiliary infrastructures will be prepared,
considering (1) to make full use of existing resources, (2) to save energy and27space, and yet (3) to achieve high performances. We plan to reuse cables andcooling system with minimum changings, while all power supplies that havebeen used for the current SPring-8 storage ring since 1990's are to be
replaced with new technologies for meeting required specifications. Aiming
at the power factor of 99 % and the power efficiency of 94 %, new power
supplies will be equipped with the switching technology. It will be beneficial
not only to save energy, but also to reduce housing sizes as well as loads tocooling system.
It has been observed at SPring-8 that the output currents of power
supplies drift in hours to months due to ambient air temperature shift. By
introducing a feedback, we expect to suppress the drift and ripple within 10
ppm.
Similar to the current SPring-8 storage ring, magnets are divided
into families (see Table I-1.4.5); 10 for quadrupoles at normal sections, 16
for quadrupoles at long straight sections (indicated in 'lss' in the table), 8
each for sextupoles at normal and long straight sections, and 3 for octupoles.
Since the new lattice has two matching cells for a high-βx injection point,
another 20 families of quadrupoles will be added (indicated in 'is' in thetable). Each family is driven by a single power supply, and is possibly
accompanied with auxiliary supplies where independent tuning isnecessary.
To apply permanent magnets for dipoles will result in the
reduction of power consumption; currently dipole magnets of the SPring-8
storage ring consume about 1.1 MW. The smaller bore diameters ofquadrupole and sextupole magnets than the present ones also serves toreduce the power. Thus, the total power consumption for the new magnet
settings is expected to be reduced by factor of two through optimizations of
the magnet designs.
Table I-1.4.5: List of power supplies.
Q

Qis

Qlss

S

Sis

O

Number of PS

10

20

16

8

8

3

Maximum current [A]

360

360

190

265

265

110

28

I-1.4.6. Alignment
In order to secure enough dynamic aperture, magnet alignment is
one of key issues in designing the new ring. The required goal of thealignment is 25 µm (rms) on a girder, and 75 µm (rms) between girders (seeSection I-1.3.4.). Since better alignment helps obtain smaller COD andlarger apertures, even better alignment, such as 10 µm (rms) on a girder,
would be preferred. For the purpose, we plan to apply a combination ofseveral alignment schemes based on laser schemes [Zhang2012] and avibration wire method for the magnet alignment.
The vibrating wire method can be advantageous over othercommon schemes in a sense that there is no need to transfer a magnetic
center to some fiducial point prior to the alignment [Jain2008,
Temnykh1997, Fukami2014]; each magnet can be aligned while a wire
directly senses the deviation of the magnetic center. We expect that magnets
will be aligned within 20 µm (rms) or less by the wire method at the initialinstallation of magnets, then long-term drifts after the installation will bemonitored by a laser that tracks fiducial points. The alignment between
girders will be carried out by laser trackers as well [Zhang1995, Matsui1995,
Tsumaki2002].
In a practical alignment procedure, not only to precisely measure amagnetic center but also to move a magnet with a good resolution and toobtain the good repeatability of magnet segments are essential. For that,mechanical structures such as a thickness of yokes, a stage for the fiducial
point, and a magnetic support are carefully designed, taking these issues
into account.

29References[Chavanne2014] J. Chavanne and G.Le Bec, Proc. of IPAC'14, Dresden,
Germany, TUZB01, p.968.
[Fukami2014] K. Fukami et al., Proc. of IPAC'14, Dresden, Germany,
MOPRO081, p.277.
[Jain2008] A. Jain, et al., Proc. of the International Workshop on
Accelerator Alignment 2008 (IWAA2008), Tsukuba, Japan, p.1.
[Matsui1995] S. Matsui, et al., Proc. of the International Workshop on
Accelerator Alignment 1995 (IWAA1995), Tsukuba, Japan (1995), p.174.
[Ohnishi1996] J. Ohnishi, et al., IEEE Transactions on Magnetics 32 (1996)
3069.
[Temnykh1997] A. Temnykh, et al., Nucl. Instrum. Meth. A 399 (1997) 185.
[Tsumaki2002] K. Tsumaki et al., Proc. of the International Workshop on
Accelerator Alignment 2002 (IWAA2002), Hyogo, Japan, p.210.
[Watanabe2014] T. Watanabe, et al., Proc. of IPAC'14, Dresden, Germany,
TUPRO092, p.1253.
[Zhang1995] C. Zhang, et al., Proc. of the International Workshop on
Accelerator Alignment 1995 (IWAA1995), Tsukuba, Japan (1995), p.185.
[Zhang2012] C. Zhang, et al., Proc. of the International Workshop on
Accelerator Alignment 2012 (IWAA2012), Batavia, U.S. (2012).

30

I-1.5. Vacuum System
I-1.5.1. Design Concept
The design of a proper vacuum system, satisfying a lattice designfor the targeted high-coherence ring of the upgrade project, holds the key tothe success, which provides stable beam operation and highly brilliant
synchrotron light. Several technical methods are applied to the new latticedesign, which significantly affect the ring vacuum design. Not only
arrangement of a large number of multi-pole magnets but also increase in
number of photon absorbers, resulting from the multi-bend achromat
configuration, brings severe space constraints. The constraints also have asubstantial impact on designing vacuum components, such as bellows andgate valves. The extremely strong focusing of the magnets requires theminiaturization of vacuum chamber, which correlates the vacuum pumping
design due to the low vacuum conductance. Furthermore, two otherparticular factors should be considered as well as the above requirements.
One is the time constraints issue. All the replacement work, namely
dismantlement,

installation,alignment

andevacuation

toultra-high-vacuum, should be done within a year followed by the beamcommssioning and making the vacuum system fully outgassed as soon as
possible. The other is that the new light extraction design for bending
magnet beamlines should fit into the existing SPring-8 beamlines, althoughboth light source point and light axis angle shall be naturally changed.
Based on the above circumstances, some design concepts are presented.
Baking strategy
It is apparently impossible to carry out the in-situ baking and thefollowing non-evaporable getter (NEG) activation throughout a range ofthe ring after the installation because of the extremely severe time
restrictions. On the other hand, even if the off-line baking is carried out
in advance, to expose the vacuum surface to the air after the
installation for vacuum connections and followed by conducting only
the NEG activation without in-situ baking would be unreasonable.
Hence, we plan to establish a highly performed and reasonable31

procedure to realize the ultra-high vacuum (UHV) system in the tunnel
within a limited amount of time, by completing the baking and NEG
activation of a long integral chamber off-line before the black out withenough time to spare, and just by transporting and installing in thering with UHV.


Pumping strategy
Vacuum pumping will be mainly provided by discrete NEG cartridges
integrated into a ConFlat Flange (CF), some of which combine a sputter
ion pump (SIP) for the evacuation of CH4 and noble gases.

Vacuum chamber strategy
The main chambers will be made from extruded aluminum alloy.
Bending radiation is going to be intercepted by only discrete photon
absorbers without directly irradiating any inner wall of vacuum
chambers. As a result, an antechamber is necessary.

Light Extraction strategy
The main prerequisite is to continue utilizing all the existing beamlines
without making any alterations on shielding concrete structure of theexisting SPring-8 storage ring tunnel.



Maintenance strategy
Once a serious failure happens to a vacuum component, we plan toexchange entirely the troubled vacuum section delimited by transport
gate valves (TGV; see section I-1.5.5.4). On the other hand, in case of aminor failure, ultra-high vacuum after the replacement work will beachieved under pure N2 purge, followed by non-baking or partial bakingof SIP and NEG re-activation.

I-1.5.2. Vacuum Chamber
I-1.5.2.1. Chamber Materials
Extruded aluminum alloy chamber (equivalent to A6063-T5),
which has been used successfully at SPring-8, will be adopted because of thesuperior properties listed below:
32

・Easy forming of complex cross section by the extrusion;
・Low temperature rise caused by the wake field because of the highelectrical conductivity;
・High thermal conductivity resulting in relatively uniform temperature
distribution against heat generation from the baking and the wake field;
・Low radio-activation.
I-1.5.2.2. Structural Design
Straight section chambers
The cross-section of the straight section chamber (SSC) is shown in Fig.
I-1.5.1. The beam chamber (30 mm × 16 mm racetrack), which corresponds
to the multi-pole magnets with a bore radius of 17 mm, is connected with anantechamber through a slot of 5 mm width for transporting the photon
beam. The antechamber also works to enhance the vacuum conductance.
The chamber has a cooling water channel to remove the heat generated by
the wake field and the scattering light. The FEM analysis shows the
maximum equivalent stress caused by the atmospheric pressure is well
below the yield stress of 125 MPa at 150°C (Fig. I-1.5.2).

Fig. I-1.5.1: Extruded cross-section of the straight section chamber.

Fig. I-1.5.2: FEM structural analysis result of the straight section chamber.
33

Bending section chamber
The bending section chamber (BSC), whose cross-section is shownin Fig. I-1.5.3, will be manufactured by bending after the extrusion. The
antechamber will be equipped with vacuum ports for inserting a photon
absorber and the NEG pump.

Fig. I-1.5.3: Extruded cross-section of the bending section chamber.
Impedance of vacuum chambers
RF shielding such as a RF-finger or a RF-contact should be set
inside bellows, gate valves and gaps between flanges to smooth thecross-section change for low impedance. When the RF-finger and the
RF-contact are made of beryllium copper or stainless steal, they should becoated with silver or something high electrical conductance to decrease
impedance. The change in the shape of the beam aperture should bedesigned to be a gentle tapered transition structure.
I-1.5.2.3. Manufacturing and Treatment
The extruded chambers will be machined to prepare grooves of
weld joint at both ends and some ports for the vacuum pumps and gauges.
The detailed cleaning procedure for the UHV will be decided based on the
results of R&D. To realize our baking strategy mentioned in section I-1.5.1,
a vacuum unit cell would be divided into two sections, as shown in Fig.
I-1.5.4, each of which has a half-cell length of about 12 m and mainly
consists of SSCs and BSCs. To handle the severe space constraints issue,each section shall be a welded integral structure.

34

I-1.5.2.4. Evacuation and Baking with NEG Activation
After all vacuum components and two TGVs are attached, the 12
m-long integral chamber (LIC) will be evacuated to UHV individually by
off-line baking and the following NEG activation in advance.

Fig. I-1.5.4: Distribution map of photon absorbers and gate valves for aunit cell.
35

I-1.5.2.5. Installation and Alignment
After the baking, rough alignment of the LIC will be conducted
off-line. Then, we are going to transport it to the ring tunnel under UHV
and align it precisely inside the quadrupole and sextupole magnets whose
upper half is removed. After the positional relation with the magnets is
confirmed, C-type bending magnets will be installed from a lateral direction.
As there are two LICs per unit cell, we are going to install a short joining
chamber including the BM3 chamber to connect the LICs through the TGVs.
We also install another joining chamber, which connects the LIC to the ID
chamber. After the connections are finished in the tunnel, the joining
chambers will be baked locally when necessary. Then, by opening the TGVs,
the total vacuum system for a unit cell will be ready without exposing thevacuum surface to the air. In this way, the LICs will not be baked in thetunnel.
I-1.5.3. Photon Absorbers
I-1.5.3.1. Distribution
The total radiation power from bending magnets is 297.9 kW (6.77
kW per unit cell), all of which but that to photon beamlines will be handled
only by three types of discrete photon absorbers. As shown in Fig. I-1.5.4, aunit cell has two crotches (CR), eight absorbers (AB) and fivebending-chamber-absorbers (BCA). CR and AB are arranged in position
between the multi-pole magnets so that the radiation may not irradiate
unwanted vacuum chamber walls. The difference between CR and AB is thepresence or absence of the light extraction window. Figure I-1.5.5 (up) shows
as an example the ray trace of BM1 radiation. CR1, laid out between Q04
and SD1, has a window for the extraction of ID radiation with a certain
sweep angle. Radiation spread cut by AB1 will go through to the next
bending section chamber (BSC) as a leakage radiation. As shown in Fig.
I-1.5.5 (down), a continued diagram from the previous one, the leakage
radiation from BM1 would spread very widely resulting in increase of theantechamber width. As a countermeasure, a supplementary absorber of36

BCA will be inserted into the downstream end of each BSC, which
contributes to the miniaturization of the antechamber in a straight section
chamber (SSC). Following this approach, the distance between the electron
orbit and the end of antechamber can be kept to be 60 mm, as shown in Fig.
I-1.5.1. The characters from A to C and from D to F shown in Fig. I-1.5.5
indicate longitudinally varying magnetic field regions in BM1 and BM2,
respectively, as specified in Table I-1.4.2.

Fig. I-1.5.5: Ray trace of the BM1 (up) and the BM2 (down) radiations.
I-1.5.3.2. Material
The photon absorbers are going to be manufactured using Glidcop
or oxygen free copper (OFC) as the situation demands based on a FEM
analysis result. Basically, OFC will be used insofar as the maximum
temperature is less than 200°C and in addition the maximum equivalentstress does not exceed yield point. For every case in excess of these criteria,
Glidcop will be selected. As to the thermal limitation of a heat-absorbing
37

body made of Glidcop, we plan to use an evaluation method based on a
low-cycle

fatiguelife

prediction

by

using

elasto-plastic

analysis[Takahashi2008]. And the allowable number of cycles to failure will be set to
be more than 10,000 cycles taking into consideration the top-up operation.
I-1.5.3.3. Heat Load
Table I-1.5.1 shows power distributions on the photon absorbers fora unit cell. When compared to the existing SPring-8 case, the change of thebeam energy from 8 GeV to 6 GeV should decrease the heat load, for the TP
and PPD are proportional to the fourth and fifth power of the beam energy,
respectively. As a result, the total absorbed power decreases significantly,
whereas the peak power density doesn’t so remarkably decrease, mainly
because the distance from the light source point becomes shorter. FigureI-1.5.6 shows spatial distributions of effective power density of AB5 as a
typical example. AB5 is irradiated with three different light sources,
namely BM4_E, BM4_F and BM3, as shown in Fig. I-1.5.7.
Table I-1.5.1: Power distributions on the photon absorbers for a unit cell. TP
and PPD stand for the total absorbed power and the peak power density,
respectively. PPD is calculated under normal incidence angle. The numbers
specified in columns of “SPring-8” show the maximum values for a crotch of
the existing SPring-8 storage ring for reference.

38

Fig. I-1.5.6: Spatial distributions of effective power density of AB5. Threelines correspond to each light source, namely BM4_E, BM4_F and BM3,
respectively.

Fig. I-1.5.7: Ray trace of the BM4 radiations.
I-1.5.3.4. Structure and Analysis
The photon absorbers are supposed to be attached to vacuum
chambers with flanged connection (CF2 3�4 or CF4 1�2 ). Considering thenecessity of accurate positioning, another type of flanged connection with ametal seal such as Helicoflex is also a candidate. Although the photon

absorbers are roughly divided into two types, namely CR&AB type and BCA
39

type, downsizing design with proper cooling ability is a common target.
Most of CR&AB can be installed horizontally into its exclusive chamber,which has the same cross section as SSC, whereas AB1 and AB5 should bevertically so as to prevent the interference with the light extraction line. On
the other hand, BCA must be installed into BSC so that it is necessary to
make the thickness of heat-absorbing body thin enough. In addition, BCA4
needs to have a beam extraction window, which would introduce BM3
radiation with a certain sweep angle to the existing SPring-8 BM2 beamline(B2-BL). Basically simple grazing angle configuration for reducing effective
power density will be applied to the irradiated area for either type. As forthe cooling water design, we have to keep it in mind that a cooling channel
should not be arranged at the just downstream side of the irradiated area inthe same plane as the electron orbit in order to avoid corrosion of copper
resulting from the interaction between cooling water and synchrotron
radiation. To keep the velocity of cooling water as low as possible within thethermally allowable range is also important as a countermeasure to severe
vibration issue. Figure I-1.5.8 shows a schematic drawing of BCA4 as an
example. Depending on the results of thermal and thermo-mechanicalanalyses by using FEM, which are ongoing, the irradiated area and thecooling channel configurations are to be optimized.

Fig. I-1.5.8: External (left) and cross-sectional (right) views of schematic drawings of
BCA4.

40

I-1.5.4. Pressure in the Vacuum System
I-1.5.4.1. Beam Lifetime
In case of an ultra low emittance ring, not the gas scattering
lifetime (τg) decided by vacuum pressure but the Touschek lifetime (τT) byintrabeam scattering naturally dominates the beam lifetime (τtotal). The
correlation among them is given by equation (I-1.5.1),
1=

𝜏𝑡𝑜𝑡𝑎𝑙

1

𝜏𝑔

+

1

(I-1.5.1)

𝜏𝑇

As for the new ring, the τT is estimated to be about as small as 12 h. As theτtotal is aimed to be kept at 10 h of a 20 % reduction from the 12 h, we set atargeted τg to 60 h. As is well known, the τg depends on three collision
scattering processes, namely Rutherford scattering (τR), Bremsstrahlung(τB) and Möller scattering (τM). τg are written as
1

𝜏𝑔

=

1

𝜏𝑅

+

1

𝜏𝐵

+

1

𝜏𝑀

= 𝑐𝑁 ∑𝑖�𝜎𝑅𝑖 + 𝜎𝐵𝑖 + 𝑍𝑖 𝜎𝑀 �(I-1.5.2)

where c is a speed of light, N is a number of molecules, σj (j = Ri, Bi, Mi) is across-section of each scattering process, and 𝑍𝑖 is atomic number ofmolecules species i. In the case of narrow vacuum chamber of inneraperture, Rutherford scattering becomes dominant. Cross-section of
Rutherford scattering is𝜎𝑅𝑖 =4𝜋𝑍𝑖2 𝑟𝑒2

(I-1.5.3)

𝛾2 𝜃𝑐2where re = 2.818×10-15 and γ is a Lorentz factor. In case of elliptical crosssection as inner shape of vacuum chamber, 1/θc2 is
1

2 =

𝜃𝑐

〈𝛽𝑥 〉𝛽𝑥𝑚
2𝑎𝑥2+〈𝛽𝑥 〉𝛽𝑥𝑚(I-1.5.4)

2
2𝑎𝑦where <βx,y>, 𝑎𝑥,𝑦 , βmx,y (x; horizontal, y; vertical) are an average ofbetatron function, the half-size of the minimum aperture and the betatronfunction at that aperture, respectively.

Table I-1.5.2 shows parameters for the calculation of τg based on
the cross section of the vacuum chamber shown in Fig. I-1.5.1. It should be41

mnoted that 𝑎𝑦 and β y naturally vary depending on the in-vacuum ID gap.
Figure I-1.5.9 shows the expected relationship between the gas scattering

lifetime 𝜏𝑔𝑖 and the partial pressure 𝑃𝑖 of typical residual gases when ID

gap is fully opened. The products of 𝜏𝑔𝑖 (h) and partial pressure 𝑃𝑖 (Pa) for
typical residual gases are constant values. These values are shown in Table
I-1.5.3 for the calculation of beam lifetime in the section of I-1.5.4.2.3.

Table I-1.5.2: Parameters of the new ring for the calculation of gas
scattering lifetime when the conditions of ID gap are fully opened and 5 mm,
respectively.
Parameter

ID gap: fully opened

ID gap: 5 mm

Min. vertical aperture (half-size); 𝑎𝑦 [m]0.008

0.0025

Min. horizontal aperture (half-size); 𝑎𝑥 [m]

0.015

0.015

βmy [m] at 𝑎𝑦

27

4.08

βmx [m] at 𝑎𝑥

30

30

Average <βy> [m]14

14

Average <βx> [m]

7

7

γc/γ

0.02

0.02

Fig. I-1.5.9: Relationship between the gas scattering lifetime and the partialpressure of typical residual gases by the parameters (Table I-1.5.2) of the
new ring in the case of fully opened ID gap.
42

Table I-1.5.3: The products of gas scattering lifetime 𝜏𝑔𝑖 (h) and partialpressure (𝑃𝑖 (Pa)) for typical residual gases at 6 GeV.
𝜏𝑔𝑖 × 𝑃𝑖 (Pa·h)

H2

CH4

H2O

CO

CO2

ID gap: fully opened

1.29 × 10-4

1.08 × 10-5

7.15 × 10-6

4.78 × 10-6

2.95 × 10-6

ID gap: 5mm

1.14 × 10-4

9.37 × 10-6

6.15 × 10-6

4.12 × 10-6

2.55 × 10-6

I-1.5.4.2. Pressure Calculation
I-1.5.4.2.1. Evaluation of Outgassing Rate
It is important to evaluate the outgassing rate based on the photon
stimulated desorption (PSD) for a pressure calculation. The yield coefficient(η), indicating the intensity of PSD, depends on the material and theaccumulated photon dose. As the main materials for the new ring are used
to be same as the extruded aluminum alloy and Glidcop used in the existing
SPring-8 storage ring, measurement results at the SPring-8 [Oishi2014] areapplicable to the pressure calculation of the new ring. Using the data on the
total pressure, partial pressure and pumping speed obtained from SPring-8
experience, we could evaluate the η related to the photon dose. It is
assumed that the pumping speeds for NEG and SIP are the initial value
from off-line measurements and the nominal value, respectively. FigureI-1.5.10 shows the PSD yield coefficients ( 𝜂𝑖 ) as a function of theaccumulated photon dose. The expressions of the ηi for each residual
gaseous species are also shown in Fig. I-1.5.10. 𝜂𝑖 is known to decrease

with photon dose as D-a. In our case of existing SPring-8, exponent “a” isbetween 0.81 and 0.91 for a variety of gaseous species. These values are
consistent with the previous reports ≤ (2/3a ≤ 1) [Anashin1998],[Gröbner1994]. 𝜂𝑖 of 6 GeV new ring are assumed to be similar with that of

the existing SPring-8. The estimated values of 𝜂𝑖 to be used in thecalculation of the pressure distribution in the section of I-1.5.4.2.3 are listed

in Table I-1.5.4. The photon dose is converted into the accumulated beamdose (A·h) of the 6 GeV new ring. We paid attention to H2 and CO having
large outgassing rate, CO2 having short gas scattering lifetime, and CH443being not able to be evacuated by NEG. The partial pressure of H2O after
baking is so small that we neglect H2O.
On the other hand, the outgassing rate based on the thermal
desorption (TD) was set to 6.7 × 10-9 (Pa·m3/s/m2), which is same as thedesign value for the existing SPring-8 storage ring [Ohkuma1994]. And thecomposition ratios of H2 and CO are assumed to be 80% and 20%,
respectively.

Fig. I-1.5.10: PSD yield coefficients (𝜂𝑖 : 𝑖 = H2, CH4, CO, CO2) as a functionof accumulated photon dose.

Table I-1.5.4: Yield coefficients for typical residual gases used in thecalculation of pressure distribution in section I-1.5.4.2.3.
Beam dose (A·h)

ηH2

ηCH4

ηCO

ηCO2

20

8.6 × 10-5

9.7 × 10-7

1.0 × 10-5

1.8 × 10-6100

2.3 × 10-5

2.5 × 10-7

2.6 × 10-6

4.2 × 10-7400

7.6 × 10-6

7.5 × 10-8

7.7 × 10-7

1.2 × 10-7

44

I-1.5.4.2.2. Arrangement of Vacuum Pumps
A suitable arrangement of pumping systems was examined so as tokeep the τg at about 60 h at 100 mA after a beam dose of 400 A·h. This inturn means that it will be necessary to operate at a lower beam current oroperate with a shorter τg in the early stage of the commissioning. NEG and
SIP will be arranged at all the CR and AB where the PSD should be excited.
Four different pumping systems with a combination of pumping speed foreach residual gas are arranged, as specified in Table I-1.5.5. Expecting the
SIP to evacuate only CH4, we selected relatively small-sized one. Although
we have a plan to insert distributed ion pump (DIP) into the BSC near the
BCA to increase evacuating ability of CH4, the pumping speed of DIP was
not taken into account. We are ready to re-activate all the NEG cartridges
when the beam dose reaches about 20 A·h in order to restore the pumping
ability when the deterioration of the effective pumping speed is recognizedbeyond the accumulated beam dose of 20 A·h.
Conductance of each vacuum chamber calculated for each residual
gas is used for the pressure calculation.
Table I-1.5.5: Assumed pumping speed and position.
Pumping

H2

CH4

CO

CO2

system(m3/s)(m3/s)(m3/s)(m3/s)

1

0.36

0.01

0.17

0.10

Position
CR1, 2, AB1, 2, 3, 4, 6,
7, BCA4

2

0.12

0.01

0.05

0.03

AB8, BCA1, 3

3

0.47

0.01

0.22

0.13

AB5

4

0.23

0.01

0.1

0.06

BCA2, 5I-1.5.4.2.3. Pressure Distribution and Beam Lifetime
Using the outgassing rate, pumping speed and conductancementioned above, partial pressures for each residual gas were calculated.
Figures I-1.5.11 and I-1.5.12 show the expected pressure distributions perunit cell except ID section at 100 mA when the beam doses reach 20 A·h and45

400 A·h, respectively. The average pressures of each gaseous species are PH2= 1.2 × 10-6 (1.3 × 10-7), PCH4 = 2.9 × 10-7 (2.3 × 10-8), PCO = 4.1 × 10-7 (5.3 ×
10-8) and PCO2 = 8.7 × 10-8 (5.8 × 10-9) Pa by the photon stimulated
desorption and the thermal desorption with the stored beam current of 100
mA at beam dose of 20 A·h (400 A·h). The each gas scattering lifetime isestimated from the average pressure of each gaseous species using therelationship of Table I-1.5.3. Then, the overall τg in a unit cell was
calculated using the following equation,
1

𝜏𝑔

= ∑𝑖

1

𝜏𝑔𝑖

.

i= H2, CH4, CO and CO2(I-1.5.5)

In this way, at a beam dose of 20 A·h, the τg was estimated at 6.6 h
resulting in the τtotal of 4.3 h derived from equation (I-1.5.1). Even if ID gap
is closed to 5 mm, they just slightly decrease to 5.7 h and 3.8 h, respectively.
Typical results are summarized in Table I-1.5.6 and all the results
are shown in Fig. I-1.5.13. As a matter of course, the lifetime increases withincreasing beam dose. When the beam dose reaches 400 A·h, the τg willreach 62 h, which exceeds our target of 60 h. It will indeed decrease to 54 hwhen ID gap is closed to 5 mm, but this reduction has little influence on theτtotal. Furthermore, when the beam dose reaches 1000 A·h, as the τg willreach about 100 h even at an ID gap of 5 mm, which is more than seventimes the τT of 12 h, it can be said that the τg has no impact on the τtotal.
Consequently, it is confirmed that the targeted τg can be achieved by theassumed vacuum pumping system.

46

Fig. I-1.5.11: Expected pressure distribution in a unit cell with the stored
beam current of 100 mA at beam dose of 20 A·h.

Fig. I-1.5.12: Expected pressure distribution in a unit cell with the stored
beam current of 100 mA at beam dose of 400 A·h.
47

Fig. I-1.5.13: The gas scattering lifetime and the total beam lifetime withthe stored beam current of 100 mA as a function of the beam dose. Averagepressure at 100 mA is also shown here.
Table I-1.5.6: Calculation results of the gas scattering lifetime and the beam
lifetime with the average pressure for beam doses of 20, 100 and 400 A·h.
GFO means that ID gap is fully opened.

τtotal (h)τg (h)

Beam dose

Average(A·h)

pressure (Pa)

ID gapID gapID gapID gap

GFO

5 mm

GFO

5 mm

20

2.0 × 10-6

6.6

5.7

4.3

3.8

100

5.5 × 10-7

24

21

8.0

7.6

400

2.1 × 10-7

62

54

10

9.8

48

I-1.5.5. Vacuum Components
I-1.5.5.1. UHV Pump
We are going to select discrete NEG cartridges integrated into CF
flanges as main UHV pumps to increase maintainability. Small-sized SIPs
will also be installed for the evacuation of CH4, which cannot be evacuatedby NEG pumps. And, insertion of the DIP element into the BSC is underconsideration. Distributed NEG strips, which have been used in the existing
SPring-8 storage ring, could not be installed inside the antechamberbecause they would interfere with synchrotron radiation due to thesmallness of the chamber. The vacuum conductance of the chamber is sosmall that small-sized NEG cartridges will be installed nearby every photon
absorbers. Half a year after the first commissioning stage, we plan to
conduct re-activation of all the NEG cartridges in a shortest possible period
within a month. In these days, NEG coating chamber has come to be
employed in some synchrotron radiation facilities [Herbeaux2008], because
it works effectively for a small chamber with low vacuum conductance.
However, it would inevitably lead us to re-activate entire vacuum chambers
composing the 1.5 km storage ring resulting in unacceptable enormous time
for the activation procedure. In addition, we are concerned that it hasessentially low sorption capacity so that the pumping ability would decreasein a short period of time [Benvenuti2001], [Bender2010], followed byfrequent re-coating would be necessary compared with the NEG cartridges.
Furthermore, according to the pressure distribution estimation (see section
I-1.5.4), it was confirmed that enough ultimate pressure for the ultra low
emittance ring with the top-up operation could be achieved by arranging
local pumping systems effectively nearby all the photon absorbers.
Therefore, we are negative about adapting NEG coating chambers to thenew machine.
I-1.5.5.2. Roughing Pump
Existing vacuum pump units in our facility, each of which consistsof a turbo molecular pump (TMP) of 0.25 m3/s and a scroll pump, will be49

used continuously as a roughing vacuum pump during the NEG activation.
We are going to prepare five vacuum ports per unit cell except ID for theconnection with the pump units. Outgassing rate desorbed from the NEG
cartridge during the activation was calculated from the pumping speed of
TMP against H2 and pressure at the TMP position obtained at SPring-8. As
shown in Fig.I-1.5.14, a predicted pressure distribution of H2 over the range
of a unit cell except ID during the activation, the maximum pressure can be
kept less than 1 × 10-2 Pa as well as the SPring-8 case.

Fig. I-1.5.14: Calculated pressure distribution of H2 in a unit cell during the
NEG activation (TGVs are closed).
I-1.5.5.3. Ring Gate Valve (RGV)
All-metal pneumatic RF-shielded RGV is going to be installed
between the ID chamber and the LIC for the isolation, as shown in Fig.
I-1.5.4. They will be completely controlled by a vacuum interlock system.
I-1.5.5.4. Transport Gate Valve (TGV)As mentioned above in section I-1.5.2.4, we need the TGVs at bothends of LIC, which has a role to seal the chamber and keep the UHV during
the baking, transportation and installation. Once after installation andadjacent vacuum chambers are connected, they are opened manually andprobably never close again without maintenance for a serious vacuum
trouble. After opened, RF contact should be inserted in TGV. The target of50

designing the TGV is miniaturization and weight reduction with simple
configuration.
I-1.5.5.5. Bellows
Bellows with the RF shielding will be arranged at both ends of the
BSC, and near the BPM. Both aluminum formed bellows and stainless steel
welded bellows are under consideration. We will make a decision based on
the R&D results, because innovative miniaturization is required.
I-1.5.6. Light Extraction Design
In the existing SPring-8 storage ring of DBA configuration, there
are three types of beamlines (ID-BL, B1-BL and B2-BL) whose light sources
are ID, BM1 and BM2, respectively. As the new lattice is designed so thatthe center of ID (light source point) may coincide with the present one, any
modification would not be required to the existing light axis of ID-BL. As forthe bending magnet beamlines, we plan to utilize BM1 and BM3 in the newlattice as light sources for the existing B1-BL and B2-BL, respectively. As
shown in Fig. I-1.5.7, the radiation from BM3 will be introduced to B2-BL
through a light extraction window of BCA4. On the other hand, as thealteration from DBA to five-bend achromat (FBA) inevitably changes bothlight source point and light axis angle, the existing and new light axes
should intersect somewhere. A new light source point, which originates thecenter of the new light axis, will be decided so as to match the intersection
point with the existing center point of exit shielding wall, as shown in Fig.
I-1.5.15. Consequently, any alterations on shielding concrete structure of
the tunnel would not be necessary.

Fig. I-1.5.15: Positional relation between the existing and new light axes forthe bending magnet beamlines.
51References[Anashin1998] V. Anashin et al., Proc. of the European Particle Accelerator
Conference 1998, Stockholm, Sweden, (2008) pp. 2163-2165.
[Bender2010] M. Bender et al., J. Nuclear Instruments and Method in

Physics Research B268 (2010) pp. 1986-1990.
[Benvenuti2001] C. Benvenuti et al., Vacuum 60 (2001) pp. 57-65.
[Gröbner1994] O. Gröbner, et al., J. Vac.Sci. Technol A12(3), May, Jun 1994,
pp.846-853.
[Herbeaux2008] C. Herbeaux, et al., Proc. of the European Particle
Accelerator for Conference 2008, Genoa, Italy, (2008) pp. 3696-3698.
[Ohkuma1994] H. Ohkuma, et al., Proc. 9th Meeting on Ultra High Vacuum
Techniques Accelerators and Storage Rings, KEK, Jpn., (1994), pp. 29-45.
[Oishi2014] M. Oishi, et al., submitted to J. Vac. Soc. Jpn. (in Japanese).
[Takahashi2008] S. Takahashi, et al., J. Synchrotron Rad. 15, (2008)
144-150.

52

I-1.6. RF System
I-1.6.1. Introduction
The role of an RF acceleration system of a storage ring is togenerate a sufficient beam-accelerating voltage and compensate forbeam-energy loss caused by synchrotron radiation in bending magnets andinsertion devices. The RF system of the SPring-8 storage ring has stably
generated a voltage of 16 MV at a frequency of 508.58 MHz and accelerated
anelectron

beam

with

a

current

of

100

mA

since

1997

for

synchrotron-radiation users. The necessary RF power is generated by
klystrons in four RF stations and supplied to 32 bell-shaped single-cellcavitiesinthestraight

sectionsoftheSPring-8

storagering[Kawashima2008].
Table I-1.6.1 shows the parameters related to the RF system of theSPring-8-II. In upgrading the storage ring for the SPring-8-II, a beamenergy is lowered to 6 GeV and energy losses by radiation in bending
magnets and insertion devices are 3 MeV/turn and 2 MeV/turn, respectively.
In consequence, a needed RF voltage decreases to 7 MV. Since the strengthof radiation damping is weakened, we should be careful as to instabilities
arising from parasitic impedances of cavities.
Since the momentum compaction factor exchanges the beamenergy with the longitudinal beam position, the energy and the position
become sensitive to the amplitude, phase, and frequency changes of the RF
system. Hence, the stability of the acceleration RF field is also important for
the SPring-8-II storage ring. We set requirements for the stabilities of thebeam energy and the longitudinal beam position to < 1 × 10−4 and < 1 ps,

respectively, which are well below the natural energy spread and thenatural bunch length in Table I-1.6.1. From these requirements, thedemanded stability for the acceleration RF field is derived, as listed in Table
I-1.6.2. Thus, a precise low-level RF (LLRF) system to regulate the RF
voltage and phase in the acceleration cavity is indispensable.
Taking these demands and upgrading cost-effectiveness into
account, the present high-power RF components with the excellent
53performance and durability are reused for the next high-power RF system.
Although the performance of the present LLRF system is comparable to therequirement, the design of the present LLRF system is out-of-date and theelectronic components are hard to maintain. Therefore, an entirely new
digital LLRF system is designed to improve the stability in beam
acceleration and to handle a low-emittance beam precisely.
Table I-1.6.1: Parameters of the SPring-8-II RF system.
Beam energy
6 GeV
Beam current

100 mA

Beam accelerating frequency

508.762 MHz

Radiation energy loss per turn

5.0 MeVin bending magnets

3.0 MeVin insertion devices

2.0 MeV

Beam accelerating voltage

7 MV

Circumference

1435.43 m

Harmonic number

2436

Beam revolution frequency

208.851 kHz

Over-voltage ratio

1.4

Natural energy spread (∆E/E)

0.093%

Synchrotron frequency

670 HzBetatron function at the cavity position
Horizontal

5.50 m

Vertical3.00 mMomentum compaction factor

3.27×10-5

Bunch length

6 ps rms

RF stations

4

RF cavities

16

Shunt impedance of the bell-shaped cavity

6 MΩ

Total wall loss in cavities

510 kW

Beam loading power

500 kW

54

Table I-1.6.2: Requirements for the SPring-8-II RF system.
Accelerating voltage stability and AM noise< 1 × 10-3

(Integral of DC – 200kHz)
Phase stability and phase noise (Integral of< 0.1 degree

DC – 200kHz)
Phase noise and AM noise at the offset< -100 dBc/Hz

frequency near the synchrotron frequency
Accelerating frequency stability< 3 × 10-9

I-1.6.2. High-Power RF System
I-1.6.2.1. Configuration of High-Power RF Components
An energy loss by radiation is up to 5 MeV/turn and the maximal
power to beam loading is 500 kW at a beam current of 100 mA. We need an
accelerating voltage of 7 MV in order to compensate the losses and have asufficient quantum lifetime. Since the SPring-8-II employs multi-bending
optics, the straight sections available for the cavities are about 20% shorter
than those of the present storage ring. The number of RF cavities in thesections is reduced to half and the existing RF system is rearranged into one
driving four cavities in each station. The 16 cavities consume an RF powerof 510 kW to generate the accelerating voltage. The RF system with the four
stations is so redundant that the beam acceleration of 7 MV can continue by
raising RF power of the three stations by 55% in case one station fails.
The plan of using six cavities in one section or the total of 18
cavities has also been investigated to carry out the acceleration with three
RF stations. This plan is also redundant against one station failure.
However, it is barely capable to install the six cavities with incidental
equipment such as vacuum chambers and further shorting the sections bychange in optics design or other ring components is difficult to be accepted.
Thus we employ the four-station system affording to install 16 cavities andhaving the redundancy described above.

55

Figure I-1.6.1 shows the configuration of the new RF station. The
main RF power source is a klystron with a rated output of 1 MW, anefficiency of better than 60% and a gain of more than 50 dB. The klystron isdriven by a 90 kV DC power-supply with a voltage ripple of less than 0.5%
peak-to-peak to decrease noises on the RF power output of the klystron. A
waveguide system from the klystron to the four cavities is shown in Fig.
I-1.6.1. An RF power of 250 kW generated by the klystron is transmitted
and distributed to the cavities. The existing rectangular waveguides,
magic-tees, a 1 MW circulator and dummy loads are available andrearranged for the configuration.

Fig. I-1.6.1: Configuration of the RF station.
I-1.6.2.2. RF Cavities
We have suppressed coupled-bunch instabilities arising from theimpedances of parasitic resonant modes in the bell-shaped cavities by using
a tuner for changing the frequencies of higher-order modes and powerfulbunch-by-bunch feedback system [Ego1997, Nakamura2005]. As the othercountermeasure against the instabilities, we are developing a new RF
cavity with its harmful parasitic resonances damped. Figure I-1.6.2 shows
the cavity structure. The TM020 mode is selected as a beam-accelerating
mode since it has a shunt impedance sufficient for beam acceleration and56

brings a compact damping structure to the cavity. The
estimated shunt impedance and unloaded Q are 6.8 MΩ
and 60,300, respectively. The cavity has two slots along
the magnetic node of the TM020 mode and materials
absorbing RF waves are fitted in them. Though the TM020
mode is insensible to the slots, the parasitic resonances
have electromagnetic fields intruding into them and are
damped sufficiently. Figure I-1.6.3 shows the estimatedimpedance distributions of all the monopole and dipole
modes trapped in the cavity. The impedances of parasitic
modes are reduced and less than theFig. I-1.6.2: RF cavity
with its harmful parsitic
resonances damped.

thresholds of the instabilities.

Fig. I-1.6.3: Impedance distributions of resonant modes trapped into thecavity calculated with MAFIA: (a) monopoles and (b) dipoles. The solid
redlines show the thresholds of the instabilities.
I-1.6.2.3. Solid-State Amplifier Option
Thanks to a recent progress of the semiconductor technology,
hundred-kW class solid-state amplifiers (SSA) in an UHF frequency band
were

fabricatedinseveralaccelerator

facilities

andcompanies[Marchand2007, Pardine2012, Jacob2013, Broere2011]. The SSA is an57

option for the high power RF source for the SPring-8-II storage ring insteadof the present klystrons.

Fig. I-1.6.4: Schematic block diagram of the SSA unit (left) and the layoutof the RF system (right).
Figure I-1.6.4 shows the configuration of the SSA. The amplifier
element is a high power MOS-FET. Recently a laterally-diffused-type
MOS-FET (LDMOS) with a nominal output of 1 kW at 500 MHz iscommercially available [Freescale]. Many MOS-FET modules are integrated
in the SSA unit and their output powers are combined by power combiners
with low insertion loss. In order to protect MOS-FET devices from abnormal
reflective powers, circulators are installed between MOS-FET modules andthe power combiners. Since the total power efficiency of the SSA unit isexpected to be less than 50%, water cooling of the MOS-FET modules isnecessary. The requisite output power for the SSA unit is 100 kW to drive
one RF cavity. The direct connection between the SSA unit and the RF
cavity allows us to control RF amplitude and phase independently without
the high power phase shifter.
The SSA and related RF system have following advantages anddisadvantages, compared to conventional klystrons.
Advantages:
1) Free from the long-term degradation of the cathode in the klystron.
2) Easy maintenance due to elimination of high voltage handling.
583) High modularity of the amplifier elements associated withredundancy and flexibility for the required RF power changes.
Disadvantages:
4) Low power efficiency less than 50 % for class-AB amplifier and theresultant high operational cost. Improvement of the power efficiency
should be studied with introducing the other class operation such asclass-D.
5) High cost for development and production of the SSA, modification ofelectrical power lines and the cooling water system.
I-1.6.3. Low-Level RF System
The low-level RF (LLRF) system is demanded to have following
functions in order to stabilize the phase and amplitude of the accelerating
RF field.
1) Generate an appropriate RF signal for the klystron from a reference RF
signal.
2) Detect RF signals in the RF cavity and the waveguide precisely.
3) Control a RF cavity tuner and a klystron anode power supply.
4) Construct an optimum feedback loop among these functions.
Some of the requirements for the LLRF system are already shownin Table I-1.6.2. Although these requirements can be satisfied by thepresent LLRF system, the present system consists of out-of-datecomponents, such as a diode amplitude detector, a mixer phase detector, andan analog feedback loop. Therefore, we are developing a new digital LLRF
system with modern RF modulation/detection technique and a digital
feedback loop. Since the number of analog components in the digital LLRF
system is small, environmental perturbations, such as temperature drift,
can be reduced. The digital LLRF system also has high adaptability to amodern digital feedback control.
Figure I-1.6.5 shows a schematic diagram of the digital LLRF
system at each RF station. The reference RF signal is modulated by an IQ
modulator and the RF signal is fed into the klystron. RF signals from the
klystron output, the RF cavity input, the pickup of an RF cavity, etc. are59

detected by ADCs and the phase and amplitude are obtained from a digital
IQ demodulator. The detected RF data are fed back to the IQ modulator, theRF cavity tuner and the klystron anode power supply.

Fig. I-1.6.5: Schematic view of the LLRF system at an RF station.
For an RF modulation, an IQ modulator with baseband DACs isemployed. There are some other methods, such as a direct digitalsynthesizer, an IQ modulator with an up converter and an intermediate
frequency (IF) DAC, and so on. However, the phase noises of these methods
are the larger than the employed method. Although an IQ modulator with abaseband input has some errors on the IQ balance etc., the errors can becorrected by the RF feedback loop.
There are many demodulation methods to detect the amplitude
and phase, such as an analog IQ demodulation with baseband ADCs, adigital IQ demodulation with a down convertor and an IF ADC, and so on[Schilcher2008]. Among them, we use a digital IQ demodulation with anunder-sampling scheme. The RF signal (508.8 MHz) is directly detected by afast ADC with a sampling frequency (e.g. 156 MHz) lower than the input RF
frequency. The amplitude and phase of the RF signal can be obtained from adata stream sampled by the ADC. This configuration is very simple anddoes not use mixers, which is a temperature sensitive element. In addition,
the hardware of this demodulator can be commonly used for the beam
60

position monitor electronics. Since a required RF amplitude stability is
1 × 10−3 , the ADC is needed to have a resolution of 1 × 10−4 , corresponding
to a 16-bit level. Furthermore, an ADC clock jitter should be as small as 100

fs. This performance can be achieved by the present ADC technology, suchas a 16-bit ADC module utilized in SACLA.
Perturbation sources to the amplitude and phase of an RF field in
the cavity are the ripple of a klystron high voltage power supply (HVPS), achange of the electrical length of the waveguide between the klystron andthe cavity, a change of the cavity resonant frequency by a cavity
temperature change, a drift of the IQ modulator and so on. A noise
component with an offset frequency higher than several 10 kHz is
suppressed by the bandwidth of the accelerating cavity [Ego1996]. There
are two feedback control loops to stabilize the amplitude and phase of thecavity. The first one is a klystron loop and it suppresses the ripple caused by
the HVPS for the klystron, etc. The bandwidth of the klystron loop isseveral kHz. The second one is a cavity loop to suppress the effect from thetemperature drift, etc. The bandwidth of the cavity loop is several 10 Hz.
The response time of the feedback loop can be optimized by software
modification of adjusting the bandwidth of a digital filter with balancing anoise level.
In addition to the stabilization of the acceleration RF field, we
implement a suppression function of coherent synchrotron oscillation to the
LLRF system. A dedicated feedback system to suppress the coherent
synchrotron oscillation is in operation at the present storage ring
[Ohshima2001]. This system will be upgraded for the SPring-8-II storage
ring. A signal from the beam position monitor is transferred to the LLRF
system, and an appropriate RF signal to suppress the coherent synchrotron
oscillation is generated. Since the synchrotron frequency of the new storage
ring is near the harmonics of the power line cycle (720Hz), the influence
from the ripple of the klystron HVPS can be large. Even if such an effectexists, coherent synchrotron oscillation can be sufficiently reduced by thissuppression scheme.
61I-1.6.4. Synchronization and Timing
Since RF stations and other RF electronics are distributed along
the storage ring, a reference RF signal of 508.8 MHz is demanded to bedelivered to the RF electronics. The phase stability of the acceleration RF
signal is required to be less than 1 ps, which is small enough compared to abunch length of 6 ps. Therefore, a low-noise master oscillator and a stable
RF distribution system are necessary for the synchronization between RF
stations. A revolution signal of 208.85 kHz is also needed to be distributed
together with a reference RF signal in order to trigger the electronics. In
addition, SACLA is newly shared as an injector instead of the booster
synchrotron of SPring-8. We need the pulse-by-pulse synchronization
between the storage ring and SACLA, since a low-emittance and ultra-shortpulse-width beam produced by SACLA must be conveyed and injected
precisely to the storage ring with a small dynamic aperture.
I-1.6.4.1. Master Oscillator
The reference RF signal of an accelerating frequency is supplied
from a master oscillator located at the E station. Requirements for theoscillator are as follows;
1) High frequency stability and a resolution of Δf/f < 3x10-9 to maintain thestored beam energy within one tenth of the natural energy spread.
2) Large tuning range of Δ f/f > 10-6 to cope with a change of thecircumference caused by a seasonal change of ground temperature(~7x10-7/year [Takao2000]) and by the daily tidal deformation (~10-8/day[Date1999]).
3) Low SSB phase noise near the synchrotron frequency not to excite large
coherent synchrotron oscillation. (< -100 dBc/Hz around an offset frequencyof 670 Hz)
The signal generator used at the present storage ring has a frequency
stability of better than 10-10 and satisfies all the requirements. Therefore,
the present signal generator is a candidate for the next ring.

62

I-1.6.4.2. Distribution of the Reference RF Signal
The reference RF signal is delivered to four RF stations, several
tens of beam position monitor units and some synchrotron radiation users
located along the ring. The optical transmission scheme is selected becauseof low transmission loss compared to that of a coaxial cable, and its
electrical isolation capability between the RF stations. The allowance of thephase drift between the RF stations is set to be 1 ps. To reduce thermal
optical fiber length drift, therefore, we use a phase-stabilized optical fiber (<5 ps/km/K).
For connections from an optical transmitter to optical receivers fortransmitting the reference RF signals, a bus topology or a star topology iscommonly used for a network system. For the bus topology, the number of
optical fibers can be reduced. However, the signal-to-noise (S/N) ratio isrelatively small, since only a small portion of transmitted optical power at a
brunched-off point using an optical coupler can be extracted to each receiver.
Furthermore, optical power handled with one optical fiber is limited,
because of non-linear response, where the power is over a certain thresholdlevel. In this case, it is difficult to achieve the required phase noise and AM
noise performance. In our case, therefore, a star topology is used because it
has better S/N ratio by increasing the optical signal power at the receiver,as shown in Fig. I-1.6.6. In addition, the star topology has an ability tointroduce an optical fiber length stabilization system as a future option.

Fig. I-1.6.6: A schematic view of the distribution system of reference andtiming signals.
63

The revolution signal is generated by dividing the reference RF
signal with the harmonic number and used to trigger the BPM electronics,
the experimental apparatus of synchrotron radiation users and so on. The
timing accuracy is required to be a few ps level, which is comparable to thereference RF signal. We use a Wavelength Division Multiplexing (WDM)
based system, because it has a proved result of low time jitter of 5 ps p-p at
SACLA [Otake2011].
I-1.6.4.3. Synchronization with SACLA
In order to inject an electron beam from SACLA to the SPring-8-II
storage ring, the beam timing of SACLA must be synchronized with thedesired bucket of the storage ring. In addition, the injection to the storage
ring must be done in parallel with the SACLA user operation. Since beam
parameters, such as a beam energy, a bunch charge and a bunch length are
different between the injection and the SACLA operation, a pulse-by-pulse
control system for changing the beam parameters conducted by changing
machine conditions, such as RF phases, is required. In this section, we
describe the synchronization scheme between the storage ring and SACLA
and the pulse-by-pulse control system for SACLA.
I-1.6.4.3.1. Synchronization Scheme
The ratio between the acceleration frequency of the storage ring,508.762 MHz, and the fundamental reference clock frequency of SACLA,
238 MHz, is 295/138. Therefore, if the RF phase of SACLA is locked to thatof the storage ring, SACLA is synchronized with the storage ring at every
295 buckets (580 ns, corresponding to 1.72 MHz). Since the harmonic
number, 2436, is relatively prime to 295, all the buckets can besynchronized with SACLA in 1.41 ms (= 580 ns x 2436). When a beam
injection is requested by the storage ring, the SACLA accelerator isnecessary to be triggered at appropriate timing for a desired bucket.

64

Fig. I-1.6.7: Schematic diagram of the synchronization system.
To realize these functions, we are developing a synchronization
module for SACLA. A schematic diagram of the synchronization between
SACLA and the storage ring is shown in Fig. I-1.6.7. An acceleration RF
signal of 508.762 MHz, a revolution trigger frequency of 208.85 kHz and an
injection bucket number are transmitted from the storage ring to the
SACLA timing system. The phase difference between the 238 MHz
reference clock of SACLA and the 508.762 MHz RF signal of the storage ringare detected and a 10 MHz reference signal for the SACLA master oscillator
is generated in order to close the phase-locked loop. In addition, thesynchronization module generates trigger signals for SACLA. When a beam
injection is requested, the module calculates a trigger timing for a desired
bucket and generates an appropriate trigger signal.
The synchronization accuracy is required to be 1 ps. This accuracy
comes from the bunch length of a beam stored in the storage ring, ~ 6 ps
rms. In SACLA, synchronization accuracy of the accelerator components
was achieved to be less than 1 ps. Therefore, the required timing accuracy is
feasible by using the present technology, such as a low phase-noise master
oscillator [Otake2012] and an optical RF transmission system with alength-stabilized fiber link [Maesaka2009].
One of the problems to synchronization between SACLA and thestorage ring is frequency tuning due to the circumference drift of thestorage ring. The tuning range is approximately 10-8/day. We performed anexperiment to change SACLA’s RF frequency of 10-7 level and any
65deterioration of X-ray laser intensity was not observed. Thus, the RF
frequency of SACLA can follow the frequency shift of the storage ring by thetuning.
I-1.6.4.3.2. Pulse-by-Pulse Trigger and RF Control System for SACLA
In order to inject an electron beam to the storage ring in parallelwith the SACLA user operation, we are developing a pulse-by-pulse trigger
and RF control system. When this system receives an injection request
during the SACLA operation, a certain trigger pulse for the SACLA
operation is replaced with an injection trigger pulse. In addition, theinjection trigger pulse is distributed to every accelerator unit of SACLA,
and the acceleration RF amplitude and phase of the units are switched fromthe preset to appropriate values in time-sharing to the storage ring
injection.
In order to switch the acceleration condition pulse-by-pulse, event
information for each trigger pulse is demanded for the accelerator unit.
Therefore, we considered following three methods for the event distribution.
1) Event code is transmitted together with the trigger pulse.
2) Event information is distributed with the existing Ethernet network.
3) Event information is distributed with a newly-built reflective memory
network.
A schematic diagram of the method 1) is shown in Fig. I-1.6.8. The
synchronization module, described in the previous section, generates thetrigger pulse with the event code. The trigger signal is distributed to the
Trigger Delay Unit (TDU) for each accelerator unit in SACLA. The TDU
provides appropriate trigger pulses to accelerator components according to
the event code. In addition, the TDU tells the event code to D/A and A/Dconverter boards for RF phase and amplitude control and the desired
acceleration RF manipulation signal is generated.
The difference among the three methods is just a route of the event
information. For the methods 1) and 2), no additional hardware is neededfrom the present trigger system situation. For the method 2), however, thelatency of the Ethernet network strongly depends on a network traffic66

Fig. I-1.6.8: Schematic diagram of the pulse-by-pulse trigger and RF
control system for SACLA.
amount. Therefore, the method 2) is not reliable for the event distribution.
For the method 1), although the firmware upgrades of the TDU, D/A andA/D boards are required, the event code is certainly transmitted. The
method 3) is also reliable, but an additional cost to build a new network.
Consequently, we design the pulse-by-pulse trigger and RF control system
based on the method 1). The event code can be overlaid to the trigger signal,
for example, by reference to the WWV Digital Time Code Format [NIST].
The reliability of the event distribution on a VME bus is also
important, because various data together with the event code aretransferred on the VME bus. At this moment, an event-synchronized dataacquisition system [Yamaga2009] is stably running on the VME bus and all
the data from the A/D boards are acquired without a missing event.
Therefore, the event distribution on the VME bus is considered to befeasible.
One can worry about the deterioration of the electron beam quality
with the pulse-by-pulse control, because quadrupole magnets etc. areunchanged while the beam energy is changed. We demonstrated
pulse-by-pulse switching of both of an electron beam energy and an X-ray
laser wavelength was simultaneously generated [Hara2013]. Consequently,
electron beam quality is preserved under the condition of the pulse-by-pulse
RF control.

67References[Broere2011] J. Broere, et. al., Proceedings of IPAC2011, MOPC137, (2011).
[Date1999] S. Date, N. Kumagai, "A long-term observation of the DC
component of the horizontal COD in the storage ring of SPring-8", Nuc.

Instr. Meth. A421 (1999) 417-422.
[Ego1996] H. Ego, et. al., Nucl. Instr. and Meth. A 383 (1996) pp. 325-336.
[Ego1997] H. Ego, et al., Nucl. Instr. and Meth. A 400 (1997) p.195.
[Freescale]For

example,freescale

MRFE6VP61K25H,

http://www.freescale.com/[Hara2013] T. Hara et al., Phys. Rev. ST Accel. and Beams, 16, 080701(2013).
[Jacob2013] J. Jacob, et. al., Proceedings of IPAC2013, WEPFI004, (2013).
[Kawashima2008] Y. Kawashima, et. al., Proceedings of the 11th EPAC,
Genoa (2008) p.1485.
[Maesaka2009] H. Maesaka et al., Proceedings of ICALEPCS’09, 85-89(2009).
[Marchand2007] P. Marchand, et. al., Phys. Rev. ST Accel. and Beams, 10,112001, (2007).
[Nakamura2005] T. Nakamura and K. Kobayashi, Proceedings of
ICALEPCS’05, PO2.022-2, (2005).
[NIST] http://www.nist.gov/pml/div688/grp40/wwv.cfm[Ohshima2001] T. Ohshima and N. Kumagai, "Suppression of Coherent
Synchrotron Oscillation of the SPring-8 Storage Ring", Proc. of PAC2001,
p.1975-1977.
[Otake2011]

Y.

Otake,"State-of-the-Art

RF

Distribution

andSynchronization Techniques", Proc. of FEL2011, p.633-640.
[Otake2012] Y. Otake et al., Nucl. Instrum. Meth. A 696, 151-163 (2012).
[Pardine2012] C. Pardine, et. al., Proceedings of IPAC2012, THPPC052,(2012).
[Schilcher2008] T. Schilcher, "RF applications in digital signal processing",
Proceedings of CERN Accelerator School 2007, Sigtuna, Sweden p.249-283.
[Takao2000] M. Takao T. Shimada, "Long Term Variation of the
Circumference of the SPring-8 Storage Ring", Proc. of EPAC2000, p.
68

1572-1574.
[Yamaga2009] M. Yamaga et al., Proceedings of ICALEPCS’09, 69-71(2009).

69

I-1.7. Beam Instrumentation and Instability Feedback
The key instrumentation for the SPring-8-II storage ring is a beam
position monitor (BPM) system to accurately measure the center-of-mass of
small-sized electron beams. The resolution of measuring closed orbit
distortion (COD) should be determined by requirements for achievingsmooth beam commissioning and stable beam operation with a design
performance. The BPM system should also have an ability to measure asingle pass (SP) trajectory with a required resolution, which isindispensable for smooth beam commissioning. To observe the designed
small electron emittance, high-resolution diagnostics is also important.
Since we are planning to inject high-quality beams from the SACLA linear
accelerator to the upgraded ring, appropriate beam monitors along theinjection transport line are necessary for diagnosing the injection beam
quality. Instrumentations for the ring and the injection transport are listed
in Table I-1.7.1 and I-1.7.2, respectively. Details of the major instruments
are described below.
Table I-1.7.1: Beam instrumentation for the SPring-8-II storage ring.
Quantity
Closed Orbit Distortion
Single pass trajectory
Beam profile, emittancecoupling

Method/instrumentation
BPM
BPM
T.B.D.

Bunch by bunch betatronand synchrotron motion

Bunch
by
feedback system

Beam current
Injected beam charge
Betatron Tune
Synchronous phase(average)
Charge and synchronous
phase of each bunchSpecification/resolution
0.05 µm RMS*1)

100 µm RMS*2)< 5 µmbunch Suppression of transverseand longitudinal
instabilities
DCCT
0.01 % (at 100 mA)
Fast CT
5 pC or 1% of charge
T.B.D.
< 0.01
IQ detector or BPM
Fast oscilloscope*1) at 1kHz BW, 10kHz repetition ( at 100 mA)*2) at turn-by-turn ( at 20 µA)
70bunch charge < 1 %bunch phase < 1 deg.
( at 0.1mA / bunch )Table I-1.7.2: Beam instrumentations for the injection transport.
Quantity
Beam

Method / instrumentationenergy Screen monitor
YAG:Ce )size,

spreadSpecification / resolution(OTR, < 10 µm

Beam emittanceQ-scan method with an 60 pm rad
OTR screen

Beam energy spread

OTR screen< 0.01 %Bunch length

Streak camera, EO sample< 0.7 psBeam positions

Stripline BPMs

< 30 µm ( at 80 pC )Chargeofinjection CT< 10 pC

beam

I-1.7.1. Beam Position Monitor
The target resolution of a BPM system is listed in the upper part ofTable I-1.7.4. The resolution for the COD measurement is determined toachieve a stability of the closed orbit within one tenth of a beam size (5 µm)
for a band-width of several hundreds hertz. For the SP trajectorymeasurement, a 100-µm resolution is required for a turn-by-turnmeasurement of an injected beam orbit, since the initial betatron oscillation
amplitude of the injected beam at BPM positions is estimated to be ~1mm.
The BPM system should also have a long-term stability and a low
dependence with the beam current as listed in Table I-1.7.3.
The position detection sensitivity of BPM electrodes and thevoltage resolution of BPM electronics mainly determine the position
resolution. The position sensitivity is determined by a layout of the BPM
electrodes on a vacuum chamber and its cross-sectional geometry. The
position sensitivities for the horizontal and vertical directions of the BPM
electrodes are given by
𝜕𝑈𝜕𝑉(I-1.7.1)

𝑆𝑥 = 𝜕𝑥 and 𝑆𝑦 = 𝜕𝑦 ,where the non-dimensional parameters U and V are defined as
1 𝑉 −𝑉𝑉 −𝑉

1 𝑉 −𝑉𝑉 −𝑉

𝑈 = 2 �𝑉1 +𝑉2 + 𝑉4 +𝑉3 � and 𝑉 = 2 �𝑉1 +𝑉4 + 𝑉2 +𝑉3 �,
1

2

4

3

71

1

4

2

3

(I-1.7.2)

with the four signal voltages V1, V2, V3 and V4 from the electrodes. By using
measured voltages and the position sensitivities, the horizontal and verticalbeam positions are calculated as 𝑥 = 1⁄𝑆𝑥 𝑈 and 𝑦 = 1⁄𝑆𝑦 𝑉. As a result,the position resolution of the BPM system can be determined from theseequations.

Table I-1.7.3: Features of the BPM system.
Property

Quantity

Condition

Beam current dependence< 5 µmfrom 0.1 mA to 100 mA of
stored beam current

Long-term stability< 5 µmfor weeks

Accuracy with respect to the centers ofneighboring quadrupole / sextupole magnets< 100 µm

N.A.

I-1.7.1.1. BPM ElectrodeThe geometrical dimension of a vacuum chamber of the SPring-8-II
storage ring is shown in Fig. I-1.7.1 and its vertical aperture is 16 mm. The“shorted-stripline” type electrode [Nakamura2005A] of the BPM could beselected, since larger signal strength at a detection frequency of 508 MHz
can be expected over a conventional round-shape button type. In Fig. I-1.7.1,
an example of the electrode under study is schematically shown. In the
figure, the length, the width of the electrode, the gap between the electrodeand the chamber, the horizontal separation of each electrode are 25 mm, 5
mm, 1 mm and 10 mm, respectively. The position sensitivities of the BPM
electrodes are calculated as 0.14 / mm for both the horizontal and verticaldirections. Then a signal-to-noise ratio (SNR) of 30 dB is required for the SP
trajectory measurement and 96 dB is demanded for the COD measurement,in order to achieve the target resolution.
As for the SP trajectory measurement at a beam current of 20 µA,
the SNR is determined by a signal strength detected with the BPM
electrode. With this electrode, a signal strength of 10 dB larger than that of
the button-type is expected. Similarly, a signal strength of -5 dBm is
expected for the COD measurement at a stored beam current of 100 mA.
These values are calculated by numerical simulations based on theabove-mentioned equations and by using the geometrical dimension.
72

Fig. I-1.7.1: Schematic view of shorted-stripline type electrode.
I-1.7.1.2. BPM Electronics
Both the SP trajectory measurement and the COD measurement
should be handled by common electronics, wherein signals from fourelectrodes are processed in parallel and sampled by individual fast ADCs.
Beam

positionsarecalculated

from

the

sampled

data

on

a

field-programmable-gate-array (FPGA) and sent to computers through afast data link, such as a reflective memory [GEIP].
In the BPM system, a long-term position-detection stability of lessthan 5 µm can be guaranteed by a periodic calibration of each signal path,
which includes RF switches, amplifiers and ADCs as shown in Fig. I-1.7.2.
For the calibration, a reproducibility of the RF switch better than 0.15 % isrequired. For example, the present technology, such as a solid-state switchof ADG918 [Fujita2006], could ensure the requirement.
We plan to employ the under-sampling method, in which the BPM
signals are sampled through a 10 MHz band pass filter at a lower frequency
than a detection frequency of 508 MHz (Fig.I-1.7.2). In this method,resolution is mainly determined by a temporal jitter of an ADC samplingclock. Suppose the clock jitter to be 0.5 ps, the total SNR is restricted to 56
dB, corresponding to a position resolution of 5 µm. By limiting a frequency
band-width of up to 1 kHz for the COD measurement, an effective position
resolution of 0.05 µm can be obtained. In this case, the total SNR become
about 100 dB by the frequency band limitation. For the SP trajectory73measurement, its small signal level restricts the position resolution to about
30-µm. The temporal jitter of the reference signal for the ADC samplingclock distributed by a LLRF system is better than 0.1 ps (see the section
describing the RF system). So a position resolution of 0.01 µm at 1kHz
band-width is achievable for the COD measurement at a beam current of
100 mA, while for the SP trajectory measurement, position resolution is also
restricted by the signal strength, then a 30-µm resolution is expected at the
injection beam current. In Table 3.7.4, the required and achievable SNRs
with signal levels are summarized.

Fig. I-1.7.2: Block diagram of the BPM electronics.
A merit of the under-sampling method is as follows; since adown-converter and a local oscillator are not needed, current dependence
caused by the non-linearity of a down-converter can be improved. The
demerit is that a very low temporal jitter in the ADC clock is needed, since
the jitter affects the resolution deterioration of the electronics.
Alternative

to

the

under-sampling

method

could

be

the

IF-sampling method with a down-converter. The temporal requirement on
the clock jitter is relaxed, however, the IF-sampling method would not be aproper solution, since the non-linearity of the down-converter spoils thecurrent dependence of the electronics.
74

Table I-1.7.4: Summary of the COD and the SP trajectory measurement.
COD
Required resolution

0.05

µm

1kHz

BW,

SP trajectory

RMS

repetitionat

100 µm RMS at

10kHz turn-by-turn ( at 20 µA)
(at

100

mA)
Required SNR (dB)9630

Noise figure of electronics

15

15

Signal power (dBm)

-5

N.A.

Noise (dBm)-144-104 (for 10 MHz(dB)

band-width)
SNR of electronics

70

70

Total SNR (dB)70

40

Achievable resolution

1 µm

30 µm

Achievable resolution with

0.01 µm at 1kHz

N.A.

band-width limitation

band-width

due to 0.1 ps jitter

I-1.7.1.3. BPM Calibration
The calibration procedure for the mechanical and electromagnetic
positions of the BPM consist of the following three steps:
1. The surface of each electrode is aligned precisely to the datum surface
of a BPM unit. The tolerance can be better that 10 microns.
2. The detection gain of each electrode is measured by assembling theelectrodes in a dummy chamber with a reference signal to be fed from astandard signal generator, as which simulates the beam. The accuracy
would be 0.01 dB, corresponding to a position error of order of 10
microns.
3. After installation, the position of each electrode is surveyed withrespect to the centers of neighboring quadrupole / sextupole magnets
with an accuracy of several tens of microns.
75

By these procedures, we can expect the detection accuracy of the BPM isbetter than 100 µm.
I-1.7.2. Beam Emittance Diagnostics at Storage Ring
For non-destructive diagnostics of the electron beam emittance,
the transverse beam size should be measured by using synchrotron
radiation emitted from a bending magnet. The

BM3 or BM4 of the fivebending magnets lattice of the SPring-8-II ring are possible candidates aslight sources. The use of the BM3 or the BM4 allows us to install emittancemeasurement instrumentations without confliction among other accelerator
equipment and the photon beamlines for user experiments. The smallest
electron beam size in the bending magnets is 5 µm in RMS for the verticalplane at the source point of BM3 (see Table I-1.7.5). To observe the beamsize, X-ray imaging by using a pinhole or a focusing element, such as a
Fresnel zone plate [Takano2006], could be employed. The whole
instrumentations for the beam emittance measurement is placed inside anaccelerator tunnel, and, in this placement, the beam emittance is regularly
monitored for machine tuning and user experiments.
Table I-1.7.5: Electron beam sizes in RMS at BM3 and ID source points.
Source( βx*εx )1/2(µm) ηx*σ/E(µm) σx (µm)σy (µm)Ec (keV)

BM3

11.3

0.9

11.3

5.0

22.8

ID

26.5

0

26.5

6.2

I-1.7.3. Diagnostics of Injection Beam at Transport Line
It is indispensable to diagnose the emittance, the transverse size,
the energy spread and the bunch length of an electron beam near to theinjection point of the storage ring.
Since, as described in the section of injector, the emittance has tobe less than 560 pm rad for a stable and highly-efficient beam injection, an
emittance measurement resolution of less than one tenth of allowable
emittance, i.e. 60 pm rad is required. The beam size should be diagnosed76

with a resolution of less than 12 µm in RMS, which is one sixth of allowablebeam size at the injection point. The energy spread and the bunch length of
the injection beam should be less than 0.1 % and 7 ps from the designed
energy spread and bunch length of the stored beam, respectively. Then the
resolution of the diagnostics should sufficiently be less than them.
The emittance is evaluated by the Q-scan method using an OTR
screen (A spatial resolution of less than 10 μm in RMS). This emittanceevaluation is already working at a dispersion free section around the middle
part of the beam transport line, XSBT, and its designed resolution is 60 pm
rad. So the existing method is applicable to our emittance evaluation.
The beam size and the energy spread are also diagnosed by using
an OTR screen. Suppose a dispersion function near the injection point of thebeam transport line is 0.5 m, a spatial resolution of 10 µm is enough for a
resolution of energy-spread measurement of within 0.01 %.
For a bunch length measurement system, a streak camera with an
OTR radiator already realizes a temporal resolution of less than 300 fs in
FWHM [Otake2013] and EO sampling works as a non-destructive andreal-time temporal monitor using inorganic Pockels EO crystals, which
could archive a temporal resolution of less than 200 fs in FWHM
[Okayasu2013]. An enough resolution is obtained in both methods.
The beam trajectory near the injection point is measured bystrip-line BPMs with a spatial resolution of 30 μm in RMS at a bunchcharge of 80 pC. These measured positions are used for tuning of the beam
injection.
I-1.7.4. Suppression of Beam Instabilities
In the transverse plane, resistive-wall impedance becomes adominant source of electron beam instabilities because of a small-verticalvacuum chamber aperture of 16 mm and narrower gaps of 5 mm at aminimum in in-vacuum undulators. The growth rate of transverse
instability due to the impedance is estimated to be 10 times faster than thatof the present ring. The present transverse bunch-by-bunch feedback (BBF)
system in the SPring-8 ring [Nakamura2005B] has enough capability to77

suppress such strong instability. In the longitudinal direction, alongitudinal BBF system with a newly developed high-energy efficiency
kicker [Nakamura2011, Masaki2013] is under development to curesuspected coupled-bunch-instability due to the HOMs of acceleration
cavities.
References[Fujita2006] T. Fujita et al., EPAC2006, pp. 1130-1132, Edinburgh, Scotland(2006).
[GEIP] http://ge-ip.com[Masaki2013] M. Masaki et al., IBIC2013, pp. 407-410, (2013).
[Nakamura2005A] T. Nakamura, DIPAC2005, pp. 304-306, Lyon, France(2005).
[Nakamura2005B]T. Nakamura, K. Kobayashi, ICALEPCS’05, PO02.022-2(2005).
[Nakamura2011] T. Nakamura, IPAC2011, pp. 493-495 (2011).
[Okayasu2013] Y. Okayasu et al., Phys. Rev. ST Accel Beams 16, 052801(2013).
[Otake2013] Y. Otake et al., Phys. Rev. ST Accel. Beams 16, 042802 (2013).
[Takano2006] S. Takano et al., Nucl. Instrum. Methods Phys. Res. A556,
357 (2006).

78

I-1.8 Injector
I-1.8.1. Introduction
The use of SACLA as an injector of SPring-8-II has been consideredto achieve good injection efficiency, low operation cost and minimum energy
consumption. The SPring-8-II storage ring requires a low emittance injector
due to its small dynamic aperture. Since the emittance of the injection beam
is determined from its equilibrium state of the circulating electron beam ina synchrotron booster, it is hard to obtain a low emittance beam with theexisting SPring-8 injector system composed of a linac and a FODO
synchrotron. In addition, despite of a low injection frequency during top-up
operation, it is necessary to maintain the injector system in a warmed-up
condition, and it results in an increase of operation cost and energy
consumption. On the other hand, the linac of SACLA is always running for
its own user experiments independently from SPring-8-II. Therefore if theinjection beam is distributed from SACLA, it will enable to save the energy
consumption and operation cost for idling the accelerator.
The nominal beam energy of SPring-8-II is expected to be 6 GeV,
whereas the beam energies used for the XFEL operation are around 5~8.5
GeV depending on the photon energy requested from the user experiment.
In addition the electron bunch is highly compressed and its peak current
reaches several kA, which is not necessary but even harmful for the
injection beam due to the emittance degradation at a beam transport line.
To achieve the SACLA XFEL operation and the beam injection toSPring-8-II in parallel, the control of the beam energy and the peak current
is indispensable on a bunch-to-bunch basis.
I-1.8.2. Requirements on the Injection Beam
To avoid the beam loss, the injected beam should stay inside thedynamic aperture of the storage ring. The electron beam will be injected in ahorizontal plane using an in-vacuum septum magnet at SPring-8-II, and atentative normalized dynamic aperture 𝑥𝑑 of 2 mm1/2 is assumed as in Eq.
(I-1.8.1).

79

𝑥𝑑

�𝛽𝑥

1

(I-1.8.1)= ±2𝑚𝑚2

𝛽𝑥 is the horizontal betatron function. Supposing the betatron function at

the injection point to be 25 m, 10 mm is obtained as beam acceptance at the
injection point. But the horizontal acceptance of the injection beam isdecided to be 3.3 mm considering a margin for various errors, such as theerror and mismatch of the betatron function, the beam energy spread, theinjection orbit fluctuation and the design and fabrication tolerances of aseptum magnet. This margin is also necessary at this design stage forpreparing future modification of the dynamic aperture and the betatronfunction during the optimization process of the lattice.
Figure I-1.8.1 is a schematic cross-sectional view of the injection
point. During the beam injection, a horizontal orbit bump will be formed bypulsed kicker magnets to move the stored beam closer to the septum. In
order to avoid the injection beam loss, the injected electrons should staywithin 3.3 mm from the center of the stored beam. Assuming 2.0 mm
thickness of the septum, 99 % injection efficiency can be obtained for the
injection beam with emittance less than 560 pm-rad (6.6 mm-mrad innormalized emittance).
The nominal electron bunch charge of SACLA is about 200 pC andthe maximum beam repetition is 60 Hz. If 100 pC- 10 Hz injection is applied,
it will take about 500 sec to fill out the storage ring with a 500 nC charge
corresponding to 100 mA stored-beam current. For the top-up operation, it
is necessary to refill 130 pC every 10 sec in case of 10 hour lifetime.

3.3 mmStored beam
0.265

Septum0.2

2.0

Injection beam

0.1 0.735

Fig. I-1.8.1: Schematic cross-sectional view of the injection point.
80I-1.8.3. Transport LineThe beam transport line from SACLA to the exit of the existing
synchrotron booster had been already constructed together with the SACLA
facility. The foremost concern about the beam transport is emittancedegradation due to CSR effects. The lattice of this XSBT (XFEL to
Synchrotron Beam Transport) line is based on DBA to mitigate emittancedegradation. After XSBT, SSBT (Synchrotron to Storage ring BeamTransport) connects the exit of the synchrotron booster and the storage ring.
SSBT has been used for the beam injection from the synchrotron booster tothe current SPring-8 storage ring. The lattice of SSBT is based on FODO
and not optimized for the transport of low emittance beams. The layout andlattice of XSBT and SSBT are shown in Figs. I-1.8.2 and I-1.8.3 respectively.
The emittance increase by CSR largely depends on the beam
parameters, particularly the peak current and the energy spread. The
kilo-ampere electron bunches used for the XFEL operation, whose bunch
length is less than 10 fs (FWHM), will certainly not satisfy the required
emittance of the beam injection. To prevent the emittance degradation at
the transport line, the peak current should be reduced down to a few
hundred amperes or less by lengthening the electron bunch.
Figure I-1.8.4 is an example of the emittance degradation throughthe beam transport line calculated with ELEGANT for the case of 8 GeV
beams. In Fig. I-1.8.4 (a), 100 pm-rad initial emittance is assumed with
0.01% energy spread and a peak current of about 100 A without any energy
chirp. Since there are both horizontal and vertical bending magnets in thetransport line, the emittance is increased in both planes. Figure I-1.8.4 (b) is
the case with energy chirp of 0.064 % over the full bunch length. The
electron bunch is elongated by R56 of the first part of the transport line, andit results in the reduction of the peak current for the rest of the transport
line. Thus the CSR effect and the emittance degradation are suppressed.
The beam transport through XSBT has been already tested.
Further beam transport to the storage ring and the evaluation of the beamemittance are planned before the construction of SPring-8-II.
81

Fig. I-1.8.2: Layout of the transport line, XSBT and SSBT.

Fig. I-1.8.3: Lattice of XSBT and SSBT.

82

Fig. I-1.8.4: Emittance degradation at the transport line. The plots are thehorizontal and vertical phase spaces at the transport exit. (a) the case of no
initial energy chirp, and (b) the case with a 0.064 % energy chirp over thefull bunch length. Initial beam parameters are 8 GeV, 500 fs (RMS) bunch
length, 100 pC charge, 0.01 % (RMS) energy spread and 100 pm-rad
emittance in both horizontal and vertical phase spaces.
I-1.8.4. Bunch-to-Bunch Control in SACLA Linac
To maintain the parallel operation of XFEL and the beam injection,
it is necessary to control the beam parameters on a bunch-to-bunch basis.
Figure I-1.8.5 is a schematic configuration of SACLA. The electron beam isgenerated by a thermionic pulsed electron gun. The initial bunch length isabout 1 ns. For the XFEL operation, the electron bunch is compressed toless than 10 fs (FWHM) by velocity bunching and three bunch compressors(BC1~3) corresponding to the peak currents of more than 5 kA. The beamenergy is adjusted between 5~8.5 GeV depending on the XFEL photon
energy requested from users.
83

Since it is necessary to keep the energy of the electron bunches at 6
GeV for the beam injection to SPring-8-II, the concept of multi-energy
operation was proposed to share the bunches between XFEL and the beam
injection [Hara2013]. In the multi-energy operation, the RF pulse
repetitions of specific RF units are changed to control the beam energy frombunch to bunch.
The multi-energy operation has been successfully demonstrated at
SACLA. Figure I-1.8.6 shows the energy of the electron bunches measured
at a chicane located upstream of the BL3 undulators. In the demonstration,
the repetition of the electron bunch was 10 Hz and 8 RF units were operated
at 5 Hz. Since the beam energy gain of one RF unit is about 130 MeV, thebeam energy is alternately changed from bunch to bunch between 8.0 GeV
and 6.9 GeV. Once a kicker magnet is installed, the energy controlled
electron bunches are distributed to the XFEL beamlines and the transportline.
For the beam injection to SPring-8-II, it is also necessary toelongate the bunch length, which is possible by controlling the RF phase to
reduce the energy chirp at the bunch compressors. Since the electron beam
should be injected on-demand timing from the storage ring during the
top-up operation, the parameter of the RF units should be changed at any
requested timing. In order to achieve the bunch-to-bunch control of thebeam energy, the bunch length and the energy chirp, a new timing andlow-level RF systems are under study.
At SACLA, a bunch-to-bunch beam distribution system using akicker magnet and a DC twin-septum magnet is planned to be installed in
the winter shutdown of 2014 for the multi-beamline XFEL operation. The
electron bunch distribution with controlled energy and length at arbitrary
timing will be further investigated and examined using the electron beam.
Reference[Hara2013] T. Hara et al., Phys. Rev. ST Accel. Beams 16 (2013) 080701.

84Fig. I-1.8.5: Schematic configuration of the SACLA accelerator.

Fig. I-1.8.6: The multi-energy operation demonstrated at SACLA. In thisexample, the electron bunch repetition is 10 Hz and the beam energy isalternately changed between 8.0 GeV and 6.9 GeV.

85

I-1.9 Storage Ring Lattice Data
In this appendix we give a full detail of the current version of lattice.
Machine parameters for this lattice are summarized in Table I-1.2.1.
The ring is composed of four zones of A, B, C and D:
Ring = [A, B, C, D]
where
A = [C01, C02, C03, ..., C12]
B = [C13, C14, C15, ..., C24]
C = [C25, C26, C27, ..., C36]
D = [C37, C38, C39, ..., C48]
and C01 and C48 are injection cells, C06, C18, C30 and C42 are longstraight cells, and others are normal cells.

The structure of each cell isgiven below.
_______________________________________________________
Normal Cell (29.903888 m)_______________________________________________________
Bending(Sector)
L[m]

ρ[m]

B[T]
Quadrupole
B'/(Bρ)[m-2]

B'[T/m]_______________________________________________________-

2.341944Q010.240000-

0.120000

Q020.550000-

0.120000

Q03

0.300000-

0.220000-1.06803060-21.37539964+2.08677656+41.76442411-2.19818951-43.99422570

B1A 0.350000

34.40181418

0.5817671593

B1B

67.56262251

0.2962265964

0.70000086B1C

0.700000-

0.120000

Q04

0.200000-

0.120000

S1

0.300000-

0.420000

120.89241670

0.1655508779-1.93398472-38.70647180

O1 0.160000-

0.120000

S20.180000-

0.120000Q05

0.300000-

0.120000

S30.180000-

0.420000+2.76586723 +55.35564000

O2 0.160000-

0.120000

S40.300000-

0.120000

Q06

0.200000-

0.120000-2.27291040

B2D

0.700000

90.73380781

0.2205776016

B2E

0.700000

50.70806566

0.3946876192

B2F

0.350000

25.81971064

0.7751382651-

0.120000

Q07

0.700000-

0.270000Q08

0.700000-

0.270000

Q09

0.650000-

0.270000

Q10

0.300000-0.290000

B3G

0.420000

21.00000000-45.48967806+2.13318029+42.69314120-2.22567622-44.54434047+2.40641274+48.16157330-0.71315042-14.27288248

0.9530402720

87-

0.290000

Q10

0.300000-

0.270000

Q09

0.650000-

0.270000Q08

0.700000-

0.270000

Q07

0.700000-

0.120000

B4F

0.350000

25.81971064

0.7751382651

B4E

0.700000

50.70806566

0.3946876192

B4D

0.700000

90.73380781

0.2205776016-

0.120000

Q06

0.200000-

0.120000

S50.300000-0.700000

S60.180000-

0.120000Q05

0.300000-

0.120000

S70.180000-

0.420000-0.71315042-14.27288248+2.40641274+48.16157330-2.22567622-44.54434047+2.13318029+42.69314120-2.27291040-45.48967806+2.76586723+55.35564000-1.93398472-38.70647180

O3 0.160000-

0.120000

S80.300000-

0.120000

Q04

0.200000-

0.120000

B5C

0.700000

120.89241670

0.1655508779

B5B

0.700000

67.56262251

0.2962265964

B5A 0.350000

34.40181418

0.5817671593-

0.220000

88Q03

0.300000-

0.120000

Q020.550000-

0.120000Q010.240000-

2.341944-2.19818951-43.99422570+2.08677656+41.76442411-1.06803060-21.37539964______________________________________________________________________________________________________________
Injection Cell C48 (29.904413 m)_______________________________________________________
Bending(Sector)
L[m]

ρ [m]

B[T]
Quadrupole
B'/(Bρ)[m-2]

B'[T/m]_______________________________________________________-

2.341944

QI20

0.240000-

0.120000

QI19

0.550000-

0.120000

QI18

0.300000-

0.220000-1.21466785-24.31017494+1.91654724+38.35748076-2.30128753-46.05761356

B1A 0.350000

34.40181418

0.5817671593

B1B

0.700000

67.56262251

0.2962265964

B1C

0.700000

120.89241670

0.1655508779-

0.120000

QI17

0.200000-

0.120000

SI8

0.300000-

0.420000-1.90993525

89-38.22514941

-

0.160000-

0.120000

SI7

0.180000-

0.120000

QI16

0.300000-

0.120000

SI6

0.180000-

0.420000-

0.160000-

0.120000

SI50.300000-

0.120000

QI15

0.200000-

0.120000+2.75763345+55.19085040-2.24450828-44.92124241

B2D

0.700000

90.73380781

0.2205776016

B2E

0.700000

50.70806566

0.3946876192

B2F

0.350000

25.81971064

0.7751382651-

0.120000

QI14

0.700000-

0.270000

QI13

0.700000-

0.270000

QI12

0.650000-

0.270000

QI11

0.300000-0.290000

B3G

0.420000-0.290000

QI10

0.300000-

0.270000

QI09

0.650000-

0.270000

QI08

0.700000

21.00000000+2.14178819+42.86541838-2.27587537-45.54901851+2.39692076+47.97160227-0.70898412-14.18949879

0.9530402720-0.71189002-14.24765702+2.38393836+47.71177452-2.22146192-44.45999612

90-0.270000

QI07

0.700000-

0.120000+2.18961061

B4H

0.350000

79.42787792

0.2519750777

B4I

0.700000

47.29911256

0.4231336410

B4J

0.700000

44.22204260

0.4525762388-

0.120000

QI06

0.200000-

0.120000

SI40.300000-0.700000

SI3

0.180000-

0.120000

QI05

0.300000-

0.120000

SI2

0.180000-

0.420000-

0.160000-

0.120000

SI1

0.300000-

0.120000

QI04

0.200000-

0.120000-0.700000

B5K-2.65899088-53.21663322+2.69996357+54.03665432-0.19671332-3.93699004

0.700000

32.78980994

0.6103678475

B5L 0.350000

69.76861887

0.2868602824-

0.220000

QI03

0.300000-

0.120000

QI02

0.550000-

0.120000

QI01

0.240000-

2.342469+43.82252892-2.14666533-42.96302871+0.20587177+4.12028584+1.98760982+39.77971627

91

______________________________________________________________________________________________________________
Injection Cell C01 (29.904413 m; Mirror of C48)_______________________________________________________
Bending(Sector)
L[m]

ρ [m]

B[T]
Quadrupole
B'/(Bρ)[m-2]

B'[T/m]_______________________________________________________-

2.342469

QI01

0.240000-

0.120000

QI02

0.550000-

0.120000

QI03

0.300000-

0.220000+1.98760982+39.77971627+0.20587177+4.12028584-2.14666533-42.96302871

B1L 0.350000

69.76861887

0.2868602824

B1K

32.78980994

0.6103678475

0.700000-0.700000-

0.120000

QI04

0.200000-

0.120000

SI1

0.300000-

0.420000-

0.160000-

0.120000

SI2

0.180000-

0.120000

QI05

0.300000-

0.120000-0.19671332-3.93699004+2.69996357+54.03665432

92

SI3

0.180000-

0.420000-

0.160000-

0.120000

SI40.300000-

0.120000

QI06

0.200000-

0.120000

B2J

0.700000

44.22204260

0.4525762388

B2I

0.700000

47.29911256

0.4231336410

B2H

0.350000

79.42787792

0.2519750777-

0.120000

QI07

0.700000-

0.270000

QI08

0.700000-

0.270000

QI09

0.650000-

0.270000

QI10

0.300000-0.290000

B3G

0.420000-2.65899088

21.00000000-53.21663322+2.18961061+43.82252892-2.22146192-44.45999612+2.38393836+47.71177452-0.71189002-14.24765702

0.9530402720-

0.290000

QI11

0.300000-

0.270000

QI12

0.650000-

0.270000

QI13

0.700000-

0.270000

QI14

0.700000-

0.120000

B4F

0.350000

25.81971064

0.7751382651

B4E

0.700000

50.70806566

0.3946876192

B4D

0.700000

90.73380781

0.2205776016-0.70898412-14.18949879+2.39692076+47.97160227-2.27587537-45.54901851+2.14178819+42.86541838

93-0.120000

QI15

0.200000-

0.120000

SI50.300000-0.700000

SI6

0.180000-

0.120000

QI16

0.300000-

0.120000

SI7

0.180000-

0.420000-

0.160000-

0.120000

SI8

0.300000-

0.120000

QI17

0.200000-

0.120000

B5C

0.700000

120.89241670

0.1655508779

B5B

0.700000

67.56262251

0.2962265964

B5A 0.350000

34.40181418

0.5817671593-

0.220000

QI18

0.300000-

0.120000

QI19

0.550000-

0.120000

QI20

0.240000-

2.341944-2.24450828-44.92124241+2.75763345+55.19085040-1.90993525-38.22514941-2.30128753-46.05761356+1.91654724+38.35748076-1.21466785-24.31017494______________________________________________________________________________________________________________Long Straight Cell C06, C18, C30, C42 (29.915600 m)_______________________________________________________

94

Bending(Sector)
L[m]

ρ [m]

B[T]
Quadrupole
B'/(Bρ)[m-2]

B'[T/m]_______________________________________________________-

2.437800

QL1

0.500000-

0.500000

QL2

0.500000-

0.400000

QL3

0.300000-

0.400000

QL4

0.500000-

1.941100-

1.941100QL40.500000-

0.400000

QL3

0.300000-

0.400000

QL2

0.500000-

0.500000

QL1

0.500000-

2.437800-

2.437800

QL1

0.500000-

0.500000

QL2

0.500000-

0.400000

QL3

0.300000-

0.400000

QL4

0.500000-+0.98004213+19.61441198-1.44051710-28.83028698-0.07608812-1.52281589+0.68442003+13.69787688+0.68442003+13.69787688-0.07608812-1.52281589-1.44051710-28.83028698+0.98004213+19.61441198+0.98004213+19.61441198-1.44051710-28.83028698-0.07608812-1.52281589+0.68442003+13.69787688

1.94110095-1.941100QL40.500000-

0.400000

QL3

0.300000-

0.400000

QL2

0.500000-

0.500000

QL1

0.500000-

2.437800+0.68442003+13.69787688-0.07608812-1.52281589-1.44051710-28.83028698+0.98004213+19.61441198______________________________________________________________________________________________________________
Sextupole (typical)
L[m]B"L/(Bρ)/2[m-2]

B"/(Bρ)[m-3]

B"[Tm-2]_______________________________________________________
S1

0.300000-19.60-130.66667-2615.1425

S2

0.180000+9.332024+103.68916+2075.2188

S3

0.180000+7.28+80.88889+1618.8977

S4

0.300000-10.83-72.20000-1444.9997

S5

0.300000( = S4 )S60.180000( = S3 )S70.180000( = S2 )S80.300000( = S1 )SI10.300000-19.60-130.66667-2615.1425

SI2

0.180000+9.332024+103.68916+2075.2188

SI3

0.180000+7.21+80.11111+1603.3314

SI4

0.300000-10.70-71.333333-1427.6543

SI5

0.300000( = SI4 )SI6

0.180000( = SI3 )SI7

0.180000( = SI2 )

96

SI8

0.300000( = SI1 )______________________________________________________________________________________________________________
Octupole (typical)
L[m]B(3)L/(Bρ)/6[m-3]

B(3)/(Bρ)[m-4]

B(3)[Tm-3]_______________________________________________________
O1 0.160000-43.3543-1625.79-32538.2

O2 0.160000-57.4151-2153.07-43091.1

O3 0.160000-58.2721-2185.20-43734.2_______________________________________________________

97

PART-I
I-2 Light Sources
I-2.1.
I-2.2.
I-2.3.
I-2.4.
I-2.5.
I-2.6.

Basic Policy of ID Specification
Hard X-ray Beamline
Soft X-ray Beamline
Expected Light Source Performances
Facilitating the Mass Production and Installation of IDs
Summary98

In this chapter, specifications of insertion devices (IDs) to beinstalled in the upgraded storage ring are described and light source
performances available in SPring-8-II are discussed. Firstly, the basic policyof how to define the specifications of IDs is described from the practicalpoint of view. Secondly, detailed specifications of IDs in several typicalbeamlines are presented together with expected light source performances,which are computed with a more rigorous method than the traditional one
based on a Gaussian approximation. Finally, a couple of technical issues are
addressed toward mass production of IDs, which are crucial for the scheduleand budget of SPring-8-II. Note that all the computations shown in thefollowing sections are carried out with the SR calculation code SPECTRA[Tanaka2001].
I-2.1. Basic Policy of ID Specification
The insertion device (ID), in which the high-quality electron beam
generates synchrotron radiation (SR), is one of the key components in
SPring-8-II. Although there exist a lot of IDs currently installed in theexisting storage ring of SPring-8, most of them cannot be used as is in
SPring-8-II because of two reasons.
Firstly, the straight sections for installation of IDs in the upgradedstorage ring will be shortened to around 4.7 m, nearly 1 m shorter than thecurrent value in SPring-8, and thus the IDs should be shorter as well. This
is to accommodate a lot of magnets required for the 5BA (5-bend achromat)
lattice instead of the current DBA (double-bend achromat) lattice. Secondly,
the electron energy is reduced from 8 GeV to 6 GeV to improve theemittance of the electron beam. In order to make sure that the tunable
range of SR is comparable to that in SPring-8, the magnetic design of IDs
should be revised.
In order to meet the requirements above, we have to at least
refurbish the existing IDs to fit the new straight sections, or more preferably,
replace them with new ones. In designing the new IDs, we have to keep inmind that the emittance of the electron beam will be reduced by a factor ofnearly 20, in order to take full advantage of the performance of the99upgraded storage ring.
I-2.1.1. Consideration of Light Source Performance
In order to consider the specifications of IDs, we need to answer afundamental question: what kind of quality is important as the SR lightsource?
The performance of a light source is defined by a lot of parameters.
Among them, we have three important characteristics of radiation in SR
beamlines: brilliance (or brightness), total flux, and transverse coherence,
which are directly related to the number of photons to be actually delivered
to the sample.
In order to discuss the practical performance of a specific SR
beamline, we need to consider another important property of radiation, i.e.,
the radiation power, which denotes the photon intensity obtained byintegrating the photon flux over the entire energy range. In mostexperiments using monochromatic photon beams, only the photons
contained in a narrow bandwidth (typically of the order of 10-3~10-4) aroundthe target photon energy are extracted by the monochromator and guided tothe sample. All the other photons outside the monochromator bandwidth
are usually absorbed by front-end components and optical elements in thebeamline and then turn to heat sources. If the cooling capability of aparticular component is not sufficient, it can be significantly damaged or atleast deformed.
The heat load problem explained above is especially serious for the
optical elements such as focusing mirrors and monochromators, because thedeformation of these devices results in a fluctuation of the photon beam
position and size at the sample, and a variation of the energy resolution. As
a result, the effective brilliance and flux can be significantly reduced. In thissense, specifications of IDs should be defined not only to increase thebrilliance and flux but also to reduce the radiation power or heat load,
especially on the optical elements.

100I-2.1.2. Undulators or Wigglers?
It is well known that IDs can be classified into two types, i.e.,
undulators and wigglers. Although the magnetic structures of these two
devices are identical in the point that they both generate periodic magnetic
fields with the period of 𝜆𝑢 and peak field of 𝐵 , the radiation
characteristics are completely different from each other. Namely, undulator

radiation (UR) is coherent, while wiggler radiation is incoherent (WR). As aresult, the brilliance of the former is always much higher than that of thelatter, and thus we usually adopt an undulator for most applications, aslong as it is available as a light source. To be specific, we need to consider
the energy region ℏω𝑢 in which the undulator effectively works. Roughly

speaking, it scales asω𝑢 ∝ 𝐸 2 /𝜆𝑢 ,(I-2.1.1)

where 𝐸 is the electron energy. Apart from technical issues such asmachining and assembling of tiny magnets, the magnet period 𝜆𝑢 has alower limit determined by a practical application. This is related to the peakfield 𝐵 available in an ID, which scales as

𝐵 ∝ exp (−𝜋𝑔/𝜆𝑢 ),(I-2.1.2)

where 𝑔 is the magnet gap. The above formula means that the peak field
exponentially decays as the ratio 𝑔/𝜆𝑢 , and thus 𝜆𝑢 cannot be much

shorter than the minimum gap available in the storage ring, or more

specifically, that at the straight section where the ID is installed. The lower
limit of 𝜆𝑢 in turn determines the upper limit of the photon energy reached

by an undulator. If the target photon energy is too high to be cover by any
undulators, we have to turn to a wiggler, whose effective energy region ℏω𝑤

scales asω𝑤 ∝ 𝐸 2 𝐵.

(I-2.1.3)

Note that the peak field 𝐵 should be strong enough to reach the target
energy region, which usually imposes a longer period at the expense of thenumber of periods.

In SPring-8, all IDs are of undulator type except one exception in
BL08W, thanks to the high electron energy of 8 GeV. This in turn means101

that we may have a debate concerning the selection of ID type in
SPring-8-II, because the electron energy will be reduced to 6 GeV, meaningthat the effective energy region is shifted to lower energies by a factor of(6/8)2 ~ 0.56 as long as the magnetic period is kept constant. As a result, we
may have to consider a number of wigglers in SPring-8-II instead ofundulators to compensate the electron energy reduction. This kind of debateis, however, completely unnecessary because of a number of definite reasonsdescribed in the following sections.
I-2.1.3. Total Flux, Brilliance and CoherenceFirst of all, let us compare the characteristics of UR and WR interms of the total flux, brilliance and degree of coherence, which are directly
related to the number of photons actually available.

Fig. I-2.1.1: Computed spectrum of total flux in BL08W and BL09XU in
SPring-8. The fundamental photon energy is 10 keV in the latter.
The total flux refers to the total number of photons in unit
bandwidth emitted over the whole solid angle per unit time. If we do nothave to take care of the dimension of the sample, the total flux specifies thepossible maximum number of photons available in the beamline. Roughly
speaking, the total flux just depends on the ID length and hardly depends
102

on the ID type, as long as the parameters such as the magnetic period andstrength are appropriately set. For example, Fig. I-2.1.1 shows thecomputed spectrum of total flux in BL08W (wiggler) and BL09XU(undulator) in SPring-8. Note that the fundamental photon energy in
BL09XU has been assumed to be 10 keV, at which the total flux is found to
be of the order of 1015 in both beamlines.
The above discussion on the total flux seems contradictory to thewell-known fact that the brilliance of UR is much higher than that of WR.
This is associated with the coherent properties of radiation as described
below.
It has been mentioned in section I-2.1.2 that UR is coherent. To be
more specific, the transverse coherence of radiation emitted from a single
electron moving in an undulator is nearly perfect. As a result, most photons
in UR are localized and contained in the coherent volume in the phase space.
In other words, the optical emittance of UR is comparable to that of thediffraction-limited light.
On the other hand, photons in WR, which is an incoherent lightsource, are distributed widely throughout the phase space. In other words,
the optical emittance is much larger than that of the diffraction-limited
light, and that of UR.
Now let us recall the fact that the brilliance is given by the
maximum photon density in the 4-dimensional phase space and is thusroughly given as the total flux divided by the optical emittance. It is easy tounderstand that the brilliance of WR is much lower than that of UR,
typically by a few orders of magnitude. In practice, we have to consider theeffects due to the finite emittance and energy spread of the electron beam,
which always deteriorates the coherent properties of UR and degrades thebrilliance. In other words, the low emittance electron beam is much more
effective for UR than for WR.
I-2.1.4. Heat Load Reduction
Secondly, let us consider the selection of ID type in terms of how to
reduce the heat load on optical elements. For this purpose, we need to103consider the spectral and spatial profiles of UR in comparison to WR.
In the front-end section located upstream of the beamline, acomponent called XY slit is installed, which intercepts the radiation emitted
to large off-axis angles and thus reduces the heat load to be brought tooptical elements. This kind of heat reduction scheme, when applied to UR,
works fine because the spatial profile of radiation power observed at the
XY-slit position is much broader than that of the photon flux at thefundamental energy (and high-harmonic energies) of UR.
An example is shown in Fig. I-2.1.2 where spatial profiles of thephoton flux at the fundamental energy of 10 keV is plotted, together withthat of the radiation power, which are computed using the parameters of
BL09XU in SPring-8, but without the effects due to the electron beam. It isclear from this example that most of the heat load on optical elements canbe reduced by adjusting the aperture of the XY slit appropriately, without
significantly sacrificing the flux available in the beamline. Note that this is
not the case for WR, because the spatial profiles of the photon flux andradiation power are close to each other and thus narrowing the slit aperture
to reduce the heat load simultaneously results in the loss of available flux.

Fig. I-2.1.2: Computed profiles of the photon flux at the fundamental
energy of 10 keV and radiation power in BL09XU in SPring-8. Note thatthe effects due to the finite emittance and energy spread are neglected.

104

It is worth noting here that the validity of the above discussions
largely depends on the electron beam emittance. If it is much larger thanthe emittance of diffraction-limited light at the target photon energy, theheat load reduction using the XY slit is not so advantageous because theflux profile is broadened by the convolution with the electron beam
distribution. This in turn means that the low emittance electron beam is
directly linked to the effective reduction of the heat load of UR.
Another important point to be mentioned is the difference inspectral property between UR and WR. The spectrum of WR is essentiallybroadband as in the case of BM radiation and does not largely depend on
the electron beam quality. On the other hand, the spectrum of UR is quasi
monochromatic, if the aperture of the XY slit is adjusted appropriately. Its
bandwidth gets narrower for smaller emittance and energy spread of theelectron beam.
Summarizing the above discussions, undulators are superior to
wigglers in terms of not only the brilliance but also the heat load reduction.
I-2.1.5. Shortest Magnetic Period Available in SPring-8-II
As explained in the former two sections, undulators are much moreadvantageous than wigglers especially in the low-emittance storage ring tobe achieved in SPring-8-II. Now we have to consider how to compensate theelectron energy reduction, or more specifically, how we can shorten themagnetic period of undulators.
In fact, the shortest magnetic period to be available in SPring-8-II
can be much shorter than that in SPring-8, because the possible minimum
gap is expected to be narrower because of two factors. One is the shorter ID
length and the other is the shorter vertical betatron function at the center ofthe straight section. As a result, the betatron function at the end of an ID,
which determined the available minimum gap, becomes shorter than thecurrent value by nearly 40 %. Based on the operational experience with theexisting storage ring, the minimum gap available in SPring-8-II has been
determined to be 5 mm, which is much narrower than the current minimum
gap of 7 mm and thus the magnetic period of undulator can be shorter
105

accordingly. It should be also mentioned that the shorter period results in alarger number of periods, and thus higher flux and brilliance as well.
The bottom line is that undulators are supposed to be the main IDs
in SPring-8-II and the choice of wigglers should not be made unless theusers request special optical properties that cannot be provided byundulators, such as a white spectrum or broad spatial profile.
In the following sections, specifications of undulators to beinstalled in several typical beamlines are presented together with thecomputation results of expected light source performances.
I-2.2. Hard X-ray Beamline
Firstly, specifications of undulators for hard x-ray beamlines(HXBLs) are discussed. In SPring-8, in-vacuum undulators (IVUs)[Hara1998a] have been aggressively adopted in pursuit of shortening themagnetic period. We will keep this concept as well in SPring-8-II, or rather,will explore utilization of more advanced form of IVUs, i.e., the cryogenic
permanent magnet undulators (CPMUs) [Hara2004], toward further
shortening of the magnetic period.
I-2.2.1. Selection of Permanent Magnet Material
Before discussing the specifications for HXBLs, we need toconsider the permanent magnet (PM) material for undulators in
SPring-8-II.
Concerning the selection of PMs for IVUs, we have to keep in mind
that they will undergo high-temperature bake-out processes and will be
continuously placed in a high dose environment inside the storage ring.
These two factors can lead to significant demagnetization of PMs anddegradation of undulator performance. In order to avoid such a problem, we
have to make sure that the coercivity of PM material, which is related toresistivity against demagnetization, should be high enough. In SPring-8, we
have a criterion that the coercivity of PMs for IVUs should be at least higher
than 2000 kA/m. In addition, all the PMs are annealed at a temperatureslightly higher than that at the bake-out process in advance of assembly.
106This thermal process improves the resistivity against demagnetization due
to two sources, i.e., high temperature at the bake-out process and radiation
damage during operation [Bizen2003]. Although we have never encountereda serious problem due to radiation damage of PMs in IVUs in SPring-8, we
have to carefully examine if the above criteria and schemes are reallysufficient, based on the operational experience of IVUs in SPring-8.
One good example may be the long-term change of integrated fieldobserved specifically in the IVU for BL10XU, which might be probably due
to radiation-induced demagnetization [Tanaka2011a]. After some inspection,
it was found that the IVU was misaligned in the vertical direction, leading
to reduction of the effective gap. Namely, the IVU had been operated at theeffective minimum gap of 6.5 mm for many years, which was the narrowest
aperture in the ring. As a result, a large fraction of electron loss took placeat the entrance and damaged the PMs. Note that the demagnetization was
localized at the entrance, and the light source performances such as the
spectral and spatial profiles were not deteriorated at all.
The above experience suggests that the selection and thermal
treatment of PMs are valid, if the condition in SPring-8-II is similar to thatin SPring-8. The impact of the narrower minimum gap supposed in
SPring-8-II may not be large, because it has been determined according to
the shorter ID length and vertical betatron function. If we keep the criterionon the coercivity (> 2000 kA/m), we can make use of PMs with the averageremanence of 1.25 T, which is about 10 % higher than that of PMs for IVUs
in SPring-8. This improvement comes from the year-by-year advancementof PM technology such as the Dysprosium-diffusion technique.
As for the selection of PMs for CPMUs, we do not have to take careof the radiation damage too much, because the coercivity is drastically
improved by cooling down the PMs to a cryogenic temperature. The only
condition that the PM material should satisfy is that the PMs are notdemagnetized during assembly at room temperature, when a strong
demagnetizing field can be applied to the PMs. With this in mind, we canmake use of PMs with the average remanence of 1.5 T at a cryogenic
temperature.
107

Note that both of the above values, 1.25 T for IVUs and 1.5 T for
CPMUs, are rather conservative, and PM materials with higher remanence
may be available, according to the advancement of PM technology.
I-2.2.2. Magnetic Configuration and Period
Having specified the remanence of PM material, let us discuss thepossible magnetic period of IVUs for SPring-8-II. There are two familiar PM
configurations to generate the sinusoidal field distribution necessary forundulators: Halbach and hybrid. For short-period undulators, the latter
configuration is more advantageous in terms of the achievable field strengthand feasibility of field correction, which is thus adopted in IVUs for HXBLs.

Fig. I-2.2.1: (a) Maximum K value at the minimum gap and (b) energy
range available by the fundamental radiation, computed as a function of
the period length.

108

In order to select the magnetic period, we have to consider the
tunable range of UR under the boundary conditions such as the minimum
gap and remanence of PM material. In other words, we need to estimate themaximum field strength, or more specifically, the maximum K value
generated at the minimum gap, which has been carried out by numericalcomputation with the code RADIA [Chubar1998], with an assumption thatthe design of individual PM units are conventional, i.e., no special technique
is applied to enhance the field strength such as additional PMs or inclined
magnetization.
The computation results are shown in Fig. I-2.2.1 (a), where themaximum K values 𝐾𝑚𝑎𝑥 are plotted as a function of the magnetic period

for two different values of remanence (Br). The lowest and highest

fundamental photon energies of UR, ℏ𝜔1𝐿 and ℏ𝜔1𝐻 , are calculated bysubstituting 𝐾 = 𝐾𝑚𝑎𝑥 and 𝐾 = 0 into the well-known formula of thefundamental photon energy of UR,

4𝜋𝛾2 𝑐𝜔1 = 𝜆 (1+𝐾2 /2),𝑢(I-2.2.1)

where 𝛾 = 1.174 × 104 is the Lorentz factor of the 6-GeV electron beam.
The results are shown in Fig. I-2.2.1 (b) for the two values of Br. For

example, the selection of IVU (Br = 1.25 T) with 𝜆𝑢 = 18 mm results in
𝐾𝑚𝑎𝑥 = 1.66, and the fundamental radiation ranges from 8 keV to 19 keV.

Based on these results, the selection of the magnetic period is

discussed for several typical HXBLs in the following sections.
I-2.2.3. Standard Undulator for Wide Tunability

Firstly, we consider the magnetic period of “standard undulator”,
which is supposed to be the main light sources in SPring-8-II. The mostimportant specification of the standard undulator is that it should be widely
tunable to be adapted to a variety of SR applications. This means that thetuning curves of respective harmonics of UR connect continuously with eachother without no energy gap. Here, the tuning curve of a specific harmonic
denotes the relation between the available photon flux or brilliance and theharmonic photon energy.
109One criterion to realize the above condition is, roughly speaking,that the maximum K value is larger than 2.3 as indicated by the dashed line
in Fig. I-2.2.1 (a), so that the tuning curves of the fundamental and 3rd
harmonic are connected smoothly. In addition, the tunable range should go
below 5 keV, which is another criterion for the standard HXBL adopted in
SPring-8. We find that the magnetic period of the standard undulator
should be longer than 22 mm for IVUs and 20 mm for CPMUs, in order tosatisfy the above conditions.
Now let us discuss the utilization of CPMUs in SPring-8-II. We
have two points to be concerned about in adopting the CPMUs as standard
undulators: initial and running cost of the device, and feasibility of mass
production.
We first consider the cost of the device. In order to cool down thePMs to a cryogenic temperature, we have two possible solutions. One is toutilize refrigerators, and the other is to circulate liquid nitrogen (LN2). Notethat the LN2 circulation system will be also required for cooling themonochromator in each beamline. It is thus possible to reduce the initialand running costs in the latter solution (LN2 circulation), if we can sharethe infrastructure to provide LN2. If this is not possible, we have to install alarge number of refrigerators, which increase not only the initial but also
the running cost, because they require regular maintenance every year or
more frequently. This is not a realistic scenario.
Next, we consider the feasibility of mass production. In
manufacturing IDs, we have a process called field correction, in which themagnetic errors of IDs are corrected so that the resultant radiation
characteristics are close to ideal ones. The errors can be roughly divided into
two types, i.e., local and global errors. The former refers to the short-range
field errors arising from the machining error of individual magnet units andthe magnetization error of non-ideal PMs. The latter refers to thelong-range field errors arising from the gradual gap variation along thelongitudinal axis, which is usually caused by misalignment of mechanicalcomponents of IDs.
In the case of CPMUs, the local error can be corrected by the110normal procedure at room temperature, while the global error should becorrected based on the field distribution measured at the cryogenic
temperature [Tanaka2009]. As a result, the field correction process of
CPMUs usually takes a much longer time than normal IVUs. This may be a
big obstacle to the mass production of CPMUs as standard undulators
within the limited period of construction.
The final decision should be made based on the performancecomparison together with the technical issues discussed above.
I-2.2.4. Short-Period Undulator for a Specific Energy Range
If the energy range of interest can be confined to a narrower one,the magnetic period can be shorter to improve the brilliance. This concept
has been already applied to several beamlines in SPring-8. For example in
BL35XU, an IVU with a magnetic period of 20 mm is installed, which covers
the photon energy range from 14.4 keV to 26 keV.
We can apply the same concept to HXBLs in SPring-8-II, which are
dedicated to a specific energy range. It is supposed that the number of such“dedicated” beamlines can be limited and thus utilization of CPMUs isvalidated.
As an example, let us consider the case when the energy range ofinterest is between 10 keV and 20 keV. Figure I-2.2.1 (b) indicates that a
CPMU with the magnetic period of 16 mm is enough to cover the whole
range. As shown later, the brilliance in this energy range is enhance by afactor of 2~3 compared to the standard undulator.
I-2.2.5. Adaptation to Higher-Energy Region
In SPring-8 BL08W, the only wiggler is installed as a light source
to provide hard x-ray photons with energies up to 300 keV. As mentioned in
section I-2.1.2, wigglers will not be adopted in SPring-8-II unless there exist
a special reason for that. For hard x-ray applications like those in BL08W,
utilization of high-harmonic UR is strongly recommended instead of WR.
If a variable polarization option is requested as in BL08W, we needto consider a novel magnetic configuration together with its operation,111

because the phasing motion, i.e., moving the undulator magnets in thelongitudinal direction to control the polarization state, is not feasible under
the ultra-high vacuum condition, as long as the conventional magnetic
configuration is assumed. Note that the polarization control based on a
dedicated undulator is not necessary in the energy region below 30 keV,
where a polarization control technique based on a crystal phase retarderusing the Bragg diffraction has been realized and routinely used.
I-2.3. Soft X-ray Beamline
Next, specifications of undulators for soft x-ray beamlines (SXBLs)
are discussed. In SPring-8-II, the energy range of SXBL is defined to be
between 250 eV and 2 keV.
I-2.3.1. Design Consideration
The reduced electron beam energy of 6 GeV in SPring-8-II is still sohigh that a high K value is required to lower the fundamental energy of UR
down to 250 eV. Assuming a conventional undulator, this results in asignificant increase in the intensity of high harmonic radiation, which turns
to heat sources in optical elements. The undulator specification should be
defined to avoid such a heat load problem especially in SXBLs. In addition,
the undulator should have a capability to control or at least select thepolarization state unlike those in HXBLs, because the polarization control
technique, such as the crystal phase retarder in the hard x-ray region, is not
yet established in the soft x-ray region. In general, the so-called APPLE
undulator [Sasaki1993] can be selected to produce a variety of polarization
states. It should be noted, however, that the heat load in the linear
polarization mode of this type of undulator can be significantly large when ahigh K value is applied, and thus it is impractical to be adopted in SXBLs in
SPring-8-II for polarization control.
In order to solve the problem, special undulators designed toreduce the heat load have been installed in SPring-8 and successfully
operated for many years: the helical undulators [Hara1998b] for circularpolarization and figure-8 undulators [Tanaka1995] for linear polarization,
112

which are supposed to be the main IDs for SXBLs as well in SPring-8-II. In
the former, the helicity of circular polarization can be flipped by the phasing
motion. In the latter, the vertical or horizontal linear polarization can beselected by changing the undulator gap together with an adequate selectionof the photon energy. It is not possible, however, to switch from the linear tocircular polarization only with these undulators, and thus each SXBL in
SPring-8 is currently confined to either of the linear- or circular-polarization
application except a few exceptions.
In order to solve the problem, a new undulator scheme called
helical-8 undulator has been proposed [Tanaka2011b], which enables thepolarization control with the heat load being kept low. In this scheme, it ispossible to switch the device from the helical undulator to the figure-8
undulator by means of a simple mechanical motion. Furthermore, inclined
linear polarization with an arbitrary angle is available in the figure-8
undulator mode. Feasibility of this new undulator scheme has been recently
tested using a prototype device constructed based on a preliminary design,and the results have been found to be promising [Kuroda2013]. Thus,
introduction of this undulator scheme is seriously considered in SPring-8-II
SXBLs, toward a flexible control of polarization states.
Now let us consider the practical design of undulators for SXBLs.
Assuming that the undulator should be equipped with the mechanicalsystem to control the phasing motion, the in-vacuum structure is not
feasible. We assume that the minimum gap of out-vacuum devices inSPring-8-II is 12 mm, which has been deduced by extrapolating the current
value of 20 mm in SPring-8, based on the consideration on the in-vacuum
devices.
As an example, we consider the possible period length in the caseof helical undulators based on the current design [Hara1998b] adopted in
SPring-8. Note that this design can be easily modified to the other possible
solutions, figure-8 and helical-8 undulators.

113

Fig. I-2.3.1: Maximum K value and lowest photon energy as a function of
the period length, computed for the helical undulators for SXBLs.
The computation results are shown in Fig. I-2.3.1, in which the
maximum K value and corresponding fundamental photon energy areplotted as a function of the undulator period. In order to satisfy thecondition of the minimum photon energy of 250 eV, the undulator period
should be longer than 77 mm.
I-2.3.2. Polarization Control Option
Although the helicity can be flipped by means of phasing motion of
the helical (or helical-8) undulators, its switching speed cannot exceed 0.1
Hz at maximum, because it is carried out mechanically. In several
experiments such as those with magnetic circular dichroism, thesignal-to-noise ratio of the measurement strongly depends on the switching
speed. For the purpose of improving the speed, a fast helicity switchingsystem, composed of two undulators with opposite helicity and five kicker
magnets, has been developed in SPring-8 and implemented in BL25SU[Shirasawa2003], which is currently operated at the maximum switching
speed of 10 Hz. In order to install the same system in the normal straight114

section of SPring-8-II, which is about 1 m shorter than that of SPring-8, weneed to make the kicker magnets much more compact than the current
ones.
Although the above helicity switching system with kicker magnets
has been operated for more than 10 years and is reliable enough, it is noteasy to achieve a switching speed higher than the current maximum valueof 10 Hz. In order to aim at a much higher switching speed, a new helicity
switching system called a segmented crossed undulator [Tanaka2002],
which has been installed and under commissioning in SPring-8 BL07LSU,
is considered. It is composed of several undulator segments, half of which
produce linear polarization and the rest produce vertical polarization, andelectromagnet phase shifters placed in between. In order to apply thisconcept to the normal straight section of SPring-8-II, downsizing theelectromagnet phase shifters and shortening the magnetic period are
crucial, because all these components should be packed into the limited
space of 4.7 m. If necessary, the possibility of modifying them to in-vacuum
devices is to be explored.
I-2.4. Expected Light Source Performances
In this section, light source performances expected in the typicalbeamlines described in the previous sections are presented, together with
the BM beamlines. The accelerator and undulator parameters to be used in
the computations are summarized in Table I-2.4.1 and Table I-2.4.2.
As shown in Table I-2.4.2, two sets of parameters have been
prepared for HXBLs. “HXBL-A” is an alternative to the standard undulator
beamline of SPring-8, while “HXBL-B” is a beamline dedicated to theconfined energy region (10 keV ~ 20 keV).
The light source performances have been computed with theseparameters and compared with those in SPring-8 BL03XU (standard
beamline), BL35XU, BL25SU (soft x-ray beamline), and BM2 (BMbeamline).

115

Table I-2.4.1: Beam and Twiss parameters at straight and bending sections
in SPring-8-II.
Beam Parameters

Twiss Parameters

Energy (GeV)

6

Straight

BM2

Average Current (mA)

100

βx (m)

5.5

0.93

Natural Emittance149

βy (m)

3.0

1.88

Coupling Constant

0.1

αx

0

-0.029Energy Spread

0.00093αy0-0.015ηx (m)

00.0016ηx’

00.0013(m.rad)Table I-2.4.2: Light source specifications in typical beamlines.
Beamline ID Type

PeriodMin. Gap

Max. K(mm)(mm)(Field)Length (m)HXBL-A

IVU

22

5

2.33

3.6

HXBL-B

CPMU

16

5

1.61

3.6

SXBL

Helical-U

80

12

4.45

3.6

BM2

BM

-

-

(0.95 T)-In what follows, the hard X-ray beamlines both in SPring-8-II and SPring-8
are collectively called HXBLs, the soft X-ray beamlines are called SXBLs,
and bending magnet beamlines are called BMBLs.
I-2.4.1. Reconsidering the Light Source Properties
Before describing the actual light source performances expected in
SPring-8-II beamlines, let us first reconsider the numerical methods toestimate the properties of SR, which is important to design a number ofcomponents to be installed in the beamline. For this purpose, several
numerical codes have been developed up to now, in which the numericalmethods can be generally divided into two types.
116

One is a rigorous method based on the Maxwell equation andspecial theory of relativity, which does not require any assumptions andapproximations except common ones in SR facilities such as that theelectron beam is relativistic. The numerical process in this method is rather
straightforward: the electric field of radiation emitted from a single electron
is computed first, then its square is integrated over time to compute theradiation power density, or it is Fourier-transformed to compute the photon
flux density. The effects due to the finite emittance and energy spread of theelectron beam is taken into account by computing the convolution integral.
Computations of photon flux and radiation power are usually carried out
based on this direct method.
The other is an approximate method, which is based on someapproximations and assumptions in addition to the direct numerical
processes explained above. For example, the brilliance and transverse
coherence of SR are in general computed in this manner, because theseproperties cannot be directly computed from the electric field of radiation,
and a rather complicated set of mathematical operations is required toderive their exact forms. In order to avoid such a complicated process, theSR emitted from a single electron is approximated by a coherent Gaussianbeam, whose angular divergence is defined to reproduce the angular profile
of SR, and source size is determined from the theory of diffraction limit[Kim1989]. Once such an approximation is made, it is easy to compute theconvolution with the electron beam, whose distribution function is also
given as a Gaussian function.
Although the Gaussian approximation explained above has been
widely used to evaluate the performances of light sources in many SR
facilities, the computation results should be regarded only as a roughestimation. In particular, the accuracy and reliability of this method are notclear for SR facilities that accommodate the low-emittance storage ring like
SPring-8-II.
In order to solve the above problems, numerical methods have been
recently developed to significantly reduce the numerical cost, which is basedon the formulation of complex amplitude of radiation right at the source
117point (center of the SR source), and efficient computation of the Wigner
function and its convolution with the electron beam [Tanaka2014]. The
methods have been implemented in SPECTRA for a more rigorous
characterization of SR.
In the following sections, the light source properties expected intypical beamlines in SPring-8-II, in comparison with the current values inSPring-8, are presented in terms of the brilliance, total flux, heat load,source distribution and transverse coherence.
I-2.4.2. Brilliance
The brilliance is defined as the maximum value of the phase-spaceden sity, i.e., the photon flux density represented in the 4-dimensional phasespace spanned by the positional and angular coordinates 𝒓 = (𝑥, 𝑦) and𝜽 = (𝑥 ′ , 𝑦′). Although the meaning of the phase-space density is clear ingeometrical optics, we have to consider its computation in the framework ofwave optics for a rigorous characterization of SR. In practice, themathematical form to compute the phase-space density is given by theWinger function 𝑊(𝒓, 𝜽) defined by𝑊(𝒓, 𝜽) = 𝜆2 ∫ 𝑑𝜽′ ℰ ∗ (𝜽 + 𝜽′ /2)ℰ(𝜽 − 𝜽′ /2)exp (−𝑖𝑘𝜽′ ∙ 𝒓)
1

(I-2.4.1)

where ℰ(𝜽) is the complex amplitude of radiation. The phase-space density

is proportional to the Wigner function and the proportional coefficient
should be defined so that the resultant phase-space density has the proper
unit.
Note that the above definition is for the special case of radiation
emitted from a single electron. In practical, we have to take a convolution
with the distribution function of the electron beam. Then the brilliance isgiven by the on-axis Wigner function 𝑊(𝟎, 𝟎) multiplied by the
proportional coefficient.

Figure I-2.4.1 shows the brilliance curves expected in the typicalbeamlines in SPring-8-II in comparison with those currently available inthe corresponding beamlines in SPring-8. Note that the brilliance curvemeans a graphical plot to show the available brilliance as a function of the118photon energy. This is exactly the same as a spectrum in BMBLs, while itgives the maximum brilliance at a given photon energy retrieved from anumber of spectra for different K values. Note that brilliance curves in
HXBLs are composed of many harmonics. For example in HXBL-A, thefundamental radiation covers from 5 keV to 13keV, the 3rd harmonic
radiation from 13 keV to 26 keV, and so on. On the other hand, only thefundamental radiation covers the whole range in SXBLs.

Fig. I-2.4.1: Expected brilliance curves in typical beamlines in SPring-8-II
in comparison with those in SPring-8.
The solid lines indicate the brilliance curves in SPring-8-II, while
the dashed lines those in SPring-8. Note that the horizontal apertures in
BMBLs are assumed to be 0.1 mrad in both cases.
The brilliance enhancement in HXBLs by upgrading from SPring-8
to SPring-8-II depends on the photon energy. Around 10 keV, theenhancement factor reaches nearly 20, which gradually drops for higher
energy regions. We find a similar trend in the BMBLs. On the other hand,the enhancement is nearly constant in the whole energy range in SXBLs.
These trends of the brilliance enhancement are attributable to thedifference in the total flux. Refer to section I-2.4.3 for details.
119Fig. I-2.4.2: Expected total flux in typical beamlines in SPring-8-II incomparison with those in SPring-8.
I-2.4.3. Total Flux
Figure I-2.4.2 shows comparison of total flux curves, computed in
the same manner as the brilliance curve. Because the total flux is obtainedby integrating the angular flux density over the whole solid angle, it
depends mainly on the electron energy and beam current, and does notdepend on the emittance of the electron beam. As a result, the difference
between SPring-8-II and SPring-8 is not so significant as in the case of thebrilliance.
It should be mentioned that the total flux available in SPring-8-IIin the high energy regions above several 10 keV is less than that inSPring-8. This is attributable to the fact that the total flux rapidly
decreases as the photon energy above the critical energy of SR, which is
proportional to the square of the electron energy. The reduction of theelectron energy thus gives rise to the loss of flux in the high energy region
as well as the improvement of the electron beam quality.
120

I-2.4.4. Heat Load
In addition to the light source performances described above, theheat load should be also investigated as explained in section 0, in particular,
in the undulator beamline. Figure I-2.4.3 (a) shows the total radiation
power as a function of the fundamental energy in the undulator beamlines,
indicating that the maximum total power in SPring-8-II is of the same orderof SPring-8, and thus the high heat load component such as the XY slit orabsorber in the front end section may not be necessarily upgraded.
Concerning the heat load on optical elements such as the mirror
and monochromator, we need to check the partial radiation power passing
through the XY slit to eliminate the off-axis power. The calculation results
are plotted in Fig. I-2.4.3 (b) as a function of the fundamental energy. The
aperture size of the XY slit is assumed to be four times the photon beam sizeat the fundamental energy so as not to lose the available flux too much. The
heat loads in the SPring-8-II beamlines are found to be reduced by a factorof ~4 in HXBLs and ~2 in SXBLs compared to those in the corresponding
beamlines in SPring-8. This will greatly contribute to the improvement ofstability of optical elements.
From the above discussions, it is now clear that the heat load
reduction using the XY slit works better in SPring-8-II than in SPring-8
thanks to the better electron beam emittance.

121Fig. I-2.4.3 (a): Total and (b): partial radiation power as a function of thefundamental photon energy in typical undulator beamlines in SPring-8-II
in comparison with those in SPring-8. Note that the vertical axes havedifferent scales.
I-2.4.5. Source Distribution
The source distribution refers to the distribution function of thephase-space density at the source point, i.e., the center of the SR source,which is directly related to the Wigner function 𝑊(𝒓, 𝜽). Because 𝑊(𝒓, 𝜽) isa function of four coordinate variables and is not convenient to visualize, we
evaluate the projected Wigner functions defined as𝑊ℎ �𝑥, 𝑥 ′ � = ∬ 𝑊�𝑥, 𝑦, 𝑥 ′ , 𝑦 ′ �𝑑𝑦𝑑𝑦 ′ ,𝑊𝑣 (𝑦, 𝑦′) = ∬ 𝑊�𝑥, 𝑦, 𝑥 ′ , 𝑦 ′ �𝑑𝑥𝑑𝑥′,(I-2.4.2)(I-2.4.3)

to show the source distribution, instead of visualizing the Winger function122

𝑊(𝒓, 𝜽). The functions 𝑊ℎ and 𝑊𝑣 give the source distributions in thehorizontal and vertical phase spaces, if 𝑊(𝒓, 𝜽) can be approximated by thedecoupled form defined by

𝑊𝑑 =

𝑊ℎ 𝑊𝑣
𝐹

,

(I-2.4.4)

where 𝐹 denotes the effective total flux obtained by integrating 𝑊(𝒓, 𝜽)over the whole phase. In practice, this approximation is usually valid under
the practical condition that the electron beam emittance is larger than, or atleast comparable to the optical emittance of diffraction-limited light[Tanaka2014]. In what follows, the source distributions in the horizontal
and vertical phase spaces are evaluated using these two functions.
Figures. I-2.4.4~I-2.4.6 show the comparisons of the source
distributions for the three beamline HXBL-A, SXBL, and BM2 in
SPring-8-II, with those for BL03XU, BL25SU, and BM2 in SPring-8,
respectively. In each figure, (a) and (c) denote the distributions in thehorizontal and vertical phase space for the SPring-8-II beamlines, while (b)and (d) denote those for the SPring-8 beamlines.
Figure I-2.4.4 shows the source distributions for HXBLs at thephoton energy of 10 keV. The comparison between (a) and (b) indicates asignificant improvement of the source size (width in the x direction) and aslight improvement of the angular divergence (width in the x’ direction) in
the horizontal direction, which comes from the reduction of the horizontal
emittance and betatron function. On the other hand, the vertical source size
and angular divergence becomes slightly larger, as found from thecomparison between (c) and (d). This reflects the larger coupling constant(and vertical emittance) of SPring-8-II than that of SPring-8.

123

Fig. I-2.4.4: Source distributions for HXBLs at 10 keV.
It is interesting to note that the vertical source distribution isslightly different from a Gaussian function as opposed to the horizontal one.
This derives from the fact that the Wigner function of UR from a single
electron is far from a Gaussian function. Because of the low emittance in thevertical direction, the original non-Gaussian profile survives even after
convolution with the Gaussian distribution function of the electron beam.
Figure I-2.4.5 shows the source distributions for SXBLs at thephoton energy of 500 eV. As in the case of HXBLs, the horizontal source size
is significantly improved, while the other properties do not change largely.
This means that the electron beam properties except the horizontal beam
size do not contribute to the source distribution in this condition. Also notethat the non-Gaussian nature of the vertical source distribution as shown in(c) and (d) is more pronounced. This may have to be taken into account for
designing the optical elements in the SXBLs.

124

Fig. I-2.4.5: Source distributions for SXBLs at 500 eV.

Fig. I-2.4.6: Source distributions for BMBLs at 10 keV.
125

Figure I-2.4.6 shows the source distributions for BMBLs at thephoton energy of 10 keV with the horizontal aperture of 0.1 mrad. The
comparison between (a) and (b) indicates a significant improvement of theangular divergence as well as the source size in the horizontal direction. Not
only the emittance reduction but also the modification of the betatronfunction contributes to this improvement. In the vertical direction, thesource size is reduced as well, while the angular divergence grows. Notethat both of them are not related to the emittance change. The former comesfrom the shorter betatron function, while the latter comes from the reduced
electron energy: the angular divergence of BM radiation is inversely
proportional to the Lorentz factor.
I-2.4.6. Transverse Coherence
The source distribution presented in the former section can beextended to a more quantitative discussion in terms of the transverse
coherence.
In general, the degree of transverse coherence 𝜇 is given as afunction of two different points 𝒓1 and 𝒓2 and thus is not convenient todescribe the light source performance. We thus introduce the overall degreeof coherence ζ instead of 𝜇(𝒓1 , 𝒓2 ) , in order to denote the coherence

property in a specific beamline, which is actually defined as a spatial

average of 𝜇 2 (𝒓1 , 𝒓2 ), and can be computed using the Wigner functions asfollows [Bazarov2012]𝜆 2

ζ = � � ∬ 𝑊 2 (𝒓, 𝜽)𝑑𝒓𝑑𝜽.
𝐹(I-2.4.5)

Assuming that the Wigner function can be decoupled, ζ is also
given as a decoupled form ζ = ζ𝑥 ζ𝑦 , whereζ𝑥 = 𝐹2 ∬ 𝑊ℎ 2 �𝑥, 𝑥 ′ �𝑑𝑥𝑑𝑥 ′ ,(I-2.4.6)

ζ𝑦 = 𝐹2 ∬ 𝑊𝑣 2 (𝑦, 𝑦′)𝑑𝑦𝑑𝑦′,(I-2.4.7)

𝜆

and

𝜆

are referred to as horizontal and vertical degrees of coherence.
126

Figure I-2.4.7 (a) and (b) show the degrees of horizontal andvertical coherence computed using the above formulas with the same
conditions as those used in Fig. I-2.4.1. We can expect a significant
improvement of the horizontal coherence by a factor of nearly 20 inundulator beamlines, and 10 in BMBLs. The vertical coherence in SXBLs do
not change at all and that in HXBLs slightly gets worse, while that in
BMBLs is improved by a factor of 2 because of the shorter betatron function
in SPring-8-II, which are already addressed in section I-2.4.5.

Fig. I-2.4.7: Expected degree of (a) horizontal and (b) vertical coherence intypical beamlines in SPring-8-II in comparison with those in SPring-8.
I-2.5. Facilitating the Mass Production and Installation of IDs
As already mentioned in section I-2.1.1, we need to replace a lot of
existing IDs because of the shorter straight sections in the upgraded storage
ring. This means that we have to construct a lot of new IDs, or at least
127

refurbish existing IDs, within a limited period for upgrading. It is thus
important to consider the method to facilitate the mass production andinstallation of IDs, which also helps to save the total cost of upgrading.
I-2.5.1. Issues on the Current ID Design
Let us first discuss existing issues on the current design of IDs,
especially IVUs, the structure of which is schematically illustrated in Fig.
I-2.5.1.
The magnetic array that generates the sinusoidal field distribution
is composed of a lot of magnet units mounted on the inner beam. The
magnet units are regularly arranged along the longitudinal axis withalternating polarity. The inner beam located inside the vacuum chamber isusually made of aluminum alloy and is precisely machined to secure the flat
surface. It is connected to the outer beam in air through a number of
support shafts. The outer beam is driven by mechanical systems to allow for
the gap motion. The overall system is supported by the pillars mounted on
the common base.

Fig. I-2.5.1: Schematic drawing of the structure of IVUs based on thecurrent design.
128

Now the technical issues coming from the structure of IVUs basedon the above design are addressed. See also Fig. I-2.5.2 for reference.

Fig. I-2.5.2: Diagram to explain the existing issues concerning the current
ID design.
It is well known that the attractive force between the top andbottom magnetic arrays, which actually depends on the gap and period of
the ID, typically reaches several tons. Such a strong attractive force
imposes two requirements on the specifications of IVUs. One is to avoid thedeformation of magnetic arrays, which spoils the gap uniformity along thelongitudinal axis and gives rise to a large global field error. The other is tosupport the magnetic arrays under a large mechanical load.
The former requirement can be met by increasing the number offixation points and distributing the mechanical load along the inner beam
as uniformly as possible (a-1). This in turn grows the number of components
such as the support shafts (a-2), which imposes tighter tolerance ofmachining and assembling the components (a-3), and requires acomplicated structure (a-4).
The latter requirement can be met by adopting a mechanical frame
which is rigid enough to hold and move the magnetic arrays precisely under129

the strong attractive force (b-1). As a result, the mechanical frame becomes
necessarily large and massive (b-2), which requires complicated procedures
for alignment and transportation of the device (b-3).
I-2.5.2. Structural Reform
All the problems explained in the previous section obviously comefrom the strong attractive force. In other words, we can expect a better
solution, if the attractive force can be cancelled by some means. To be more
specific, the mechanical frame can be much lighter, simpler, and morecost-effective, because it does not have to be too rigid any longer. Such astructural reform of IDs will contribute to shortening the lead time for themass production and facilitating the transportation and alignment.
The key issue toward the structural reform is the development of
the system to cancel the attractive force over the whole operational gap
range. We need to generate a repulsive force which depends on the gap
identically to the attractive force by the ID magnet array, which is actuallyan exponential function of the gap.

(a)

(b)

Fig. I-2.5.3: Possible solutions to cancel the attractive force of ID magneticarrays: (a) mechanical and (b) magnetic.

130

Figure I-2.5.3 (a) shows a mechanical system based on springs
attached to the both ends of the magnetic array to cancel the attractive force.
Note that the repulsive force of the springs should be nonlinear(exponential), which is not easily fabricated. An alternative way based on
the combination of a number of springs with different properties[Marcouillé2010] may help. In any case, we have a concern about thelong-term stability of springs. In addition, precise field measurement, whichis indispensable for the field correction process of IDs, is another challenge
under the enclosed environment.
Another way as shown in Fig. I-2.5.3 (b) is to take advantage of themagnetic system, in which additional magnetic arrays to generate arepulsive force are attached to the main (central) magnetic array. The
additional magnetic arrays should have the same magnetic properties as
the main one, so that the attractive force is cancelled. In principle, thismethod will completely cancel the attractive force. It should be noted,
however, that we need four more magnetic arrays in addition to the twomain ones, which considerably increases not only the total cost but also thenumber of processes for manufacturing the IDs.
In order to overcome the above difficulty toward realization of theattractive force cancellation system, a new approach is under investigation,
which is based on the magnetic method but is much more cost-effective than
the above scheme. In a couple of years, we will demonstrate the new systemand then build a prototype device based on it, toward the structural reformof IDs in prospect of utilization in SPring-8-II.
I-2.5.3. Revision of the Field Correction Procedure
The field correction is an important process to eliminate theunwanted effects due to magnetic errors, which requires a precise
measurement of the field distribution. For this purpose, the ID is usuallytransported to the field measurement laboratory which is equipped with anumber of magnetic sensors and several multi-axis actuation stages to movethe sensors precisely.

Then alignment of the ID and measurement

instrument is made to perform a precise and reliable measurement.
131In the mass production of IDs for SPring-8-II, we need to repeat
the field correction process so many times. To be specific, after completingthe field correction, the ID is taken away from the measurement laboratory
to be installed in the accelerator tunnel, or moved to the storage area. Then
next ID is installed in the laboratory to continue the field correction.
In the above process, we waste a lot of time: during the installation
and alignment of the ID in the laboratory, the measurement apparatus is
not available and the field correction process stops.
One idea to reduce the waste of time in the field correction process
is to install a kind of mechanical frame dedicated to the field correction in
the measurement laboratory. After completing the field correction of a
specific ID, we replace only the magnetic arrays without moving themechanical frame. Then the time for installation and alignment is no longer
necessary, which will significantly save the lead time for mass production ofIDs.
In the above scheme, we have a concern that the global field error
is induced in the final manufacturing process in which the magnetic array isassembled in the other mechanical frame to be actually used. The alignment
accuracy of the support shafts that hold the magnetic array cannot be better
than several microns at the best of times, which can lead to a long-range
gap variation along the longitudinal axis. This kind of global error can giverise to relatively a large phase error and significant degradation of theavailable flux.
In order to solve the above problem, it is necessary to measure thefield distribution after assembly and correct the global error if necessary.
This requires a portable field measurement system and adjustable support
shafts, which are already developed in SPring-8 and already applied to thefield correction in CPMUs [Tanaka2009].
I-2.6. Summary
In this report, we have described the specifications andperformances of new IDs to be replaced with the existing ones. We haveanother option to keep on using them after a minor or major revision.
132

The minor revision refers to the refurbishment only to fit the newstraight section. The magnetic arrays, vacuum equipment, and mechanicalcomponents are disassembled first and then assembled again with reduced
number of components to shorten the whole length. In the major revision,
the magnetic arrays are renewed to have the optimized period in addition to
the refurbishment above.
It should be noted that we are not sure about the total cost andlead time necessary for the above revision, even minor one, because thestructure of the existing IDs is quite complicated as already mentioned andrequires a careful alignment to restore the original performance. The final
decision should be made comprehensively with all the related issues taken
into account.
References[Bazarov2012] I. V. Bazarov, Phys. Rev. ST-AB, 15, 050703 (2012).
[Bizen2003]

T. Bizen et al., Nucl. Instr. and Meth. A515, 850 (2003).

[Chubar1998] O. Chubar et al., J. Synchrotron Radiation 5, 481 (1998).
[Hara1998a]

T. Hara et al., J. Synchrotron Rad., 5, 403 (1998).

[Hara1998b]

T. Hara et al., J. Synchrotron Rad., 5, 426 (1998).

[Hara2004]

T. Hara et al., Phys. Rev. ST-AB 7, 050702 (2004).

[Kim1989]

K. J. Kim, AIP Conf. Proc. 184, 565 (1989).

[Kuroda2013] A. Kuroda et al., Proc. 10th PASJ Meeting, 897 (2013).
[Marcouillé2010] O. Marcouillé et al., Proc. IPAC’10, 3104 (2010).
[Sasaki1993] S. Sasaki et al., Nucl. Instrum. and Meth., A331, 763 (1993).
[Shirasawa2003] K. Shirasawa et al. Proc. SRI2003, 191 (2003).
[Tanaka1995] T. Tanaka et al., Nucl. Instrum. and Meth., A364, 368 (1995).
[Tanaka2001] T. Tanaka et al., J. Synchrotron Rad., 8 (2001) 1221.
[Tanaka2002] T. Tanaka et al., Nucl. Instrum. Meth., A490, 583 (2002).
[Tanaka2009] T. Tanaka et al., Phys. Rev. ST-AB, 12, 120702 (2009).
[Tanaka2011a] T. Tanaka et al, presented in 24th JSSRR Annual Meeting,
Tsukuba (2011) (unpublished).
[Tanaka2011b] T. Tanaka et al., Nucl. Instrum. and Meth., A659, 537 (2011).
[Tanaka2014] T. Tanaka, Phys. Rev. ST-AB, accepted for publication.
133

PART-I
I-3 Control System
I-3.1. Design Overview
I-3.2. Computing and Network
I-3.3. Accelerator Equipment Controls
I-3.4. Beamline Control
I-3.5. Safety Interlock134I-3. Control System
I-3.1. Design Overview
The accelerator control system for the SPring-8-II storage ring isdesigned on the basis of the current control framework, MADOCA[Tanaka1997], with adding the state-of-the-art technologies. The MADOCA
control framework was developed originally at SPring-8, and has beenstably working for more than seventeen years as a keystone in order tocontrol accelerators and beamlines. Recently, MADOCA messaging scheme
was upgraded with ZeroQM middleware to handle variable data structurewith no limitation of a carrying data size [Matsumoto2013]. And also, the
new scheme supports Windows operation system (OS) in addition to theUNIX OS families. Application field of MADOCA is now extended to theexperimental data-acquisition system in SACLA/XFEL in SPring-8.
The cutting edge technologies on the information and theelectronics systems in the market have been making progress rapidly. In thedesign of SPring-8-II control system, we keep the reliable control
components with good record in our operation experience. On the other
hand, we will not use some components such as fairly out-of-datemaintainability or less performance below the required level. Thee-tiercontrol architecture is kept on the bases of distributed control architectureof MADOCA with considering continuity of the facility sustainability. The
resources ever developed so far in SPring-8 will be re-used as much as
possible both on software and hardware for the compatibility and quick
construction of the control and operation system.
We keep the relational database management system (RDBMS) forthe machine configuration data, and also adopt Key-Value Store (KVS) to
handle the machine logging data [Kago2013]. A high-speed network withwide bandwidth and low latency plays a middleware physical layer between
operator consoles and equipment device controllers. The network is built byusing Ethernet technology that is properly segmented and aggregated.
Firewalls are suitably installed for interconnection between network
segments to guarantee access control and security with sufficient
135bandwidth. Control components are selected to fit each accelerator
sub-system on the classification of three levels of control processing
performance; i.e. slow, medium and fast. We deploy the commercial
off-the-shelf (COTS) products as the building blocks of the control system asmuch as possible in order to minimize construction cost and developmenttask/risk.
A programmable logic controller (PLC) will be used for the slow
control subsystems because of easy installation and maintenance. The
VMEbus systems still survive as the well-established system that has been
proven during the long duty operation in the current SPring-8. The VME
will be adopted as a medium speed controller with many of re-usable
software libraries ever built, which will reduce the production time andeffort.
Recently, the demands on higher performance of digital dataprocessing has been growing up, especially at low level RF (LLRF) control,
waveform data handling/analysis and experimental data processing. The
telecommunication packet transaction in the market needs ultra-fast
digital-data processing at the huge scale of communication digital data.
Hence, the data handling in this field requires high-bandwidth cutting-edge
technology

necessarily.

Theemerging

technology

such

as

TCA

(Telecommunications and Computing Architecture) is promising as a
candidate of the high-end control platform. The TCA system carries
advanced mezzanine cards (AMC) that are expected to satisfy the high level
requirements of the digital data processing.
In this section, the design of control system of accelerator,
beamline, computing, network, utilities and safety interlock are described.
I-3.2. Computing and Network
Reliability is important for the computing and network system as
the elements of the controls, such as database and file servers, operation
consoles, and backbone networks. To enhance reliability, we introduce
virtualization technology for various layers of the computing systems
including communication over network, operating system, middleware, and136

application software beyond the hardware physical boundaries. By applying
the virtualization technology, we can suppress the system proliferation, andexpect cost effectiveness and low power consumption consequently.
I-3.2.1. Server System
To achieve 24h365d availability as high enough, computer
hardware will be settled concentrating into a computer room, which isequipped

with

CVCF

power

supplysystemandprotected

byvoltage-drop-compensated electrical power.
Database Server:
The Sybase, a commercial enterprise RDBMS, has been storing
many signal points since 1997. The signals are not only the status ofaccelerator components, but also the facility status such as air conditioner,
cooling water, and electrical powers. The RDBMS has been playing theessential role for the data management in SPring-8 by the standard SQL
data-access to the relational schema.
Recently, Key-Value Store has been growing up at the market
especially for the big data handling. The KVS that provides NoSQL data
access, is suitable for handling of the chronological logging data. Hence, theKVS will be used to manage the on-line and archive database of themachine status logging. The RDBMS is kept to manage the machine
configuration data. A promising database scheme by using KVS will consistof a combination of Redis and Cassandra that is under development inSPring-8 [Kago2013]. The KVS computer system will be composed of agroup of PC assuring redundancy by the quorum determination method.
File Server:
Both high IOPS performance and high capacity are required forfile server, because the file server stores many file types, such as source code
repository, binaries including middleware libraries, and disk image ofvirtual machines.

To satisfy the requirements, the file server will behierarchical storage using both non-volatile memory and high-capacity137magnetic hard disk.
Most of computing system will be virtualized using hypervisor
and/or container technologies. By using virtualization, we can reduce thenumber of hardware, and improve availability simultaneously.
All network connections in the computer room will be based on
10-giga-bit Ethernet (10GbE) or wider bandwidth Ethernet.

In the case of

low-latency control performance such as fast feedback or correction, theInfiniBand will be an alternative choice for interconnection.
I-3.2.2. Central Control RoomThin client and virtual desktop technology will be used for operator
consoles. Using the thin client with virtualized computers in the computerroom, we can improve electric-power consumption. The virtualization
technology also improves availability of the operator consoles. A large-sized
screen, display wall, will be kept at the front of the control room. Operating
status and alarms are displayed on the screen for information sharing by
operators.
I-3.2.3. Network System
The site-wide network system will be applied virtualization
technology to reduce physical devices and to improve reliability. A 10GbE orwider bandwidth Ethernet will be used for trunk route of physical network.
Logical networks, such as accelerator control, experimental control, andexperimental data transfer, are segmented and established on the physicalnetwork. Performance of each logical network will be guaranteed using
quality-of-service technology. For the access control, firewalls will be
installed suitably between the logical networks.
I-3.2.4. Software
The Linux OS family and Solaris that we have been using for thelong time have proven its reliability by ever-lasting good records. The
continuity of the same OS will be the keystone of the compatibility for abunch of running application software. For the core software frameworks, C
138

and C++ standardized computer languages are chosen to guarantee
long-term development.
In addition to the C and the C++ languages, script languages andvisualized languages, such as Python and LabVIEW, will be offered to users
for beamline components and experimental station control [Furukawa2013].
By using the latest MADOCA II framework, users can use Windows PC
directly to operate the beamline components seamlessly, which had not been
supported. To maintain code quality, static and dynamic program analysis
tools will be offered. All versions of source codes and binaries are archived in
the file server.
I-3.3. Accelerator Equipment Controls
An interface from the control system to the accelerator equipment
will be a mixture of the Programmable Logic Controller and the High Speed
Serial Bus System such as MTCA.4 or ATCA, and the VME system. Eachplatform will be adopted on the concept of right platform in the right place.
The PLC will be used for the slow control system for rigid control such as
PID control and local device interlocks. The sequence control of PLC is firm
and robust enough to support device interlock and radiation safety interlock
too. The PLC can be used with Devicenet and/or EtherCAT field bus
interface if it is necessary [EtherCAT_URL]. For the high-speed signal
processing such as the low-level RF control and the beam monitor system,and trigger signal distribution, we will use the xTCA platform implemented
with the A/D, D/A and FPGA, where the TCA platform family is noted as
xTCA. One standard of the xTCA, micro-TCA, showed good performance asapplied to the measurement of the electron beam emittance by using
interferometer [Kiyomichi2013].

The VME system will be re-used for slow

or medium speed control. We have many of well-established software
libraries that are necessary to control the wide variety of VME I/O boards
by using device drivers and application program interfaces.
I-3.3.1. Vacuum System Control
To control the vacuum system including device interlock, a PLC
139system will be used. Each PLC station will be linked with an optical fiber asshown in Fig. I-3.3.1. The FL-net will be a communication interface between
an upper controller and PLC stations [Fukui2004]. The upper controller canbe either a PLC master station or VMEbus system that is connected to theaccelerator control Ethernet for uplink, towards the operation consoles atthe central control room.

Fig. I-3.3.1: Vacuum system control with PLC system.
I-3.3.2. Magnet Power Supply Control
The local interlocks, cooling water flow and magnet temperature,
will be measured and controlled by using PLCs. An optical-linked remote
I/O (RIO) system is used to control the magnet power supplies to avoid
electromagnetic interference. The main magnets, such as bending magnet(BM), quadrupole magnet (QM) and sextupole magnet (SX), will becontrolled in the slow control category by using PLC RIO systems as shownin Fig. I-3.3.2. A PLC master station and intelligent interface boards inpower supplies are connected by optical fibres. The interface board has a
140

FPGA chip that stores a pre-defined control sequence of a power supply. The
sequence is reconfigurable by installing an operation algorithm on the chip,and can be driven by the control commands issued from operator consoles.
The response time is a millisecond or less when there are assuming two or
more magnet power supplies in operation at the same time.

Fig. I-3.3.2: Magnet power-supply control by intelligent interface cards.
For a set of permanent magnets that are optional design of BM, themovable outer plate will be controlled by PLC with the pulse motor
controller via the field bus to reduce the number of cablings. Because of thetemperature dependence of magnetic field, we will measure magnet
temperature with high precision of 0.01 degree. We need a fast control
system for the power supplies of steering magnets (ST) and auxiliary power
supplies of QM because of COD correction and tune correction of stored
electron beams with feedback rate of 100Hz or faster.
I-3.3.3. RF System Control
Figure I-3.3.3 shows a schematic view of the RF control system. A
signal processing board will be designed with the high speed AD / DA andFPGA with the function of a digital down conversion to provide theamplitude and phase control for the acceleration cavities. The xTCA
platform is suitable to handle the RF digital signals of low-level control
components with better performance than the current NIM and VMEbus
141

composite system. We’ll use the intelligent motor controller for the tuner
control via the high-speed field bus, for example Ethercat. Other part of the
RF control, such as klystron power supplies and device protection interlocks
will be built with PLCs. The slow control of RF high-power system is goodenough. And high-power interlocks need robustness and reliability rather
than processing speed. The communication between the rest of the control
system and PLCs will be handled via FL-net for the subsysteminterconnection and data uplink.

Fig. I-3.3.3: RF control scheme.
I-3.3.4. Control System of Beam Position Monitors
The same signal processing board designed for the RF system will
be used for the signal handling of beam position monitors (BPM) as well.
For the global feedback of the stored electron beams, a shared memory
network (reflective memory network, RFM) will be adopted to perform
high-speed feedback to the COD correction as shown in Fig. I-3.3.4. The
beam position data that are taken by the BPM readout systems, and the set
values of the ST power supplies will be used for COD correction to achieve142the better beam optics. The BPM readout systems have to be interconnectedmutually by the same high speed RFM network in order to act on within theminimum synchronization latency. The beam current monitor, tune monitor
and emittance monitor will be built by using the fast platform such as
xTCA.

Fig. I-3.3.4: BPM data readout and ST magnet control scheme by usingreflective memory network.
I-3.3.5. Facility Utility
The accelerator operation conditions are sensitive to theenvironment such as air temperature, stability of power line voltage andcooling water temperature and so on. We will monitor them with required
sampling rate and resolution from the viewpoint of the accelerator control
so that the signals of the facility utilities have to be stored into the same
database with the accelerator control system.

143

I-3.4. Beamline Control
A beamline control system, which controls an insertion device (ID),frontend components and a monochromator, is installed for each beamline[Ohata2001]. The beamline control system consists of two VME systems anda PC for operation. One VME system controls ID and the other one is for thecontrol of the rest of beamline components. The control framework
MADOCA connects the VME controllers and the PC over the network.
However, it is difficult to synchronize the ID gap and the monochromatorusing the Ethernet.

Fig. I-3.4.1: A design of beamline control system for synchronizing the ID,
the monochromator, and detectors.
The following new functions are required for SPring-8-II in order
to use the brilliant light effectively:(a) Synchronous control of the gap of the ID and the monochromator.
(b) Synchronous control of the monochromator and the detector.
144

The function (a) enables the effective use of the SR light becauseuser does not have to wait for satisfying both the monochromator and ID
gap when user changes the photon energy. The ID gap is always kept by theappropriate value with the monochromator. The function (b) enables thequick-scan with changing the monochromator settings, which shorten atime required for the measurement of energy distribution such as XAFS
experiment.
In order to realize the new functions, we designed a new beamline
control system as shown in Fig. I-3.4.1. We will install a synchronous
control system that consists of a BL sync master, some of slave stations anda fast field bus. The slaves control the ID gap, steering magnets for the ID,
pulse motors of the monochromator and triggers for the detectors. The
length of the filed bus is necessary for about 100m, and the system has toachieve the synchronization of the gap and monochromator control within
1ms.
I-3.5. Safety Interlock
A safety interlock system protects users from radiation hazards
induced by electron beam and synchrotron radiations. The schematic viewof the safety interlock system for SPring-8-II is shown in Fig. I-3.5.1 The
system is composed of accelerator safety interlock systems and beamline
interlock systems. The accelerator safety interlock system based on the“area interlock” concept established in 2010 will be utilized [Saji2009]. Eacharea interlock system monitors the status of safety components and controls
the permission signal for the RF located in each area according to the safety
condition of the area. The gun permission signal is sent to the gun safety
interlock. When all gun permissions are established, the gun safety
interlock system permits the gun operation.
The beamline interlock system controls the doors of experimental
hutches, beam shutters, and so on [Matushita2001]. When the safety
condition is not established, the beamline interlock system closes and locks
the beam shutter. In the case of emergency, the beamline interlock system
145

turns off the safety signal that connects to the SR area safety interlock
system, and the SR area safety interlock system stops the gun and RF
permissions immediately.

Fig. I-3.5.1: Interlock system for SPring-8-II.
The PLC systems and custom circuits dedicated for high-speed
processing for a fast beam abort have been utilized for these interlock
system since 1997. The troubles of the interlock systems were mainly due tothe simple malfunctions of the custom circuits in SPring-8 and SACLA. In
SPring-8-II, the custom circuits will be replaced to the up-to-date PLC
systems, which provide high-speed processing better than now, in order toincrease stability of the interlock system.

146

References[EtherCAT_URL] http://www.ethercat.org/en/technology.html[Fukui2004] T. Fukui et al., Proc. of PCaPAC2005, Hayama, Japan, 2004.
[Furukawa2013] Y. Furukawa et al., Proc. of ICALEPCS2013, San
Francisco, USA, 410 (2013).
[Kago2013] M. Kago and A. Yamashita, Proc. of ICALEPCS2013, San
Francisco, USA, 532 (2013).
[Kiyomichi2013] A. Kiyomichi et al., Proc. of ICALEPCS2013, San
Francisco, USA, 786 (2013).
[Matsumoto2013] T. Matsumoto et al., Proc. of ICALEPCS2013, San
Francisco, USA, 944 (2013).
[Matushita2001] T. Matsushita et al., Nucl. Inst. and Meth. in Physics Res.
A, 467-468 (2001) 816.
[Ohata2001] T. Ohata et al., Nucl. Instrum. Methods 467 (2001) 820.
[Saji2009] C. Saji et al., Proc. of ICALEPCS2009 (2009); M. Kago et al.,Proc. of ICALEPCS2009 (2009); M. Kago et al., Proc. of IPAC’10, (2010).
[Tanaka1997] R. Tanaka et al., Proc. of ICALEPCS1997, Beijing, China, 1(1997).

147

PART-I
I-4 Safety Issues
I-4.1.
I-4.2.
I-4.3.
I-4.4.
I-4.5.

Introduction
Radiation Shielding
Safety System
Induced Activity and the Radioactive Waste
Summary148

I-4.1. Introduction
Since 1997, SPring-8 has been operated constantly with 8GeV
electrons without serious radiation problems, and the top-up operation forthe steady stored current at the storage ring has been conducted from2004[Tanaka2004], [Asano2007] . Radiation safety systems and radiation
shielding have been operated and placed without significant changes fromthe beginning of the operation till now. In addition, the injection efficiency
has been much improved rather than the initial establishments. However itis required to reassess the radiation shielding condition in occasion of the
upgrade of SPring-8 to obtain the low emittance of the diffraction limit with
6 GeV electrons without changing the bulk shield, because of the
expectation of severe conditions due to the larger quantity of the electron
injection required to correct the deficit for prospective much shorter life
time. Skyshine dose re-estimation is also required because the dosecriterion at the site boundary has also a strong influence on the shielding
conditions. A large amount of radioactive waste due to the dismantling of
the existing storage ring will be produced so that the precious estimation of
the induced activities is required strongly.
During the past 10 years, the simulation techniques have been
improved drastically by using Monte Carlo methods, therefore thereassessments of shielding design should be performed by using these
updated methods as much as possible [Asano2013] .
I-4.2. Radiation Shielding
For the SPring-8-II design, the upgrade of the bulk shield of thering must be avoided as much as possible. Figure I-4.2.1 shows the design ofthe storage ring of SPring-8, and the shield thicknesses of the side wall are
1 m and 1.65 m ordinary concretes for normal section and injection section,
respectively. The ratchet walls of the storage ring are made of 0.65 m
ordinary concrete and 0.15 m lead. The largest possible number of theaccelerated electron should be injected into the ring on the conditions. To dothis, it is required to reassess the beam loss scenario with the shorter life
time on the basis of the present design criteria and detailed analyses.
149

Fig. I-4.2.1: Drawing of the storage ring building and the experimental hall.
I-4.2.1. Design Criteria
On the basis of ALARA principle and the Japanese law for theprevention from radiation hazards, the design criteria of the SPring-8 site
was authorized as follows [Sasamoto1994] ,(1) For radiation workers, the dose must be less than 50 mSv for every year
(25 μSv/h for 40 hours per week) and 100 mSv for every five years in thelaw so that the design criteria were decided to be 8μSv/h ( 25μSv/h *1/3)(2) At the boundary of radiation controlled area, the dose must be less than
1.3mSv fo r e ve ry thre e mo nths (2.5μSv/h ) and the design crite ria are
the same.
(3) At the site boundary, the dose must be less than 250μSv for every threemonths, and the design criteria of the external dose were decided to be
50μSv for every year.
I-4.2.2. Design Methods
For the shielding design of SPring-8, we employed the Swanson’s[Swanson1979] and Jenkins’ [Jenkins1979] formula for forward and lateral
directions of the bulk shield, respectively. After the beginning of theoperation, the shielding calculation methods have been improved drastically150such as Monte Carlo simulation codes [Asano2013]. In addition to theseabove mentioned formula, we have been mainly using the EGS4
[Nelson1985] and FLUKA [Fasso2005] Monte Carlo codes, and the SHIELD
11 [Nelson2005] empirical adjustment code, and the calculation results ofthe codes are shown in Fig. I-4.2.2. As shown in the figures, the target
geometry is sensitive so that it must be treated carefully.
10

–5

10

–5

10

–6

10

–7

10

–8

6GeV electron
1m shield (O.C.)

Leakage neutron dose (pSv/electron)

Leakage gamma dose (pSv/electron)

6GeV electron
1m shield (O.C.)

60

80

100

120

10

–6

10

–7

10

–8

Scattering angle(deg.)

60

80

100

120

Scattering angle(deg.)

Fig. I-4.2.2: Inter-comparison of leakage dose calculations between Jenkins’
formula, SHIELD 11 and FLUKA. Left and right hand figures are for
gamma and neutron leakage dose, respectively. Black solid lines are theresults of Jenkins calculations, Blue lines are SHIELD 11 and blue
diamonds are SHIELD11 without considering the self-shielding of thetarget. Red dots are FLUKA calculations.

Target and its size wereassumed as iron and 20cm thick. The shield wall is made of ordinary
concrete with 1m in thickness.
I-4.2.3. Beam Loss Scenario
The present capacity of the beam operation of the storage ring is asfollows: maximum electron energy, maximum stored power, and maximum
stored electrons are 8 GeV, 7.6 kJ, and 2.4 x1013, respectively. And we have
now four operation modes, these are tuning, stored injection, storage withdecay, and top-up with steady stored current modes. For each mode,
maximum permissible electron injection numbers are as follows, 1.8x1014
e/w and 1.5x1015 e/3M for tuning, 3.8x1014 e/w and 3.1x1015e/3M for stored151

injection, and 6.3x1013 e/w for top-up operation mode. The beam loss
scenario based on the permitted operation is indicated as Table I-4.2.1.
Table I-4.2.1: Beam loss scenario for SPring-8 storage ring during 10 hours.
Operation

Tuning mode

Stored injection modecondition

2mA/15min (10h)10mA/6min (10h)SR user mode
Decay mode

Top-up mode

10h (life)
Loss points

injection

5points

injection

44 points

44 points

Any points

Loss rate

80%

20%

0.9x80%0.9x20%100%(10h)

20%

-

>80%

Injection

0%10%efficiency

At the beginning of the SPring-8 project, we had no information
about the injection efficiency for the 8 GeV class machines so that we
estimated the low injection efficiency of 10% conservatively. However, many
experiences and the data have been accumulated until now, therefore therelatively new synchrotron facilities employ higher injection efficiencies as
indicated in Table I-4.2.2. Figure I-4.2.3 shows the measurement data of theinjection efficiency at the beginning of the top-up operation of the SPring-8
storage ring [Asano2007]. As shown in Table I-4.2.2 and Fig. I-4.2.3, theinjection efficiency of SPring-8-II can be set up to be higher values.
Table I-4.2.2: Rough summary of normal beam loss scenario during normal
operation of storage rings. * ;design injection efficiency for normal
operation.
Facility

EnergyCurrent

(GeV)

(mA)

Electron beam loss rate
Injection Region Non-injection region& efficiency*

BESSY-II(Germany)

1.9

300

40% loss*30%

152

30%/24points

TLS

1.5

200

35% loss(Taiwan)
SSRL

*30%
3

500

(U.S.A.)
DIAMOND

35%ring uniformly

3500

Frequency probability ( percents )(U.K.)

16mW

56mW/28points,*75%

50mW at Dump

40%

10%

*50%

at any points: top–up: normalincluding tuning

30

20

10

0
80

90

Injection efficiency ( percents )Fig. I-4.2.3: Measurement data of the injection efficiency distributions
during normal and top-up operations at SPring-8. These data were
measured from 24th June to 8th July, 2004.
I-4.2.4. Beamline Shielding
For the beamline shielding, we must consider three radiation
sources, one is high energy photons and associated neutrons due to highenergy electron beam loss, one is the gas bremsstrahlung and associatedneutrons, and the other is synchrotron radiation. Of them, high energy
photons and associated neutrons directly depend on electron beam loss. Gas
bremsstrahlung depends directly on the physical straight section length of153the storage ring and the vacuum pressure. On the assumption of the same
vacuum pressure, the straight section of SPring-8-II will be shorter than the
19m of SPring-8 so that the effect of the gas bremsstrahlung will be lessthan that of present conditions. The ID parameters of typical beamlines for
SPring-8 and the current SPring-8-II in hand are listed in Table I-4.2.3. andthe spectra are shown in Fig. I-4.2.4.
Table I-4.2.3: Typical ID parameters for SPring-8 and SPring-8-II(provisional).
SPring-8 (BL37XU)

SPring-8-II

Electron energy (GeV)

8

6Max. K value

2.6

2.2

Magnetic field strength (T)

0.87

1.24

Period140

190

Period length (mm)32

19Total flux (photons/eV):SPring–8(Bl37XU)

10

14

10

12

10

10

:SPring–8–II

10

0

1

2

10
10
Photon energy (keV)

10

3Fig. I-4.2.4: Synchrotron radiation source spectra for SPring-8 typicalundulator beamline and a provisional SPring-8-II undulator beamline at
100 mA stored currents.

154

The leakage doses outside the optics hutches between the SPring-8 andSPring8-II typical beamlines are shown in Fig. I-4.2.5 by using STAC8[Asano1994]. In the cases, the geometry and shield materials of optics
hutches are set as the SPring-8 BL-37XU optics hutch. As shown in the
figure, the leakage dose at the SPring-8-II optics hutch outside is lower
than that of SPring-8 because of the lower intensity photons of the higher
energy component of the spectrum as shown in Fig. I-4.2.4. Therefore, there
are some margins for the hutch shielding against the synchrotron radiation,

Leakage dose rate (microSv/h)generally.

10:SPring–8(Bl37XU)

1:SPring–8–II

10

10

0

–1

0

50

100

150

Scattering angle (deg.)

Fig. I-4.2.5: Leakage dose distribution depending on the scattering
angle outside the optics hutch of SPring-8 and SPring-8-II. (The sidewall are made of 2.3 cm lead sandwiched by 0.5 cm iron plates)
I-4.2.5. Skyshine
For the skyshine dose calculations of SPring-8, we used theStevenson-Thomas’ formula [Stevenson1984] on the assumption of the 1/E
spectrum of leakage neutrons. In addition, other facilities are in the site
such as the SACLA X-ray free electron laser facility and the NewSUBARU
small synchrotron radiation ring so that we must consider all of thecontributions to the dose at the site boundary. Table I-4.2.4 shows thecurrent estimated doses at the nearest site boundary. Figure I-4.2.6 shows155the skyshine dose distributions including the direct radiation. In thesimulations by FLUKA, the thickness of the shield wall and the geometry of
the ring are almost the same as the SPring-8 that is 240m radius of the ringand the 1m shield thickness of ordinary concrete.

The dose at the siteboundary will be reduced to be about 70% of that corresponding to reduce
the stored electron energy from 8 GeV to 6 GeV.
Table I-4.2.4: Current dose estimation at the nearest site boundary.
SACLA

NewSUBARU

Total

Design criteria(μSv/3M)(μSv/3M)(μSv/3M)(μSv/3M)(μSv/3M)

3.56

5.20

1.37

10.1

12.5

10

–5

10

–6

10

–7

2

2

Skyshine dose*R (pSv*m /electron)

SPring-8

400
600
800
1000
Distance from the center of the ring (m)Fig. I-4.2.6: Skyshine dose calculations including direct radiation for
SPring-8 storage ring. Red and blue lines indicate the neutron doses due to
8 GeV and 6 GeV electron beam losses at any point of the ring, respectively.
Black and green lines are gamma doses due to 8GeV and 6 GeV electron
beam loss, respectively.
156

I-4.3. Safety System
The current radiation safety system at SPring-8 is shown in Fig.
I-4.3.1, and it does not need to change the system, fundamentally. At least,
the access control procedure to experimental hutches is better not to change
to avoid the confusion of users.
Terminal

Data server

Beamline
Beamline
PLC

Accelerator
PLC

Main Beam
Shutter

Electron beam control(Injector/Booster/storage ring)

Optics/Exp.hutchaccess control

Access control
Personal key/Door

SR beam control

Emergency button

Emergency button

Radiation monitoring

Fig. I-4.3.1: Conceptual main frame of radiation safety system at SPring-8.
(PLC: Programmable Logic Controller; Full arrows mean to connect theinterlock system)
I-4.4. Induced Activity and the Radioactive Waste
About 20 years, the storage ring will have been operating until thereconstruction of the ring. Almost the components of the ring will be
replaced on the program in a short time of about a year and the hugeamount of radioactive waste will be produced so that the countermeasures
such as radioactivity measurement system and the stock system must be
taken fully. The materials of the main components are iron, copper andaluminum. The induced activities of these materials are listed in Tables
I-4.4.1, 4.2, and 4.3 [Swanson1979] after 20 years operation and the coolingtime of one week. In the tables, the units of activities are indicated by
GBq/kW of electron beam loss rate. The electron beam loss is eccentrically157

located such as the injection area and the points with high betatronfunctions so that we must check the components made of iron and copper
installed into these points, carefully.
Table I-4.4.1: Main saturation activity induced by high energy electrons (iron).
DaughterHalf life

nuclideDominant

Saturation activity

1 week cooling

production(GBq/kW)(GBq/kW)

Sc-46

83.9d

(γ,sp)

7.40E+00

6.98E+00

V-48

16d

(γ,sp)

1.50E+01

1.11E+01

Cr-51

27.8d

(γ,sp)

1.50E+01

1.26E+01

Mn-52

5.6d

(γ,np)

1.30E+00

5.47E-01

Mn-52m

21.1min(γ,np)

1.30E+00

-

Mn-54

303d

(γ,np)

2.20E+01

2.17E+01

Mn-56

2.576h(γ,p)

1.20E+00

8.67E-43

Fe-52

8.2h

(γ,2n)

2.10E+00

1.43E-06

Fe-53

8.51min(γ,n)

2.70E+01

-

Fe-55

2.6a

(γ,n)

4.90E+02

4.85E+02

Fe-59

45.6d

(n, γ)*--*Neutron capture reaction
Table I-4.4.2: Main saturation activity induced by high energy electrons (copper).
DaughterHalf life

nuclideDominant

Saturation activity

1

week

cooling

production(GBq/kW)(GBq/kW)

Co-58

71.3d

(γ,sp)

2.40E+01

2.24E+01

Co-58m9.2h(γ,sp)

2.40E+01

1.88E-01

Co-60

5.263a(γ,n2p)2.40E+01

2.22E+01

Ni-63

92a

(γ,np)

1.70E+01

2.38E+00

Cu-61

3.32h

(γ,2n)

3.20E+01

8.41E-01

Cu-62

9.76min(γ,n)

4.07E+02

-

Cu-64

12.8h

(γ,n)

1.85E+02

9.86E+01

Cu-66

5.1min(n,γ)*-*Neutron capture reaction
158-

Table I-4.4.3: Main saturation activity induced by high energy electrons(Aluminum).
DaughterHalf life

nuclideDominant

Saturation

production(GBq/kW)

activity

1weekcooling(GBq/kW)

Be-7

53.6d

(γ,sp)

4.80E+00

4.38E+00

C-11

20.3min(γ,sp)

1.90E+00-

N-13

9.96min(γ,sp)

5.0E-01

-

O-15

123s

(γ,sp)

2.50E+01-

F-18

109.7min(γ,sp)

5.20E+00

1.14E-27

Ne-24

3.38min(γ,3p)

1.10E-01-

Na-22

2.62min(γ,3n2p)

9.30E+00-

Na-24

14.96h(γ,1n2p)

1.00E+01

8.77E-05

Al-25

7.24s

(γ,2n)

1.40E+00-

Al-26

7.40E+05a(γ, n)

3.30E+02

6.18E-03

Al-26m6.37s(γ, n)

3.30E+02-

Mg-27

9.46min(γ,π+)

5.90E-01-

I-4.5. Summary
In comparison with the current status of the SPring-8 ring, theimproved points and the things to keep mind of the safety have been picked
up to the SPring-8-II project. There are many safety research items,
especially the beam loss scenario has most ambiguities and uncertainties.
In addition, the consensus of the beam loss scenario should be required
among the inside and outside the site including the regulatory committee ofthe government. Target conditions at the ring are also important toestimate the accurate leakage dose. The management of radioactive wastes
including the estimation of the induced activity is urgent issues. The
re-estimation of the skyshine dose is one of the key issues including otherfacilities in the site. All of these must be tied up before starting theconstruction of the SPring-8-II ring.

159

References[Asano1994] Y.Asano & N.Sasamoto,, Radiation Physics and Chemistry
Vol.44 No1/2 p.133-137 (1994).
[Asano2007] Yoshihiro Asano & Tetsuya Takagi, Radiation measurements
Vol.41 p.s236-241 (2007).
[Asano2013] Yoshihiro Asano, J. Radiation Protection & Environments
Vol.35 p.135-144 (2013). ISSN 0972-0464[Fasso2005]

A.

Fasso

et

al.,

al.,IFFN/TC_5/11; A. Fasso et al.,CERN-2005-10(2005);

A.

Fasso

et

SLAC-Report-773.

[Jenkins1979] T. M. Jenkins, Nuclear Instr. & Methods Vol.159 p.265-288(1979).
[Nelson1985] W. R. Nelson, H. Hirayama and D. W. O. Rogers, SLAC-Report265 (1985).
[Nelson2005] W. R. Nelson & T. M. Jenkins, SLAC-Report 737 (2005).
[Sasamoto1994] N. Sasamoto and Y. Asano, Proceedings of the 1st
specialist's meeting on Shielding aspects of Accelerators, Targets andIrradiation Facilities: SATIF 1994 OECD NEA Documents p.235-245(1994).
[Stevenson1984] G. H. Stevenson & R. H. Thomas, Health Physics Vol.46
No.1 p.115-122(1984).
[Swanson1979] W. P. Swanson, IAEA Technical Reports No.188 (1979).
[Tanaka2004] H. Tanaka et al., Proceedings of 9th European Particle
Accelerator Conference, p.222-224 Lucerne, Swiss (2004).

160

PART-II
II-1 Overview

161II-1. Overview
Over the last half century, synchrotron radiation (SR) light sources
have steadily evolved with significant increases in brilliance. The advent of
3rd generation synchrotron facilities (e.g., ESRF, APS, and SPring-8 in the
1990s) together with the emergence of new experimental techniques
drastically expanded the ranges for applications. These facilities, as well as
subsequent medium-sized light sources, are now regarded as vital scientificinfrastructure required to address essential problems facing society tod ay.
A recent

remarkable milestone in optical science is thedevelopment of XFEL light sources. At SPring-8, we inaugurated SACLA,
the first compact XFEL facility, in 2012. SACLA produces ultrabrilliant,femtosecond X-ray pulses with excellent transverse coherence. These
characteristics are completely different from those of SR sources that havemodest peak intensity with quasi-CW pulse patterns. A new scheme,“measurement-before-destruction” (MBD), has been exploited as a key
concept for experiments using XFEL. This technique offers atomic-scalespatial information with femtosecond snapshots, while the destructive
property requires a huge number of repetitive measurements in order tounderstand the whole processes. This scheme is thus suited for elucidating
fundamental characteristics using many replica samples, rather than for
tracing spatiotemporal correlations and evolutions for a single complex
target. For the latter purpose, SR is still an indispensable X-ray probe. The
wide gap between XFEL and SR, however, has recently been recognized as aserious challenge that must be overcome in order to reveal the fundamental
nature of materials of interest.
Our upgrade plan for the SPring-8 facility calls for bridging thisgap by making drastic enhancements to SR capabilities. A brighter SR
source naturally allows us to access (i) very complex and minute targets (ii)working in real environments (iii) with improved resolution (iv) using
combinative

experimental

techniques.

Moreover,theintense,highrepetition-rate source enables (v) drastic reduction of the time constant of
measurement from seconds to the millisecond or even microsecond range for162

a broad range of applications. Based on these advanced capabilities, we plan
to build a new scheme, “measurement-during-change”, as a standard
experimental concept for SPring-8-II, which is distinct from the “static”
observation available in the existing SPring-8 beamlines or from the MBD
scheme used at SACLA. Practical utilization of X-ray nanoscopy combinedwith diffraction/scattering/spectroscopic techniques will enable us to reveal
the complex functions of heterogeneous systems in catalytic and energy
materials, to illuminate fluctuating dynamics in strongly correlated systems,
to clarify distinct the characteristics of matter in extreme conditions, and toinvestigate essential reactions among biomolecules. Understanding thesefundamental questions will provide promising clues on improving
sustainability, major progress in information technology, management ofdisaster, and control & prevention of diseases. Several interesting scientificcases for ultralow-emittance SR sources proposed at the present stage canbe found in the special issue “Diffraction-Limited Storage Rings and New
Science Opportunities” in the Journal of Synchrotron Radiation [JSR2014].
To realize the potential of the upgraded facility, we must fully
exploit the capabilities of the SPring-8-II light source. This necessitates
upgrading the whole experimental system, including the beamlines, theend-stations, and the detector system [Yabashi2014].
For the beamlines, a primary challenge at the start of SPring-8
was how to manage high heat loads above 400 W for the most upstream
optics. For SPring-8-II, the heat load will be mitigated, as described in Part
I. A key requirement for SPring-8-II is speckle-free properties for all optics,
including mirrors, crystals, and windows, to preserve coherent X-ray
wavefronts. Another mandate is to reduce instability. To generate a small
focusing spot using a current SPring-8 beamline, one utilizes a secondary
slit as a virtual source, which removes an unwanted influence of instability,
originating mainly from a double-crystal monochromator (DCM). However,this geometry results in the loss of considerable amounts of available
photon flux for the samples. If we could remove the secondary slit atSPring-8-II, we would be able to keep the flux of ~1014 photons/s from the163source within an ultra-small spot of ~150×230 nm2. The flux density in thisfocusing condition is three orders of magnitude higher than that in theexisting SPring-8 beamline. To achieve stabilization, it is increasing
important to damp vibration of the cooling system, as well as to utilize
precise X-ray diagnostics. Beamline design is described in more detail in
Sect. II-2.
For end-stations, precise and fast data acquisition is essential for
efficient utilization of SPring-8-II. This requires a detailed synchronization
scheme among motions of sample stages, operand parameters, detectors,
and data analysis. However, the technological development needed to meetthe requirements is massive. Thus, core systems, which are extracted fromvarious types of experimental setups, should be designed and constructed ina centralized and standardized way. Applications for each experimental
instrument are conducted as variants of the core systems. This strategy isin contrast to that performed at end-stations of the existing SPring-8 facility,
where development of experimental instruments is conducted rather
independently at each place. Since the design work of the core systems isstill in a very early stage, we do not include a detailed description in thisreport. However, in Sect. II-3 we present a design strategy for the detector
system, which is the most important component of the core systems.
To reach this goal, we are able to exploit the experience gained andresources developed for SACLA utilization. At SACLA beamlines,
state-of-the-art X-ray optical components are operated for advanced
applications such as quantum X-ray optics, coherent diffractive imaging(CDI), and high-energy density science. Non-destructive beam monitors
provide X-ray beam parameters in a shot-by-shot manner, allowing users to
conduct reliable analysis on experimental data [Tono2013]. In the SACLA
end-stations, standardized experimental systems are designed, constructed,
and operated to address the majority of user applications. A good example isthe DAPHNIS system, which was developed as a diffraction/scatteringexperimental platform. The system consists of a compact sample chamber,various types of sample injectors, and a short-work-distance octal MPCCD
164

detector [Kameshima2014; Tono2015]. Additionally, the DAQ system wasdeveloped for handling and analyzing massive data streams with a rate of~4 Gbps and a volume of ~10 TB/day. Based on these prototype systems, we
intend to design core experimental platforms for synthetic utilization ofSPring-8-II.
Finally, we describe possible scenarios of how we intend to carryout the beamline upgrade. We could consider some alternatives dependingon the available resources. If we have sufficient resources, we could upgrade
all the beamlines and end-stations simultaneously with the accelerator
upgrade. In case our resources are rather limited, we will start the upgrade
for a select number of beamlines. Even in this case, one could continue toutilize the existing beamlines without major changes, whereas theperformance is limited. To maintain compatibility between the existing andupgraded beamlines, we will preserve the beam axes for almost all
beamlines, as described in Sect. I, except for realignment of the bending
magnet B2 beamlines due to the small changes of the beam axes. This
scenario allows us to balance resources over an extended term, rather thanconcentrating them into a short period.

References[JSR2014] Special issue on Diffraction-Limited Storage Rings and New

Science Opportunities, J. Synchrotron Rad. 21, Part 5 (2014).
[Kameshima2014] T. Kameshima et al., Rev. Sci. Instrum. 85 (2014) 033110.
[Tono2013] K. Tono et al., New. J. Phys. 12 (2013) 083035.
[Tono2015] K. Tono et al., submitted.
[Yabashi2014] M. Yabashi et al., J. Synchrotron Rad. 21 (2014) 976-985.

165

PART-II
II-2 Beamline DesignII-2.1.
II-2.2.
II-2.3.
II-2.4.

High-heat-load optics
X-ray optics with wavefront preservation
Photon diagnostics
New experimental schemes with innovative optics166II-2. Beamline Design
In this chapter, we describe X-ray optics and beam diagnostics,
mostly used with standard X-ray undulators, regarding key technologies
and issues for the beamlines at the upgraded storage ring, SPring-8-II.
1) Based on the expected heat load at the front-end, estimated in chapter I-2
of Part-I, we first consider requirements for the high-heat-load optics: adouble-crystal monochromator and a mirror. We discuss how vibration
effects at the monochromator and mirror affect the source size anddivergence, as well as the criterion of angular vibration.
2) The X-ray optics used for monochromatization, focusing, collimation, andpolarization control are increasingly important for advanced applications at
SPring-8-II. When using coherent X-rays from SPring-8-II, it is critical tomaintain high quality for every optical element in order to preserve a
coherent wavefront and to suppress unwanted speckles. Demands for the
key optical elements are described.
3) Furthermore, X-ray optics are useful for diagnostics of radiation
characteristics (e.g., source size, transverse coherence, wavefront, energy
spectrum, and pulse duration) and stability, which are required for
performing advanced experiments with useful feedback for operation of theaccelerator.
4) Finally, several innovative-optics technologies will be presented.

Note: This chapter is partly based on the article in the Journal of
Synchrotron Radiation "Optics for coherent X-ray applications" [Yabashi
2014]. Reproduced with permission of from the International Union of
Crystallography (http://dx.doi.org/10.1107/S1600577514016415).

167

II-2.1. High-heat-load optics
II-2.1.1. Double-crystal monochromators (DCMs)
We propose to use short-period in-vacuum undulators (e.g., λu of 22mm) to produce high photon energies at 6-GeV operation for the standard
X-ray undulator beamline. In this case, one can keep the maximum K
parameter over 2.3, which continuously covers a wide wavelength range by
combining odd harmonics, with a minimum gap of 5 mm. We expect anincrease of brilliance by a factor of 20 (Fig. II-2.1.1). Figures II-2.1.2 (a) and(b) show the comparison of the beam profiles of the central cone of thefundamental peaks observed at 30 m from the source. The horizontal beam
size for the new design is more concentrated due to the smaller horizontal
emittance. The vertical beam size is slightly larger due to the shorter lengthof the undulator. The smaller horizontal emittance helps to reduce the heatload on the first optical element, because one can control the aperture size ofa pre-slit for the first optics without losing photon flux with moderate power.
In the case of SPring-8-II, an aperture size of 1.1 mm(H) × 0.97 mm(V) for
the frontend pre-slit is large enough to accommodate the central cone of the
X-ray beam, which suppresses the maximum heat load below 300 W at astored current of 100 mA. This is a typical amount of power, which is
sufficiently controlled at the existing SPring-8 beamlines. Thus we could
adopt the same cooling scheme for the first crystal of DCMs at SPring-8-II:
indirect cooling using liquid nitrogen (LN2) circulation. In order to suppress
thermal deformation of the silicon crystals, LN2 cooling has been widely
applied for DCMs [Mochizuki 2001]. Figure II-2.1.3 shows the photon flux
available using odd harmonics at K= 2.3. The photon flux with a Si 111
double-crystal monochromator (DCM) reaches ~1014 photons/s for a photon
energy range below 15 keV, which is ~50% higher than that of the existing
SPring-8 beamlines.

168

Fig. II-2.1.1: Increase of brilliance at the standard X-ray undulator at
SPring-8-II, compared with that of SPring-8, for 10-keV X-rays with
fundamental peaks.

Fig. II-2.1.2: Central cones of the fundamental peaks observed at 30 m fromthe source at K= 2.3.
(a) SPring-8 (8 GeV, λu= 32 mm, N= 140, I= 100 mA),(b) SPring-8-II (6 GeV, λu= 22 mm, N= 163, I= 100 mA).

169

Fig. II-2.1.3: Photon flux available using odd harmonics at K= 2.3. The
front-end slit is located at 30 m from the source, and the slit sizes are 1.3
mm(H) x 0.46 mm(V) for SPring-8, and 1.1 mm(H) x 0.97 mm(V) for
SPring-8-II.
II-2.1.2. Mirror
For several applications, we will use a high-heat-load mirror for
the first optics of the beamline. A water-cooled or LN2-cooled mirror isapplicable for higher harmonic rejection, beam collimation, and beamfocusing.
The surface figure errors induced during the mirror fabrication
process or resulting from mirror-holding are controlled to be less than 1 nm.
This helps avoid inhomogeneity or speckles from the mirror. Requirements
for the speckle-free mirror are described in section II-2.2.1. Special caremust be taken when clamping and cooling the mirror, so as not to deformthe mirror.
II-2.1.3. Vibration suppression of reflection optics
Better stabilization of high-heat load DCMs and mirrors is a170

critical challenge for utilization of SPring-8-II. In the case of LN2 cooling,
the bumping of vaporized nitrogen could be a cause of vibration, in addition
to typical sources such as the pulsations of coolant pumps and theturbulence within flow channels. To suppress short-term fluctuations (i.e.,
vibrations), the characteristics of the flexible tubes used for the flow
channel are of great importance [Yamazaki 2013]. This is because thecorrugation of a flexible tube can easily produce unwanted turbulence in theflow of LN2.
We recently developed a low-vibration flexible tube with low
turbulence. A standard flexible tube made of stainless steel is fitted with
soft sleeves of alumina fiber to serve as a smooth inner lining. This innertube can remain pliable at LN2 temperature levels under radiation. FigureII-2.1.4 shows a trend of 12-keV X-ray intensity after the LN2-cooled DCM
using (a) standard tubes and (b) the aforementioned low-vibration tubes,
measured at an undulator beamline of SPring-8 with a sampling rate of 1
kHz. The angular fluctuation (peak-peak) was reduced from 5 μrad to 0.75
μrad with a corresponding decrease of intensity fluctuation from 5.3% to
2.0%. Even smaller angular fluctuation, however, is needed to reduce thegrowth of virtual source size and divergence caused by vibration.
Minute vibrations in the reflection optics cause a virtual increaseof the source size and divergence [Goto 2014]. Figure II-2.1.5 shows thescheme, indicating how beam size and divergence grow due to vibrations.
The virtual source size and divergence will be:Σ x′, y′ σ v′ z 22

S x, y = Σ x, y +222Σ x′, y′ + σ v′22,(II-2.1.1)

S x′, y′ = Σ x′, y′ + σ v′ ,2

2

(II-2.1.2)

2

where, z : the distance between the source and reflection optics, Σ x, y : thesource size (convolution of electron and photon size), Σ x′, y′ : the source
divergence (convolution of electron and photon divergence), and σ v′ : theeffective (rms) angular vibration of the reflected beam due to angular171

vibration of the optics. σ v′ is twice the angular vibration of the optics suchas total reflection mirror and symmetrical-Bragg-case diffraction of perfect
crystals. We assume that σ v′ is very small, so that the reflectivity is
maintained. In addition to increasing the effective beam size at thedownstream location, the virtual beam waist shifts along the optical axis.
The distance between the virtual beam waist and the reflection optics
becomes:ς=Σ x′, y′2

Σ x′, y′ + σ v′
2

2

z.

(II-2.1.3)

The beam size observed with non-focusing optics will be expressed byconvolution of the intrinsic beam size and the contribution of vibration:
S L = Σ x , y + Σ x′, y ' L2 + σ v′ (L − z )222(II-2.1.4)

2where, L is the distance from the source,Σ x , y + Σ x′, y ' L2 provides the22

intrinsic beam size, and σ v′ (L − z ) is the contribution of vibration.
Using these equations, we can evaluate the criterion of vibration
suppression, compared with the source size and divergence, particularly inthe vertical direction. We must suppress the angular vibration as

σ v′ << Σ x′ or σ v′ << Σ y′ . For example, we consider a vibration effect for DCM
located at 40 m from the vertical beam size by eq. II-2.1.1. A 10% increase in
the vertical size (in the case of Σ y = 6 μm , Σ y′ = 4.2 μrad , and S y = 6 .6 μm )corresponds to σ v ′ = 0.07 μrad . This is 1/10~1/5 of the present value. Thus,
vibration suppression becomes more critical.

172

Fig. II-2.1.4: A time trace of the X-ray
intensity transmitted by the LN2-cooled
DCM. The first crystal was rotated withsteps of 1 μrad at a time interval of 5 s. The
LN2 flow channels for the two crystals
inside the DCM are connected with (a) astandard flexible tube and (b) a
low-vibration flexible tube. Vibration of 0.75
μrad (peak-to-peak) corresponds to σ v′ of
0.5 μrad, assuming sinusoidal vibration.

Reproduced

withpermission

of

the

International Union of Crystallography(http://dx.doi.org/10.1107/S16005775140164
15).

Fig. II-2.1.5: Phase ellipse change due to the angular vibration of thereflection optics, at (a) the light source, (b) the virtual beam waist, and (c)the reflection optics.
173

II-2.2. X-ray optics with wavefront preservation
High-quality optical elements are required for preserving thecoherent wavefront at SPring-8-II and for suppressing unwanted speckles.
In this section, we describe status of key optical elements: reflective mirrors,
diamond crystals, and beryllium windows.
II-2.2.1. Reflective mirrors
X-ray reflective mirrors are essential optical elements fordesigning various optical systems for X-ray focusing [Kirkpatrick 1948],
imaging [Matsuyama 2012b], and interferometry. In particular, anultra-precise figure shape is required for mirrors to control coherent
wavefronts while suppressing unwanted speckles. The phase error φ of the
wavefront in the reflected X-ray beam is expressed by(II-2.2.1)

φ = 4πd sin θ λwhere d and θ are the height error and the incident angle, respectively. For
the condition where θ= 5 mrad and λ=0.8 Å, a height error of d= 8 nm
generates a substantial phase error of φ = 2π , which indicates that a highdegree of accuracy at the nanometer level is required to avoid deteriorationof the wavefront. In addition, figure errors with a spatial period in the
sub-millimeter region should be sufficiently suppressed in order to produce
a uniform intensity profile [Mimura 2004, Yamauchi 2005]. To achieve thislevel of accuracy, several machining techniques have been developed, suchas elastic emission machining (EEM) [Yamauchi 2002], and surface
metrologies such as an interferometer [Yamauchi 2003, Mimura 2005a].
Recently, it has become possible to achieve a high degree of accuracy and asmall degree of roughness of 1 nm (peak-to-valley) and 0.2 nm (root meansquare), respectively. Figure II-2.2.1 shows the result of achieving a flat
intensity profile without speckles [Mori 2001, Yamauchi 2005] by combining
EEM with a micro-stitching interferometer (MSI) [Yamauchi 2003].
Measurements were taken at the 1-km beamline [Ishikawa 2001] using174

15-keV photons. The profiles agreed well with those expected from themeasured surface figures and wave-optical simulations.

Fig. II-2.2.1: Reflected beam images from flat mirrors measured at the 1 km
beamline of SPring-8 for (a) a non-EEM-processed area, and (b) an EEM
processed area. The photon energy was 15 keV, the mirror length was 70
mm, the glancing angle was 1.2 mrad, the incident slit width was 100 mm,
and the distance between the mirror center and the camera was 166 mm.

Reproduced with permission of from the International Union of
Crystallography (http://dx.doi.org/10.1107/S1600577514016415).
II-2.2.2. Diamond crystals
High-pressure high-temperature (HPHT) synthetic type IIa
diamond crystals [Sumiya 2012] are used for many synchrotron radiation
applications due to their excellent thermal properties: low thermal
expansion and high heat conductivity (300 K), high X-ray transmittance,and radiation durability. Consequently, SPring-8-II will use these diamond
crystals for the high-heat-load monochromator, beam splitter, and phase
plates.
We assessed the <001>-growth IIa diamond crystals supplied by
Sumitomo Electric Industries, Ltd. at the 1-km beamline [Tamasaku 2005]
and they appeared to be nearly perfect. The main limitation with the175

diamond crystals was the non-uniformity of the intensity profiles of thereflected beam that was enhanced at the experimental station, located at a
distance over 10 m from the monochromator when they were used for the
double-crystal monochromator [Yabashi 2007]. A beam profile with speckles
presents serious problems for coherent X-ray applications. A non-uniform
profile with fringes up to 50% (rms) was observed in the X-ray beam images
from the double-crystal monochromator at BL39XU at SPring-8. We
performed simple simulations based on Fresnel diffraction from crystal
segments, divided by the growth sector boundaries. When we assumed
phase shifts due to a lattice inclination of 0.5 μrad or more, or a lattice step
due to stacking faults, it caused a similar non-uniformity in theexperimental results [Goto 2012]. The simple model suggests that a smaller
phase shift in the whole region in the crystal is essential for wavefront
preservation. Diamond crystals with better quality and a larger domain are
required for practical use at the SPring-8-II facility.
II-2.2.3. X-ray windows
Beryllium foil will be used for vacuum windows at the hard X-ray
beamlines at SPring-8-II. The transmission of X-rays requires that thewindows have high quality, without any speckles or non-uniformity.
After systematic studies, we found that speckles with bright spots
resulted from internal voids, which were several to dozens of microns indiameter, for conventional beryllium foils fabricated by powder sintering(Electrofusion, PF-60) and a high-purity ingot foil (Electrofusion, IF-1)[Goto 2004]. A beryllium foil fabricated using physical-vapour-deposition(PVD) has fewer or even no internal voids [Goto 2007]. Figure II-2.2.2 shows
the transmission X-ray images for PF-60, IF-1, and PVD beryllium foils
measured at the 1-km beamline with a wavelength of 0.1 nm. They were
supplied by the Electrofusion and polished to 0.1 μm (rms) or less. A
two-dimensional X-ray detector with a spatial resolution of 0.5 μm was used
for the observation. The sample-detector distance was set to 1.4 m. Fig.
II-2.2.2 (d) shows the Fresnel diffraction patterns calculated for spherical176

vo ids with diame te rs ranging fro m 3 to 15 μm. The phase shift re sulting
from the voids in the beryllium was -0.14 radians/μm for 0.1-nm X-rays,
resulting in bright-spot diffraction. The experimental results show that
PF-60 and IF-1 have voids with a 10-μm-diameter and a 5-μm-diameter,
respectively. The concentration of the voids, estimated to be 103-104/mm3 forthese materials [Goto 2004, Goto 2007], is the primary cause for speckles
when using conventional beryllium windows.
Although PVD beryllium foil is still under development at several
manufacturers [Goto 2011], it will be adopted as a part of the speckle-free
optics to be used at SPring-8-II.
A CVD diamond window is an alternative material for speckle-freewindows. The diamond window offers advantages including a larger
tolerance for high heat loads and a reduced small-angle scattering [Jaski2007]. Although the Bragg diffractions from grains that accidentally satisfy
the Bragg condition leave dark spots in the transmitted beam [Goto 2007],
this problem could be mitigated by using, for example, nano-crystal
diamonds with a smaller grain size.

Fig. II-2.2.2: Transmission X-ray images through beryllium foils obtained at
the 1 km beamline of SPring-8 at a wavelength of 0.1 nm: (a) powder foil
(PF-60) of 250 mm thickness, (b) ingot foil (IF-1) of 250 mm thickness, and(c) PVD foil of 100 mm thickness. The distance between the beryllium foils
and the detector is 1.4 m. (d) Calculated Fresnel diffraction patterns fromspherical voids with diameters of 3, 5, 10 and 15 mm.

Reproduced with permission of from the International Union of
Crystallography (http://dx.doi.org/10.1107/S1600577514016415).
177

II-2.3. Photon diagnostics
SPring-8-II is capable of generating a small intense spot with adiameter of ~100 nm and a photon flux of more than 1013 photons/s by direct
demagnification of the source. Achieving this capability requires highstability for the source and every optical element, particularly in thereflection geometry, because one has to retract the aperture as thesecondary source. The target level is less than 0.1 μrad in both the verticaland horizontal directions (see II-2.1.3).
An important step is performing precise diagnostics of the beam
properties. We present useful online diagnostics tools and diagnostics ofhigher-order coherence with Hanbury-Brown and Twiss experiments in the
X-ray region, which can provide access to ideal transverse coherence profiles
and source sizes, even under conditions of optical instability.
II-2.3.1. Online diagnostics
Online photon diagnostics are a prerequisite for new light sources,
not only for experiments but also for the stable operation of accelerators andbeamlines. In many cases, only a small fraction of X-rays are sampled for
diagnostics through interactions between a transparent material and the
X-rays. Such materials must have the “speckle-free” quality for wavefront
preservation, as scientific applications intended for SPring-8-II rely on ahigh coherent flux, e.g., coherent X-ray imaging and X-ray photon
correlation spectroscopy. For the in-line photon diagnostic systems at
SPring-8-II, some good candidates are found in the XFEL beamlines, which
also require the preservation of coherent wavefronts. In-line intensity andprofile diagnostics have already been in operation at SACLA [Ishikawa
2012, Tono 2013].
One example is an in-line intensity/position monitor, in which
scattered X-rays from a thin foil are detected using quadrant photodiodes asshown in Fig. II-2.3.1(a) [Alkire 2000, Tono 2011]. The foil, only 15 μm thick,is composed of small diamond crystals with an average grain size of only a
few dozen nanometers. The beam position is obtained from the178difference-over-the-sums of the horizontal and vertical pairs of photodiodes.
The beam displacement in the horizontal direction, Δx, is expressed as,

I −I ∆x = α  R L 
 IR + IL 

(II-2.3.1)

where IR and IL represent the signal intensities of the right and left
photodiodes, respectively, and α is a proportionality factor. An error in theposition measurement, being sufficiently smaller than 1/10 of the beam sizeof ~300 μm, would be promising for accurate monitoring of X-ray beams at
SPring-8-II.
A thin diamond foil is also applied to the diagnostics of the spatial
profile. It is well known that diamonds with impurities and/or defects give
fluorescence in the visible-light wavelength range. The chemical vapor
deposition(CVD)

providesimpurity-doped

diamond

foils

with

a

well-controlled impurity concentration. The diagnostic system at SACLA
includes 30-μm-thick foils of boron-doped CVD diamonds that emit
fluorescence in the visible spectrum. The surface of each foil was polished tohave an average roughness less than 50 nm. This level of surface quality is
sufficient for avoiding the deterioration of the coherent wavefront of the
XFEL.
The above monitors owe their ability for online measurement to
the high X-ray transmittance and speckle-free quality of the diamond foils,
as evaluated at the 1-km beamline [Goto 2007]. Figure II-2.3.1(b) shows atransmission coherent-X-ray image of the nano-crystal diamond foil, used
for the intensity/position monitor. This test indicated the absence ofsignificant voids, impurities, and surface roughness in the foil. The
speckle-free quality has been successfully demonstrated in operation at
SACLA.

179

Fig. II-2.3.1: (a) Schematic for the beam position monitor, (b) Transmitted
coherent X-ray image of the nanocrystal diamond foil in the monitor.
Signal intensities over the 450×450 pixels have a standard deviation of only
2% of the average. Cross-sectional profiles along the dotted lines are shown.
II-2.3.2. Diagnostics of 2nd-order coherence and applications.

Reproduced with permission of from the International Union of
Crystallography (http://dx.doi.org/10.1107/S1600577514016415).
For chaotic sources such as synchrotron light sources, transverse
coherence length is reciprocally proportional to the source size. The
coherence length can deteriorate due to possible instability of the source
and/or optics, enlarging the effective source size as seen from the sample
position. However, the 2nd order coherence measurement, first proposed by
Hanbury-Brown and Twiss (HBT), is free from such fluctuation andinstability, because the characteristic time to measure the coherence isdetermined by the time window of the coincidence circuit for the CW source[Hanbury-Brown 1956a, 1956b] or by the pulse duration for pulsed sources
including synchrotrons [Ikonen 1992]. A comparison with usual 1st ordercoherence measurements such as a Young’s double slit provides a good
indication of the stability level and its influence on coherence. FigureII-2.3.2 shows the results of an X-ray HBT experiment performed at
SPring-8’s BL29XU to determine the transverse coherence length and thesource size [Yabashi 2001a, Yabashi 2004]. Here the excess ratio R of thecoincidence is plotted as a function of the vertical slit width. Since R is givenby the ratio of the temporal coherence time to the pulse duration, a180high-resolution

X-ray

monochromator,which

combinesfourhighly-asymmetric reflections, with a bandwidth of 120 μeV at a photon
energy of 14.4 keV was applied to enhance R [Yabashi 2001b]. From thisprofile, the coherence length at a distance of 53.3 m from the source wasdetermined to be 161.3±5.0 μm, which corresponds to the vertical source
size and the vertical emittance of 4.5±0.1 μm and 3.6±0.2 pm.rad,respectively, which agreed well with expectations. The HBT experiments
will provide accurate and useful information on coherence and source
properties for SPring-8-II.

Fig. II-2.3.2: The excess coincidence ratio R as a function of vertical slit
width. The solid circles are experimental data, while the solid curve is thebest fit for a Gaussian coherence profile with a coherence length of 161.3
mm.

Reproduced with permission of the International Union of Crystallography

II-2.4. New experimental schemes with innovative optics
In this section we discuss new opportunities for experiments
using innovative X-ray optics. First, we mention the possibilities ofnano-focusing using a K-B mirror system for direct focusing of the lightsource. Then, we introduce two-dimensional focusing using a single mirror
element. Next is an application of adaptive optics to enable variablefocusing size while maintaining the focus position, which facilitates181

combinative analyses in an end-station. Last is a new optical scheme foremission spectroscopy, which combines a parabolic mirror and crystal
optics.
II-2.4.1. K-B mirror system for nano-focusing
The EEM techniques for mirror fabrication and high precision
metrology were further applied to the development of focusing mirrors withaspherical surfaces. We successfully fabricated an elliptical focusing mirror
togeneratenearlydiffraction-limited

focusing,using

wave-opticalsimulation [Yamauchi 2002b] and a relative angle determinable stitchinginterferometer (RADSI) [Mimura 2005a]. We constructed a two-dimensional
focusing system with a beam size of 36 nm(V) × 48 nm(H) in the
Kirkpatrick-Baez geometry at a photon energy of 15 keV [Mimura 2005b].
Using these techniques, we are able to design various nano-beam focusing
systems.
The source sizes at SPring-8-II are as small as 24.0 μm(H)× 5.6μm(V) (rms). This facilitates the generation of intense nano-beams by using
a focusing optics scheme that directly demagnifies the source without
requiring a small aperture for a secondary virtual source. For example,
production of a small spot of 230 nm(H) × 150 nm(V) is possible with a fluxof 5×1013 photons/s, which corresponds to a flux density as high as ~1021
photons/s/mm2. In this estimation, we assume the use of reflective focusing
mirrors in the Kirkpatrick-Baez (KB) geometry [Kirkpatrick & Baez 1948],
which is located 75 m from the source with focal lengths of 0.3 m(H) and
0.75 m (V) (Table II-2.4.1(a)). Rigorous control of vibration and temperatureare essential to achieve high performance in the focusing system. A similar
scheme will be adopted as for DCM and other pre-optics (II-2.1.3).
When we need X-ray beams smaller than 100 nm x 100 nm fornano- applications, we have the flexibility to configure the KB mirror
system accordingly. Typical focusing conditions are shown in Table II-2.4.1(b). We obtain a photon flux of 1013 ph/s, even with photon flux loss within
limited acceptance at the mirrors. The potential of direct focusing opens the182possibility of combining a nano-beam with ultra-high energy resolution
measurement, or other types of high resolution measurement, with which
we may eliminate photon flux.
Table II-2.4.1: Examples of parameters for the KB mirror for direct focusingof the light source. (a): configured for high flux with a large mirror
acceptance, (b): configured for 100-nm focusing. For the focal size columns,“Geometrical” indicate the sizes given by the geometrical reduction ratios ofthe source sizes, while “Diffraction” means those given by limited numericalapertures (NAs) of the focusing optics.
Wavelength
Source-mirror distance
Mirror length (V)Glancing angle (V)Mirror length (H)Glancing angle (H)Mirror-Mirror gap
Mirror(V)-focus distance
Mirror(H)-focus distanceWorking distance
Focal size (FWHM)
Geometrical
VerticalHorizontal
Diffraction
VerticalHorizontal
Total
VerticalHorizontal

183

(a)
0.1 nm
75 m
400 mm3 mrad400 mm3 mrad
50 mm
750 mm
300 mm
100 mm(b)
0.1 nm
75 m
300 mm3 mrad100 mm3 mrad
50 mm
370 mm
120 mm
70 mm

141 nm
227 nm

70 nm
91 nm

55 nm
22 nm

36 nm
35 nm

152 nm
228 nm

79 nm
97 nm

II-2.4.2. Mirror systems for larger NAs
Further increasing NA by using a graded multilayer coating allows
for much smaller focusing, with a sub-10 nm level. To achieve the higher
accuracy required for this condition, we introduced adaptive optics, i.e., adeformable mirror that compensates for the excess error of the wavefront byapplying the phase retrieval method [Mimura 2010]. We successfully
generated a 2D focused spot with a size of less than 10 nm at 20 keV[Yamauchi 2011].
A two-stage focusing system, which consists of two KB focusing
systems, was originally developed for the tight focusing of XFEL pulses. The
first KB system expands the beam size, while the second one generates asmall focus with a large NA. We successfully generated ultra-intense X-ray
pulses of 1020 W/cm2 with a size of 30 nm× 55 nm for 9.9-keV XFEL pulses
from SACLA [Mimura 2014]. We note that this system is also widely
applicable for generating small spots for beamlines with limited length atSPring-8-II.
II-2.4.3. Two-dimensional focusing using a single mirror element
Single-mirror focusing, using a mirror such as an ellipsoidal mirror
or a Wolter mirror, promises high potential for nano-focusing. Recent
breakthroughs with precision machining technology have enabled theirdevelopment [Motoyama 2014], overcoming the difficulties of complicatedand extremely steep profiles. As a prototype, we fabricated an ellipsoidal
focusing mirror that has a ring-shaped aperture for the water-window
wavelength region [Takei 2013]. The fabrication system consists of mandrel
fabrication, surface replication, metrology and refiguring. An objective forthe mirror is to focus soft X-ray FEL light to the sub-10 nm region without
chromatic aberrations [Motoyama 2013].
For hard X-ray ranges, a grazing-incidence ellipsoidal mirror,
which has a very steep curve in the transverse direction to the incident
X-rays, is also under development. An advanced stitching interferometer
was developed for profiling the three dimensional shape by combining the184

MSI and the RADSI [Yumoto 2010]. An upgraded EEM [Takei 2014] system
will also be combined for fabrication.
II-2.4.4. Variable-sized focusing with adaptive optics
Combinative analysis using a flexible focusing size remains
challenging, because it is more difficult to change the optical parameters in
an X-ray focusing system. An adaptive focusing device [Susini 1995] is apromising candidate for expanding flexibility. 100-nm focusing using
piezoelectric deformable mirrors was achieved with bimorphstructure atSPring-8 [Nakamori 2013], using techniques for the at-wavelength
wavefront measurement, such as grating interferometry [Matsuyama
2012c].
A drawback of the single adaptive element applied for diffraction
limited focusing is the change of the focus position along the optical axis. To
overcome this problem, we propose an innovative adaptive focusing opticalsystem comprised of four deformable mirrors arranged in a two-stage
Kirkpatrick-Baez configuration, as shown in Fig. II-2.4.1 [Matsuyama
2012a, Kimura 2013]. This optical system can control the spot size at a fixedsample position while maintaining the diffraction-limited focusing bycontrolling the numerical aperture. We plan to use this system to delivercoherent X-rays with a controllable beam size ranging from one micrometerto a few dozen nanometers for various types of microscopy.

Fig. II-2.4.1: Schematic of a one-dimensional adaptive focusing system. The
NA and final focus size are controlled by changing the configuration of thetwo deformable mirrors (DM1 and DM2). The inset shows an example ofcoherent diffraction imaging (CDI) and scanning microscopy. Reproduced
185

with

permission

of

the

International

Union

of

Crystallography(http://dx.doi.org/10.1107/S1600577514016415).
II-2.4.5. Optics for emission spectroscopy with parallel beam geometry
Advancesinfabricating

asphericalmirrorsurfaceswithunprecedented precision are opening up new optical configurations, e.g., amirror-based spectrometer, for various types of emission spectroscopy
measurements including inelastic X-ray scattering and X-ray nonlinear
optics.
Conventionally, cylindrically or spherically bent crystals have been
widely used in the Rowland or the von Hamos geometries to disperse
divergent X-rays from the samples (Figs. II-2.4.2 (a) and (b)). However, thecurvature radius tends to become longer in order to avoid distortion, which
enlarges the size of the spectrometer, e.g., ~10-m long. Furthermore, theoptical layout is quite limited (for example, multicrystal optics, which is
widely used in X-ray monochromators to control the bandwidth, cannot beapplied). Now it becomes possible to collimate the divergent beam withaspherical mirrors. Once the beam is made parallel, the spectrometer can be
designed more flexibly. A multicrystal spectrometer may be used to achieve
sub-meV resolutions. Mirror-based spectrometers may become even morecompact.
Here, we present a prototypical mirror-based spectrometer, which
consists of a parabolic mirror and a channel-cut analyzer crystal (Fig.
II-2.4.2(c)), designed for a future X-ray parametric down-conversion
experiment [Tamasaku 2011]. In this nonlinear optical phenomenon, adown-converted X-ray photon is emitted into a small solid angle, e.g., about
1×10-5 sr, and has a typical bandwidth of about 1 eV around 10 keV. The
parabolic mirror is 400-mm long with an effective width of 4 mm, and is
manufactured on a fused silica block. The surface is coated with Pt film
having a thickness of 60 nm. The roughness and the figure error aremeasured to be 0.12 nm (rms) and 0.7 nm (rms), respectively. The distance
between the sample and the mirror center is set to be 400 mm, and the186glancing angle at the center is 4 mrad. Thus, the acceptance angle of themirror is 1.6 mrad within the reflecting plane, and 10 mrad along thesurface. A Ge 220 channel-cut monochromator is set just downstream fromthe mirror.
Figure II-2.4.3 shows the spectrum of the scattered X-rays from adiamond sample measured at BL19LXU at SPring-8. The pump photon
energy is 8 keV. The peak at the origin is the Rayleigh line and the broad
peak around 50 eV is due to Compton scattering. We also plot a spectrum
measured with a conventional spectrometer, which consists of acylindrically bent Ge 220 monochromator set 1.5 m away from the sample(Fig. II-2.4.2(b)). Compared to the conventional one, the mirror-based
spectrometer shows better performance, revealing sharper structures,
especially near the strong elastic peaks. The results provide clear evidence
for the advantages of using an aspheric mirror. The prototypicalmirror-based spectrometer can be easily upgraded using the KB setup tocollimate the beam two dimensionally for higher energy resolution. To covera larger solid angle, a multilayer coating can be used, although thephoton-energy range must be fixed. Such innovative spectrometers are fullycompatible with the beam characteristics of DLSRs, which allow for asmaller focus size on the sample and a higher photon flux with a narrower
bandwidth.

187

Fig. II-2.4.2: Spectrometer design for divergent X-rays. The conventional
set-up uses (a) spherically and (b) cylindrically bent crystals. (c) Aparabolic mirror collimates the beam prior to entering a channel-cut
monochromator. S: sample, C: crystal, CC: channel-cut crystal, M:
mirror, D: detector.

Reproduced

withpermission

of

the

International

Union

of

Crystallography (http://dx.doi.org/10.1107/S1600577514016415).

Fig. II-2.4.3: Energy spectra of X-rays scattered by a diamond sample.
The pump photon energy is 8 keV. The red and blue lines are measured
by the mirror-based and the bent-crystal spectrometers, respectively.

Reproduced

withpermission

of

the

International

Union

Crystallography (http://dx.doi.org/10.1107/S1600577514016415).

188

of

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190

PART-II
II-3 Detector System
II-3.1.

SensorsII-3.2.

Sensor Variants

II-3.3. Detector System
II-3.4. Infrastructure to operate coherently with the Detector System191

II-3. Detector System
X-ray imaging detectors will play a key role in the development of
new measurement technologies that are required to fully exploit the
upgraded capabilities of SPring-8-II. Each scientific study (and beamline)
requires specific detector functions. However, the expected development
cost for state-of-the-art X-ray imaging detector systems is so high that
establishing dedicated development programs for each individual scientificcase is no longer practical. As discussed in Sec. II-1, we must therefore
develop a core detector system to cover the baseline scientific needs and wemust develop the ability to create alternate configurations of the detector
system. The core detector system has functions to detect, visualize, store
and analyze the data from the X-ray images. First we discuss the sensors
and next aspects of the system design.
II-3.1. Sensors
X-rays are detected by sensors. We address four critical demands
on sensors, namely, (i) higher quantum efficiency even in the photon energy
range of 20-30 keV, (ii) a count rate over 20 Mphotons/pixel/s, (iii) extreme
X-ray radiation hardness, and (iv) a fast frame rate, preferably over 209
kHz to synchronize with the revolution frequency of the synchrotron.
(i) Higher quantum efficiency even in the photon energy range of 20-30 keV
Silicon is the most widely used material in imaging sensors andelectronics. Direct-detection silicon sensors have been proven to offer many
advantages such as uniformity, high frame rate, and single-photon
detection capability. To cover the higher photon energy regime, however,silicon becomes transparent and quantum efficiency (Q.E.) starts to drop
significantly at around 15 keV. Non-silicon material such as CdTe,
Germanium or GaAs can provide higher quantum efficiency owing to theirhigher density, although they have drawbacks such as non-uniformity,
lower stability, a lower operation temperature, and/or greater difficulty in
manufacturing. To overcome these difficulties, several groups are currently

192conducting research. Another potential way to increase the Q.E. is using alayered silicon sensor.
In the SPring-8 upgrade plan, we will pursue development using
layered silicon sensors. The baseline target is to assemble 3 layers of 0.65
mm thick sensors to form total sensitive thickness of 2 mm, whichcorresponds to 94% Q.E. at 20 keV, and 60% Q.E. at 30 keV. Each sensor ismade using a quasi-monolithic sensor approach in order to maintain thequantum efficiency. The manufacturing process is assumed to be an
extension of the monolithic CMOS sensor, SOPHIAS, under development
for SACLA [Hatsui2013]. Two types of sensor modules are considered: asingle-layer module and a triple-layer module. The baseline implementation
uses flip chip bonding to a custom ceramic package. In this case, the layer
separation will be about 0.5-1.0 mm. This geometry results in a large
parallax. The development of a parallax calibration method must be carriedout as well. We will also investigate direct stacking of three sensors layers
without spacing in order to reduce the parallax.
(ii) A count rate over 20 Mphotons/pixel/s
In many experiments, the maximum count rate is one of thebottlenecks in the overall throughput of the instrument system. This
happens especially in experiments that study dynamics, where good
statistics are required within a short exposure time. In the conventional
photon-counting pixel detector, a maximum count rate is limited by thein-pixel analog circuitry. The typical count rate was reported to be below 5
Mphotons/pixel/s. Rate correction with a retrigger in-pixel circuit wasreportedly able to calibrate the rate over 10 Mphotons/pixel/s [Trueb2013].
The maximum count rate has a trade-off with noise, power dissipation, andpixel size, and is difficult to improve significantly without degrading theother performance metrics. To overcome this limit, we will develop an
integration pixel with multiple storage cells. We set the target peak countrate as 35 Mphotons/pixel/s for a photon energy of 12.4 keV. Reading out at
the frame rate of 21 kHz with a peak signal of 1840 photons/frame gives anative maximum count rate of 38 Mphotons/pixel/s. By differentiating the193exposure time for each storage cell, we can effectively increase the peakcount rate further. By combining the fast frame rate with alternating shortand long exposure times of 1.9 and 47 µs, the effective count rate can beincreased by 25 times to 950 Mphotons/pixels/s at the expense of a small
increase in the photon shot noise. The sensor will have a global shutter line.
The time constant to open and close the electronic shutter will be below 10
ns. The minimum exposure time will be about 100 ns. Thanks to theexcellent effective count rate, this sensor can also be used in many of thediffraction experiments where we currently use one-dimensional strip
sensors.
(iii) Extreme X-ray radiation hardness
The X-ray radiation hardness is governed mainly by the structureof the sensor itself. Silicon dioxide is the major source of degradation as it
undergoes a charge buildup after X-ray absorption. We will develop amonolithic sensor process that can resist up to 100 Mrad @ 10 keV by
combining industry standard CMOS technology. With a short exposure time,such as 100 ns at a frame rate of 21 kHz, most of the X-rays illuminating
the detector are just degrading the sensors. In this case, synchronizing the
X-ray shutter to the bunch cycle, which removes the unwanted X-ray beams,
will significantly reduce the X-ray dose onto the sensor. The effectiveness of
this type of shutter configuration will be investigated in each scientificstudy.
(iv) A fast frame rate, preferably at 209 kHz
The frame rate is determined by the sensor architecture and the
production

process.

With

anavailable

complementary

metal–oxide–semiconductor (CMOS) technology the feasible frame rateusing a 0.3 Mpixel sensor was estimated to be about 21 kHz (Table I). The
rate is limited by the analog voltage settling time for the signal transferfrom the pixel to the on-chip periphery circuitry, the conversion speed of thecolumn analog-to-digital converter (ADC) in the periphery, the bandwidthof the digital outputs, and power dissipation. Achieving a frame rate of 209
kHz is feasible using an identical design with a region-of-interest operation
194mode, where about one-tenth of the image section is readout. In the readout
mode of the full image section, all of the pixel data is read out at a frame
rate of 21 kHz. In both the operation modes, the resulting data rate becomes
184 Gbps for a single sensor chip.
II-3.2. Sensor Variants
Once the sensor platform has been established, sensor variants(such as higher time resolution up to 10 ns at the expense of the peak signal,
spectroscopy imaging sensor with energy resolution less than 120 eV, or asmaller pixel for 0.5 µm resolution) can be developed at significantly lower
cost. The sensor and other components discussed in the next section should
be developed by taking into account maximum reuse of the design blocks for
future development of these sensor variants.
II-3.3. Detector System
An ideal implementation reads out, visualizes, and stores all of thedata. However, this type of scheme is not practical simply due to the large
data size. A sensor running at 21 kfps with 0.3 Mpixels of bit depth of 30
bits/pixel yields a data rate of 182 Gbps, reaching a data rate of 82.8
TBytes/hour/sensor (Table II-3-1). To cope with data of this size, we willdevelop proximity electronics, located near the sensor chip. The proximity
electronics will have a data memory of 4 GBytes that can store data of aboutfour thousand frames. The field-programmable gate array (FPGA) of theproximity electronics can conduct on-the-fly data analysis. Typicaloperation modes are listed below.
(a) Video mode
Some experiments demand high dynamic ranges, but dataacquisition can be slow. In these cases, the sensor chip operates at a frame
rate maximum of 21 kHz, and the proximity electronics sum up multiple
frames of data into low frequency image data. The presumed frame rate inthis mode is a maximum of 1 kHz.
(b) Burst mode
195

Some experiments demand acquisition of all of the image data. In
these cases, all of the frame data is first buffered within the proximity
electronics, and then transferred to the downstream computing server
nodes. In this burst mode, X-ray detection and data transfer to the server
are done sequentially. The highest bandwidth between the proximity
electronics and the server will be implemented by optical link through thehigh bandwidth serial port of the FPGA. An optical link with 20 Gbps
bandwidth will be used for a single sensor chip. The corresponding duty
ratio is about 10 %. Data is then cashed on the main memory of the local PC
cluster and analyzed by central processing units (CPUs). Analyzed data are
stored locally using a wide bandwidth data cache. The burst mode iseffective, for example, in destructive measurements, where samples are
periodically exchanged.
(c) On-the-fly data analysis mode
Some experiments have efficient analysis algorithms which
reduces the data rate without losing critical information. One of thesimplest cases is pump-probe experiments, where repetition of identicalexperimental conditions is assumed. Summing the adequate frame data ineach cycle drastically reduces the data size. Other on-the-fly algorithms will
be investigated in order to fulfill other scientific cases.
II-3.4. Infrastructure to operate coherently with the Detector System
Frequently, X-ray image data is analyzed together with other
instrument data, such as motor position and timing data of the pump
optical laser. From the analysis point of view, X-ray image data is better
formatted with a meta-data section containing all the experimental
conditions including the other instrument data. Because the frame rate of
the X-ray imaging detector is so high, efficient implementation should be
investigated. The baseline implementation will have a parameter
configuration function. Experiments will be conducted according to their
specific parameter configuration, which will be set prior to the dataacquisition. The parameter configuration will be sent to the X-ray imaging
196

detector system before the data acquisition starts, and the X-ray imaging
system will write the other instrument data into the meta-data section
according to the parameter configuration. To complement the parameter
configuration scheme, a monitor function will be built to watch over the
instruments in order to detect any failure. The synchronization of the X-ray
imaging detector system and other instruments should be designed andimplemented in a systematic way in order to reduce the costs ofdevelopment, maintenance, and learning. The implementation details must
carefully designed in terms of communication speed, failure patterns,
mismatches between the configuration and execution data (as for highprecision motors), and dependency on the efficiency of input/output
throughput on the data format.
The X-ray imaging detector system will have a local data cache andcomputation power to diagnose the data quality during the experiment. In
some end-stations, where the data volume is lower, the detector system will
be able to run as a stand-alone system. However, for many of the beamlines,
the local data cache is not large enough and we plan to move large data sets
to a global computation infrastructure through seamless archiving software.
The computation resources for post analysis should be implemented at the
global computation infrastructure. The details of the functions of the global
computation infrastructure must be carefully defined and developed.
Synergies with the SACLA data analysis framework should also be
considered in order to minimize construction and operations costs.

197

Table II-3-1. Draft specifications for the X-ray Imaging Detector for SPring-8-II

Physical

Value

Unit

Comments

Pixel size

70

µm

Square pixel

Pixel Format

768x384

Pixel Number

0.295

Mpixels/sensor

Peak signal

1840

photons/frame

1 photon@ 12.4 keV = 3.4
ke-Sensor

1 LSB

60

e-rms

Noise

60

e-rms

Bit depth

30

bits

Two 12 bit ADCs for high andlow gain circuitries with anoffset level of 6 bits

Performance
Max Frame Rate

20.88

kHz

Data Size

1.11

MB/frameMax. Raw Count38

Mphotons/s

184

Gbps

Raw data

Rate
Data Bandwidth(Gbps)
Output Data

Detector System

Values between sensor toproximity electronics

up to 20Gbps/sensor

Data can be reduced using

bandwidth fromanalysis algorithms

the proximity

conducted in the proximity

electronics

electronics.

Tiling Option

Yes

N/A

198

References[Hatsui2013] T. Hatsui, M. Omodani, T. Kudo, K. Kobayashi, T. Imamura，T. Ohmoto，A. Iwata, S. Ono，Y. Kirihara，T. Kameshima, H. Kasai, N. Miura,
N. Kuriyama, M. Okihara, Y. Nagatomo, M. Nagasaki, T. Watanabe, M.
Yabashi, Proc. Int. Image Sensor Workshop, (2013) Art. Num. 3.05. Online:
http://www.imagesensors.org/Past%20Workshops/2013%20Workshop/2013%20Papers/03-5_058_hatsui_paper.pdf[Trueb2013] P. Trueb et al., Journal of Physics: Conf. Ser. 425 (2013)
062002.

199

Appendix
Contributing Authors
Yoshihiro Asano

Takashi Ohshima

Teruhiko Bizen

Masaya Oishi

Hiroyasu Ego

Yuichi Okayasu

Takahiro Fujita

Yuji Otake

Kenji Fukami

Shigeki Sasaki

Toru Fukui

Yoshito Shimosaki

Shunji Goto

Kouichi Soutome

Toru Hara

Takashi Sugimoto

Takaki Hatsui

Sunao Takahashi

Takahiro Inagaki

Shiro Takano

Tetsuya Ishikawa

Masaru Takao

Kimitaka Kaneki

Hideki Takebe

Hirokazu Maesaka

Kenji Tamasaku

Mitsuhiro Masaki

Hitoshi Tanaka

Tomohiro Matsushita

Ryotaro Tanaka

Satoshi Matsuyama

Takashi Tanaka

Hidekazu Mimura

Kensuke Tono

Chikaori Mitsuda

Takahiro Watanabe

Takeshi Nakamura

Makina Yabashi

Haruhiko Ohashi

Kazuto Yamauchi

Haruo Ohkuma

200