This document was created by collaborative work between
JASRI/SPring-8 and Harima RIKEN.
We would like to thank all contributors......May, 2005

Copyright © RIKEN, Japan, May 2005. All rights reserved.
SCSS XFEL, R&D GroupRIKEN Harima Institute/SPring-8
1-1-1, Kouto, Mikazuki, Sayo, Hyogo JAPAN 679-5148
Edited by Takashi Tanaka and Tsumoru Shintake

TABLE OF CONTENTS
Table of Contents .......................................................................................................................................... 1
Introduction ................................................................................................................................................... 2
Scientific Case ............................................................................................................................................... 5
Accelerator Design Overview ..................................................................................................................... 10
Optimization of Parameters ......................................................................................................................... 20
Electron Injector .......................................................................................................................................... 25
C-band Main Accelerator ............................................................................................................................ 33
Undulator Line............................................................................................................................................. 42
Undulator to Experimental Area: ................................................................................................................ 50
X-Ray Transport System ............................................................................................................................. 50
Beam Trajectory control and Alignment ..................................................................................................... 55
FEL Commissioning.................................................................................................................................... 66
SCSS Control System Conceptual Design .................................................................................................. 73
Time Base and RF Signal Generation, Transmission and Distribution System .......................................... 80
Radiation Safety Issues................................................................................................................................ 89

INTRODUCTION

The first practical application of synchrotron
radiation began in the later part of the 1960’s. The
electromagnetic wave (light) produced when highenergy electrons moving at almost the speed of lighthave their motion deflected by magnetic fields orotherwise, is what we call synchrotron radiation. If
the electron beam is of sufficiently high energy, we
can obtain an extremely intense light source
covering a broad range of wave lengths from the RF
up to soft X-rays. The initial period of the 1960’s
was when the synchrotron radiation available forapplications was generated as a side effect in the
beam bending magnets of electron beam
accelerators being used for high energy physics
experiments. As light sources, the synchrotron
radiation of that period was so incoherent that it
didn’t make sense to even discuss its coherence
properties. However, by the 1990’s large scale
synchrotron radiation light sources, as typified by
the 3rd generation SPring-8, had come online. By
this generation of machines, the production ofsynchrotron radiation had transitioned from abending magnet side-effect, to being the output of
purpose-built specialized insertion devices such as
undulators. An undulator has a linear array ofpermanent magnets placed such that their polar
orientations are changed in a cyclic fashion. Whenhigh energy electron are injected into an undulator,
they execute a repeated meandering wavy motion
such that the intensity spectrum of the resulting
synchrotron radiation is strongly peaked at aparticular wavelength (monochromatic) with alaser-like directivity.
In other words, whencompared with the synchrotron radiation from beam
orbit bending devices, the coherence property(brightness characteristic) of undulator sources isoverwhelmingly superior, and scientific studies
which capitalize on these special features haveadvanced in rapid progression.
While the coherence properties of these sources
are certainly superior to that obtainable fromdeflection sources, when compared with theproperties taken for granted with lasers, even they
appear to be not far from incandescent lamps. For
example, it has been said that less than one
millionth of the incident light remained after the
optical processing needed to obtain coherent
radiation for an X-ray diffraction microscopy proof
of concept experiment was done with the highbrightness radiation from SPring-8.
Theexperiment observed colon bacillus, but the 1000
second exposure time for even a single image wasso long at that it would be out of the question to try
to observe the dynamics of living organisms thatway.
Nevertheless, that proof of concept
experiment did point out a very important shortcoming and potentiality. The reason why the longexposure time was needed was that the intensity of
the coherent X-rays contained in the SPring-8
synchrotron radiation is entirely too weak. But, if
the light intensity could be raised 5 or 6 orders ofmagnitude, it should be possible to make atomic
level observations of living systems withmillisecond time resolution.
.When even SPring-8, the world’s most intensesource of X-rays is unable to provide entirely oreven largely coherent X-rays, then the only
remaining possible source has to be a source basedon the induced emission radiation principle of
XFEL, the X-ray Free Electron Laser. Naturally,
the use of XFEL will enable closure in research into
the interaction of light and matter in experimental
regimes heretofore inaccessible, and the potential
for single molecule structural analysis, atomic level
X-ray microscopy, measurements down to subpicosecond resolution is under serious discussion.
So that going forward, just as happened so often in
the past when a new light source became available,
we can look for progress in nanotechnology,
biological, medical, information science, materials
science, all fields of fundamental and essential
importance.

The above has presented the motivations behind
this present project. The project itself is named
SCSS, short for the SPring-8 Compact SASE
Source, and the name directly reflects the above
concepts and considerations. The reason why wechoose this can be seen from the following
background. In the decade of the 1990’s, one after
another 3rd generation large scale synchrotron
radiation sources such as ESRF, APS and SPring-8
came online. As scientific results began pouringforth from these facilities as they exploited thespecial characteristics of high brilliance X-rays forprotein structural analysis and the like, more andmore interest in using high brilliance X-rays in everbroader scientific fields grew, to the point where it
became impossible for the just the three large
facilities to keep up with the world-wide demands
of users. Further, when we consider the applicationof high brilliance X-rays to bio and industrial fields,
it is natural to expect each country to want to haveits own synchrotron radiation facility. However,when we take the current economic situation into
consideration, we immediately come to theconclusion that it is not practical at all for eachcountry to attempt construction of its own 6 GeV orhigher energy large scale facility.
The large scale synchrotron radiation facilities
established in the late 1980’s, such as SPring-8, allhave their design concepts based on the undulator
technology of the time, a technology which fromtoday’s standpoint can only be called undeveloped.
Even for what was called short period, at best about
all that could be practically realized was a 4cm
period length. In order to obtain high brilliance Xrays with undulators of this period, the beam energy
was raised from 6GeV to 8GeV for these machines.
But, in 1990 the in-vacuum undulator, a giant
advance in synchrotron radiation insertion devices,
was born at KEK. This undulator does not have avacuum duct inside the magnet gap, and so an
extremely small gap can be obtained, limited only
by how tightly it can be closed before theaccelerator operation would be impacted. As aresult, even in an undulator with a very short perioda sufficiently strong magnetic field can be achieved.

After that, this undulator technology was muchfurther developed at SPring-8. In particular, thesuccessful joint BNL-NSLS development of an
11mm period, 3.2mm pole piece gap, in-vacuum
undulator has completely changed the world viewof what 3rd generation synchrotron radiation sources
can be, and thus creating what we might call the
New Third Generation Source or perhaps the 3rdGeneration Source Plus. In its main features, thisconcept combines an in-vacuum short-period
undulator with a low emittance, rather low energy
accelerator, so that a facility which while providing
performance comparable to the large scale facilities
like SPring-8, could be realized at low cost and mid
sized scale. The first facility based on this designconcept was the SLS (Swiss Light Source); the invacuum short period undulator and beam line
installed there has already begun producing
spectacular results in the field of structural biology.
Furthermore, mid-scale facilities of DIAMOND(UK), SOLEIL (France), CLS (Canada), AS(Australia) and SSRF (China) are all currently
under construction.
The same considerations can also be applied to
XFEL light sources. The first two projects, The
European X-Ray Laser Project XFEL (Germany,
E=20GeV) and the Linac Coherent Light Source
LCLS (USA, E=14.5GeV) as seen from theirenergy setpoints clearly belong to the large scale
facility category.
It is obvious that with the
exception of the great powers, their scale is suchthat they could not be constructed by every country
independently. Nevertheless, just as we havealready seen 3rd generation sources go from large
scale to mid-scale facilities around the world, in thenear future we may well expect a similar tide in theworld of XFEL sources as well. The SCSS projectis designed to be the leader in this next wave. One
of its design features is the adoption of the invacuum short period undulator, just as in the New
3rd Generation synchrotron radiation sources, thisallows reducing the beam energy needed. But, thisin turn causes a serious problem for the design ofthe linac. The allowable beam emittance forrealizing an XFEL is determined not by the beam

energy, but rather by the target wavelength of thea linac where adiabatic damping reduces theemittance in proportion to the beam energy, when
the target wavelength is fixed, when reducing theenergy setpoint, the accompanying normalized
emittance also has to be reduced proportionally. In
other words, this design concept would be rendered
impossible if we were restricted to the currently
available photocathode electron guns which can notachieve a good enough emittance characteristic.
Therefore it is necessary to develop an electron gun
with a new principle of operation that can realize anemittance value a factor of 1/2 ~ 1/3 lower.
In order to realize a mid-scale XFEL, it is alsoessential to realize a linac with a higher acceleration
gradient. Fortunately, in Japan the technology for apractical working C-band accelerator structure isalready available. An acceleration gradient of 40
MeV/m can be expected of the acceleration
structures. Reiterating the essential components ofthe mid-scale XFEL concept, we see that the
combination of in-vacuum short period undulators,
a linac with a rather low energy but with highradiation to be produced by the XFEL. Therefore inacceleration gradient, and an extremely low
emittance electron gun, could make the realizationof a low cost, mid-scale machine with performancethat could compare favorably with the alreadyongoing German and US large scale projects. The
SCSS project was design based on the above
concepts, and has already successfully completeddevelopment of the important components.
For a site for the SCSS facility, we are planningon the space adjoining the 1km beam line at SPring8. There are 3 reasons for choosing the SPring-8
site for SCSS: 1) it would complement the lightsource characteristics of the current SPring-8 8GeV
storage ring, 3rd generation synchrotron radiation
source, 2) it would be possible to simultaneously
use the synchrotron radiation from the SPring-8
storage ring and the SCSS, and 3) the site provides
a bedrock foundation with enough strength toassure a stable nanometer light source.

SCIENTIFIC CASE
Toward the utilization of the SCSS, several users working groups are being organized to discuss
innovative sciences at the SCSS. Several Research and Developments for the use of coherent xrays and/or short pulsed x-rays are ongoing using the 1 km and 27 m undulator beamlines of theSPring-8. These include optics and optical elements, single particle imaging and timing devices.

INTRODUCTION
It is widely recognized that the XFEL lightproperties are utterly different from those of the 3rd
generation synchrotron light. Therefore, sciences tobe explored with XFEL should not be mere
analytical continuation from sciences with the 3rd
generation synchrotron light sources. Recalling theinitial planning stage of the SPring-8 utilization,
many of the scientific proposals were simple
extensions from what were going on 2nd generation
sources. However, we know that experiences withthe 3rd generation light have been giving birth tounprecedented sciences that were not expected
before seeing the real light. Although the scientificcase discussed at the moment may be similar, it is
not useless to survey and compile the foreseeable
scientific trends of XFEL.
On the other hand, accumulated experiences byusing coherent x-rays at the 1000 m beam line andhigh power load at the 27 m undulator beam line of
the SPring-8 will surely help develop the
instrumentation needed to handle the XFEL beams.
The properties of x-ray beams obtained from theSPring-8 Compact SASE Source (SCSS) are similar
to those from other XFEL sources. Nevertheless, wehope the SCSS will be one of the predecessors ofcompact XFEL in the world, since we may require
more similar sources when the XFELs are proved to
be extremely useful. The size and cost of thecompact XFEL would enable many countries toafford the similar sources.

GENERAL CONSIDERATIONSXFEL X-rays
SASE-based XFEL will give us unprecedented
short-pulsed, spatially coherent and super-intense xrays. These extraordinary properties will open up ahuge number of new possibilities in vast scientificfields as has already discussed in detail in theconceptual design report [1] for LCLS and thetechnical design report [2] for TESLA-XFEL,
which is now reorganized as the European XFEL
facility.
At the moment, we have started organizing ausers association which gathers potential users fromwide range of the scientific fields. A series ofworkshops will be held to discuss the targets withXFEL in each scientific field, as well as to enhancecommunication among different fields.

Features of SCSS
Unique feature of SCSS compared with otherexisting XFEL projects is that it will be constructedadjacent to an operating 3rd generation SR. This will
give various opportunities of simultaneous use of
XFEL and SR beams. One of the simply
conceivable examples may be an XFEL-pump andSR-probe where SR can be hard x-rays for structure
dynamics or soft x-rays for electronics dynamics.
XFEL x-rays can interact with electrons in thestorage ring to give rise to inverse Compton
scattering, or can modulate a short portion ofelectron bunches to produce new phenomena. To
use the SCSS linac as an injector of SPring-8
storage ring will open up several new possibilities
of producing x-rays with quite different nature fromthose delivered by the present SPring-8.

USERS ORGANIZATION
In view of the huge potential of XFEL, we areplanning to organize users working groups invarious fields. These include following scientificworking groups:• Atomic and Molecular Physics• Condensed Matter Physics• Materials Sciences• Chemistry• Life and Medical Sciences• Optical and Laser Sciences
Each working group are being asked to organize
several workshops where more detailed sciencecases are to be discussed.
Meanwhile, the developments of some technicalissues applicable to interdisciplinary scientificfields are highly demanded. These include:• Sub-ps to fs timing techniques• Imaging techniques• Nanometer focusing techniques• Detectors• Optics
These technical developments will be discussed
either in the scientific working groups or in
dedicatedly organized technical working groups.
In addition to the discussion among domestic
scientists, international collaboration should beorganized to facilitate information exchange. The
tri-party workshop held by ESRF, APS and SPring8 for the information exchange among large scale
3rd generation synchrotron radiation facilities would
be a model for establishing a new international
collaboration scheme among XFEL facilities.

for coherent beam is highly demanded. We areproceeding of the development of slit apertures, Be
windows giving no interference fringes, supersmooth x-ray mirrors and high-perfection synthetic
diamond crystals.

Ion Beam Figuring (IBF)In performing coherence-related experimentseven at the SPring-8 beam lines, speckles from slit
and/or pinhole apertures always become a problem.
Conventional machining techniques can hardly
make a smooth figure required for coherent x-rays.
The speckles observed are a result of interference.
Therefore, it is difficult to subtract the backgroundin the intensity regime since the background itself isa result of interference between unwanted radiation
and real signals. To minimize the unwanted
radiation, or at least to make the background easily
treated, the surface finish of the slits or pinholes
before the sample should be as smooth as possible.
For this purpose, we are collaborating with a groupof the University of Hyogo to finish the aperturesurfaces using the Ion Beam Figuring (IBF)technique. Figure 1 illustrates a Ta pinhole (a) madewith laser machining and (b) with IBF.
Considerable improvement is observed both for the
shape and surface roughness. We are planning to
make x-ray tests soon.

ONGOING R&DS
Coherence preservation is the first-priority issue
in developing optics for XFEL. By using partial
coherent x-rays available at 1000 m beamline andhigh-brightness x-rays from the 27 m undulator of
the SPring-8, several R&Ds for XFEL optics areunderway. In fact, any optical elements in thecoherent x-ray beam can be origins of unwanted
interferencing contamination of the beam.
Therefore, further development of optical elements

Fig. 1 SEM pictures of pinholes in Ta plates: laser
drilled with 10 mm diameter (left); IBF machinedwith 13 mm diameter (right).

Be Window Development
One of the astonishing observations at undulator
beamlines of the SPring-8 was the interference
fringes caused by the imperfections of Be windows.

At the beginning, we attributed the origin of thefringes to surface imperfections. Accordingly, we
have replaced the windows by super-polished Be
foils with 0.1 µm Ra. The beam images after thewindows became more uniform with thereplacement, but interference fringes still remained.
We have examined several types of Be foils withphase contrast x-ray imaging, and found that Be
materials contain many void defects that produce
interference fringe patterns [3]. Since Be foils canbe an important candidate for the vacuum sealing
windows of XFEL, further improvement in quality
is highly demanded. There may be anotherapproach not to use any windows but differential
pumping system. However, we have a clear
evidence at the 27 m undulator beam line that thefiltering of lower energy components with Be
window helps preventing the first optical elementsfrom deteriorating with the radiation.

Super-Smooth X-Ray Mirrors
X-ray mirrors are another possible origin ofinterference speckles when illuminated with highly
coherent x-rays. Super-smooth x-ray mirrors that
produce no interference speckles are being
developed in collaboration with a group of Osaka
University. An atomic-level surface figuring
technique called Elastic Emission Machining(EEM) [4] combined with a new surface figure
metrology called micro-stitching interferometer [5]has enabled to fabricate super-smooth mirrors withless than 1 nm peak-to valley figure error even foraspherical shaped mirrors when applied to single
crystal silicon crystals. A Kirkpatric-Baez mirror
composed of uncoated elliptical mirrors gave afocal size of 180 nm×90 nm at the 1000 m end
station [6]. A new coating technique that does not
introduce deterioration in surface quality is being
developed to increase the usable glancing angle,
thus enabling to fabricate aspherical mirrors withlarger numerical apertures. As a result, apreliminarily fabricated elliptical mirrors gave afocal size of 40 nm×40 nm.
In parallel with the development of the supersmooth mirrors, a computer code based on the

wave-optical theory was developed for thesimulation of x-ray reflection properties from roughmirror surfaces. Conventional ray-tracing programs
based on the geometrical optics, such as SHADOW,
cannot fully predict the reflection properties ofrough mirrors when the incident x-rays are coherent.
The newly developed code numerically solves theFresnel-Kirchhof integral with measured surface
figure data as a boundary condition.

Synthetic Single Crystal DiamondSingle crystals of diamond may be one of the few
candidates that can overcome the unprecedented
high power of XFEL, though the detailed
interaction scheme is not yet fully revealed.
Actually, heat load at some of the 3rd generation
undulator beam lines is already above the limit ofcryogenic cooling of silicon (~500W/mm2), so that
diamonds are seriously considered as the first
optical elements in such beam lines [7]. However,the current crystal quality of the diamond is not ashigh as that of the best silicon. In particular, it isstill an open question whether the diamond crystal
can preserve x-ray coherence.
The SPring-8 optics group has been collaboratingwith Sumitomo Electric Industries Co. Ltd toward
the growth of diamond crystals with higher quality
for more than ten years. Various types ofcharacterization made at the SPring-8 havecontributed to the improvement in crystal growing
processes at Sumitomo. The characterization
techniques
includex-ray
rocking
curvemeasurements, x-ray topography, triple-crystal
diffractometry, surface-figure profiling and x-ray
fluorescence analysis. The long collaboration has
led to the current availability of high quality
crystals with a small number of stacking faults anddislocations [8].

Single Particle Imaging
Visualizing the arrangement of atoms has playeda crucial role in understanding the microscopic
world. There are already a few ways of imaging
atomic structures, but each has its limitations.
Scanning probe microscopes are limited to imaging

atomic structures at surface. Transmission electron
microscopes can resolve atoms but only for samples
thinner than ~ 30 nm. Crystallography can reveal
the globally averaged 3D atomic structure based on
the diffraction phenomenon, but requires crystals.
These limitations can in principle be overcome bycoherent diffraction imaging that is based upon theprinciple of using coherent x-ray scattering in
combination with a method of direct phase recovery
called oversampling [9]. Coherent diffraction
imaging has been successfully applied to 2D and
3D imaging of nanoscale materials [10,11] andbiological samples [12]. A highest spatial resolutionof 7 nm has been achieved, while the ultimate
resolution is only limited by the X-ray wavelengths.
By using the 3rd generation synchrotron radiation
sources, better-designed instruments and morerobust image reconstruction algorithms, we expect
to improve the spatial resolution to the 1 nm levelwithin the next few years. Meanwhile, we will also
pursue its applications in materials and nanoscience. We will focus on 3D imaging of porous
silica particles, GaN semiconductors, quantum dots
and mineralized bone composite. The ability toimage the internal pore structures in three
dimensions, 3D morphology of GaN and its alloys
in nanocrystal form, and 3D internal structures ofquantum dots, coupled with computational methods
such as molecular dynamics and ab initio
calculations, will profoundly expand ourunderstanding of the critical structural andmorphological features required to make superior
catalysts,
adsorbents,
electrodes
andsemiconductors. Understanding the mineralcomponent of bone such as the size, shape andarrangement of the calcium apatite crystals in acollagen matrix will be of fundamental importancein biology and medicine. In the long run, in a
combination of brighter X-ray sources and pixel
array detectors with higher quantum efficiency anda higher dynamic range, coherent diffraction
imaging could potentially be used to determine the
3D structure of single particles down to the single
atom level [13].

Time-Resolved Scattering/Spectroscopy
Several devices for the ps timing measurement at
the SPring-8 beam lines have been developed.
However, it would be difficult to use the existing
devices for much faster timing measurement at theSCSS. A time-to-space converter (TSC) [14] could
be one of the solutions. A post-processing systemusing TSC is under discussion.

REFERENCES[1] “Linac Coherent Light Source (LCLS) Conceptual
Design Report”, SLAC-R-593, April 2002.
[2] “TESLA Technical Design Report, PART V, The XRay Free Electron Laser”, ed. G. Materlik and Th.
Tschentscher, March 2001.
[3] S. Goto et al. “Characterization of Beryllium
Windows Using Coherent X-Rays at 1-km Beamline,”
AIP Proc. vol. 705, 2004, pp. 400-403.
[4] Y. Mori, Y. Yamauchi and K. Endo, “Mechanism ofAtomic Removal in Elastic Emission Machining,”
Precis. Eng. 10, 1988, pp.24-28.
[5] K. Yamauchi et al. “Microstitching Interferometry for
X-Ray Reflective Optics,” Rev. Sci. Instrum. 74, 2003,
pp. 2894-2898.
[6] K. Yamauchi et al. “Two-Dimensional Submicron
Focusing of Hard X-Rays by Two Elliptical Mirrors
Fabricated by Plasma Chemical Vaporization
Machining and Elastic Emission Machining,” Jpn. J.
Appl. Phys. 42, 2003, pp. 7129-7134.
[7] http://www.esrf.fr/Conferences/DiamondWorkshop/[8] K. Tamasaku et al., “Goniometric and Topographic
Characterization of Synthetic IIa Diamonds,” XTOP
2004 Abstacts, 2004, p.88. Proceedings will appear in
J. Phys. D if the paper is selected by the committee.
[9] J. Miao et al., “Extending the Methodology of X-Ray
Crystallography to Allow Imaging of MicrometerSized Non-Crystalline Specimens,” Nature, 400, 1999,
pp. 342-344.
[10] J. Miao et al., “High Resolution 3D X-Ray
Diffraction Microscopy,” Phys. Rev. Lett. 89, 2002,
art. no. 088303.
[11] J. Miao et al., “Phase Retrieval of Diffraction
Patterns from Non-Crystalline Samples using
Oversampling Method,” Phys. Rev. B, 67, 2003, art.
no. 174104.

[12] J. Miao et al., “Imaging Whole Escherichia Coli
Bacteria by using Single-Particle X-Ray Diffraction,”
Proc. Natl. Acad. Sci. 100, 2003, pp. 110-112.
[13] J. Miao, K. O. Hodgson and D. Sayre, “AnApproach to Three-Dimensional Structure Analysis of
Biomolecule by using Single-Molecule Diffraction

Image,” Proc. Natl. Acad. Sci. 98, 2001, pp. 66416645.
[14] Y. Tanaka et al. “Time-to-Space Converter for
Ultrashort Pulsed X-Ray Experiments,” AIP Proc. vol.
705, 2004, pp. 1379-1382.

ACCELERATOR DESIGN OVERVIEWIn SCSS project, three key technologies enable to construct a SASE mode FEL within availablesite length of 800 m at SPring-8. They are (1) low emittance electron injector using single crystal
thermionic cathode and velocity bunching scheme, (2) high gradient C-band main accelerator, (3)short period undulator of in-vacuum type. By using shorter undulator period of 15 mm, we canlower the electron beam energy as low as 6 GeV to generate coherent X-ray radiation at 0.1 nm
wavelength. The C-band rf system can generate 32 MV/m accelerating gradient using 50 MWclass klystrons. 6 GeV (or 8 GeV) beam energy is available in accelerator length of 240 m (or 320m). Including undulator length of nearly 100 m and X-ray beam line of 200 m, the total facility
length becomes 750 m, which fits the open space at SPring-8 site.
When the SPring-8 was constructed, the west north
XFEL MACHINE LAYOUT
side hill was open cut, from where a large amountof rock were removed and buried in this site. About
Figure 1 shows the machine layout of X-ray FEL.
ten years have past after constructed, thus we
The facility building is designed to house the higher
believe the basement is steady enough. Detailenergy machine up to 8 GeV. The electron gun,survey is scheduled from this year.
injector system located left end, and the electron
In Fig. 1, the green coloured area under the
beam will be accelerated in the S-band and C-band
undulatorlines and X-ray beam delivery, wasmain accelerator from left to right, reaches tooriginally hill, rock base, and it is quite steady. In
maximum energy at 350 m downstream. In thethe X-ray FEL machine, most sensitive part to thebeam switchyard, the electron beam will beposition movement is the undulator line and userswitched into different undulator lines in pulse-tofacility, both of them are located mostly original
pulse time sharing mode, or shift-to-shift slow
rock base. Therefore, ground motion due toswitching mode, or distribute bunch-by-bunch byseasonal changes, or change due to weather (rain or
fast sweeping along multi-beam line.
dry high temperature) will be negligibly small.
The proposed site is nest to the 1 km X-ray beamline at SPring-8, see Fig. 2, where 800 m long and
100 wide area has been reserved for future upgrade.

Fig. 1 XFEL machine layout and facility building. Total length fits available site length of 800 m.

(3) short period undulator of invacuum type. By using shorter
undulator period of 15 mm, we
can lower the electron beamenergy as low as 6 GeV togenerate coherent X-ray radiation
at 0.1 nm wavelength. The Cband rf system can generate 32
MV/m accelerating gradientusing 50 MW class klystrons. 6
GeV (or 8 GeV) beam energy isavailable in accelerator length of
240 m (or 320 m). Including
undulator length of nearly 100 m
and X-ray beam line of 200 m,
Fig. 2 Proposed site for XFEL machine at SPring-8.
the total facility length becomes
SCSS CONCEPT
750 m, which fits the open space at SPring-8 site.
Figure 3 shows the key technology in SCSS
project. In SCSS project, three key technologies
enable to construct a SASE mode FEL within
available site length of 800 m at SPring-8. They are(1) low emittance electron injector using single
crystal thermionic cathode and velocity bunching
scheme, (2) high gradient C-band main accelerator,

ELECTRON SOURCE DESIGNAs discussed in the later section on the machineparameter, XFEL requires high peak current andlow emittance beam. In SCSS, it is 3 kA and 0.85
π.mm.mrad. We may not generate this beam
directly from any kind of cathodes, therefore we

Fig. 3 Three key technologies in SCSS enable compact SASE-FEL machine.

Beamemittance fromthe cathode
Peak current
available from
cathodeEmittance
deterioration

Bunch lengthfrom cathode

Merit

Demerit

Table-1 Comparison of rf-gun and thermionic cathode electron sources.
Thermionic cathode electron gun, followed by
Rf-gunbuncher.
with photocathodeand short pulse laser< 1 π.mm.mrad< 1 π .mm.mrad
depend on cathode size.
depend on cathode size.
>100 A

a few Ampere max.
current density< 100 A/cm2

rf field inside rf-gun creates correlated
beam emittance growth, part of which canbe compensated by a solenoid field.

There is no big deterioration in the gun. The rffield inside pre-buncher and buncher systemcreates correlated beam emittance growth. Bychoosing lower rf frequency, it may lowered verysmall.
Long, basically DC beam. The control grid can
not be used, since it breaks the emittance badly.
We need beam chopper and pre-buncher andbuncher system.
Stable electron emission property for long period
of operation. Low field on cathode, and no darkcurrent emission.
Complicated system
Need beam chopper, pre-bunch and buncher
system. However, a laser system for the cathodeas rf-gun is not required, and also its associatedfacility, manpower are not required.

~ 1 psec, depends on laser pulse. Very shortpulse <0.1 fsec is also possible, but too
short pulse causes emittance growth inside
the rf-gun.
Direct generation of high current and shortbunch from cathode using short pulse laser.
High dark current
To extract short pulse beam from thecathode, very high accelerating field is
required to suppress the space chare field
effect. High dark current emission from thecathode runs through the accelerator, andcause radiation damage on undulator
magnet.

must rely on bunch compression scheme using
magnetic chicane. As discussed in later section ofstart-to-end simulation, in the magnetic chicane
bunch compressor, the coherent synchrotron
radiation (CSR) breaks the emittance in case of highcompression ratio. By using two stages
compression scheme, we may tolerate this effect. In
SCSS, the total compression ratio is 75. In thiscompression ratio, no much emittance growth isexpected. The peak current requirement becomes 40
A before the bunch compressor.
To generate 40 A peak current, there are two
possible choices, (1) rf-gun with photo-cathodeilluminated by short-pulse laser, (2) thermioniccathode followed by traditional pre-buncher andbuncher system. They are compared in Table-1. We

chose the thermionic cathode rather than the rf-gun.
This is mainly due to superior stability and system
reliability in the thermionic cathode. To an operateuser facility, the first priority is the higher beam
availability, which will be supported by highreliability and stability of the machine. From thispoint of view, we decided not to use the rf-gun
system, which is not fully reliable yet, and the laser
system requires manpower and frequent
maintenance.
About the dark current, it will be emitted fromthe copper surface in the accelerating structure, andit can be transported down through the linac
together with the main beam. 2D phase-space plot(r, r’) of the dark-current emission has an empty
hole at the center, since all of these trajectory

started from the copper surface, while the main
beam stays at the center, thus it can be fully
eliminated by using properly designed collimator.
On the other hand, the dark-current from thecathode surface occupies the same phase space as
the main beam, thus is can not be eliminated withcollimators, and cause radiation damage on theundulator magnet. The rf-gun needs very highelectric field on the cathode in order to quickly
accelerate the bunched electron beam, therefore
higher dark-current emission will be generated.

CHOICE OF TECHNOLOGY FOR THE
MAIN AC CELERATOR
Some of the recently started new FEL projects
have decided to employ the super conducting
accelerator technology, mostly running at 1.3 GHz
L-bandfrequency.
Advantages
inthesuperconducting accelerator are(1)
Super conducting rf-cavity is able to run
as CW mode, where the rf power to thebeam energy efficiency becomes higher.
(2)
After intensive R&D for TELA e+e- linear
collider project, it has been proven that thesuper conducting rf cavity can handle highaccelerating gradient as high as 30 MV/mor even higher at long pulse mode.
However, we have to note that linear
collider design assumes pulse mode
operation, while XFEL requires CW mode
operation for super conducting cavity toprovide higher average beam power,
where the accelerating gradient is still
limited around 10 MV/m.
(3)
Peripheral hardware components (klystron,
coupler, tuner, cryogenic system etc.) havebeen well developed and some of them are
now available from industry.
However, we did not chose the super conducting
technology in SCSS project, because(1) Project target is to generate coherent X-ray atwavelength ~1 Å, which is not a soft X-ray,thus the beam energy has to be as high as 6
GeV by using the short period in-vacuum
undulator of 15 mm period.

(2) To accelerate beam up to 6 GeV by thepresent super-conducting technology, themachine size beam too large. The achievable
accelerating gradient in today’s superconducting accelerator will be to be 12
MV/m (CEBAF upgrade design), the active
accelerator length becomes 500 m andaccelerator length will be ~600 m including
focusing system and beam diagnostics, which
does not fit our site. Also, the total
construction cost of the main accelerator(accelerator, refrigerator and facility tunnel)
becomes much higher than normal
conducting design.
(3) Our major interests in X-FEL beam is its
high peak brightness rather than high average
brightness derived from CW beam. Therefore,
the maximum pulse repetition rate of 60 Hz
is acceptable for most scientific cases. For
the multi-beam-line option, we accelerate themulti-bunch beam in each pulse anddistribute each bunch to the multi-beam-lines
by scanning kicker magnet.
(4) Actual hardware implementation of the super
conducting accelerator system is much more
complex than the normal conducting
accelerator system. We need many specialists(scientist and technician) not only for the
installation but also for the maintenance.
(5) The normal conducting accelerator system
has been widely used and its technology has
been well established. Thus we can construct
the normal conducting accelerator system
within a reasonable cost and maintain it withminimum manpower.
When we look a long-term perspective for 10
years or even longer, after the SCSS, there might beneeds for the super conducting option in XFEL,
because of its intrinsic superior performance: betterbeam stability and CW mode operation.
We propose SCSS X-FEL project as a short to
middle-term project, i.e., from the case study tobeam commissioning must fit within five years (twoyears engineering design and short term R&D, three
years facility and machine construction). Only thenormal conducting technology fits this project atpresent.
If we do this project in correct time-scale, many
new findings will be made in this machine quickly.
Therefore, normal conducting accelerator design isthe best choice. For details, see the later section.

UNDULATOR
There are several special requirements on
undulator for the SASE-FEL, and discussed below.
(1) Very long undulator line: ~ 100 m. We use
segmented undulators of 4.5 m long, andinstall in the line with focusing element and
BPM in between. We have technology
established in the storage ring undulators at
RIKEN/SPring-8. We are studying on
maintenance issue and production cost.
(2) Small gap: < 5 mm. This small gap size has
been sometimes used in storage ring
undulators, thus technically manageable. The
various wakefield effects become larger

when the gap is small, and careful theoreticalstudies have been carried out in manylaboratories. The wakefield is a kind of spacecharge effect, and the influence on theelectron bunch is always collective effect, i.e.
it cause projected emittance growth, but it
does not deteriorate the slice emittance.
(3) Very tight tolerance on electron beam
trajectory straightness, it is less than 10 µm
transverse for 10 m travel distance. As
discussed later section, we have establishedtechnology to control the beam trajectory inthis precision.
For details on undulator design, see later section.

OPTIMIZATION OF PARAMETERS
In designing each component in the FEL facility of SCSS, various parameters such as the electronenergy, peak current, energy spread, undulator period and field strength, etc., should be carefully
determined in order to realize lasing at 1 Å wavelength with the scale of facility as compact as
possible. In practice, we need to optimise these values because they are closely related with eachother.

UNDULATOR PARAMETER
The wavelength λ of FEL radiation is deteminedby the equation

λu ⎛ K 2 ⎞⎟,λ = 2 ⎜⎜1 +
2 ⎟⎠
2γ ⎝
where γ is the Lorentz factor of the electron beamand λu is the periodic length of the undulator. The
parameter K, called a deflection parameter, is adimensionless magnetic field strength of theundulator and given asK=P ∝ exp(z / Lg ),eB0 λu,2πmc

where e and m are the electron charge and mass, B0
the peak field of the undulator, and c the speed oflight. Assuming a pure Halbach configuration, thepeak field is roughly estimated as

B0 = 1.8 B r exp(−πg / λu ) × α ,where Br is the remanent field of the permanent
magnet material, g the magnetic gap and α a 3Electron Energy (GeV)

16
14
12
10
8
6
4
2
25
Pe
rio 20
dic
Le 15
ng
th

dimensional geometrical factor of permanent
magnet blocks. Using these formulae, the relation
between the parameters λu, γ, and g can be plotted
as shown in Fig 1. In principle, it is possible to takeany combination of (λu, g, γ) indicated in the figure
to realize 1Å FEL. It should be noted, however, that
the FEL amplification efficiency is also dependent
on these parameters, which actually gives a scheme
to find the optimum set of these parameters.
Under 1-dimensional approximation, the FEL
radiation power is amplified according to theequation
with

Lg =

λu
,
4π 3 ρ
1/ 3⎡ γλ2 re ne⎤
F1 ( K )⎥ ,
ρ=⎢
⎣ 8π
⎦
2
K
F1 ( K ) =×2
1+ K 2 / 2

(

)

2

⎡ ⎛ K2 /4 ⎞⎛ K 2 / 4 ⎞⎤⎜⎟⎢J 0 ⎜
2⎟ − J 1 ⎜⎜ 1 + K 2 / 2 ⎟⎟⎥ ,
K
+
1
/
2
⎝
⎠
⎠⎦
⎣ ⎝

λu 10

(m
m)

52

18 20
14 16
12
8 10 ap (mm)
G
4 6

22

Figure. 1 Electron energy as a function of the periodic
length and gap of the undulator to provide 1 Å radiation.

where re is the classical radius of electron, ne theelectron density, J0 and J1 the Bessel functions of
0th and 1st order. The parameter Lg is called gain
length and has a dimension of length, while ρ iscalled FEL parameter and is dimensionless. Thus ρ
can be used to estimate the performance of FEL
system. Apart from the wavelength λ to be fixed at1Å, ρ is dependent on three parameters, γ, ne, and K.
Among them, ne is independent of the other two
parameters. Thus a figure of merit, q, for
optimisation of parameters is the quantity q=(γF1)1/3.

The contour plot of this quantity is plotted as afunction of λu and g in Fig. 2 together with that of
the electron energy. In terms of the FEL
amplification, larger value of q is preferable, while
lower electron energy results in a more compact
facility and lower total cost. Thus, we have chosen
the three parameters, the electron energy, periodic
length, and nominal gap at point indicated by thecross mark in the figure, i.e., E = 6 GeV, λu = 15mm, g = 3.5 mm. The K value in this case iscalculated as K=1.3.

Periodic Length(mm)

25

11
10

9 8 7

Figureof Merit

6

20

5
15
4
10

1.0
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50

η = F (λu , γ , Lg , λ , β , ε n , I p ,σ E ),being the scaling function composed of the above 8
input arguments and 19 fitting parameters. Amongthe arguments, the average betatron function β,
normalized emittance εn, peak current Ip, and energy
spread σΕ are still unknown and should bedetermined.
It is easy to understand that Lg,3d is a monotonous
function of εn, Ip, σE, i.e., lower values of εn and σΕ
and a higher value of Ip results in a shorter gain
length. On the other hand, there is an optimum
value of β to obtain shorter Lg,3d as shown in Fig. 3.
Thus, β can be optimised for each set of εn, Ip, andσE.
Now we can investigate the requirements on the
beam parameters to realize the 1Å FEL. Figure4(a)~(d) shows contour plots of saturation length asa function of Ip and σE for different values of thenatural emittance. The saturation length is given by35
2 4 6 8 10 12 14 16 18 20 22
90

Figure. 2 Contour plot of the figure of merit (colour plot)
and electron energy (indicated by numbers) for 1Å
wavelength.

Gain Length Lg,3dGap(mm)Lg ,3d = Lg (1 + η ),
with

85

80

75

REQUIREMENTS ON THE BEAM
PARAMETER
Having optimised several parameters for 1Å
wavelength, the requirement on the electron beam
parameters, the emittance, current, and energy
spread, are investigated. We can use the universal
scaling function for the FEL gain length (Lg,3d) withthe 3-dimensional effects and energy spread of theelectron beam taken into account, which has been
derived from rigorous analysis on the FEL process[1]. Using the function, the gain length is expressedas-4

Ip=4kA,σE=1x10 ,εn=1.0πmm.mrad

70

10

15

20

25
β (m)

30

35

40

Figure. 3 Example of β dependence of Lg,3d. Other
parameters necessary to calculate the scaling function
are indicated in the figure.

⎛ P ⎞Lsat = L g ln⎜⎜ sat ⎟⎟.
⎝ Pin / 9 ⎠with

Pin = ρ 2 cE / λand(a) εn=0.8πmm.mrad(b) εn=1.0πmm.mrad(c) εn=1.2πmm.mrad(d) εn=1.4πmm.mrad-3

Energy Spread (10 )

0.25
0.20
0.15
0.10

0.05
1.5 2.0 2.5 3.0 3.5 4.0 4.51.5 2.0 2.5 3.0 3.5 4.0 4.51.5 2.0 2.5 3.0 3.5 4.0 4.51.5 2.0 2.5 3.0 3.5 4.0

Peak Current (kA)

Peak Current (kA)

Peak Current (kA)

Peak Current (kA)

50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Saturation Length (mFigure. 4 Contour plots of the saturation length as functions of Ip and σE for different values of the naturalemittance.

2⎛ Lg ⎞⎟ PbeamPsat = 1.6 ρ ⎜⎜L⎟⎝ g ,3d ⎠being the effective input noise power and saturation
radiation power, respectively [1]. The total length of
the undulator should be longer than Lsat to ensure
saturation of the FEL radiation power. In each set ofparameters εn, Ip and σE, β are optimised to give theshortest length of Lg,3d.

START TO END SIMULATION
Machine layout
Schematic layout of the 6 GeV machine is shownin Fig. 5. In order to estimate electron beam
parameters and radiation performance, start to end
simulation is carried out using three codes,
PARMELA for the injector, ELEGANT for themain LINACs and bunch compressors and
GENESIS for the undulator.

Expected electron beam parameters at injector
The initial condition of the simulation is based on
the results of the emittance measurements of theelectron gun. Since PARMELA can not treat a DC
electron gun, the simulation does not fully
reproduce the results of the emittance measurement.

In the simulation, 500 keV energy is immediately
given to the electrons at the start and no DC
acceleration process at the gun cathode isconsidered. Therefore, the effect of the space charge
would be underestimated. Figure 6 compares themeasured and simulated profiles of the electron
beam under the same set up used for the emittancemeasurements. The effect of the space charge,
which bends the edges of the distribution, isdifferent between Fig. 6 (a) and (b). However, theinitial condition of the simulation is determined soas that the divergences of the electron beam at x = 0
are consistent with each other as shown in Fig. 6 (c).
In a real machine, this initial condition can beachieved by eliminating the edges of the
distribution (Fig. 6 (a)) using a pin-hole slit. In thiscase, the emittance of the beam core neglecting thebent edges is about 0.5 π mm-mrad and this beamcore and 1 A beam current is used as the initialcondition of the simulation.
A schematic of the SCSS injector is shown in Fig.
7. As described in the previous chapter, an electron
pulse with 1~2 ns duration is produced from µsec
cathode emission by a chopper installed between
the electron gun and a 238 MHz PB (Pre-Buncher).
In the 238 MHz PB, the energy of the electron
bunch is modulated as a function of its longitudinal
position. This energy modulation turns into avelocity difference of the electrons. When thephases of the RF fields in the 238 MHz PB, 476

Fig. 5 Schematic layout of the 6 GeV machine

MHz booster and S-band LINACs are properlychosen, the length of the bunch can be compressed
as it moves toward downstream. Thus the peakcurrent of the electron bunch can be increased.
The energy and current profiles of the electron
bunch at various locations in the injector are shownin Fig. 7 (a) ~ (d). The head of the bunchcorresponds to the smaller time scale in Fig. 7 (a) ~(d). At the end of the injector, the peak current close
to 40 A is obtained with the beam energy of 50 MeV.
The emittance is another important parameter ofthe electron beam. Since a wavepacket of theemitted light in the undulator can interact only witha small fraction of the electron bunch, local
property of the electron bunch is important for the
SASE radiation. The local emittance of the electron
bunch, so-called a slice emittance, is calculated andshown in Fig. 8. Since the electron beam energy inthe injector does not fully get to a relativistic
regime, the emittance of the beam is degraded dueto the space charge effect as the peak current
increases. The slice emittance at the end of theinjector is estimated to be about 0.85 π mm-mrad asshown in Fig. 8 (c).

Main LINACs and bunch compressors
There are two stages of bunch compressors (BCs)in the current machine layout (Fig. 5) [2]. The firstBC is located at the beam energy of ~ 450 MeV.
After the injector, the electron beam is accelerated
up to 482 MeV in S-band LINACs. Before the firstBC, the electron beam is decelerated in an X-band
harmonic cavity as shown in Fig. 9 in order tocorrect the nonlinearity of the energy slope due tothe RF curvature.
Between the first and second BCs, the beam isaccelerated in C-band LINACs. The schematiclayout and parameters of the second BC is
described in Fig. 10. The configuration of the four
bending magnets are the same for the two BCs, but
the bending angle and compression factor of thesecond BC are set to be smaller in order to reduce
the CSR effect [3, 4]. Figure 11 compares thenormalized slice emittance at the two BCs, and it isFig. 6 Phase space profiles of the electron bunch, (a) measured and (b) calculated with PARMELA. The
divergence of the electron beam at x = 0 (mm) is plotted in (c).

shown that there is no significant emittancedegradation at the two BCs.
After the second BC, the electron bunch isaccelerated up to 6 GeV with on crest phase of the
C-band LINACs, and no further compression is
occurred at the final chicane. Final projected
emittance is about 3 π mm-mrad at the end of theaccelerator.
Figure 12 is the electron beam lattice used forthe simulation. The beta function is minimized at
the forth bending magnets of the two BCs, and an
FODO lattice is applied in the C-band LINACs andthe undulator section.

Electron energy
Normalized slice emittancePeak current
Slice energy spread
Undulator period
Undulator section length
Undulator gap (nominal)
K value (nominal)
β function at undulators
Radiation wavelength
FEL parameter
3-D gain length

6.135 GeV
~ 0.85 π mm.mrad~ 3 kA~ 4 × 10-5
15 mm
4.5 m
3.5 mm
1.3~ 30 m
~ 0.1 nm
2.8 × 10-4
3.7 m

Table 1. Slice beam parameters of the center part ofthe electron bunch (blue part in Fig. 13 (b)), together
with the parameters of the SASE radiation and theundulator.

Fig. 7 Velocity bunching at the injector. Electron beam energy and bunch profile (peak current profile) are
shown in red and blue lines respectively in (a) at gun exit, (b) at pre-buncher exit, (c) at S-band entrance and(d) at injector end (after energy filter).

Fig. 8 Normalized slice emittance and bunch profile (peak current profile) are shown in red and blue lines
respectively in (a) at gun exit, (b) at S-band entrance and (c) at injector end (after energy filter).

Radiation from undulators
The final electron beam emittance, peak current
profile and longitudinal beam energy distribution at
the entrance of the undulator section are shown in
Fig. 13 (a) and (b). In order to estimate the SASE
radiation, a center part of the electron bunch, whichis shown in blue in Fig. 13 (b), is sliced out andused for the calculation of the SASE radiation. The
slice paratemers of this center part of the electron
bunch is summarized in table 1.
Figure 14 is the simulated evolution of the SASE
radiation using the transverse electron beam
distribution contained in the blue part of Fig. 13 (b).
The SASE radiation is calculated in a time
independent mode. From Fig. 14, the expectedsatuation length is 80 ~ 100 m including the space
between the undulator segments.

REFERENCE[1] M. Xie, Proceedings of PAC 1995, p. 183.
[2] Y. Kim, private communication.
[3] Y. Kim et al., Nucl. Instr. and Meth., A528
(2004) 421.
[4] Y. Kim et al., "Start-to-End Simulations for PAL
XFEL Project", Proceedings of FEL 2004.

Fig. 9 Schematic layout and parameters of the first bunch compressor (BC-1). (a) and (b) are the electron bunch
profiles before and after the BC-1 respectively.

Fig. 10 Schematic layout and parameters of the second bunch compressor (BC-2). (a) and (b) are theelectron bunch profiles before and after the BC-2 respectively.

Fig. 11 Variation of normalized slice emittance at the two BCs, (a) at the injector end (~ 50 MeV), (b)after the first BC (~ 450 MeV) and (c) after the second BC (~ 800 MeV). Horizontal and vertical emittances
ae shown in red and blue lines respectively.

Fig. 12 Electron beam lattice of the 6 GeV accelerator, horizontal and vertical beta functions andhorizontal dispersion are shown in red, blue and black lines respectively.

Fig. 13 Electron beam parameters at the entrance of the undulator, (a) normalized slice emittances(horizontal in red, vertical in blue) and peak current profile (in green), (b) longitudinal beam energy
distribution.are shown in red, blue and black lines respectively.

Fig. 14 Evolution of SASE radiation (in black) emitted from the blue part in Fig. 13 (b). Calculation isdone in a time independent mode. Horizontal and vertical RMS electron beam sizes are shown in red andblue lines.

ELECTRON INJECTOR
In order to operate X-ray FEL in saturation region, the electron injector system is required to
produce a very stable beam with low slice emittance (0.85π mm.mrad) and high peak current (40A). We decided to use a thermionic high-voltage gun using a single-crystal CeB6 cathode.
Recently, a prototype gun was constructed and the normalized rms emittance of 1.1π mm.mrad
has been experimentally demonstrated. This result is very hopeful for the X-ray FEL. In thissection, we describe the design of the CeB6 gun and the injector system.

CHOICE OF THERMIONIC GUNIn the X-ray FEL theory, it is well known that thequality of the internal structure of the bunchedbeam dominates the FEL gain, that is, the sliced
emittance of the beam should be very low and thepeak current should be of the order of kA to saturate
SASE-FEL. Moreover from the FEL application
point of view, the FEL light should be stable forlong periods of operation.
Because the electron beam generated by the gun
is directly injected to the long undulator andgenerates X-ray beam, any fluctuation of theelectron bunch in transverse position, timing, size,
charge, etc., will directly affect on X-ray lasing.
This situation is quite different from the storage
ring type machine. Therefore, to produce a stable
X-ray FEL light, the stability of the electron gun isessential.

As we know well, the rf-photocathode gun has a
big advantage to generate a short bunch with highpeak current from the cathode directly. The rfphotocathode gun has been well studied andadvancedrecently.
Usinganemittancecompensation solenoid, the emittance below 3π
mm.mrad at 0.3 nC charge has been demonstrated[1]. However, the beam parameter is very sensitive
to the conditions of the laser pulse and thephotocathode surface. Generally speaking, the laser
system introduces additional fluctuation source ofthe system. And, it is not easy to make and keep theuniform quantum efficiency of the photocathode,because it is very sensitive to vacuum and a darkcurrent in the rf-cavity. A lot of efforts will beneeded to stabilize these items.
For this reason, we decided to use a thermioniccathode followed by a buncher system [2]. The
overall layout of the injector system is shown in Fig.

Fig. 1 : Injector system using thermionic cathode.

1. Basically, this is a traditional injector system formany types of electron accelerators. The stability
and long lifetime has been well accomplished in thepresent injectors, however, the typical emittance is~30π mm.mrad or larger. In order to make theemittance low, we have added the following
upgrade.
1) Small size cathode. The initial emittance of thegun dominates its cathode size. We use a single
crystal CeB6 cathode with 3 mm diameter.
Theoretical emittance is 0.4π mm.mrad at ~1400°C.
High beam current of >1 A can be produced fromthe CeB6 at this temperature for long lifetime.
2) Elimination of a control grid from the cathode.
The emittance of the traditional thermionic cathodeis broken due to the electric field distortion causedby the grid mesh.
3) Applying 500 kV on the cathode. In order tominimize emittance growth due to space charge, thegun voltage must be higher. We use 500 kV pulse
with a few µsec width.
4) Fast beam deflector. To form a nsec single
bunch from the long pulse generated by the gun, we
use a beam deflector with a fast pulser after the gun.
5) Adiabatic bunching and acceleration. In orderto minimize emittance growth due to rf-field, a
lower frequency is desirable. We use a 238 MHz
sub-harmonic pre-buncher, followed by a 1.6 m
drift section, then a 476 MHz booster cavity raises

the beam energy up to 1.1 MeV. S-band linac
system is used to accelerate the bunch to 50 MeV
before C-band main linac.
The design beam parameter at the gun exit andthe end of the injector system is summarized inTable 1.
Table1 : The design beam parameter at the gun exit
and the end of the injector system

Gun exit
Beam energy
Peak current
Pulse width (FWHM)
Repetition rate
Normalized emittance (rms)
Injector end
Beam energy
Peak current
Bunch Length (FWHM)
Normalized emittance (rms)

500 keV>1 A
1.6 µsec
60 Hz
0.4π mm.mrad

50 MeV
40 A
12 ps
0.85π mm.mrad

THE CeB6 GUN
We have designed and constructed a 500 kVelectron gun with a CeB6 cathode [3]. A side viewof the prototype gun with an emittance monitor isshown in Fig. 2.

Fig. 2 : Prototype of the CeB6 gun and emittance monitor.

CeB6 CathodeThe normalized rms emittance of electrons
emitted from a hot cathode is described byr2

ε n ,rms = c

k BT
,
mec 2

where rc is the cathode radius, kB is Boltzman's
constant, and T is the cathode temperature. In orderto obtain the required emittance of 0.4π mm.mrad at
the temperature of ~1400°C, the cathode diameter
becomes 3 mm. On the other hand, the beam
current is required to be >1 A, thus the emission
density
becomes>14 A/cm2. Rare-earth
hexaborides, that is, LaB6 or CeB6 can emit such a
dense current for long lifetime. Recently, a single
crystal CeB6 cathode is widely used for electron
microscope and the stability has been demonstrated[4]. It is reported that CeB6 is very strong against
carbon contamination compared with LaB6. The
operational temperature of CeB6 becomes lower
than that of LaB6 because it has lower work
function (~2.4 eV).
A single crystal is preferable to obtain low
emittance. Its surface can be extremely flat(roughness≤1 µm) with low porous after its material
evaporation [5]. The emission density becomes
uniform because the crystal direction is matched on
the whole surface.
For the above reasons, we decided to use thesingle-crystal CeB6 cathode with crystal face [100].
Fig. 3 shows the cathode assembly and the heating
cathode in the test chamber. The CeB6 crystal ismounted in a graphite sleeve. This produces auniform electric field on the whole cathode surface.
This is quite important for elimination of halo inbeam emission from the cathode edge, which cancause damage to the undulator magnets.
We use a graphite heater rather than theconventional metallic filament made of tungsten or
the like. Graphite is mechanically and chemicallystable even at very high temperatures and does notevaporate like other metals. Since its electric
resistance does not change much as a function oftemperature, it is easy to control the heater power.
The heater resistance is 0.18 Ω.

A tantalum cylinder covers the graphite heater toshield the thermal radiation from its surface. A baseplate of the cathode assembly is made of silicone
nitride, which is mechanically strong against
thermal stress.
The cathode was heated up to ~1400°C with 210
W of heater power in the test chamber (right side in
Fig. 3). The reference temperature was measured at
the graphite sleeve surrounding the cathode bymeans of a radiation monitor. We need more study
to determine the cathode temperature precisely. Up
to now, it has been operated for 10000 hours
without failures.

Fig. 3 : CeB6 cathode assembly.

Accelerating ElectrodeThe accelerating electrode was carefully
designed and manufactured because it affects theinitial condition of low energy beam. We chose aflat Wehnelt rather than the common Pierce-type
electrode. The reasons for this are as follows: (1)The Pierce electrode was originally designed to
produce a parallel beam whose space charge field isbalanced by a focusing electric field. However, if
the cathode is not exactly centered due tomisalignment of cathode mount or shifts in cathodeposition due to heating, the asymmetric focusing
field acts on the beam. This may cause emittance
growth. The flat Wehnelt does not have such aneffect. (2) We planned to vary the beam current overa wide range in order to tune the accelerator system.
The gun would be operated in a temperature limited
region. The Pierce electrode is not suitable for suchan operation, because at a low current, the beam isover-focused, however, the flat Wehnelt does notover-focus the beam.
Space charge limited current is described by
I scl =

42e S 3 / 2ε0V ⋅F ,
9me d 2

manufactured the electrode. The ultra-clean
stainless steel is used as the electrode’s material.
The surface was chemically etched and rinsed withultra-pure water to remove the hydrocarbon
contamination which causes discharge.

where V is gap voltage, d is gap distance of theelectrode and S is cathode area. In the case that thecathode radius is much smaller than d, the limited
current becomes higher than that of an infinite
parallel electrode. In order to take into account thiseffect, we introduced the enhancement factor, F, inFig. 4 : Beam trajectory at gun electrode.
Child’s law. By performing analytical evaluationand computer simulation using EGUN code, it was
High-voltage tankfound that the enhancement factor is a function of
Circuit diagram of the high-voltage tank is shown
rc/d. We set the gap distance to be 50 mm. In thisin Fig. 5. Basically, it follows a conventionalcase, F becomes 4.5 and the limited current is 10.5
klystron tank. Nowadays, 500 kV pulse is
A. It is much higher than the required current,
technically feasible in high power devices, such as
namely, the cathode is operated in temperature
X-band klystrons. We use the same model of the Climited region. The electric field near cathodeband klystron modulator [6] to feed -24 kV pulsed
surface becomes higher than that in space charge
voltage to the high-voltage tank. The primary pulse
limited region. Since the beam near cathode isis step-up to –500 kV by a pulse transformer, whose
immediately accelerated, it is expected thatturn ratio is 1:21. In order to match the impedance
emittance dilution due to space charge is minimized.
of the gun to the modulator PFN circuit, a 1.9 kΩ
We performed a computer simulation for 500 keV,dummy load is connected in parallel with the
1 A beam using the EGUN code. As shown in Fig. 4,
cathode. AC power for the cathode heater is fed
the beam trajectory does not diverge much at theelectrode gap. Whenthe mesh size becomes
small, the phase spaceplot becomes a straight
line, and the emittancewithout initial thermal
motion converges to
less
than
0.1π
mm.mrad below amesh size of 0.05 mm.
The maximum fieldon the Wehnelt edge iscalculated to be 26
MV/m at 500 kV using
POISSON code. To
avoid
discharge
problem at this highfield, we carefully
Fig. 5 : Circuit diagram of the high-voltage tank.

through the secondary winding of the pulse
transformer. Since the heater current is very high(>30 A), the step-down transformer, whose turnratio is 7:1, is used to reduce the power loss intransmission line from the power supply to thecathode .
Since we need to apply a -500 kV pulse voltage
to the cathode, all the high-voltage components,
namely, the ceramic insulator, pulse transformer,
dummy load, etc., are immersed in insulating oil toeliminate discharge problems. Before operation, thehigh-voltage tank is pumped out to eliminate gases
remaining in the oil and the high-voltage
component .

was 0.25 mm for the upstream slit and 0.1 mm forthe downstream slit. From the phase space profile,
we analysed the normalized rms emittance, definedas
ε n, rms = βγ < x 2 >< x' 2 > − < xx' > 2 ,where <x2>, <x'2> and <xx'2> denote mean squarevalues weighted by current. The result was 1.1π
mm. mrad. The beam parameter at the gun exit
demonstrated in the experiment is summarized inTable 2.

First experimental results
We have performed the first experiment on the
CeB6 gun [7]. Fig. 6 shows the waveform of the gun
voltage and the beam current. The beam current
was measured by a current transformer (CT) locatedin the beam line right after the gun. The beamenergy is 500 keV, and the peak current is 1 A. The
flat-top portion of the pulse is about 0.8 µsec, whichis enough to generate a 1 nsec bunch.
Fig. 7 : Phase space profile of 500 keV, 1 A beam.
Table 2 : Demonstrated beam parameter at gun exit

Beam energy
Peak current
Pulse width (FWHM)
Repetition rate
Normalized emittance (rms)

500 keV
1A
3 µsec10 Hz
1.1π mm.mrad

Improvements for real injector
Fig. 6 : Waveform of the gun voltage and the beam
current.

We measured the beam emittance by the socalled double-slits method. The upstream x-slit arelocated at 50 cm downstream from the cathode,followed by a 60 cm drift space and thedownstream slit. Fig. 7 shows the 2-demensional
plot of the phase space profile (x-direction)measured for the 500 keV beam with 1 A peakcurrent. The width for both the upstream anddownstream slits was set to 50 µm and the scan step

Based on the experimental results, the following
improvement will be done for the real injector
system.
1) Emittance : We achieved very small emittance,
but it was somewhat larger than the theoreticalvalue. The small tail at the profile edge, which may
be generated by the space charge effect, is a sourceof the emittance increase. The emittance without
this tail can be roughly estimated by making the
product of the rms diverging angle at the beam
center (σx’) and the rms beam radius (~r/2). Theobtained value of 0.6π mm.mrad is near thetheoretical thermal emittance. Since the nonlinear
tail comes from the edge part of the round beam, it
can be removed using a beam collimator. We
should realize the required small emittance less than
1π mm.mrad.
1) Beam current : We may need to increase thesource current to satisfy the required peak current (1A) after a collimator hole of the beam deflector,
which is used to remove the beam edge. In the casethat 2 A source current is needed, the increase of
the cathode temperature is estimated to be ~70°C.
2) Pulse width : The high-voltage pulse width
became two times longer than the design value. The
fairly big stray capacitance of the dummy load
registers causes this long duration. As a result, theheat load of the high-voltage tank became heavy.
The large size of the high-voltage tank is also
determined by the registers. In order to shorten thepulse width and to make the tank compact, we aredeveloping a dummy electron tube, instead of thedummy load register. The design of the newcompact gun is shown in Fig. 8.

voltage pulser. The principle of the deflector isshown in Fig. 9. The deflector consists of twoelectrode plates in parallel and a steering coil. The
pulser generates a high-voltage pulse (+3.5 kV)with short width (2 nsec) and fast rise time (~200
psec). The coil applies a horizontal magnetic fieldcontinuously in the beam line to deflect an unused
part of the beam and to dump it to a collimator. At
the timing that the flattop portion of the original
beam pulse comes into the deflector, the highvoltage pulse is applied to the upper electrode tocancel the magnetic force, and the beam passes
through the collimator hole. After the high-voltage
pulse passes through the deflector plate, the beam isdeflected to the dump again.

Fig. 9 : Principle of beam deflector.

The length of the deflector plates is 150 mm, andthe gap is 20 mm. The magnetic field of the coil is
13 Gauss. In this case, the vertical displacement of
the beam position at the collimator which located
150 mm downstream is 15 mm. It is enough large
compared with the beam diameter of ~5 mm. We
will prepare several sizes of the collimator holes toadjust the beam size. This collimator is also used toremove the beam edge which becomes the source ofemittance increase.

238 MHz BUNCHER

Fig. 8 : New compact gun with dummy tube.

BEAM DEFLECTOR
The single bunched beam is cut out from the longpulse by using a beam deflector and a fast high-As shown in Fig. 1, we will use a reentrant cavity
for the sub-harmonic buncher. The resonant
frequency is 238 MHz. The beam energy ismodulated by ±210 kV at the cavity, then the bunchis compressed after the following 1.6 m drift line.
The rf generated by the solid-state amplifier is
fed through four rf couplers connected at the end
plate of the cavity. The frequency is tuned precisely

by using two tuners. The dimension and thespecification of the cavity is shown in Fig. 10 and
Table 3, respectively.

growth, the magnetic lens was designed so that thedifferentiation of Bz0 becomes low. As shown in
Fig.11, the inner diameter of the yoke is chosen to
be fairly large to reduce dBz0/dz. The emittance
growth for this lens as a function of r is shown in
Fig. 12. dBz0/dz is calculated using POISSON code.
This result shows that the emittance growth isnegligible small for the beam with ~5 mm diameter.

Fig. 10 : Dimension of 238 MHz buncher cavity.
Table 3 : Specification of 238 MHz buncher cavity

Frequency
Gap voltage
Gap length
Input power (peak)rf pulse width

238 MHz280 kV (max.)
80 mm
14 kW (max.)
100 µsec

Fig. 11 : Dimension of magnetic lens.

FOCUSING LENS
The beam spreading due to transverse spacecharge force is given by

d 2r
dz

2

=

2I
,
( βγ )3 I 0 r

where I is a beam current and I0 is Alfven current. It
is obvious that the beam spreading can bedramatically reduced by increasing the beam energy.
Since the beam energy of 500 keV is enough high,we decided to use some magnetic lenses to focus
and transport the beam, rather than a series of big
solenoid coils which is often used in conventional
injectors to confine a low energy beam.
The emittance growth due to nonlinear focusing
force of the magnetic field is described by∆ε n ,rms ≈

⎛ e ⎞
⎟
⎜
96 2 ( βγ ) ⎜⎝ mec ⎟⎠1

2∫

Fig. 12 : Analytical estimation of emittance growth
due to nonlinear magnetic field. The plot is in the casethat the center field Bz0(0) is 825 Gauss.

2⎛ dBz 0 ⎞4⎜
⎟ dz ⋅ r ,⎝ dz ⎠

where Bz0 is longitudinal component of magnetic
field on the z-axis [8]. It has strong dependence on
the beam radius. In order to minimize the emittance

BOOSTER CAVITY AND S-BAND LINAC
In order to reduce the space charge induced
emittance growth of the bunched beam, the 476
MHz booster cavity is used to boost up the beam

energy from 500 keV to 1.1 MeV. A commerciallyavailable IOT-tube is used as an rf source, which
generate 100 kW peak power level stably.
Before injecting the short bunch into the C-band
main linac, the beam is accelerated to 50 MeV by
S-band accelerator system. The first column is astanding wave accelerator with alternating periodic
structure (APS). The second is a 2 m travellingwave accelerator with disk loaded structure. The rf
phase between two columns is adjusted by a phaseshifter located in the S-band circuit.

BEAM DIAGNOSTICS
At the end of the injector system, a diagnostic
section will be prepared to monitor the beam
qualities, i.e., the bunch charge, the bunch length,
the energy spread, the transverse emittance etc. The
beam is took off from the beam line by a bending
magnet which works as an energy analyser,
followed by a screen monitor. The slice emittance ismeasured at the screen by scanning a magnetic fieldof the quadrupole magnet located upstream of theenergy analyser.

REFERENCES[1] D.H. Dowell et al., “Slice Emittance
Measurement at the SLAC Gun Test Facility”,
Nucl. Instr. Meth. A 507 (2003) 327[2] T. Shintake et al., “SPring-8 Compact SASE
Source, SPIE2001, San Diego, USA, June
2001.
[3] K. Togawa et al., “CeB6 Electron Gun for theSoft X-ray FEL Project at SPring-8”,
FEL2003, Tsukuba, Japan, Sept. 2003, to be
published in Nucl. Instr. Meth. A[4] http://www.feibeamtech.com/[5] H. Kobayashi et al., Emittance Measurementfor High-Brightness Electron Guns, 1992
Linear Accelerator Conference, Ottawa,
Canada, August 1992
[6] H. Matsumoto et al., “A Closed Compact
Modulator for 50 MW C-band (5712 MHz)
Klystron” 25th International Power Modulator
Symposium and 2002 High Voltage

Workshop, Hollywood, USA, 2002, ISBN 07803-7540-8[7] K. Togawa et al., “Emittance Measurementon the CeB6 Electron Gun for the SPring-8
Compact SASE Source FEL Project”,
APAC2004, Gyeongju, Korea, March 2004
[8] K. Togawa, private note

C-BAND MAIN ACCELERATOR
We chose normal-conducting accelerator technology rather than the super conducting technology
for the SPring-8 X-FEL machine because of its lower construction cost and ready to start
construction today. The operating rf frequency is 5712 MHz in C-band, which is twice higher
than the conventional S-band 2856 MHz. Since the shunt-impedance of the rf accelerating
structure becomes higher at higher frequency, C-band system has superior performances than theconventional S-band (3GHz) system, and provides higher accelerating gradient as high as 40
MV/m for single bunch operation and 35 MV/m for multi-bunch, which makes the accelerator
length short and the facility construction cost lower. XFEL will be operated in the single bunchmode at 60 Hz for the first phase, while the C-band system is capable to accelerate multi-bunch
beam, this will be a candidate for future upgrade to multi-beam line option and higher average
brightness.
microwave component becomes smaller at higher
BRIEF HISTORY OF C-BAND
frequency, and mechanical machining becomes
ACCELERATOR
harder. Also, to generate higher accelerating field
The C-band (5712MHz) accelerator technology gradient, the rf energy density becomes higher,
was firstly proposed by Shintake in 1992 [1], and which cause potential risk of damage in high power
developed at KEK (Tsukuba, Japan) under the R&D components. After compromising between the
program in the future e+e- collider project for the energy efficiency and the technical difficulty infabrication and risk of damage, we chose the C-band
high energy physics research [2].
Energy transfer efficiency of rf energy to electron (5712 MHz) as the optimum frequency.
During the R&D period (year 1996~2000), we
energy of the accelerating structure in steady state
condition can be estimated by the shunt-impedance have developed:(1) 50 MW class pulsed high-power C-band
parameter, which scales as rf frequency:
klystron.
η ∝ r ∝ f 0.5(1)(2) A compact pulse power supply for the klystron,
When we raise the operating rf frequency twice
named “Smart-Modulator”
higher, the energy efficiency become 1.4 times better,(3) RF pulse compressor using low thermal
in steady state condition (CW or long beam pulse).
expansion material, which generates a flat output
However, in the single bunch mode, the energy
compressed power.
efficiency is inversely proportional to the stored RF(4) C-band accelerating structure using chokeenergy per unit length, which scales asmodetype higher-order-mode damper.
η ∝ 1/ w ∝ f 2(2)(5) C-band waveguide components (travellingActual machine operation mode is in between the wave type ceramic window, 3 dB hybrid coupler,
CW mode and single bunch mode, thus the etc).
efficiency scaling is(6) Cavity type beam position monitor, which has
0.5
2
high
spatial resolution (~20 nm@1nC) and high
η∝ f
f
(3)
Therefore, from the efficiency point of view, higher center position accuracy (a few µm).
frequency is desirable. This is the reason why Cband (5.7 GHz), X-band (11.4 GHz) or even higher
17 GHz was proposed as the drive frequency in thee+e- linear collider project. However, size of eachIn the year of 2002, KEK C-band group jointed to
RIKEN SPring-8 and started the SCSS R&D

program [3], where we continued
Table-1 C-band system parameter for 6 (8) GeV XFEL.
upgrading and debugging hardware Final beam energy at undulator
6
8 GeV
components.
Energy gain required for C-band
6105 8305 MeV
In 2004, we firstly tested the C- with 10% margin, injection energy 450 MeVband accelerator of choke-mode 6GeV…(6000 - 450 MeV) x1.1= 6105 MeV
damper design with SiC loads, 8GeV…(8000 - 450 MeV) x1.1= 8305 MeV
where we confirmed there is no Active length required.
191
260 m
serious problem up to 31 MV/m. 6GeV…6105 MeV / 32 MV/m = 191 m
We found rather high dark-current 8GeV…8305 MeV / 32 MV/m = 260 memission than we expected. We are Accelerator tubes
108
144
currently studying the beam 6GeV…191 m / 1.8 m = 106.111 --> 108
collimation system to eliminate the 8GeV…260 m / 1.8 m = 144.4 --> 144
dark-current.
C-band unit27
36
In 2001, the C-band R&D group 6GeV…108/4 = 27
for KEK-B upgrade was started 8GeV…144/4 = 36
separately at KEK [4]. In 2003, the Klystron and Modulator
54
72
first C-band accelerating structure 6GeV…27 x 2 = 54
of conventional disk-loaded design 8GeV…36 x 2 = 72
was installed in the existing KEK-B C-band Accelerator Length
243
324 m
injector, and tested with beam. (80% packing efficiency)
Accelerating gradient up to 40 6GeV…108 x 1.8 / 0.8 = 243 m
MV/m was achieved and energy 8GeV…144 x 1.8 / 0.8 = 324 m
gain was confirmed with electron Wall plug power for C-band accelerator
2.6
3.4 MWenergy measurement. The system is 6GeV…95 kW x 27 = 2565 --> 2.6 MW
continuously used to inject beam in 8GeV…95 kW x 36 = 3420 --> 3.4 MW
KEK-B operation. In this test, Wall Plug Power per unit at 60 pps
frequent high-voltage breakdowns Inveter capacitor charger 35 kW x 2 = 70 kW
Klystron focusing solenoide 5 kW x 2 = 10 kW
were found near the input coupler. Klystron heater
300 W x 2 = 0.6 kW
It is believed that a sharp edge of Thyratron heater
600 W x 2 = 1.2 kW
Thyratron
reservoir
50 W x 2 = 0.1 kW
coupling iris emits dark-current due
Core Bias
200 W x 2 = 0.4 kW
to high-density rf current, which Control system
0.5 kW
initiates the break-down in the main Wate temp-cotrol heater 1 kW x 5 = 5 kW
5 kW
cavity. A new version is designed Q-magnet power supply
Steering magnet
1 kW
and under fabrication, which has Vaccum pumps
total 1 kW
Total per c-band unit
95 kW
rounded corner in the coupling iris.
We started collaboration withmake feed-forward type feedback on the rf pulse
KEK-B team to share knowledge and hardware
compressor, and generates compressed power with aresource. This will be very important for design andflat top, which was initially designed to accelerate
construction of 6 GeV XFEL.
multi-bunch beam (80 bunches, total duration 200nsec) in the e+e- linear collider. XFEL will be
SYSTEM CONFIGURATION
operated in the single bunch mode at 60 Hz for the
Fig. 1 shows system diagram of one unit of Cfirst phase, while the C-band system is capable toband accelerator, and whose parameters are listed inaccelerate multi-bunch beam, this will be a
Table-1. Two 50 MW klystrons are used in parallel,
whose input rf phases are controlled by digitally toFig. 1 C-band RF acceleration system designed for 6 GeV (or 8 GeV) XFEL at SPring-8.
candidate for future upgrading of average brightness
and multi-beam line option.
As summarized in Table-I, the C-band main
accelerator generate 32 MV/m for multi-bunch. For
6 GeV (8GeV) XFEL, we use 27 (36) units of thissystem, where 54 (72) klystron, 54 (72) modulators,
and 108 (144) accelerating structure will be used.

DIFFERENT REQUIREMENTS FOR
LINEAR COLLIDER AND XFELIn the first phase R&D on C-band main
accelerator for linear collider (1995~2000), the firstpriority was to demonstrate reliable high-power
operation of the rf acceleration system. Also theenergy efficiency was one of the first priority. To
enhance energy efficiency the multi-bunch beam
acceleration scheme was introduced, and handlingtechnique of multi-bunch beam became an important
subject, where wakefield effect and beam loading
compensation were intensively studied in world
wide linear collider community. According to theconstruction cost, it was assumed that fabrication
cost will be drastically reduced by mass-production

effect and also the reliability will be increased bylearning effect.
On the other hand, in XFEL the first priority is themachine stability and reliability because it will be amulti-user facility. Especially very high energy
stability is required. If we realize energy stability inthe order of 10-5, the spectrum analysis without
monochrometer becomes possible. In the today’s
electron linear accelerators, the beam energy
stability is around 10-3, which will not be enough for
XFEL operation.
In the XFEL design work, we took the following
subject as the first priority.
Beam energy and intensity stability.
Higher reliability and maintainability.
Smaller size and lower production cost.
In SASE-FEL, the power gain is fairly small, thus
we need a very long undulator line to saturate X-ray
beam. The gain is very sensitive to beam parameter,
specially, the beam trajectory in the undulator line.
Accuracy requirement on the beam alignment inundulator line is only 4 µm. To maintain the
trajectory straight, electron beam position

Table-2. Inverter HV capacitor charger.
Maximum Charging Voltage
50 kV
Maximum Charging Rate
1.5 A
Average Output Power
35 kJ/sec
Peak Output Power
75 kJ/sec
Voltage Regulation /1 minute
0.12 %
Power Efficiency
85 %Power Factor
0.85

I out = Q / τ = CV / τ = 1.5 A (constant current)
Ppeak = I out × Vmax = 75 kJ/sec
1
Paverage = CV 2 / τ = 35 kJ/sec
2
Thanks to the today’s advancing IGBT solid-statetechnology, we can realize such power converterwith small size, as shown in Fig. 2. The total size
was reduced more than ten times from the traditional

monitoring and feedback, and HeNe laseralignment system will be implemented.
And also X-ray beam based alignment will
be used as the fine tuning. To performthese procedures, machine stability is themost important key.
The details R&D work on eachhardware components will be describedfollowing sections.

HV Capacitor Charger (Inverter
Power Supply)
We developed the switching inverter HVgenerator to charge capacitor bank in PFN(pulse forming network) of the klystronpulse power supply (klystron modulator).
Fairly high peak output power is requiredin order to charge the PFN capacitor of
Fig. 3 Pulse modulator for 50 MW klystron and electron gu.
400 nF (total) up to 50 kV in 15 msec
swing-charge capacitor-charging circuit having a(1/60 pps = 16 msec, and 1 msec rest time), peaklarge volume transformer and series inductor.
output power capability becomes 75 kJ/sec.
Table-2 shows the specification and performanceof the developed inverter power supply.

Pulse Modulator Power Supply (Klystron
Modulator)

Fig. 2 Inverter type HV capacitor charger forpulse modulator for klystron and electron gun.

The klystron modulator was the most expensive andtroublesome part among all hardware components inthe pulse electron accelerator. Traditional modulator
design was
Large size.
Large EM noise source.
Requires large amount of cooling air, blowfan (sometimes short-life).
Accumulated dust on HV parts cause leakage
current, and discharge.
Humidity in air cause HV leakage current.

In order to solve these problems, we employed metalshield box installed all HV components and filledwith insulator oil. Whit this design,
Smaller size W1.7 m x D 1.2m x H 1.2 m
Good EM shield. Metal enclosure forms,
No cooling air is required. Heat is removed
by insulation oil and cooling water pipe.
HV circuit is perfectly shielded from thehumidity and dust.
Figure 3 shows the klystron modulator, whose tankis made by stainless panels commonly used in thestandard water supply. All HV components are
suspended to the top flange, as shown in the figure.
For maintenance, by simply lifting up the top flange
we may access to the circuit components.
To realize this scheme, we have checked reliabilityof all HV circuit parts: the HV capacitor, thyratron,
diode, resistor, cable and sockets. Especially thethyratron is the most unreliable component, thus wehave carefully selected tube type and triggering
circuit [5].

Table-4 C-band 50 MW klystron parameter(TOSHIBA E3746).
Peak rf output power
50 MW
Beam to rf power efficiency
46 %Nominal pulse length
2.5 µsec
Max. pulse repetition frequency
Electron gun voltage
Electron beam current
Beam micro-perveance
Number of output cavity
Number of output ceramic window

60
350
316
1.5

Hz
kV
A

µA/V1.5
3 cell
2 Parallelby surface density of energy deposition in one pulse.
From these conditions, we find the scaling law on
the power handling capability in pulsed klystron as
Eq. 4.
Higher frequency klystron has lower power
capability. By scaling from existing klystron data atband frequency we chose, 50 MW peak and 2.5 µsec
as the basic parameter of our C-band klystron.
Table-3 is the parameter list of the developed C-

50 MW C-band KlystronThe pulse C-band klystron has been developed
under linear collider R&D at KEK collaboratingwith TOSHIBA Co., which is named E3746 tube,
and now on the TOSHIBA’s product line [6].
The klystron basic parameter is designed by scaling
existing S-band klystron, such as, SLAC 5045 and
TOSHIBA E3712 tubes. We assumed frequency
scaling law on the handling power limit,

W = Pτ ∝ f −2

(4)

The breakdown limit of the klystron in pulse mode
operation is basically limited by the following two
factors.
(1) The high voltage breakdown across theelectron gun gap, and high voltage insulator.
(2) Meltdown of copper surface of beam pipe
structure due to electron collision.
The first limit can be tolerated by reducing
the field gradient by enlarging cavity gap, or the HV
insulator. However, the second limit is tight. To
avoid feedback parasitic oscillation, we need to use
small beam pipe to cut off the higher order modes.
On the other hand, the tube meltdown limit is given

Fig. 4 TOSHIBA 50 MW C-band
klystron.

Table-5 RF pulse compressor.
Cavity dimension.
153Dx430L
RF ModeTE0,1,15
Q-factor
180 x103
Compression factor
5Power gain for flat pulse
3.1
200 nsec ramping for beam loading
compensation
250 nsec for multi-bunch beam

Peak output power
Power efficiency

300 MW60 %band klystron (TOSHIBA E3746). We designed thegun voltage at 350 kV, which is quite safety numberin the high voltage breakdown point of view, andalso in the power supply design. Since 1996, morethan ten tubes of this design have been developed
and used in high power test stands at C-band linear
collider R&D, Super-KEK-B, and SCSS XFEL at
RIKEN. There is no trouble reported, so far, and nodead tube until now (April 2005).
As listed in Table-1, total number of 54-tubes for 6
GeV XFEL ( 72 for 8 GeV).

RF Pulse Compressor
In the linear collider R&D, to accelerate multibunch beam train, beam loading compensation
inside the train was required. In the C-band design,
digital phase-modulators were implemented at the
klystron input. The phase modulation was converted
to amplitude modulation by combining two klystronpowers as shown in Fig. 1, then fed into the rf pulse
compressor. The rf pulse compressor possess threeenergy storage cavities in series in each arm, andgenerates a flat pulse output with ramping waveform
for beam loading compensation.
Three-cavity design was introduced to improveenergy efficiency. However, in XFEL, machinescale is not huge, and it is not necessary to pushenergy efficiency higher. To make simple the system,
we chose the traditional SLED design (single cavity)
for XFEL project. The parameter of the SLED type
pulse compressor is listed in Table-5.

Accelerating Structure
In order to stably accelerate multi-bunch beam, weneed to cure the beam instability due to the longrange wakefield in the accelerating structure. Since
the electron linear accelerator uses periodic structure,
this wakefield effect, specially the long range
wakefield (high-Q higher order modes) does the
major role on beam instability. In order to damp the
HOM (higher order mode), the choke-mode
structure was devised by T. Shintake in 1991 [7]. As
shown in the cut view of Fig. 5, when an electron
bunch runs through the accelerating structure, it
generates wakefield, which propagates outward anddamped with ring-shaped microwave absorber ineach cell. The rf accelerating field is monochromatic,
which is inductively reflected by the choke structure
resonating at the rf drive frequency.
The first model of the chokemode cavity was developed for Sband frequency and tested withhigh power rf in 1994. With 120
MW peak input power, the beam
was accelerated at 50 MV/m,
where no rf breakdown wasobserved inside the structure [8].

Fig. 5 The choke mode accelerating structure concept.

The first C-band accelerating structure of this typewas developed in 1998 for the linear collider project,
whose HOM damping performance was tested withdriving-prove-beams in ASSET facility at SLAC in1998. With this test measurement, we confirmed
basic performance of the choke-mode structure[9].
There are small resonance at 18 GHz was found in
the test, which was a trapped mode in the structure.
To solve this resonance problem, we slightly
modified cavity dimensions, and 1.8 m long
structure was developed in 2002 for SCSS XFEL
project as shown in Fig. 6 and Table-6.
In 2004, we firstly tested the C-band structure withhigh power rf in the high-power test stand at Spring8 site[10]. We tested the structure up to 31 MV/maverage accelerating gradient (limited by available
power, no rf pulse compressor was used). No serious
rf break down was observed during the test, and nofatal damage was found inside the structure after test.
The dark current was found to be larger than our
expectation, but it still allowable level if we
correctly implement beam collimator in several

Table-6 C-band Accelerating structure.

points in the main linac.

Digital RF Signal Control
As discussed in the previous section about the rf
pulse compressor, to obtain the flat pulse output rf

Fig. 6 C-band accelerating structure for SCSS. In each cell, the microwave absorber made by SiC was
loaded to damp higher order modes.

power, we need to compensate the impulse responseof the resonance circuit, which has normally a spikepeak at beginning and followed by exponential
decay. To do this, we apply amplitude modulationon the pulse compressor input. However, when the
klystron is operated in the linear regime, i.e., not insaturated condition, the output power is very
sensitive to the gun voltage, and temperature change,
and also aging of cathode. In the saturated condition,
it becomes very stable, and power efficient.
Therefore, we apply phase modulation on the
klystron input and combine two klystron outputs.
When phase signs are opposite each others, a sum of
two rotating vectors gives amplitude modulation.
Required speed of the phase modulation is givenby time response of the system. The system
bandwidth is limited by the accelerating structure.
When an electron bunch passes through theaccelerating structure, it gains energy from allaccelerating cells,

to the downstream end, then rest of the rf power isdamped in dummy load. When the rf power reachedto the downstream end, the electron beam comes.
The electron runs through one accelerating structure
in a few nano-seconds only, whereas it takes a few
100 nsec for rf power to fill the accelerating
structure, since the rf energy propagation speed ismuch slower than speed of the light (0.01c~0.02c).
Therefore, what electron sees is a time integration of
the rf power during the rf filling time, it is 230 nsec
in C-band accelerator.
For the digital modulation system, the sampling
speed about ten times faster than the filling time will
be enough. By taking another factor two for Shanons
sampling theorem, the required sampling frequency
becomes,

Fsample = 20 ×1
= 100 MHz
230 nsec(6)

The vertical resolution is defined by the requiredenergy resolution. Most important SASE FEL
jφn
V = Vn ⋅ e(5)application is the diffraction imaging, which requires-3
where Vn is the accelerating voltage in each cell. In energy resolution ~10 . When we use seeding laser
the disk-loaded structure, rf power is fed from injection, which will improve coherency, were-4
upstream end, and propagates down along the series energy resolution of 10 will be required. Asof cavity chain as a slow wave structure, and reach discussed above, we have 20 samples in filling time,and electron will see the average value of those, therequired resolution
is given by 20
times of 10-4, i.e.,
10-3, which is 10
bit. By taking one
bit for plus andminus sign, andone bit for a roomof dynamic range(nominal
peak
outputvoltagecomes 50% of
dynamic range),
finally12
bit
digital system isrequired.
12 bit 128 MHz
Fig. 7 Digital feed back system for C-band accelerator. By sampling rf vector voltage, DA and AD cards
computes feedback and apply to the next pulse.

∑are underdevelopment, for details refer later chapter.
In order to perform feedback on successive pulses
by self-learning from previous sampled data, it isrequired to precisely apply feedback on the same

REFERENCES[1] T. Shintake, "JLC-I", KEK Report 92-16,

December 1992, A/H/M[2] T. Shintake and H. Matsumoto, “Results from
Hardware R&D on C-band RF-System for e+eLinear Collider”, Proc. LINAC98[3] T. Shintake et.al, “SPring-8 Compact SASE
Source (SCSS)” SPIE, Sandi ego, CA 2001
[4] T. Kamitani et.al., “Development of C-band
Accelerating Section for SuperKEKB”,
LINAC2004, Lubeck Germany, Aug. 2004
[5] H. Matsumoto et. al., “A Closed Compact
Modulator for 50 MW C-band (5712 MHz)
Klystrons”, Proc. Power Modulator Conf. &High Voltage W.S., 2002
[6] T. Shintake et.al., “Development of C-band 50
MW Pulse Klystron for e+e- Linear Collider”,
PAC97, Vancouver, Canada, May 1997

timing in the rf pulse, to do this, the sample
frequency is chosen at 128 MHz and phase locked,
which is half integer of lowest rf 238 MHz in prebuncher cavity.
[7] T. Shintake, “The Choke Mode Cavity”, Jpn. J.
Appl. Phys. Vol.31, p.1567, No.11A, 1992
[8] T. Shintake et.al., "High Power Test of HOMFree Choke-Mode Damped Accelerating
Structure, LINAC94, August 21, 1994, Tsukuba,
Japan[9] T. Shintake et.al., “The First Wakefield Test on
the C-band Choke-Mode Accelerating Structure”,
PAC99, March 29 - April 2, 1999, New York
City, New York[10] T. Inagaki, “High Gradient Test of the C-band
Choke-Mode Type Accelerating Structure (InJapanese)”, Proc. of 1st Accel. Society of Japan
2004

UNDULATOR LINE
The undulator line is that section of the SCSS accelerator that actually produces the intense X-ray
beam of the FEL system; it is composed of undulators, beam position monitors (BPMs), steering
magnets, and quadrupole magnets. After being accelerated to 6 GeV by the main linac, theelectron beam is injected into the undulator where the SASE process takes place. The undulator
consists of 18 segments each 4.5 m long, between which the other components are installed. The
electron beam is focused by the quadrupole magnets to preserve optimum betatron functions overthe undulator line. The electron trajectory is monitored by the BPMs and corrected by thesteering magnets so that trajectory error does not degrade the FEL amplification.
magnetic properties are reported in the following
sections.

UNDULATORThe SCSS undulator consists of 18 segments,
each of which is 4.5 meters long. In contrast to theway undulators are typically used in synchrotron
radiation facilities, the magnet gap will not bescanned to change the photon energy, but rather be
fixed at a nominal gap (~3.5 mm). With a fixed gap,
the injected electron energy will be varied to tune
the photon energy. Nevertheless, the gap can be
opened and closed since a variable gap positioning
system is necessary for several operating utilities,
and is moreover necessary to allow a full-open state
for the initial commissioning of the electron beamand mechanical alignment of the beam position
monitors (BPMs) using the optical laser light. The
undulator parameters are summarized in Table 1.
Table 1: Main Parameters of SCSS Undulator

Periodic Length
Length/Segment
Number of Periods
Gap
Minimum
NominalMaximum
K Value
Maximum
NominalMagnet Material

15 mm
4.5 m
300
25 mm
3.5 mm
2 mm
1.8
1.3
NdFeB

A prototype undulator segment with theparameters as given above has been constructed andtested [1]. The undulator mechanical structures andMechanical Frames
The mechanical frame is composed of 4 units,each of which supports the top and bottom iron Ibeams holding the magnet jaws. Two stepping
motors drive the top and bottom I-beams
independently, which enables vertical centring forthe on-beam alignment of the undulator. The linear
vertical motion of each I-beam is transmitted by 8ball screws driven by the stepping motor, by varying
the screw starting positions, both vertical position
and tilt along the longitudinal direction can be set
independently for each unit. The beam elevation
above the floor level of the SCSS accelerator isdesigned at a rather low 800 mm in order tominimize component mechanical vibration andposition drift. Thus, the SCSS undulator is required
to have a more compact structure than that of more
standard undulators such as used in the SPring-8
storage ring.

Figure. 1 Schematic Illustration ofundulator mechanical structure

SCSS

In-vacuum Structure
Because of the short length of the magnetic period,a very narrow gap is needed to obtain sufficientmagnetic field strength. An in-vacuum structure
where the magnetic arrays are placed inside thevacuum chamber can provide a narrower gap thanthe conventional outside-vacuum structure, because
there is no vacuum chamber between the magnet
gaps. But it is also important for SCSS project that awide vertical aperture (25 mm) can be realized byfully opening the magnet gap. This will help with
the initial commissioning of the electron beam in theundulator line. In addition, a laser light beam can bepassed through entire undulator line (~80 m in total),
this will help with the necessary precision finemechanical alignment of the BPMs installed in theundulator line.
Because of the in-vacuum structure, the magnet
blocks would be exposed directly to the electron
beam if no countermeasures were taken. Therefore,
the magnet surface is first covered by a thin metal
foil made from copper to reduce the resistive wakefields which will be described later. In addition, thecopper foil has a nickel plating thus providing an
attractive force between the magnet blocks and thefoil; this improves thermal conductivity for reduced
heat deposition in the foil.

Gap full close

Gap full open
wide
aperture

Electron beam
Alignment optical laser

Figure 2: Cross sectional view of the in-vacuum
structure of the SCSS undulator.

Magnetic Structure
Because they must be installed in the vacuum,
out-gassing considerations prohibit the use of glue in
assembling the magnets. Therefore the magnet

Figure 3: Photograph of a single unit of the SCSS
undulator magnet.

pieces must be fixed together mechanically.
However, the short period (implying thin magnet
pieces) makes the clamping of each piece quitedifficult (impractically small screw size). To
overcome this problem, four magnet pieces areassembled in one block as shown in Fig. 3. If theusual Halbach configuration is adopted, the poles ofthe four magnets would be orientated as shown in
Fig 4(a). However, the magnetic field that results
after initial assembly of the magnet unit is usually
not satisfactory. Therefore it is necessary to maketrial and error changes in the assembly configuration
until the resulting field satisfies the several criteria
for proper undulator performance. At SPring-8, thefield correction is carried out with an "in-situ
sorting" technique based on rearrangement of themagnet units by flipping and swapping. However,with the Halbach scheme the allowableconfiguration possibilities are limited because oflack of symmetry. Instead, we have adopted a
configuration of magnets with its easy axis inclined
45-degrees as shown in Fig. 4(b). The result is twice
as many possibilities for flipping directions. An
example magnetic field calculation for such aconfiguration is given in [2], it shows a slightly
higher peak field is obtained when compared withthe usual Halbach type even while keeping theeffective K value the same. We also calculated themagnetic field from blocks with the SCSS undulator
dimensions and found that the difference was verysmall (~3%).

y
x

z

Figure 4: Two ways of assembling the four magnet
blocks into a single unit: (a) normal Halbach
configuration
and(b)
45-degree
inclined
configuration providing increased symmetry.

the standard method for insertion device fieldcorrection used at SPring-8. The method was
originally developed for field correction of the invacuum undulators where the common shimming
technique is not allowed because the resulting
uneven magnet surfaces created by the shimming
would cause undesirable effects on the electron
beam in the storage ring. The main purpose of the
field correction is to reduce the orbit deviation andphase error. In the FEL process, the former will
degrade the overlap between the photons andelectrons, while the latter will affect the formation of
micro bunches.
Figures 5(a) and 5(b) show the undulator
properties calculated with the magnetic fieldmeasured at the initial (just after assembly) and finalalignment and sorting. It is found that both theelectron orbit and phase error are improved
significantly. The r.m.s. phase error of 2.76 degree
obtained after the final correction would make thisundulator quite adequate for use as the light source
in a synchrotron radiation (SR) facility. In fact, it is

Horizontal
Vertical

12
10
8

(a)

Initial

6
4
2

Final

0
-2000 -1000
Phase Error (degree)

After delivery to the Spring-8 site, the beam axis
vertical and horizontal magnetic fields were
measured using a１mm thick Hall probe. The result
was that the magnetic properties were found to have
undesirable systematic errors induced by the gap
differences between units and/or tapering within asingle unit (gap misalignments). It should be notedthat a precision mechanical alignment had alreadybeen performed before starting the fieldmeasurement and corrections. Thus we see that themagnetic measurement was more sensitive to thegap misalignment than the conventional mechanicalposition sensors used for the precision mechanicalalignment. In fact, magnetic properties, especiallythe phase error, was found to be sensitive to even a
1µm gap misalignment. These gap misalignments
were carefully corrected so that to the limits ofmeasurability, the systematic error was completely
removed.
After correcting gap misalignments, field corrections through in-situ sorting were performed. This is

Displacement (µm)

Field Corrections

60
50
40
30
20
10
0
-10

0
z(m)

1000

2000

(b)
Initial(17.4)

Final(2.76)

0100 200 300 400 500 600
Pole Number

Figure 5: Results of field corrections in terms of (a)the electron trajectory and (b) phase error.

As shown in the preceding section, prototype
undulator segment as constructed was found to havegood magnetic performance as the light source in a
SR facility, i.e., as the spontaneous-radiation source.
For the next step, we need to investigate its practical
performance in an FEL system. An FEL simulation
code has been developed for such a purpose [3]; it
can take into account the effects resulting from error
components included in a practical magnetic field.
Simulations were performed for the ideal error freemagnetic fields and with the two different magnetic
fields as measured before and after correction.
The parameters used in the simulations are
summarized in Table 2. It was assumed that all theundulator segments were identical copies of the
prototype segment and had the same designed or
measured magnetic field distribution, so as to
identify clearly the effects due to the error
components of the magnetic field. The simulation
results are shown in Figs. 6(a) and 6(b) in terms of
the peak radiation power and bunching factor. The
results show that the FEL performances obtainedwith the final corrected magnetic fields are nearly
equivalent to those with ideal error free magnetic
fields, and that the field correction has significantly
improved the undulator performance as an FEL
driver.
Table 2. Parameters used in the simulations.

Electron Energy
Peak Current
Energy Spread
Normalized Emittance
Average Betatron Function

6 GeV
3 kA
1x10-40.7 π mm.mrad
30 m

EFFECTS DUE TO WAKEFIELD
Wakefields are electromagnetic fields induced by
the interaction between the electron beam and theRadiation Power (GW)FEL Simulations

1

Bunching Factor

found by a numerical computation that the spectral
intensity degradation is just 5% at the 5th harmonicand 30% at the 13th harmonic.

10
0
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
10
-7
10
0
10
10

-1

10

-2

10

-3

10

-4

10

-5

10

-6

10

-7

Ideal
Initial
Final

0
20
40
60
80
Distance from Undulator Entrance (m)Figure 6: Results of FEL simulations to check the
performance of the constructed undulator segment as
an FEL driver: (a) peak radiation power and (b)bunching factor are shown as functions of the distancefrom the undulator entrance.

surrounding environment. Depending on theorientation of the electric field, wakefields can beclassified into two types, i.e., longitudinal andtransverse wakefields. The former causes anincreased energy spread, while the latter causes
emittance growth.
It should be noted that the emittance growthcaused by a transverse wakefield only occurs whenthe electron beam has a transverse position offset. In
other words, it can be neglected if the electron beam
moves along a trajectory on the undulator axis with
only small deviations (excepting sinusoidal
oscillation.) We may expect that this condition will
be satisfied by the beam guidance system, so that the
trajectory error does not affect the FEL amplification.
Thus, the effects due to the transverse wakefields
can be neglected and only the longitudinal
wakefields affect the FEL amplification process.

The electron beam injected into the undulator
passes through a space whose narrowest dimensions
are set by the top and bottom magnet arrays covered
with the copper foil. Because the nominal gap
distance (~3.5 mm) is much shorter than the widthof the copper foil (~ 50 mm), it is reasonable toassume that the copper foil dominates theenvironment that encloses the electron beam. In sucha case, we should consider two component
wakefields induced by interaction with the copper
foil, i.e., the resistive wall and surface roughness
wakefields. While it is certainly not the case for the
SCSS in-vacuum undulator, for a first approximationin order to simplify the problem, we use theanalytical formulae for wakefields in a cylindricallysymmetric enclosure. This should give us someorder of magnitude estimates for the effect of thewakefields.

cZ 0
×
π a2L

Ws ( s ) =∫∫ dk dk kxResistive Wall Wakefield
An expression for the resistive wall wakefields
generated by a point charge is analytically derived in[4] and is given as

4cZ
Wr ( s ) = − 20 ×πa∞
⎡ exp(− sˆ)
2 x 2 exp − x 2 sˆ ⎤ˆcos 3s −
dx ⎥,
⎢
π ∫0
x6 + 8
⎣ 3
⎦

(

)

with

⎛ 2a 2 ⎞⎟⎟sˆ = s⎜⎜⎝ Z 0σ ⎠"roughness" of the surface is essentially nonsystematic random noise. In order to make a close
estimate of the wakefield caused by the surface
roughness, it would be necessary to measure thesurface profile very precisely and then solve theMaxwell equations numerically with boundary
conditions from the measured sample. In general
this is a very time consuming and non-insightful
calculation. Instead, Stupakov [5] has derived aquasi-analytical formula to describe the surface
roughness wakefield by assuming that the surface is
not like a steep mountain but rather like a gentle hillwith a very slowly varying slope (small-angle
approximation).
His expression for the surface roughness
wakefield using the small-angle approximation is [5]−1 / 3,where Z0 is the vacuum impedance, c the speed oflight, σ the conductivity of the copper foil, and s the
distance from the point charge. The parameter, a, isin general the radius of the beam pipe surrounding
the electron; here we will use the nominal half gapof the SCSS undulator.

Surface Roughness Wakefield
The functional form for the surface roughness
wakefield cannot be expressed by a simple formulasuch as the resistive wakefield because the

z

3/ 2
z

2

p (k x , k z ) w(k x , k z , s ),withw(k x , k z , s ) =q=(2qs − 1) cos qs − (2q + 1) sin qs,
2s 3 / 2

k x2 + k z2,
2 kz

where p(kx, kz) is the Fourier transform of thesurface profile in the xz plane and L is thelongitudinal length for Fourier transform. It istherefore necessary to measure the surface profile inorder to calculate the surface roughness wakefield;
and this has not yet been performed for the SCSS
undulator copper foil. Lacking data, instead we
assume that the surface profile consists of randomly
distributed bumps. Averaging over the distributionfunction, the quantity |p(kx, kz)|2 is replaced byaL
R(k x , k z )2πwhere R is the Fourier transform of the correlation
function for the surface roughness. Assuming a
Gaussian spectrum with the r.m.s. height of d andcorrelation length lc, R can be expressed as⎛ k 2 + k y2 2 ⎞l 2d 2R (k x ,k z ) = c exp⎜⎜ − xlc ⎟⎟.
2π
2⎠⎝Energy Gain by Wakefield (keV/m)

10

w(s)

5
0
-5
-10
0

2
4
6
8
Normalized Distance s

40

surface roughness(lc=50µm,d=1µm)

20bunch head

0
-20bunch tail

-40
-60

resistive

-80
-150

10

-100

-50

0

50

100

150

Position Along Bunch (µm)

Figure 7: Function w for surface roughness
wakefiled as a function of the normalized distance.

Figure 8: Examples of the energy modulation by theresistive (blue line) and surface roughness (red line)
wakefields.

Thus we finally have⎛s⎞w⎜⎜ ⎟⎟,⎝ lc ⎠

with⎛ kˆ 2 ⎞ˆˆˆˆw( s ) = ∫∫ dk x dk z k z exp⎜⎜ − ⎟⎟ ×⎝ 2⎠
kˆ 2 sˆ − kˆ cos qˆsˆ − kˆ 2 sˆ + kˆ sin qˆsˆ ,

[(

z

)

(

z

)

dependent simulations have been performed toestimate the gain degradation due to the energy
modulation brought by the resistive and surfaceroughness wakefields, the results of which are
shown in Figs 9(a) and 9(b), respectively. In each]kˆ 2
kˆ 2 = kˆx2 + kˆz2 , q =
.
2 kˆ
z

The function w dominating the surface roughness
wakefield is calculated numerically and is plotted inFig. 7. Now we can estimate the surface roughness
wakefield using the above equation and theroughness height d and correlation length lc.

Estimation of Wakefield Effects by Simulations
The resistive and surface-roughness wakefields
described above cause an energy modulation in theelectron bunch as shown in Fig. 8, where examplesof the energy gain per unit length are plotted asfunctions of the position along the bunch. For
calculation of each wakefield, the parameter a, or
the pipe radius, has been substituted by the half gap
width 1.75 mm. The correlation length lc and r.m.s.
height d to specify the surface roughness have been
assumed to be 50 µm and 1 µm, respectively. Time-

Radiation Power (GW)

−3 / 2

10

1

10

0

10

-1

10

-2

10

-3

(a) Resistive Wakefield

No Wake
300K
77K

-4

10
1
10

Radiation Power (GW)cZ d 2 ⎛ s ⎞Ws ( s ) = 5 /02 3 ⎜⎜ ⎟⎟8π al c ⎝ l c ⎠(b) Surface-Roughness Wakefield
10

0

10

-1

10

-2

10

-3

10

-4

No Wake
lc=50µm,d=1µm
lc=50µm,d=5µm
lc=50µm,d=10µm
0

30

60

90

Distance from Undulator Entrance (m)Figure 9: Results of time-dependent simulations toinvestigate the effects due to (a) the resistive and (b)surface-roughness wakefields, respectively.

LATTICE FUNCTIONSThe electron beam is focused by quadrupole fields
while it is transported through the undulator line inorder to ensure the FEL amplification process totake place. The optimum betatron function with the
SCSS beam parameters is estimated at around 20 m
on average. We have considered two kinds offocusing-magnet structures to achieve this condition(a)FODO

Undulator
Focusing

Defocusing(b)Triplet

Undulator
Focusing

Defocusing

Figure 10: Schematic illustrations of two different
focusing-magnet structure: (a) FODO lattice and (b)triplet quadrupole.

betatron functions (m)28(a) FODO

24
20
16
12
28

betatron functions (m)figure, the gain curve without any wakefield effects
is shown in the black line. We find that the effects
due to the wakefields are not too small to beneglected. In particular, the rough surface of thecopper foil to cover the magnet blocks may result ina significant gain degradation. It is therefore
important to finish the surface of the copper foil assmoothly as possible. In addition, the surface profile
should be measured to obtain information on theroughness.
As for the resistive wakefield, we can reduce theeffects by decreasing the operating temperature of
the undulator magnets. For example, theconductivity of copper at 77 K is better than that at
300 K by about one order of magnitude, which
results in a significant improvement of the output
power as shown in Fig. 9(a).
It should be noted that the above simulation
results overestimate the effects of the wakefields
because the analytical formula implemented in thesimulation are those for the cylindrical enclosures.
We need to evaluate the wakefields under theconditions of the in-vacuum undulator structure formore precise estimations of the effects due towakefields.

(b) Triplet

24

βx
βy

20
16
12
0

10 20 30 40 50 60 70 80 90

Distance from the Undulator Entrance (m)Figure 11: Betatron functions with the average valueof 20 m calculated for the (a) FODO lattice and (b)triplet quadrupole magnet.

as shown in Fig. 10. One is the FODO lattice, where
the focusing and defocusing quadrupole magnets are
installed alternatively. The other is to install the
triplet quadrupole magnets in each drift section.
The lattice functions caclucated with the above
two magnet structures that give the average betatronfunctions of 20 m in the horizontal and verticaldirections are shown in Fig. 11. The integrated field
gradient for each quadrupole-magnet structure aresummarized in Table 2. Note that the distance
between the focusing and defocusing magnets in the
triplet quadupole is assumed to be 0.2 m.
Table 2. Integrated field gradient required toachieve the average betatron function of 20 m.
FODO
TripletFocusing Field Gradient (T)
1.85
6.50
Defocusing Field Gradient (T) 1.75
3.23(×2)
The advantage of exploiting the triplet quadrupoleis that the deviation of the betatron functions from

Radiation Power (GW)

the nominal value of 20 m is very small in each
undulator segment. On the other hand, it brings astringent restriction on designing the quadrupole
magnet because higher field gradients and longer
drift sections for installation are required than that of
10

1

10

0

10

-1

10

-2

10

-3

10

-4

10

-5

10

-6

10

-7

FODO
Triplet

0

10

20

30

In order to determine which lattice should bechosen, we have performed FEL simulations withthe two lattice functions. The results are shown in
Fig. 12, where we can find little difference between
the two. It is therefore possible to choose the FODO
lattice without any performance degradation.

REFERENCES

40

50

60

70

80

90

Distance from the Undulator Entrance (m)Figure 12: Results of FEL simulations with (a) the
FODO lattice and (b) the triplet quadurpole.

the quadrupole magnets for the FODO lattice.

[1] T. Tanaka, K. Shirasawa, T. Seike and H.
Kitamura, Proc. 8th Int. Conf. Synch. Rad. Instrum.
(SRI2003), 227 (2004)[2] M. N. Smolyakov, Nucl. Instrum. Meth. A467468, 207 (2001)[3] T. Tanaka, Proc. Int. Conf. FEL (FEL2004) to be
published; see also
http://radiant.harima.riken.go.jp/simplex/[4] K.L.F. Bane and M. Sands, SLAC-PUB 95-7074[5] G. V. Stupakov, Phys. Rev. ST-AB 1, 064401(1998)UNDULATOR TO EXPERIMENTAL AREA:
X-RAY TRANSPORT SYSTEM
Abstract: The conceptual design of x-ray transport system at SCSS is described. Conventional
designs are adopted for spatial filtering components, vacuum systems, and beam shutter.
Theoretical and technological developments are progressed for fixing final specifications ofcomponents which are directly exposed in the on-axis XFEL radiation, such as mirror,
monochromator, and absorber.

Figure 1: Schematic layout of the x-ray transport system.

GENERALLAYOUT

A main purpose of the x-ray transport system is todeliver XFEL beam to the experimental station after
conditioning x-rays to demand user requirements.
This is realized by spatial/spectral filtering system
which removes unnecessary radiation especially
included in spontaneous light. The second issue is todiagnose photon beam properties (e.g. position,
direction, intensity, etc.) and to provide information
to the feedback system for the accelerator operationand to user experiment. Diagnostics tools for the
initial commissioning are also included. The third
purpose is to assure human safety. The possibility ofradiation exposure should be completely eliminatedby shielding, shutters, and the interlock system.

In addition to usual radiation safety for x-rays,
direct hit of the electron beam from the linear
accelerator to the users must be avoided. Two typesof the beamline layout are considered. One design isusage of a deflecting mirror. In case of an accident
to the electric power of the bending magnets at thebeam dump, the mirror intercepts the electron beam.
In the other design, permanent magnets are set
between the end of the undulator and the beam dump.
The permanent magnets deflect electron beamslightly but sufficient angle so that the electron beam
does not reach the beamline if the beam transport islong enough.
A schematic view based on the former design isshown in Figure 1. A front end section includes
spatial filtering components and evacuation systems
and terminates with a differential pumping system. A
main beam shutter (MBS) follows the section. A gas

Table 1: FEL x-ray beam characteristics

FEL energy range< 22 keV(8 GeV electron beam)

Pulse width

500 fs (FWHM)

Repetition

60 Hz
FEL x-ray beam characteristics at E=12.4 keV(6 GeV electron beam)

Peak power

3 GW

Average power

0.09 W

Average power density

560 kW/mrad2

Photons/pulse

7.6×1011

Beam size at the source

50 µmφ (STD)

Beam divergence

0.40 µrad (STD)

Bandwidth

6×10-4 (FWHM)

cell for attenuating beam intensity is placed withanother differential pumping system in thedownstream. A mirror and a double-crystal
monochromator (DCM) work as main opticalcomponents. Several types of diagnostics systems
are used for daily operation as well as for initialcommissioning.
The layout is basically applied for the latter case,with modification of the mirror section and theoptical axis in the downstream of the mirror.

DETAILS
Front-end section
The front-end section is designed for removing
off-axial spontaneous/thermal radiation and forkeeping high vacuum condition. The total power andpower density of the peripheral radiation is not so
extreme. Thus the design is based on that developed
for the SPring-8 undulator beamlines [1,2] except
on-axis components. A sequence of fixed masks anda front-end slit gradually decreases spatial widths of
the incident beam. These components are made ofmetals (e.g. GlidCop) in grazed angle geometry withwater cooling. They are not intended to receive on-axis XFEL radiation. Thus the minimum gap of thefront-end slit should be controlled to avoid
intercepting XFEL beam. The use of low-Z material
may be considered for smaller gap operation. The
maximum gap is determined by the aperture (~several millimeters) of the differential pumping
system located in the downstream.
The section is directly connected to undulator
chambers without any windows or apertures. Thus
vacuum level under 10-7 Pa should be kept with ion
pumping system. A fast closing shutter (FCS) is
installed at the entrance of the section in order toprotect transmission of vacuum trouble to the
upstream. The exit of the section is connected to thedownstream through a differential pumping system.
Be-windows, which is widely used for vacuum
separation in present x-ray beamlines, are not
adopted in order to avoid unwanted modulation ofbeam intensity caused from inhomogeneity of thematerial.

Main Beam shutter
The Main Beam Shutter (MBS), which stands fora pair of a photon absorber and a beam shutter, isinstalled after the front-end section of the beamline.

It works to decide whether the photon beam isintroduced to the transportation channel or not. The
photon absorber and the beam shutter intercept thethermal and the radiational constituents ofsynchrotron radiation, respectively.
As shown in Figure 2, there are two paths lined up
vertically inside the absorber body, which is made ofcopper. The upper one of the closed mode stops thephoton beam, whereas the lower one of the openmode passes it through a fixed aperture. Each pathwill be moved on the beam axis by a pneumatic
driven support on which the photon absorber ismounted. The photon beam is absorbed using
popular grazed angle geometry, because an average
power level on the absorber will not be problematic.
The resultant absorbed power will be dissipated bycooling water. However, since the peak power
density per unit solid angle is extremely huge ofabout 3 MW/mrad2, there is a possibility that wehave to reconsider this design including a selectionof the material considering a photon-induced
damage. On the other hand, Figure 3 shows theinside of the beam shutter, which is installed
downstream side of the photon absorber. Theintercepting body for radiational constituent, madeof tungsten, is connected to the pneumatic driven
cylinder.
The mechanical motions of both components shallbe linked each other by an interlock system. Whenthe signal of closing the MBS in inputted, at first thephoton absorber would travel downwards to stop thephoton beam thermally. After the limit switch
indicating the closed position of the absorber isactivated, the intercepting body of the beam shutter
would start to move on the beam axis sequentially.
In case of the signal of opening the MBS, the reverse
sequential movement would be operated.
A similar combination of the components is usedas fixed beam stoppers located after the mirror andthe first crystal of the DCM.

Figure 2: Conceptual drawing of the photon absorber,
which intercepts the thermal constituent of the beam axis,
namely intercepts the radiational constituent.

Figure 3: Internal photograph of the beam shutter. In
this case, the intercepting body, made of tungsten, is set
on the beam axis, namely intercepts the radiational
constituent.

Beam attenuation system
The beam attenuation system is composed of agas cell for attenuating beam intensity if necessary.
Several types of gases can be introduced to the cellwith controlling its pressure. The section isconnected to downstream through a differential
pumping system.

MirrorA main purpose of the mirror is to eliminate highenergy radiation which largely comes from higherorder spontaneous radiation. Figure error of mirror
surface should be minimized to suppress unwanted
intensity
modulation.
Ultrahighprecisiontechnologies for figuring surface, such as elastic
emission machining (EEM) and plasma chemicalvaporization machining (PCVM) [3], are quite
useful for the purpose. Silicon is used as a substratematerial. Coating of high-Z material is notpreferable because of increasing peak dose.
Theoretical study from acceptable dose is required
to determine final design including coating material,
incident angle, and total length.
Another role of the mirror from the viewpoint ofsafety is to separate paths of x-rays and electron
beam accidentally injecting from the accelerator.
The mirror can be removed from beam path withusage ofpermanent magnets, as previously
mentioned.

Monochromator
The double-crystal monochromator is used whenexperiments need to scan the energy or longer
temporal coherence than that achieved by the 6×10-4
bandwidth
of
the
FEL.
Otherwisethemonochromator is shifted to pass the FEL beamthrough it. If the deflecting mirror is omitted fromthe beamline layout, the γ-stopper set downstream
the monochromator is shifted together with themonochromator.
The monochromator crystal must be selected fromthe viewpoint of radiation damage and preservationof spatial coherence. Table 2 summarizes the melt
limit [4] and estimated radiation dose under normal
incidence for relevant materials. Considering theradiation damage and the available quality of crystal,
only diamonds are suitable for the monochromator
crystals. Synthetic type IIa diamonds are goodcandidate within several variations of diamonds [5].
However there are still problem of perfectness andsize at this point. Silicon crystals might be used at
higher energies than 12.4 keV, when the energy
resolution of diamonds is too high. Cryogenic
cooling of the crystals may be unnecessary owing to
the low repetition frequency of 60 Hz at SCSS [6].

Table 2: Normal incidence peak energy dose at 50 mfrom the source

Dose (eV/atom)

Material

Melt(eV/atom)

12.4 keV

6 keV

Li

0.1

0.00005

0.0002Be

0.3

0.00010.0007B

0.5

0.00030.002

graphite

0.9

0.00050.004

diamond-0.0004

0.004

Al

0.2

0.01

0.1

Si

0.4

0.02

0.1

Cu

0.3

0.3

0.2

Diagnostics system
In the front-end section, a beam position monitor(BPM) is installed to measure beam position in shotby-shot. Combination with another BPM in
downstream section enables to determine beam
direction.
An intensity monitor is set downstream themonochromator. The monitor must be transparent toavoid disturbing the beam coherence, and be
sufficiently fast to measure the intensity shot by shot.
One possible design is a combination of a gas
chamber and a pin photodiode. Elastically scattered
x-rays are monitored by the pin photodiode.
There are several insertable fluorescence screen
monitors, which are used for check of the beam axis
and rough alignment of optical components, such as
the collimator, the front-end slit, the first mirror, andthe monochromator. These screen monitors are used
only in the alignment mode, where sufficiently weakunsaturated beam is delivered using one of the
fifteen segments of the undulator. To avoid
accidental exposure to the saturated beam, the screen
monitors is operated through the interlock system.

REFERENCES[1] Y. Sakurai et al., Rev. Sci. Instrum. 66 (1995)
1771.
[2] S. Takahashi et al., Nucl. Instrum. Methods A
467-468 (2001) 758.
[3] Y. Mori et al., Proc. SPIE 4501 (2001) 30.
[4] “Linac Coherent Light Source (LCLS)
Conceptual Design Report”, SLAC-R-593, SLAC,

Stanford (2002)[5] International Workshop on “Diamond Single
Crystal For 3rd and 4th Generation X-ray Sources”,
ESRF, Grenoble (2004).

[6] L. Zhang, A. K. Freund, T.
Tschentscher,andH.
SchulteSchrepping, AIP Conference Proceedings
705 (2003) 639.

BEAM TRAJECTORY CONTROL AND ALIGNMENTOne of the most challenging issues in SASE-FEL machine at X-ray wavelength region is theaccurate beam guiding along the very long undulator line (~80 m). In order to maintain overlapof the electron beam on the X-ray beam, the electron beam trajectory has to be kept in a verystraight line. The tolerance is typically 4 micron-meter transverse for each undulator segment(4.5 m long). To achieve this accuracy, we will perform the alignment in three steps. (1) Aligncavity-type electron beam position monitor (cavity-BPM) by means of HeNe laser, (2) measureelectron beam position and correct error using corrector magnets, (3) By observing the X-ray
radiation pattern, perform fine tuning of the beam trajectory and phase matching, which isseparately discussed in the FEL commissioning part..

REQUIRED ALIGNMENT
It will be practically impossible to keep theelectron trajectory in a straight line within ten
micron-meters along whole 80 m long undulatorline. To find realistic solution, the study of
trajectory error in the undulator line was performed(see chapter of undulator line), and it was found thatthe followings criterion. The gain degradation by
the single kick errors (SKE) effect was analyticallyinvestigated in reference [1], where it was foundthat a critical angle θc, which gives the criterion forallowable SKE to maintain a certain level of the
FEL saturation power.

θc =λLg , 3 d.

(1)

With the SCSS parameters, θc is found to be 5.5µrad. The trajectory slope of the electron beam in
the undulator segments should be smaller than thisvalue. Thus, the accuracy of the BPM alignment
should be better than several microns.
The worst case is the zigzag trajectory as shownin Fig.1 (a), where the trajectory must be kept in azone width less than Lθ c . By FEL simulations, theallowable error on BPM alignment was found to be
4 µm. If the single kick errors have the samepolarity, the trajectory becomes a curved line asshown in Fig. 1(b). Even if the trajectory offset
from an ideal accelerator axis becomes very large,
we will obtain a certain amount of gain when eachsingle kick error is smaller than θc. This means that
we do not need to align all BPMs along a perfect

Fig. 1. Trajectory error model. (a) zigzag error, (b) curved trajectory error.

Fig. 2. Alignment stations in the undulator line are distributed in each 5 m separation. When we use
HeNe laser alignment system, we open the undulator gap.
straight-line, but we need to keep the angle error
smaller than θc.
Process of applying alignment procedure isequivalent to making a smooth trajectory byeliminating kick errors. We assume alignment error
budget as Table-1. To achieve this tight accuracy,
we are currently performing R&D on high accuracy
BPM using cavity type detector and laser alignment
system.
Table-1. Error budget on cavity-BPM alignment.
Error Source
Error Budget
Position
error
of
2 µm
alignment iris.
BPM resolution< 0.5 µm
Alignment iris to BPM
electrical centre.
Total

2 µm
4 µm

ALIGNMENT TOOL
To achieve required alignment tolerance, we
perform the following procedure (refer Fig.2).
(1)
Align BPM positions by using HeNe laser
with 2 µm accuracy in a straight line.
(2)
Measure electron beam position using
cavity type BPM, with resolution better
than one micron meter at 0.1 nC or higher,
and absolute accuracy better than 2 µm

referred to mechanical center, which isaligned by procedure (1).
(3)
Control beam trajectory by a corrector
magnet on upstream focusing magnet.
(4)
Fine tuning: optical beam based alignmentby monitoring radiation from undulator.
It will be probably necessary HeNe laseralignment procedure once per day, while electron
beam position feedback will be always applied
during beam run. The fine tuning (4) will be
performed after procedure (3), and correction
data will be used to refine the residual error in thealignment system.

LASER BEAM ALIGNMENT SYSTEM
HeNe Laser Beam Transport
For mechanical alignment of the cavity-BPM, we
use laser beam alignment system. To avoid beam
pointing instability due to the air turbulence, weneed to transport the HeNe laser light in vacuum. If
we prepare a separate vacuum pipe in parallel to thebeam line, same as the SLAC two mile linac, we
can stably transport a laser beam for fairly long
distance, and we can align optical instruments, suchas using a Fresnel lenses, in a straight line withfairly good accuracy, in a few tens-micron meter intransverse. However, a problem will arise when wecopy the mechanical position from the aligned
Fresnel lens to a BPM center located in the electron
beam line. Even if using invar material asmechanical structure to support the BPM and theFresnel lens, small errors will build up at eachmechanical contacts, and rotational error of thestructure will also contribute, finally, cause a large
error.
To solve this problem and achieve accuracy
better than 4 micron-meter, we conclude the bestsolution is to introduce HeNe laser beam in thesame line as electron beam, and directly aligninstrument at the same location, whose concept is
illustrated in Fig. 2. The in-vacuum undulator isvariable gap design, so that we can open the gap
when we perform the laser beam alignment. Withfully open the gap of 25 mm, we have enough room
to transport the HeNe laser beam for a long distance.
Fig. 3 shows the envelop variation along beam
propagation distance for various beam waist size,
where we assume the laser beam has Gaussian
profile. As well known, for a small beam waist,
beam diverges quickly due to the diffraction effect.
To transfer beam for a long distance, we need alarge beam size. However, a small fraction ofGaussian beam tail will be lost due to the beam pipe
aperture, and diffracted wave causes interference

Fig. 3. Laser beam envelop variation alongpropagation assuming Gaussian beam.

fringe, resulting in generating alignment error. To
avoid this effect, we have to choose laser beam size
smaller.
To make beam power profile fitting to Gaussianfunction with better than ± 1% , the noise
amplitude due to the diffraction effect has to be
0.5%, which corresponds to 2.5 × 10 −5 . To keep thediffraction intensity less than 2.5 × 10 −5 , we needto chose the beam size about five times smaller thanthe beam aperture. Since the minimum aperture is
20 mm at the cavity BPM, the laser beam size has
to be smaller than 4 mm. From Fig. 3, we cantransport 3 mm laser beam for 45 m long distance atwaist size of 2 mm. This is shorter than 80 m longundulator line in XFEL project. Therefore we need
to use chain of focusing lenses to transfer laser
beam. As we discussed in the beginning of thischapter, aim of alignment process is to makesmooth electron trajectory, therefore, if we stagger
the chain lenses we may reduce alignment error.

Laser Alignment Tool
As shown in Fig. 2, the alignment stations areprepared at each drift-apace between undulator
segments, on which the cavity-BPM and focusing
Q-magnet, and OTR profile monitor will be located.
Retractable round iris will be mounted in the same
block of the cavity-BPM. The transverse positionof the cavity-BPM can be adjusted by remotecontrol mover (see later sections for details). In
order to minimize position drift due to ambient
temperature change, the support stand is made by aspecial low-thermal expansion material (cordierite
ceramic).
When we perform alignment of cavity-BPM, we
open the undulator gap to provide enough aperture
for the laser beam pass. Inserting alignment irisand monitoring its image on CCD camera at
downstream, we can find mean center of the iris at
upstream. Adjust the iris position together with thecavity-BPM position, and bring the iris image tothe reference position on the CCD sensor. Repeat
the same process on the series of alignment
stations.

diffraction limit),

Airy Disk Image of Alignment Iris
To find a straight line of a laser beam,
traditionally the Fresnel lens was frequently used,since the image position is insensitive to tilt angle
error and easy to make a large lens. In the present
case, the required longitudinal distance of onealignment span is not so long, it is basically twotimes of the undulator magnet as seen in Fig. 2, that
is, about 10 m. In practice, to make efficient thesystem, and using a minimum number of CCD, weperform alignment for 20 m long section with one
CCD.
When a laser beam is incident on a circular
aperture of radius a, it generates diffraction image
on a screen at downstream. When Fresnel number ismuch smaller 1, the circular aperture creates Airydisk, whose intensity is given by [2]2⎡ J (αy ) ⎤I ( y) = ⎢ 1⎥ ,⎣ αy ⎦
2πa y
⋅
αy =
λ L

(2)

Fig. 4 shows example Airy disk image. Using
intensity data from a CCD camera, we calculatemean position by simply taking average as follows.

x0 =∑I x , y = ∑I y
i i

xi

i

0

yi

i

(3)

F=

a2
<1
λL

(4)

The diameter (FWHM) of the first Airy disk can
be derived from eq. (2) as follows.

DA =ρ11 λ L ρ11 a ρ11 λ L
⋅
=
⋅ =
2π a
2π F 2π F(5)or, the distance which gives DA is2⎛ 2π ⎞ FDA2
FDA2
⋅
=
⋅
L=⎜
2.7
⎟
λ
λ
⎝ ρ11 ⎠

(6)

where ρ 11 = 3.83 is the first zero of Bessel function
J1(x).
In order to avoid error field due to diffraction
wave, the Airy disk diameter has to be muchsmaller than the beam pipe diameter (20 mm). We
assume DA < 3 mm, and use F = 0.5, which gives
the minimum tail field. From eq. (6), we find L <19.1 m, and from eq. (4), a = 2.5 mm, thus the iris
diameter is 5 mm. In X-ray FEL, we prepare one
CCD sensor for each four or five segments.
To fit well with a CCD sensor with industry
standard, 1024 x 1024 pixels and 10 x 10 micron
pixel size, we zoom down the beam size by areducer lens in front of a CCD sensor. Be zooming
factor of 1/2, the Airy disk diameter becomes 1.5
mm (FWHM). The number of pixels in this image

To make the smooth image, we need a small
aperture, i.e., small Fresnel number (Fraunhofer

Fig. 4. Airy disk of 2 mm diameter aperture.
HeNe laser image on CCD located 3 m
downstream from a circular aperture of 2 mmdiameter. Small aperture cleans up the error fieldof incoming laser beam. Fresnel number is 0.53.

Fig. 5. Experimental setup of laser alignment.
Circular irises are located at 5 m and 15 m downstream
from the laser, and CCD sensor is at 20 m.

Fig. 6. Experimental test results on image response of iris movement. Each 10 µm step, ten
frames of the CCD images were averaged and its mean center was determined.
is about 150 x 150 = 22,500 pixels. Using 10 bit
digitiser (9 bit linear), and taking pixel average byeq. (3), we have very high resolution better than 20
nm (worst case).

Experimental Demonstration of Alignment
In order to demonstrate feasibility of thealignment using HeNe laser, we tested simple
measurement. As shown Fig. 5, we madeexperimental setup in air with vinyl tunnel shield.
We located a HeNe laser in one side, and measured
laser spot from other side using CCD sensor, andput circular iris with 3 mm diameter. Distance fromiris to CCD sensor is 10 m and 15 m. By moving
iris position x- and y-direction, we measured
movement of the mean center calculated by eq. (3).
By eliminating air turbulence effect, we took
multiple images for 10 times. To reduce noise, we
cut low level noise at 5% intensity. Fig. 6 shows theresult. We can clearly see 10 µm step movement,
and points are fairly linear.
We conclude this method has special resolution
better than a few µm and linear response in 100 µm
range. Further R&D will be required on(1)
Accuracy crosscheck. Comparing withother technique, such as, the laseralignment tool using interferometer
method. High precision water level will bealso useful to check vertical alignment,
and stretched wire for horizontalalignment.
(2)Long term stability test, and repeatability
test, specially the alignment iris setting
repeatability.
(3)
Test vacuum chamber effect. Laser beam
tail will be cut by limited aperture of beam
pipe (20 mm), or reflected back, andcreate interference fringe on the beam core.
Those tests will be performed in the SCSS Test
Accelerator.

Engineering Design of Alignment Circular Iris
Laser alignment system discussed above will
provide position reference at the center of circulariris. One problem is how to copy the positionreference to the center of the cavity-BPM. As
discussed next section, cavity-BPM utilizes electromagnetic resonance in a pill-box cavity, and its
electric field leaks into beam pipe for a certain
distance. It exponentially decays as distance fromthe cavity center due to cut-off effect. To make 10-3

Fig. 7. The cavity-BPM assembly with laser-alignment iris.
amplitude, we need locate the iris 50 mm awayfrom the cavity-BPM.
Figure 7 shows the alignment iris and cavityBPM mounted on transverse mover. Cavity-BPM
and alignment iris chamber are made in one block.
Half cell of the cavity-BPM and the alignment
block are fabricated from one piece of stainless rod
on a turning lathe, thus they are aligned co-axis
with accuracy better than a few micron-meter. Lefthalf cell of the cavity-BPM will be jointed bybrazing technique. With careful geometry design ofbrazing boundary, we may keep alignment between
two half cells within a few micron-meter. Modern
machining center is capable of cutting 3D structureon the left half cell with accuracy better than a few
micron-meters.
When the circular iris is inserted into thechamber, it mechanically contacts with innersurface of the beam pipe (20 mm), and pressed with
air-pressure, and automatically aligned. A test will
be required to verify repeatability of position setting.

If the cavity-BPM block axis is not well alignedwith the beam line axis, it will introduce a position
error between the iris center to the cavity-BPM
center. To make this error less than 1 µm, we needto place those instruments within 10 µ-radian angle
accuracy (10 µ-radian multiplying 100 mm spacing
gives 1 µm). We can align pitch by means of awater level. To align yaw, we can use the optical
autocollimator, which typically can measure theangle in 0.01 second range, or, 50 n-radian. In

Fig. 8. Cavity type beam-position-monitor concept.

combination
with
90-degree
reflector(commercially available penta-prism has accuracy
0.8 seconds), we may look into a flat mirror placedon the side surface of the cavity block from atelescope located upstream about one meter offset
horizontally, and align yaw within 1 second or 5 µrad.

CAVITY BPM
The highest position resolution and highest
absolute position accuracy were achieved by thecavity-BPM type electron beam position monitor.
The concept of this type of position monitor isschematically shown in Fig. 8. It uses TM110mode, whose longitudinal electric field becomeszero at the cavity axis, and varies linearly withradial position r.
E z = AJ1 (k11r ) ⋅ cos φ(7)
When the beam passes through the BPM cavity, it
generates electro-magnetic field, which issometimes called the wake field. The output voltage
has the following components.

VRF = A1qy + jA2 qy '+ jA3q + VN

(8)where,
A1qy is the beam-position signal, which is thein-phase component of TM110 mode;jA2 qy ' the beam angle signal, which is 90
degree out-of-phase from the beam current;
jA3 q the common-mode leakage of TM010
mode through the band-pass filter; andVN the thermal noise in the detector circuit.
As seen in eq.(), the beam slope and common-mode
leakage signal has 90 degree offset from the beam
position signal, which cause beam position error. In
order to reject those unwanted signal, we use thesynchronous detection circuit as shown in Fig. . The
reference cavity operating at the same frequency,
but TM010 common mode, is used to reject thoseerror signal and detect the relative phase, fromwhich, we may determine the position polarity
using the synchronous detection circuit.

Position Resolution
The output signal into 50 Ω cable connected to the
BPM cavity is derived using the equivalent circuit

Fig. 9. Cavity-BPM detection circuit for XFEL.

model as follows.
∆V50 =Z 50⋅ ∆V11 ( y ) ⋅ e − t / τ cos(ωt )cir .
( R / Q)11
Qe

(9)

+∞

∆V11 ( y ) =1⋅ qE ⋅ vdt
C11V1 −∫∞
+∞

cir .
jkz
= qω ( R / Q)11
∫ E z e dz / V1

Position Accuracy

−∞

cir .
= qω ( R / Q)11
⋅ T (θ e ) ⋅J1 (k11 y )∝y
J1. max(10)

Details of these equations are given in reference[3]. As seen these equations, near the beam axis, theoutput voltage is proportional to the beam charge
and position displacement from the axis.
Different from the button type BPM electrode, thecavity type BPM has much larger beam-to-circuit
coupling coefficient, this is due to fact that all of thewall current associated with beam interacts withcavity field. Therefore the intrinsic beam position
resolution defined by the S/N ratio to the thermal
noise in the detector circuit can be very high.
The equivalent thermal noise is given by
VN = 4kT∆BRN F ,(11)where k is the Boltzman constant,
T the absolute temperature (K),∆B the bandwidth (Hz),R the circuit resistance (Ω), andN F the noise figure of the head amplifier.
For 50 Ω impedance at room temperature, thenoise level becomes
VN = 1 × N F (nV/ Hz ) .

time and compare the position readout, the beam
position resolution was determined as 25 nm. Thisis four times larger than the theoretically expected
resolution, however, it is quite enough for practicalapplication to the XFEL project, where the required
resolution is 1 µ-meter level.

(12)

Using the cavity parameter listed in Table -2, thetheoretical resolution for an ideal setup is 0.6 nm,
assuming a noise figure of 3 dB in the headamplifier. In a practical setup, by including thesignal losses of rf-cables, magic-T, filter, attenuator,
mixer and waveform shaping loss in filters, the total
signal loss becomes –20 dB, thus the expected
resolution becomes 6 nm.
In the course of R&D program for e+e- linear
collider project, experimental test was performed in
FFTB beam line at SLAC in 1998 [4]. Using threecavity-BPMs, reading beam position at the same

In principle, the cavity type beam position monitor
has a highly accurate electrical center position to
the mechanical center of the cylinder cavity. This isdue to the TM110-mode has zero-filed point at thecavity center, whose position is defined by average
position of cylinder wall of the BPM cavity, which
can be machined on a turning lathe, thus the
Table-2 Cavity-BPM Parameter
Single bunch charge: q~ 1 nC
TM110 frequency: f11
4760 MHz
Cavity radius : a
30.0 mm
Cavity length : l
5.0 mmBeam hole diameter : D
20.0 mm
Effective cavity length : le
25 mm
Loaded Q factor : Qe
130
Circuit shunt-impedance :
22 Ω
cir .
( R / Q)11

Longitudinal impedance :410 kΩ / m 2l( R / Q)11

(numerical simulation by
MAFIA)
Induced voltage in the cavity:
V11
BPM signal output into 50 Ω:
V50Band-pass filter : ∆B
Thermal noise into 50
MHz :VN
Noise figure of detection circuit
Signal loss in cable from BP
M cavity to the detector circuit
Theoretical resolution :∆yObserved resolution at FFTB

(502 kΩ / m 2 )120µV/nC/nm
16µV/nC/nm
50 MHz
7 µV
3 dB−20 dB
6 nm
25 nm

In the machine operation, at the beginning, wetransport beam through the undulator line with theundulator gap full open, and tune the corrector
magnet to bring the beam to center position at eachcavity BPMs. Realistic initial position error will bea few mm, and finally we have to adjust the beam
within five µ-meters. Thus the detection circuit has
to have 1000-times dynamic range during thisprocess.
In order to compress the dynamic and reduce therequired number of digit in ADC, we designed adetection circuit using the logarithmic rf amplifier
as shown in Fig. 9. As shown in the sensitivity
diagram of logarithmic response of Fig. 10, thedetail position near the axis is magnified by thelogarithmic circuit. Therefore we may precisely
detect the beam position near the axis in one fixed
rf-gain and ADC-range, where no gain adjustment
is required from mm- to µm-range. The rf
logarithmic amplifier is designed to detect the rfamplitude by the source current flows into thecascaded amplifiers, thus the phase has to be
detected separately by the phase detector at the endof this cascade amplifier. A single chip logarithmic
amplifier is commercially available in low cost,originally designed for telecommunication systems.
Detection Circuit
Some of those products posses a very highIn XFEL, highest accuracy on beam trajectory
performance, that is, the logarithmic function
control is required in the undulator line, where theaccurately extends up to 70 dB (1: 3000).
maximum allowable error is five µ-meters for 1 A
A SAW-filter was implemented to delay the rf
operation as discussed in the first section in thissignal at the end of logarithmic amplifier, andchapter.
provides the phase information, which is detectedby the same ADC with different timing. In this way,phase and amplitude is digitized in the same ADC.
We should not use the rf-acceleration frequency
5712 MHz as the BPM signal, because the darkcurrent emission from the high-gradient accelerator
has the bunch train structure at rf-periods, and its
frequency spectrum has a sharp peak peaks at 5712
MHz. We chose 4760 MHz as the BPM signal,
which is the higher harmonic frequency of the subharmonic bunching frequency of 238 MHz in theelectron injector. There are 5236 MHz rf-signal
Fig. 10. Dynamic range extension by rf-logarithmic distributed in the accelerator, which is used as theamplifier.
local rf-signal to mixer in IQ-modulator and deelectrical center position is well agreed with themechanical center, in practice, within a few µ-meter
easily.
An important error source on electrical center
location is signal mixing from the common-mode
TM010, whose resonance frequency is lower thanthe BPM mode of TM110, which has 90 degree
phase offset from the BPM signal as shown in thethird term in eq. (8). In Fig. 8, the magic-T was
prepared to reject the common mode signal. If thedetection circuit is ideal, it does not cause position
error. However, performance of the magic-T is not
perfect, and the phase tuning of the synchronous
detection circuit is not perfect, thus it cause certain
amount of position error.
In order to eliminate this type of error, special
geometry was designed as the BPM cavity, where
the BPM signal is picked-up by means of antennas
located inside the longitudinal slots, which separate
the BPM signal from mixed rf-signals with thecommon mode in the cylindrical cavity.
Using a thin wire simulating a beam, the BPM
response was measure with test rf-signal, and center
position discrepancy was found 3 and 7 µm for two
BPM model cavities.

Ceramic Support Stand

Fig. 11. Stable support stand for alignment station
made by low-thermal-expansion ceramic.
modulator. Therefore, it is convenient to chose 4760
MHz as the BPM frequency, since the IF frequency
476 MHz is directly down-converted from 5712
MHz by mixing the distributed local frequency of5236 MHz.

We have to maintain cavity-BPM and focusing
element stable in few µm range. We made thespecial design as follows.
(1) Height of the beam-line from the floor as
800 mm, which is roughly half oftraditional design. Lower beam-line heightmakes position movement and vibration
smaller.
(2) We made the support stands using a low
thermal expansion material: cordierite
ceramic, which has about ten times lower
thermal expansion coefficient: 2x10−6 /degC. Thus, 1 deg change of 800 mm height
becomes 1.6 µm.
Figure 11 shows the support stand for the cavity
BPM and focusing element, which is made by thecordierite ceramic. Interior cavity of the support
stand is filled with sands to damp the vibration
associated with the cooling water system, etc.

Fig. 12. Beam alignment station. (from left to right) Cavity-BPM, alignment iris, mover, Q-magnets,
OTR profile monitor.

Fig. 12 shows the alignment station, where thecavity-BPM with the laser alignment iris on XYmover, Q-focusing magnets, and the OTR profile
monitor will be mounted on the ceramic support
stand.

REFERENCES[1] T. Tanaka, H. Kitamura and T. Shintake, Nucl.

Instrum. Meth. A528 (2004) 172.

[2] elsewhere on textbooks for classical optics.
[3] T. Shintake, “Development of Nanometer

Resolution RF-BPMs”, Proc. HEAC 1999,[4] T. Slaton and T. Shintake, “Development of
Nanometer Resolution C-band Radio Frequency
Beam Position Monitor in The Final Focus Test
Beam”, SLAC-PUB-7921, 1998.

FEL COMMISSIONING
Along with the undulator itself, a great many components must be installed throughout the 100 mof the undulator line, as a result there are many possible error sources acting to degrade the FEL
gain. It is not easy, and may not even be possible, to reduce these errors to acceptable levels
through an off-line alignment alone. It is therefore necessary to perform an on-beam fine tuningof components in the undulator line after installation and mechanical alignment, in order toreduce possible errors that will affect the FEL amplification process. Commissioning procedures
using electron and photon beams will be described in the following sections which will enables us
to adjust very finely the parameters of each component and reduce the residual errors tonegligible levels.

INTRODUCTION
The FEL process actually takes place in theundulator line, but there are many key components
composing the line: undulator, focusing magnets,
and BPMs. Even if the performance of eachcomponent itself were to be perfect, there remain
several error sources arising from their combined
positioning and interaction which may greatly affect
the FEL performance of the undulator line.
Naturally some of these errors can be considerably
reduced by employing the methods and procedures
describe in previous chapters. Initial installation andsetup procedures such as the undulator fieldcorrections and BPM alignment technique using theoptical laser can be done without an electron beam.
It is important, however, to perform an on-beamcommissioning to precisely check the performanceof each component in the FEL driver and improve
the overall performance as necessary. Let us callthis live beam process the “FEL commissioning.”

ERROR SOURCES AND TOLERANCESThe principle error sources which may degrade
the FEL gain of the undulator line are summarizedin Table 1. Before describing the procedure for FEL
commissioning that will reduce these errors, let us
consider acceptable tolerances on each error source.
It is difficult to derive an analytic expression for
FEL gain which takes these errors into account.
Therefore, we have performed simulations toinvestigate these effects numerically by using
SIMPLEX, an FEL simulation code developed at

Spring-8 [1]. For a tolerance criterion, we rather
arbitrarily look for an error level that would degrade
the output power by half. Table 1 previews thetolerances that resulted from the simulations withthe half power criterion.
Table 1. Error sources to be considered and acceptable
tolerances for each.

Error Source
Field Discrepancy

Tolerance

Phase Mismatch|∆φ| < 40 degree|Bg| < 0.4 Gauss

Environmental Field
BPM Misalignment|∆K| < 6×10-4|∆x,y| < 4 µm

Field Discrepancy between Segments
The average magnetic field of each undulator
segment is not exactly the same but deviates about anominal value because of differences between
segments in the average gap and/or in the operating
temperature. This results in a difference in the
average of the longitudinal electron velocity
between segments and therefore causes gain
degradation. In order to estimate the acceptabledeviation (∆K) from the nominal K value (=1.3),
500 sets of undulator line models were created with
the K value of each segment being distributed from
K-∆K to K+∆K randomly and uniformly, and FEL
simulations were performed for each set. The
results are shown in Fig. 1 as histograms of theoutput power for different values of ∆K. Note thatthe output radiation power is estimated at about 3
GW without any error sources. It is found from theEvents500∆K=4x10-3∆K=1.2x10

-4

-4

-4

∆K=8x10∆K=6x10250
0500

Events-4∆K=2x10∆K=1x10∆K=1.4x10-3-3∆K=1.6x10

-3

250
0
0

1

2

3

4

50

1

2

3

4

50

1

2

3

4

50

1

2

3

4

5

Output Radiation Power (GW)
Figure 1: Histograms of the output power for different values of the K-value error ∆K.

figure that the histogram has a smaller mean and alarger variance for larger ∆K value. For example,
the output power is found to be in the range from
2.5 GW to 3.5 GW when ∆K=2×10-4, while itextends from 0.5 GW to 4.5 GW for ∆K of 1.2×10-3.
This means that ∆K of 1.2×10-3 degrades theradiation power by 1/6 at worst. According to ourtolerance criterion, we find the acceptable value of∆K to be 6×10-4.

section is an integer multiple of 2π. Achieving that
insures good phase matching between the electrons
and the radiation emitted in the each segment. It has
been shown in [2] that this condition is not crucial
to an FEL system that needs only a few segments
for saturation, but this is not the case for SCSS.
Thus, the phase slippage between segments should
be adjusted to satisfy the phase-matching condition
by some means: for example by inserting a magnet
bump with an adjustable field. It should be noted,
however, that the phase slippage cannot be adjusted
perfectly to satisfy the phase matching condition,i.e., it will be some errors (mismatched phase).
In order to investigate the effects of theMismatched Phase in theDrift Section
The electron path length in the drift section
between segments should satisfy the condition thatthe phase accumulated over the distance of the drift

Events

500

o

∆φ=20

o

∆φ=30

o

∆φ=40

o

o

∆φ=60

o

∆φ=70

o

∆φ=80

250
0
500

Events

∆φ=10

∆φ=50

o

250
0
0

1

2

3

4

5

0

1

2

3

4

5

0

1

2

3

4

5

0

1

2

3

Output Radiation Power (GW)
Figure 2: Histograms of the output power for different values of the mismatched phase ∆φ.

45

Events500∆x,y=2µm∆x,y=3µm∆x,y=4µm∆x,y=5µm∆x,y=6µm∆x,y=7µm∆x,y=8µm

250
0
500

Events∆x,y=1µm

250
0
0

1

2

3

4

5

0

1

2

3

4

5

0

1

2

3

4

5

0

1

2

3

4

5

Output Radiation Power (GW)
Figure 4: Histograms of the output power for different values of the BPM alignment accuracy ∆x,y.

mismatched phase introduced in the drift sections,
500 sets of undulator line models were created withthe mismatched in phase being distributed from –∆φ
to ∆φ randomly and uniformly and simulations were
performed. The results are shown in Fig. 2 ashistograms of the output power. The statisticaltrends are similar to those in Fig. 1. According to
the criterion of tolerance, we find an acceptablevalue of –∆φ to be 40 degree.

Environmental Field
The geomagnetic field is very weak being at most
0.5 Gauss, however, it causes a parabolic electron

Gain Length Growth (%)

1000

Horizontal Field
Vertical Field

100
10
1
0.1
0.01

orbit that is not negligible in the FEL amplification
process. For example, a uniform field of 0.5 Gaussover a length of 4.5 m results in a 50 µm
displacement of a 6-GeV electron, which is larger
than the electron beam size (1σ) of SCSS. Thiscannot be corrected for by the steering magnets
installed between undulator segments. In addition,
injection of an electron beam with a vertical
position offset causes an error orbit similar to thatinduced by the geomagnetic field due to the natural
focusing of the undulator magnetic field. Let us callthese error fields the “environmental field.”
In order to estimate the effects of theenvironmental fields on the FEL amplification, we
performed simulations to calculate the gain length(Lg) as a function of the environmental-fieldstrength (Bg). The results are shown in Fig 3. The
abscissa shows the strength of the environmental
field, and the ordinate shows the gain length growth,
Lg(Bg)/Lg(0)-1, in %. Assuming an exponential
growth of the radiation power, we have an equation
to determine the acceptable value of Bg according to
our tolerance criterion as follows

L g (0)
0.0

0.2

0.4

0.6

0.8

1.0

Geomagnetic Field (Gauss)

Figure 3: Growth of gain length due to residual
uniform field.

Lg ( Bg )= 1−

Lg (0)
L

ln 2

where L is the total length of the undulator.
Substituting Lg(0) = 3.8 m (estimated from thesimulation) and L = 81 m (4.5 m× 18 segments), we
have Lg(Bg)/Lg(0)-1= 0.034, which is indicated by
the dotted line in Fig. 3. Thus, the acceptable valueof Bg is found to be 0.4 Gauss, in both horizontal
and vertical directions.

Trajectory Error
As shown in the “Undulator” section, thetrajectory error within a single undulator segment
can be significantly reduced by means of the fieldcorrection technique described before. It should be
noted, however, that this does not mean that
trajectory straightness is ensured over the entire
undulator line. In practice, the trajectory ismonitored by the BPMs located between undulator
segments and the electron beam is steered so that itgoes through the origins of the BPMs. Thus, theaccuracy of the BPM alignment is crucial to the
trajectory straightness over the undulator line. In
this case, dipole fields in the drift sections, or single
kick errors (SKE) located between undulator
segments, bring about a trajectory error. The gain
degradation by SKE effects was analyticallyinvestigated in [3] and a critical angle θc, which
gives the criterion for the SKE to be small, has been
derived as follows

θc =λL g ,3d.

With the SCSS parameters, θc is found to be 5.5µrad. The trajectory slope of the electron beam in
the undulator segments should be smaller than thisvalue. Thus, the accuracy of the BPM alignment
should be better than several microns.
For quantitative estimation of an acceptable
tolerance for the BPM alignment accuracy, FEL
simulations were performed on 500 sets ofundulator line models with the BPM alignment
errors ranging from –∆x,y to ∆x,y randomly anduniformly. The results are shown in Fig. 4. The
statistical trends are found to be similar to those in
Figs. 1 and 2. According to the criterion oftolerance, we find the acceptable value of ∆x,y to be
4 µm, being consistent with that obtained from theabove analytical formula for the critical angle.

COMMISSIONING STRATEGY
Having obtained an acceptable tolerance for eacherror source, let us now describe the details of the
FEL commissioning. A schematic illustration of theprocedure for the FEL commissioning is shown in
Fig 5. It is based on the effects of each error sourceon the optical properties of the spontaneous
undulator radiation. Only one or two undulator
segments are activated at a time, the resulting
spontaneous undulator radiation is monitored at thephoton diagnostics station located far from theundulator line. The station contains a spectrometer
to measure the spectrum of undulator radiation anda slit assembly to extract the photon beam andmeasure the spatial profile of radiation. Because theelectron beam emittance at 6 GeV (5.96×1011
m.rad) is comparable to the optical naturalemittance at 1Å (λ/4π = 7.96×10-12m.rad), thecoherent property of undulator radiation is lesssmeared than would be found in synchrotron
radiation (SR) facilities. This enables a precisionadjustment of the parameters to correct the error
sources in the undulator line under consideration. Itshould be noted that the electron emittance can be
reduced by inserting a collimator before theundulator line if the charge in the electron bunch is
not of concern; which would makes thecommissioning result more reliable.

undulator segments
gapclosedphoton diagnostics(spectrometer & slit)

Figure 5: Schematic illustration for FEL commissioning. Only one or two undulator segments are activated tomonitor the photon beam at the diagnostics station.

In the following, the procedures for correction of
the error sources as listed in Table 1 will be
discussed. The procedures are all based on themeasurement of the optical properties of
spontaneous undulator radiation from one or twosegments. The electron beam parameters to be used
are summarized in Table 2. The calculation of SR
properties is performed using SPECTRA, a SR
calculation code developed at Spring-8 [4].
Table 2. Electron beam parameters used in thecalculation.

Emittance (x,y)
5.96×10-11m.rad
Energy Spread
2.0×10-4
Average betatron function 30 m

Field Discrepancy between Segments
The average field in the undulator determines thephoton energy of the fundamental radiation. It istherefore possible to reduce the average field
differences by comparing the peak energy of
spontaneous radiation from a specific segment withthat of a reference segment. In practice, it is moreconvenient to fix the photon energy for the flux
density measurement of the spontaneous radiation,and then scan the gap of the undulator to change theK value while looking for the intensity peak.
Figure 6 shows the flux density as a function of

the K value for photon energies fixed at the peaksof the 1st, 3rd and 5th harmonics. The K value isindicated as a difference (∆K) from the nominal
value, 1.3. A FWHM value of 4.4×10-3 and 1.6×10-3
is obtained for the 1st and 3rd harmonics,
respectively. It is intrinsic to undulator radiationthat a narrower profile is obtained for the higherharmonic radiation. This is, however, no longer
valid for the 5th harmonic because the energy spreadof the electron beam broadens the peak width.
It is easy to adjust the gap of each undulator
segment so that all segments have the identical K
values. However, the accuracy of adjustment isdependent on the S/N ratio of the measurement
system. Let us assume that a 10% difference inintensity can be distinguished (10% criterion). Thenthe resolution is found to be 9×10-4 and 4×10-4 forthe 1st and 3rd harmonics, respectively. In this way,
we can reduce the error using the 3rd harmonicmeasurement to our acceptable level of 6×10-4 thathas been previously determined.

Flux Density (arb.unit)

PROCEDURE AND ACCURACY

1.0

12.4 keV (1st)37.2 keV (3rd)

(a)

0.9
0.8
0.7
-60

-40

-20

0

20

40

60

Phase Slip (degree)
Flux Density (arb.unit)o

Flux Density (arb.unit)

1.0
0.8
0.6
0.4

1st 12.4 keV
3rd 37.2 keV
5th 62.0 keV

0.2
0.0
-6

-4

-2

0

2

4

10

0

-30
o
-20
o
-10
0
o
+10
o
+20
o
+30

(b)

-1

10

-2

10

12300 12320 12340 12360 12380 12400

Photon Energy (eV)6-3Relative K Value(10 )Figure 6: Flux densities at the peak energies of the
1st , 3rd and 5th harmonic as functions of the K valueerror with respect to the nominal value.

Figure 7: Effects of the phase slip in the drift section
between segments: (a) flux densities as functions of
the phase error at two energies of the 1st and 3rd
harmonics and (b) variation of spectra around the 1st
harmonic.

Mismatched Phase in Drift Section

Flux Density(arb.unit)

The intensity of spontaneous radiation from twoconsecutive undulator segments is strongly
dependent on the phase slippage between the twosegments. Figure 7(a) shows the dependence of theflux densities on the phase slippage at the 1st and 3rd
harmonics. Using the 10% criterion, the accuracy inspecify the phase-matching condition is found to be
45 and 25 degree for the 1st and 3rd harmonics,
respectively. These values can be improved bymonitoring the variation of the spectrum as shownin Fig. 7(b), where growth of the sub-peak due tothe mismatched phase is found. Thus it is possible

1.0
0.8
0.6
0.4
0.2

No ∆Bx
0.2G

0.4G
0.6G
0.8G

to adjust more accurately the electron path in thedrift section to satisfy the phase-matching condition.

Environmental Field
The environmental field, being essentially
uniform, can be corrected by long coils surrounding
the undulator segment. There are several effects on
the optical properties of undulator radiation caused
by a uniform field, among which the off-axis peakin the angular profile is the most sensitive. Figures
8(a) and 8(b) show the effects due to theenvironmental field on the photon flux at 12.26 keV,slightly lower than the fundamental energy of
undulator radiation, 12.4 keV. The flux isnormalized by the peak values without theenvironmental field. As the strength of theenvironmental field increases, the off-axis peak ismore broadened. Thus, the environmental field canbe cancelled by adjusting the current of the longcoil so that it maximize the flux at the off-axis peak.
Using the 10% criterion (indicated by dotted lines
in the figure), the residual uniform field can be

0.0

Flux Density(arb.unit)-0.02 -0.01 0.00 0.01 0.02
Observation Angle θy(mrad)
1.0
0.8

Flux Density (arb.unit)

1.0
0.8
0.6

Fixed Energy (keV)
12.4
12.35

0.4
0.2
0.0

0.6

-16

-12

0.4
0.2
0.0

-8

-4

0

4

8

12

16

Observation Angle(µrad)θy=0.01mrad with ∆Bxθx=0.01mrad with ∆By

0.0 0.2 0.4 0.6 0.8
Field Error ∆Bx,y(Gauss)
Figure 8: Effects due to the environmental field on theradiation intensity. (a) Angular profiles for different
values of the environmental field. (b) Intensity
variation as a function of the environmental fieldstrength.

Figure 9: Angular profile for two difference photon
energies around the 1st harmonic.

reduced to 0.15 Gauss or less in both directions.

Trajectory Error
The spontaneous undulator radiation has a good
directivity if the undulator has a great number of
periods; it is therefore possible to estimate the slopeof the electron beam passing through a specificsegment and correct it by monitoring the angularprofile of the spontaneous radiation from thatsegment. Figure 9 shows the angular profile from asingle undulator segment at two different photon
energies around the 1st harmonic. The peak has anarrower width at lower photon energy, which
results in a more accurate adjustment of the
trajectory slope in the undulator segment. Using the
10% criterion, the slope error can be reduced to
0.8µrad using 12.35keV radiation. This corresponds
to the BPM alignment error of 2 mm.

REFERENCES[1] T. Tanaka, Proc. Int. Conf. FEL (FEL2004) to be
published; see also
http://radiant.harima.riken.go.jp/simplex/[2] T. Tanaka, H. Kitamura and T. Shintake, Phys.
Rev. ST. AB 5, 040701 (2002)[3] T. Tanaka, H. Kitamura and T. Shintake, Nucl.
Instrum. Meth. A528 (2004) 172[4] T. Tanaka and H. Kitamura, J. Synchrotron Rad.,8, 1221 (2001); see also
http://radiant.harima.riken.go.jp/spectra/index.html

SCSS CONTROL SYSTEM CONCEPTUAL DESIGN

1. Overall Design(1) Control Organization
The SCCS control system will use the MADOCA(Message And Database Oriented Control
Architecture) operating system which has a provenrecord at SPring-8. MADOCA operates the linac,
booster synchrotron and 8GeV storage rings, andbeam lines at SPring-8; and at two other mid-scalemachines in Japan, the NewSUBARU and HiSOR;
it is operating reliably and has shown itself to a
proven control system fully capable of supporting
operations for all these accelerators.
Because it has all the flexibility and power
needed to fill the needs of running all of the various
operational modes at an accelerator facility,
MADOCA can certainly control the combined
operations of the SCSS accelerator and its insertion
device light sources. Also, it could operate SCSS
either from the SPring-8 central control room orfrom another control room located closer to the
SCSS site. Turning to the beam line, where there
are many users and a changing and unpredictableusage environment, with MADOCA it is possible toimplement a flexible operating control system yetwith the appropriate access limitations and security
checks. The SCSS controls architecture will bebased on the standard model for accelerator control
systems, conceptually divided into 3 layers: manmachine upper level, device control layer, and low
level device interfaces. The control devices will beinterconnected by a high speed wide area network.
This is the same control architecture employed at
SPring-8 and as implemented in MADOCA.
The entire SCSS accelerator will be assigned asingle device group name, with device subgroups
identified under it appropriately for operational andmaintenance purposes. Depending on the level ofthe device and the device unit under control, control
message groups can be defined and registered with

MADOCA. In the development process, theSPring-8 Message Guide Lines – as separately
documented and as implemented in include header
files which define the C language prototype
declarations both in format and grammar should bereferenced. From the beginning, strict adherence tothese standards in matters of group names, error
codes and the like will make possible to produce anintegrated control system with is both efficient,error free and safe.

(2) Man-Machine Computer System
For the man-machine interface layer, computers
running the UNIX OS are unsurpassed in terms ofstability. Tools, Builder Wizards and Application
software packages have all achieved satisfactory
levels, both in quality and cost. From the viewpointof hardware reliability and OS support, a control
system composed of workstations running acommercial UNIX version such as HP-UX would
be desirable, but it could also be implemented in
PCs running Linux. Software development for thisproject will primarily be done in C. There is no
particular reason why C++ has to be used;
depending on the purpose, it as well as otherlanguages such as Python, FORTAN or Java would
not be precluded. Microsoft WINDOWS OS isprobably not suited for application to mainstream
accelerator controls because of its questionablestability, and proclivity to virus infections and othermalware attacks.
However, there is a sole
exception in the case of commercial subsystems formeasurement or device control that are available
only for the Windows OS. The machine operation
control panels will be made with the same GUI
builder used in standard MADOCA practice.

(3) Server Computers
Dedicated machines will be provided for the
obvious system servers: database, file, and web

servers. In particular, the device database server isabsolutely indispensable for normal accelerator
operation. Further, beam diagnosis, failure analysis
and repair strategy planning are all greatly assisted
by an excellent database server. The databaseserver will be able to support machine operation
because it will store and have access to many tables
containing parameters and device organization andinterconnection map data, setpoints and current
readout values, alarm records and so forth. In orderto make the data in the database easily accessible, a
web-server will be set up so that device status canbe inspected, and data log analysis can be done
from anywhere on site. The executable programs
for accelerator operation will be created within thesame common development environment alreadyused at SPring-8. Programs, setpoint data etc. will
be stored and maintained at a central location on afile server, from which they can be exported as NFS
files for remote access.

overall control will use VMEbus systems, thus
making advanced functionality available for
operations. When using PLCs, all of the control
applications logic should be implemented in Ladder
Logic; it would be most desirable if each device
developer or responsible person was able himself tomaintain and extend the code functionality as
needed. For any PLC to be considered for adoption,
conformance to open communications standards isnecessary, while inter-operability with othernetwork and system components is a sine qua nonfor selection.
As a general rule, GPIB will not be used for
device control. When a VMEbus crate is operated
with a GPIB interface board inserted, notinfrequently reliability problems arise. If there
happens to be measurement instrument which only
has a GPIB interface, it should be used over the
network through a GPIB-Ethernet converter.

(4-1) VMEbus System(4) Device Controllers
Besides remote control operation from the central
control room, the device control system is required
to support local control in order to facilitatebringing up the equipment, maintenance and failure
analysis. Anything which can be done with local
controls should also be able to be done remotelyfrom the central control system. Depending on thecontrol characteristics of the individual device,
either a PLC or VMEbus based system can be used.
When hardware reliability is considered to be mostimportant, PLC solutions are appropriate. Where
there is a coherent grouping of function, such as for
the C-Band klystron modulators, or vacuum
systems, there is merit in having each subgrouphaving its own PLC or VMEbus subsystem forunified control. Doing this also has the advantagethat independent operation or tuning etc. of entire
groups can more easily be done. PLCs besides
executing the specific device control fixedsequential ladder program, will also communicate
with the upper layers of the control system via a
VMEbus interface. High speed controls, andIn terms of system composition and control
software creation, the VMEbus system can provide
a great deal of flexibility. System development
becomes easy since a great variety of resources
such as CPU boards, I/O boards, device drivers are
already available and in use at SPring-8. CPUboards should use an Intel CPU, and not require aforced air cooling fan. Low power CPU versions
are better in helping to control heat dissipation. The
OS should be either Solaris8 or Solaris9, withcapability of booting from a Compact Flash card.
In the cases where high speed analog processing isrequired, such as for waveform analysis, it should
not be done by dedicated front-end electronics
circuitry, but rather a board should be developed
which can be read out by a back-end VME system.
When high speed feedback, real-time response(particularly for synchronized operation) is required,
VME based shared memory should be used. For
places where the noise environment is very sever,an optical fiber based high speed remote I/O VME
system could be used as a field bus.

(4-2) PLC, Fieldbus, etc.
PLCs are programmable control devices withreliability comparable to hardware logic. PLCs
execute a fixed control sequence; usually a local
control capability is also implemented to support
maintenance and monitoring of the controlled
device. A display panel is provided for local
control for this reason. Because a PLC is provided
for each set of subsystem devices, the amount ofsignal cable wiring is reduced. Depending on thesituation, if DeviceNet or other fieldbus is used forinterconnection, the cabling does not need todepend on the where the controlled device is
physically located and there is much more
flexibility in the way wiring can be done. Because
it is desirable to use an open standards based
communications protocol between PLCs, the FLnet as proposed by the Japan Electrical
Manufacturers Association will be used. FL-net isbased on Ethernet, using token-passing, timedivision multiplexed rotating slots, or messaging
between addressed stations. VME boards havebeen developed with FL-net interfaces so as tofacilitate using VME as a gateway between thefieldbus and the upper level Ethernet system. Using
this option for PLC-VME interconnection, PLCs
can continue to execute their control sequences
without interference, yet data is available over FLnet to be collected and processed at high speed by
the VME processor. In general PLC programs will
be implemented in “ladder logic”, but the option
exists to create applications with C or other higher
level languages, and execute them in the PLC
network computer.

2. Individual Components(1) Network
Figure 1 shows the overall SCSS control system
network. A fully redundant dual star topology setof optical fibers connects the network nodes inside

the control room to wherever the individual
controlled devices are located. The network main
trunk line is also a redundant pair of Giga-bit
Ethernet cables. As a general rule, branch line
twisted pair cable will be run from the position ofeach controlled device node back to its controlling
VME crate. But, in places where the noise
environment might be thought to be particularly bad,
optical fiber can be used.
Separately from the above control network, amaintenance network will also be laid out to theaccelerator building. Using this network, it will bepossible to do maintenance while reviewing stored
log file data, or to automate some of themaintenance procedures by going through thecontrol system.
The program software development servers will
be placed in the DMZ with a firewall interposed
between them and the control and the office
networks. Doing this, software development can be
carried out from the office network, yet control
network can be effectively cut off from the outside
world. This architecture provides security andhelps prevent the spread of computer viruses into
the control network.
Since network technology is advancing rapidly it
may be possible to later provide a wide area
network even faster than Giga-bit Ethernet, and as
accelerator operations also become moresophisticated and demanding, such an increase in
network capabilities may be required. Therefore
the fiber installation and device selection will bedone now with this eventuality in mind.

(2) Operator’s Console
HP-UX and Linux are supported for theoperator’s console OS.
Specifically, HP
workstations running HP-UX 11.0 and PC’s
running Red Hat Linux (RHL) have both been used,and preformed reliably. For SCSS, 10 or so HP
workstations running HP-UX 11.0 on the 64 bit PARISC will be provided. Operating programs will be
developed using X-Mate, a GUI creation tool. All
programs will be written in the C language, will beFigure1. Overall Network Diagram

compiled with the standard make command on the
program development HP-UX 11.0 servers, andactually run on the operator’s console machines.

(3) Server Computers
Sybase-ASE will be used for the system
relational database (RDBMS). Sybase is currently
in use by SPring-8 and there are versions for both
HP-UX and Linux. Because the Linux version ofSybase is inexpensive, an enterprise class RDBMS
can be inexpensively set up. It would even be
possible to run it on a multi-CPU PC with sufficient
memory running RHL. Nevertheless, judging fromour previous operational experience, since theoverall reliability of the system depends so
critically on this server and we know that the 64 bit
enterprise server computers from HP have provided
superior performance; HP products will be used for

SCSS.
If there are signs that the databaseprocessing is not fast enough, the situation can be
dealt with by increasing the numbers of CPUs.
Dual power supplies and network hardware will be
provided for redundancy, and data will be backed
up to mirrored RAID disk storage system. Sincehigh availability is required, there will be 2
database servers combined to provide a highavailability cluster.
The file server will use NFS to export theexecutable operations programs, parameters anddata files. Because accelerator operations would be
impeded by any failure of the file server, just as in
the database server case, it will have redundant
power supplies etc. The actual files and data to beserved will be kept on mirrored RAID disk. A
rather small scale HP server computer will be used.
The usefulness of Web technology is wellrecognized not only because data access via abrowser allows monitoring device data, but allforms of information become available for viewing
from any place. So the web server will be built on amulti-CPU server PC running RHL to makepossible display of device data during operations,device data log files and alarm files etc.

(4) Device Control Subsystems
The control of the linac, including all device
interlocks, will be done almost entirely with PLCs.
The interconnection of the PLCs and the rest of thecontrol system will be done through FL-net.
Controllers for the klystron modulators and so forthwill be designed so that they can be switchedbetween local and remote control; and the samefunctionality should be available both to remotecontrol and the local control panel. As previously
mentioned, a FL-net interface board for VME will
be developed so that VME can be used as thegateway between the PLCs and the rest of thecontrol system.
a) Vacuum System Controls
Almost everything including machine protection
interlocks can be done with PLCs. A single VME
station will use FL-net to communicate with thevacuum system control PLCs under it.
b) Magnet Power Supply Controls

Almost everything including machine protection
interlocks can be done with PLCs. A single VME
station will be used to control the beam transportline magnets; it will have an FL-net interface tocommunicate with its PLCs. The transport magnet
control will use optical fiber, and so that pulse noise
will not cause interference on signal lines, the VME
crate will have an optical transceiver board.
c) RF Controls
A VME board will be designed with high speed
AD / DA and FPGAs to provide the voltage andphase control waveforms for the acceleration
cavities. Optical fiber will be used for the VME
system data input. Other low level control, such as
for power supply control and device protection
interlocks
will
be
done
with
PLCs.
Intercommunications between the rest of the control
system and PLCs will be over FL-net. Each
klystron drives 4 accelerator structures forming asingle unit. There will be a VME crate for each unit,
and all of the low level logic, power supply control
etc. for the unit will be done on a stand-alone basisby that VME system.
d) Beam Monitor System Control
The same VME waveform control board
designed for the RF system will be used here too,with input data coming into the VME crate overoptical fiber. Using the locally acquired signals,
and a networked shared memory board, the VME

Figure 2. Magnet & Vacuum Network Node
Figure 3. RF Network Nodecontroller will be able to provide high speed
feedback to the transport line magnets. An opticaltransceiver board will be used for the beam current
monitors, etc. again using optical fiber to isolate theanalog system from the VME.

SCSS can be operated from the SPring-8 central
control room, or a control room could be located in
the SCSS facility from which operation would
equally be possible.
But before going into
production operation, it is expected that the closer
control room will be used for bringing up theindividual components and the original SCSS
commissioning. After operation from the central
SPring-8 control room starts, the local control room
could be used for maintenance purposes, etc.

3. Development Process and Methods(1) Overall Philosophy
Figure 4. Beam Monitor Network Node.

e) Insertion Device Control
Figure 5 shows main features of the insertion
device control system. Device interlock PLC data
will be collected over FL-net. Everything else,
pulse motor controllers and encoders, ADC/DAC
signals etc. will all be taken care of with the VME
system.

Figure 5. Insertion Device Network Node.

(5) Control RoomThe process of developing the SCSS control
system from initial startup until production
operation is achieved can be specified. A list of allof the control points for the devices that make up
the accelerator is to be made, along with a list ofcorresponding control commands. The MADOCA
upper level software works with abstract devices, sothat it does not have to be aware of hardware details
such as channels or slots or the like. At the stage
when the message lists are completed, we finallyhave an overall grasp of the accelerator operation.
From here, spiral development of the software canbegin keeping in step with the progress indevelopment of the various component devices.
First working bottom up, application software forthe lower level controls should be finished before
higher levels are started. This means beginningwith component device control software running on
the PLCs and VME computers.
Local control
sequencing and diagnostic logic should also be
prepared at this time, along with the control
software for each equipment device component.
As the lower level control software is completed,
operational testing of the controlled device canbegin. This test operation would be in the local
control mode, not remote control. Even whensoftware is still incomplete, rapid prototyping
techniques can be used to initiate testing. It is very
important at this stage that point by point thehardware and software controlling each operation
be tested and proper operation confirmed, focusing
on the new as yet untested portions as step by stepdevelopment proceeds. From about the midpoint insoftware development, writing the operational GUI
software for the operator’s console can begin, andthe correct operation of the GUI logic in astandalone mode with dummy returns from theoperating system and hardware can be confirmed.
Next, the MADOCA middleware can be linked in
to complete the connection to the lower level
control system and for the first time the overalltesting can begin.

(2) Development Task Assignment
In developing the SCSS control software,
assistance will be requested from the SPring-8
control group that originally developed MADOCA.
The division of labor and responsibility between the
SCSS device control designer and the appropriate
SPring-8 control group specialist will be as follows.
The device designer will also design the operational
GUI program for that device. Since the GUI
program captures essence of the device operationand collects into itself the relevant knowledgebase
about the device, it should naturally be created byperson responsible for the device, who knows it
best of all. The responsibility of the control groupis to provide and support the MADOCA
framework; and the control group point person is toassist in the integration of the device designer’s
application program into that overall framework.
It would be good if applications that run on the
PLCs could be made by the person responsible forthe device in question, but outsourcing suchdevelopment would also be alright. But even if the
fixed sequence coding and the device
commissioning logic is outsourced, in cases where
increased operational functionality might be neededat any time, or on an ongoing basis, then it isnecessary that the PLC software development
process, tools and technology be maintained by thein house staff. The control software that runs on the
VME stations can be created by the SCSS staff as
there are many example programs and experience tobuild on.
The control group will be depended upon tocreate the database administration tables, as well as
the configuration tables with device parameters,
alarm monitoring information etc.
Becausetemplates are already available, the web display
system which is driven from the database will beeasily made.

TIME BASE AND RF SIGNAL GENERATION, TRANSMISSION AND
DISTRIBUTION SYSTEM
The 6 GeV electron linear accelerator (linac) for the SCSS soft X-ray free electron laser uses
5712 MHz pulsed rf to accelerate electrons [1]. High-power klystrons installed along theaccelerator generate the pulsed rf signals. A system to distribute a reference rf signal to the
klystrons is necessary. Furthermore, a timing pulse distribution system is needed for the klystron
modulators to make the pulsed rf, and for monitor and instrumentation devices. The SCSS will
generate soft X-rays with the extremely narrow ~0.5ps pulse width 6 GeV electron beam in orderto analyze the structure of proteins and their time resolved dynamic behavior. Depending on theexperiment, in some cases, measurement precision with femto-second order timing jitter isrequired [2]. For this reason the systems which distribute the reference time standard rf signal
and trigger pulse, and which thereby determine the time accuracy of the accelerator, should havethe same femto-second order precision. This makes the low power rf system and the timing
distribution systems of the linac very important. In the following paragraphs, we describe thesystems for realizing the femto-second time accuracy, the causes of the timing errors in thesystems such as noise, and finally the rational for why the systems were chosen.

THE CAUSES OF ACCELERATORENERGY VARIATIONSIn an electron linear accelerator, the causes ofvariation in energy accelerating an electron riding at
the crest of the rf pulse fed from a single klystron
into an accelerator structure are defined asfollows:[3]1. The klystron output variation caused
primarily by a changes in the high-voltage
pulse.
2. The input frequency changes andtemperature changes of the cooling water tothe accelerator structures.
3. The effect of detuning of the rf pulse
compression cavity such as a SLED. The
detuning is may be caused by a temperaturechange of cooling water. This effect is also
due to changes of the frequency ofoperation of the accelerator.
4. The effects due to thermal expansion of therf cables, frequency instability of the master
oscillator and phase variations of the
klystron and rf solid state devices which areproportional to ambient temperaturechanges.
5. The effects of trigger timing pulse jittersand the rf phase jitters.

The low-power reference rf system and the timing
pulse distribution system must be such that the 4
and 5 terms are reduced as much a possible. In
previous accelerators, this effect has been
suppressed by using a phase stabilized coaxial cable[4] having an electrical length stability of 2 ppm/oK
to transmit the timing and reference rf signals. This
cable is very thick and heavy and not easy to workwith, and large rf power is necessary for driving thecable for a long distance ( e. g. it means large rf
signal transmission loss andlarge power
dissipation in the cable.) In the next system, weshould also employ a light cable to be easy to workand to have small power dissipation. We must
realize the stable low power rf and timing system
for SCSS.

SCSS LOW POWER RF SYSTEM(REFERENCE RF SIGNAL
TRANSMISSION)
Figure 1 shows the SCSS time base distribution
and low-power rf reference system, designed toreduce as much as possible jitter and disturbancecausing factors. The low-level rf system for driving
the high power C-band klystrons which are the- Notice V: Pulse Height
w: Pulse Widthd: Delay

AC line
60 Hz

Trigger Gener ato r
60 Hz

Gun,
ChopperAND

Buncher,
Booster

Buncher,
Booster

Local

S-bandC-band (4 units)

Local

Local

Invert er PS
InhibitInvert er PS
InhibitCW / Puse Signal
Generator
5712-476=5236MHz
2856 - 476=2380MHz
CW / Pulse Generator(V=+8V, w=2us)
1/2

Thyr at ron
Tri gger(V=8V, w=2us)
HV Moni tor
Tri gger(d=2.3us)

ON-timing

FPGA
Timmin g
Mod uleFPGA
Timmin g
Mod uleHV Moni tor
Trigger(d=4.8us)

HV Moni tor
Tri gger(d=4.8us)

Pre-Tri gger

ON-tim ing

ON-tim ing

FPGA
Timmin g
Mod uleFPGA
Timmin g
Mod uleON-timing

FPGA
Timmin g
Mod uleThyr at ron
Trigger(V=8V, w=2us)

Thyr at ron
Trigger(V=8V, w=2us)

BPM Tri gger
(d=4.8us)

HV Mo nitor
Trigger(d=4.8us)

HV Mo nitor
Trigger(d=4.8us)

Other m onitors(d=4.8us)

To Undulator
and Users

5236MHz
2380MHz

476MHzCW / Pulse Generator(V=+8V, w=2us)
1/6

476MHz238MHzCW/ Pulse Generator(V=+8V, w=2us)
1/2

238MHzIQ Modulator

El ectron Gun

Master Trigger

w=2us, d=3.8us

CW / Puse Signal
Generator
5712MHz

d=2.3us

FPGA
Timmin g
Mod uleON-tim ing

w=2us, d=3.8us

ON-tim ing

w=2us, d=3.8us

Trigger Gener ato r
60,.. . 1 Hz, or Single

Invert er PS
Inhibit

Pre-Tri gger

Pre-Tri gger

Beam Transport

Beam Chopper
Trigger Pulse

RF/Beam Moni tor
BPM etc.

IQ Moudlator

IQ Moudlator

Pre-Amp (TWT or SS)
2856 MHzIOT RF
237 MHz, Pulsed

IOT RF
476 MHz, Pulsed

S-band Klystron2856 MHzRF/Beam Moni tor
BPM etc.

RF/Beam Moni tor
BPM etc.

RF/Beam Moni tor
BPM etc.

Gate
I/Q mod. modul e(d=2.5us)
Amp. Gate?
(d=2us)
Pre-Amp (TWT or SS)
5.7 GHzStreak Camer a

C-band Kl ystron
5.7 GHz

RF/Beam Moni tor
BPM etc.

5712 MHz

Fig. 1: Fig. 1. The low power rf and timing distribution system for SCSS. The output of the Master
Trigger drives 3 logical groups of components: the Injector (Gun, Beam Chopper, Sub-Harmonic Buncher(238 MHz and 476 MHz), Buncher, and Booster accelerator 2856 MHz), the Main Accelerator complex, andthe beam transport system to the undulators. The rf signals at 5712 MHz, 2856 MHz, 476 MHz, and 238
MHz are made by frequency division and a time reference is also created by the master oscillator. Opticalfiber cables are used for transmitting the signals.

main rf sources for the SCSS is shown in Fig. 2.
The system is comprised of I. a master oscillator
system, II. a signal transmission/distribution system,
and III. a drive system delivering the signals to the
klystrons etc.
I. Master Oscillator system
The master oscillator system as shown in Fig. 3
makes rf signals with frequencies of 238 MHz 476
MHz, 2380 MHz, 5326MHz and 5712 MHz. The
238 and 476 MHz signals are for the sub-harmonicbuncher and the booster acclerator. The signals of
2380MHz and 5329MHz are necessary for the IQ
modulations at the klystron inputs. 476 MHz fromthe IQ modulator has added to it either 2380MHz

or 5326 MHz to produce the final signals at 2856
MHz and 5712 MHz which drive the klystrons.
2856 MHz is for a booster accelerator and 5712
MHz is for main accelerator.
To decrease phase noise to make the above
mentioned frequency signals, we employ frequency
division method. Because dividing by N from asource frequency can decrease the amplitude ofsignal source noise (phase noise) by up to 1/N at thedivider output. Conversely, multiplying by N
increases the noise amplitude up to N times. Notethat the noise of a commercial synthesizer signal
generator does not increase in amplitude
proportionally to an increase in frequency [5].
Therefore, we choose the frequency division

60 Hz or 120 Hz Reference Trigger Signal
Drive RF Line (238 MHz)
Drive RF Line (476 MHz)

O/E

Drive RF Line (52376 MHz)

O/E

O/E

Mo dulation Board+10dBmO/E
FP GA Delay
VME Module

Band Pass
Filter (5 .7GHz)

RF V ector Mo dulator
I

476MHzQ

238 MHzClo ck 238 or 119MHz
Trigger Signal

Me mory
FP GA

500W
Solid State Amp.

C-Ba nd High Power Klystro n

100MW

DA C

Pulse
Co mpressor

DA C

Arbitrary Wave Function
Generator (12 bit D/A)
VME Module

400MW+10dBm
5.2GHz119 MHz is divided by 1/2
from 238 MHz.
476MHz+4dBm
5.7GHzRF V ector
De tector

Band Pass

Clo ck 238 or 119MHz
Trigger Signal

Me mory
FP GA

Q

ADC

95MW

Accel. Structure

Filter (4 76 MHz)I
476MHz

x 6ch

40MW

40MW

ADC

Wave Memory(A/D
12 bit VME Module

Fig. 2. Low Level rf system for SCSS. This system mainly uses IQ modulation and detection to control thephase and amplitude of rf signals for driving the high power klystrons. The IQ detector and modulator arecontrolled by VME ADC and DAC modules interfaced to the SPring 8 control system (MADOCA) [6].

method, becausecharacteristic.

of

its

betterlow

noise

The Master Oscillator is composed of:a. a low noise 5712 MHz synthesizer signal
generator having an almost pure signal spectrum,b. frequency division and adding circuits forgenerating the above frequency signals from5712 MHz,c. and an E/O (Electrical/Optical) converter
for each signal.

pulse with the frequency spectrum with a comb likestructure, with very stable and almost purely
monochromatic frequency components. An opticalsignal of the necessary modulation frequency suchas 5712 MHz is extracted from the comb structureby using an optical filter. The wave length of thelaser light is 1520 nm and the stability of the 5712
MHz optical component is mainly dependent on
stability of the synthesizer signal generator doing
the frequency modulation.
II. Signal transmission/distribution system

For obtaining a highly stable optical signal as atime standard, we employ an optical comb
generator [7] that generates laser light modulated by
the rf signals of the previously mentioned
frequencies (Fig. 4). An electrical/optical (LiNbO3
acoustic modulator) modulator modulates the light.
The optical output of this generator has a short

The role of the signal transmission/distribution
system is to transmit and distribute the rf signals
such as 5712 MHz generated by the master
oscillator system, this is done with cables. To
decrease the energy fluctuation of the electron beamin the SCSS linac, the thermal dependence of theDivider5712 MHz Clock

F/F

F/F

5712 MHz

F/F

1/6

1/2

Low Noise
SignalSource1

δ φ < 1 / 24
δ t = 44 ps

1/2

2856 MHz476 MHz

238 MHzBand Pass
FilterBand Pass
Filter5712 - 476=5236 MHz

2856 - 476=2380 MHz
Limiter
Amplifier

Limiter
AmplifierSinusoidal Pulse
Wave
Train
2856 MHz

Sinusoidal Pulse
Wave
Train
476 MHz

Sinusoidal Pulse
Wave
Train
5712 MHz

Sinusoidal Pulse
Wave
Train
238 MHz

Sinusoidal
Wave

Sinusoidal
Wave5236 MHz

2380 MHz

Fig. 3. Master oscillator for SCSS. The signals of 238 and 476 MHz are for a sub-harmonic buncher and abooster accelerator. The 2856 and 5712 MHz rf signals are necessary for booster and main accelerators
guides. The signals of 2380MHz and 5329MHz are needed for IQ modulations at the input of klystrons.
electrical length of the cables must be reduced.

Therefore, we employ a phase stabilized opticalfiber cable [8,9] to transmit the signals. The cable islight and small power dissipation. It is a single
mode optical fiber where the electrical lengthchange due to ambient temperature changes iscompensated for by an LPC (Liquid Polymer
Crystal) coating. This LCP material is coated on theouter cladding surface of the cable. The temperature
coefficient of the cable electrical length is 2 ppm/oK
at 20 oC [9]. An optical coupler splits the signal tofeed optical fiber cables driving each klystron. If
there is not enough optical power available, an
erbium-doped optical fiber amplifier (EDFA) canbe used to amplify the optical signal.
III. Driving system
The purpose of the driving system is to convert
the optical signal as delivered by the signaltransmission system into an electrical signal, andthen to amplify it as necessary before making thephase, amplitude or pulse modulation needed by theuser device. A schematic diagram may be seen in
Fig. 2.
The details are as follows:
1. The first stage is an O/E converter. In the past,
such converters mainly used an avalanche
photo diode having a high speed pulse
response. However, this diode is noisy. At
the present time, an O/E converter with a
PIN photo diode which has low noise isavailable on the market. Some of these
diodes have cut off frequencies up to 40
GHz; they can satisfy our requirement fortransmitting 5712 MHz. Therefore, we plan
to use PIN photo diode based O/E converter.
2. After the O/E converter, there is an IQ ( Inphase/Quadrature) modulator to produce
pulse, phase and amplitude modulation.

time

Modulation
Signal Generator
OutputInput

Laser Light

Electro Optic Modulator

MirrorMirrorOptical Amplitude

Optical Cavity

n=1
n=2
n=3
n=5
n=6

f (Hz)
Frequency

Fig. 4. Schematic drawing of an optical comb generator. This instrument generates an accurate and stable
optical signal having a comb frequency structure.

Test RF signal(e. g. Beating downfrom 5712 MHz by 476 MHz)
Optical or RF sinal
5712 MHz RF singal 1

Band Pass Filter
476 MHz

O/E

I
Div.

O/E
O/E
5236 MHz
Optical or RF sinal

RF phase/
amlitude detector

Div.

Reference RF
476 MHz
Optical or RF sinal

90 deg. ShiftQ

Fig. 5. IQ modulator for generating drive signals for high-power rf sources such as the klystrons.
Figure 5 shows a schematic of themodulator. It can be used for functions
such as rf pulse modulation to drive the
klystron and make phase adjustments
between the electron beam and acceleration
rf so as to obtain the maximum energy gain.
3. Following that there is a 600W solid state
amplifier in the modulator for driving the

klystrons.
4. Besides the driving functions described sofar, it is also necessary to monitor the phaseand amplitude of the rf outputs of the
klystrons and accelerator structures. To dothis, an IQ detector as shown in Fig. 6 canbe used. The detector can also obtainphase/amplitude information from beam

master trigger pulses might have to beeither synchronized or unsynchronized withthe commercial power mains frequency of
60 Hz. But they should always be
synchronized with the 5712 MHz
acceleration frequency. This synchronization is what guarantees accelerating theelectrons at stable point on the rf crest, andensures the energy/current stability of theaccelerated electron beams.
c. The master trigger system can set any
repetition rate up to 120 Hz for
independently extracting electron beams
from the electron gun and pulsing the
klystronmodulator.
Thesystemcontinuously outputs master trigger pulses.
Functions of parts a. and b. are included in
the Master Trigger.
d. Each trigger pulse, distributed over opticalfiber from the master trigger to the various
accelerator devices, should be resynchronized to the 5712 MHz reference
clock, or one of its divisor frequencies, atthe individual driven devices. This candecrease rise time jitter in the trigger pulse.
e. The trigger distribution circuit (Fig. 7)
belonging to local devices such as the
klystron modulators should be able toarbitrarily control whether to activate orinactivate the trigger pulse at each device,
and individually set any repetition cycle
pattern. The signal distribution part of theinduced fields in the accelerator structures(e. g. for phase adjustment) and monitor thevariation in individual rf pulse energies.

TRIGGER PULSE TRANSMISSIONSYSTEMA trigger pulse transmission system using copper
coaxial cable can be difficult to use in high speed
applications because the pulse raise time is slowed
because of high frequency attenuation due to theskin effect on the inner conductor of the cable. But
without a fast raise time, an accurate trigger time tofire an instrument or data acquisition is impossible.
For these reasons, we employ an optical fiber cablesystem for transmitting trigger pulses. Figure 1
shows the overall system that is comprised of amaster trigger, an optical transmission plant, and asignal splitting and distribution system.
The necessary operational functions provided by
the system are as follows:a. A master trigger pulse train establishing
the electron beam acceleration cycle isnecessary for pulsing the electron gun andklystron modulators. This master trigger
system is always operated at its maximum
rate, 120 Hz.
b. Depending on the operational mode, theFrom D/A converter 1

Test RF signal 5712 MHz(e. g. Beating up from5236 MHz by 476 MHz)I

476 MHz Optical or RF Signal
O/E

Divider.

From D/A converter 2Band Pass
Filter for
5712 MHz

Q

O/E
5236 MHz Optical or RF Signal
Fig. 6. IQ detector for monitoring the output signals from high-power rf sources such as the klystrons.

Injector/Accelerator related signals (I-VIII)of the S-band klystron modulator.
Triggers for the IQ modulators of the input
drive rf signal to the S-band booster
accelerator.
VII. Triggers for the A/D converters used inmonitors of beam positions, rf signals, andhigh-voltage signals around the injector
accelerator.
VIII. Triggers for other data acquisition units
such as in the various PLC (Programmable
Logic Controller) devices.

I.

Main accelerator related (A-D)

triggerpulse
transmission
systemimplements the functions of parts d. and e.
Figure 8 shows the trigger sequences forthe main accelerator including the klystron(Fig. 8-A) and the injector including theelectron gun (Fig. 8-B).
The trigger items of interest are shown as follows:

II.
III.
IV.
V.

A pre-trigger and a trigger to start thethyratron of the high-voltage modulator forthe electron gun. These triggers generate ahigh voltage pulse.
A trigger to decide the repetition rate of theelectron beam.
A trigger for the deflector which
determines the beam pulse width.
Triggers for the IQ modulators andamplifiers of the 238 MHz and 476 MHz
sub-harmonic buncher and booster.
A pre-trigger and a trigger for the thyratron

VI.

A.
B.
C.

D.

Pre-triggers and triggers for the thyratronsof the C-band klystron modulators.
Triggers for the IQ modulators for the input
rf signals to the C-band main accelerator.
Triggers for A/D converters for themonitors of beam positions, rf signals, andhigh-voltage signals around the C-band
main accelerator.
Triggers for data acquisition such as PLC
devices.

Optical 238 MHz Transmission
Reference RF signal

Optical Coupler
E/O
Optical Coupler
Trigger Pulse

Optical 5712 MHz Transmission
Reference RF signal

Optical Coupler
E/O

E/O

238 MHz5712 MHz

Trigger Pulse
Distributor
FPGA

BufferTrigger Pulse
D

RF

CK

D

CK

Q

Buffer

24 bitCounterx 8 ch.

8 Channels
Measurement
D CK Q

Software Control
Inhibit, 8 Channels
Read/Wright

Modulator

Counter Preset

VME BusHardware Control
Inhibit, 8 Channels

Fig. 7. The block diagram of a trigger distribution circuit using FPGA. It controls activating timings ofdevices such as the klystron modulators, and ensures synchronization the output trigger pulse to 5712 MHz
rf signal.

High Voltage
Pule 4.7 µs Flat Top

RF Klystron
2.5 µsRF SLED-II
0.5 µs2.5 µs
4.5 µsTn = 0Tn = 0Beam

IQ Modulator
Trigger-3.5 µs
Thyratron
TriggerThyratron
Pre-trigger- 4 µsA/D ConversionStart- 5 µsEnd of HV Charge- 12 µs

A/D Conversion 20 µs- 10 µs

InverterCharging Start

Beam+10 µs- 1,000 µs

SystemStart

15,000 µs
16,000 µs
60 Hz, 16,666.6 --- µs, ~ 16,670 µs

A)

The timing sequence to activate the main accelerator components (such as the klystrons).
GUN High Voltage
Pulse 1.6 µs Flat Top
S-band RF Klystron
2.5 µs238MHz SHB RF Pulse 100 µs Flat Top

Tn = 0476MHz SHB RF Pulse 50 µs Flat Top

Beam

S-bnd IQ
ModulatorTrigger
GUN Thyratron
Trigger
GUN ThyratronPre-trigger
A/D Conversion- 3 µsStart- 4 µs
238 MHz SHB- 12 µs- 30 µs

InverterCharging Start
SystemStart

Tn = 0Beam
Beam Deflector-0.03 µs
-3.5 µs- 55 µs
End of HV Charge- 1,000 µs

15,000 µs
16,000 µs
60 Hz, 16,666.6 --- µs, ~ 16,670 µs

B) The timing sequence for the injector accelerator components(such as the electron gun).

Fig. 8. SCSS accelerator timing chart and trigger signals.

Downstream of the main accelerator:
1.

Triggers for the pulse magnet used to switch
the beam trajectory and for instruments suchas streak cameras.

To implement the above functions and thesynchronization to the 5712 MHz rf signal at themaster trigger and distribution units, we employ
fast GaAs logic IC’s and FPGA’s with counters asshown in Fig. 7.

REFERENCES[1] T. Shintake, “Status of the SCSS Project”, Asian
Particle Accelerator Conference (APAC 2004),
MOM102, March 22-26, 2004. Gyeongiu, Korea[2] Refer to the SCSS Scientific Case, This Report.
[3] H. Hanaki, “Stability of RF Signal (Low PowerRF system)”, Design Report on PF Injector

Linac Upgrade for KEKB, KEK Report 95-18,
Chap. 9.1.2, pp 188-196, 1996 in Japanese.
[4] http://www.mitsubishi-cable.co.jp/[5] Agilent technology catalog, p 149, 2004.
[6] Refer to the SCSS Control System, This Report.
[7] M.Kourogi, B.Widiyatmoko, K.Imai and
M.Ohtsu "Generation of Expanded Optical
Frequency Combs", Edited by A.N Luiten,"Frequency Measurements and Control",Springer-Verlag Berlin Heidelberg 2001, pp
315-335[8] Kakuta, Tanaka, “LCD Coated Optical Fiberwith Zero Thermal Coefficient of Transmission
Delay Time”, Sumitomo Electric Industries Ltd.,
Yokohma, p 244, Japan.
[9] S. Tanaka, “Phase Stabilized Optical Fiber”, Tec.
Rep. of Sumitomo Electric Ind. Ltd., 1989.

RADIATION SAFETY ISSUESSCSS relies on SPring-8 experience and know-how for radiation safety analysis (design of theshielding, including beamline shielding) and management (radiation safety system). Preliminary
studies confirmed that skyshine is the most critical issue for the design of the SCSS bulk shield,
because of the low annual dose limit at the site boundary and the short distance between the
beam (dumps, slits) and the site boundary. SPring-8 being a multi accelerator complex, the dosecriterion at the site boundary has also a strong influence on shielding.

BASIC CRITERIASHIELDING DESIGN

The shielding design criteria are based on the
ALARA (as low as reasonably achievable) principle,
the dose limits being set by the law regulating theprevention of radiation hazards and by SPring-8
regulations (Table 1). By the law, the effective
dose limits to radiation workers from both internaland external radiation sources are 50 mSv per year
and 100 mSv with in5 years. For the public, thedose within 3 months must not exceed 1.3 mSv atthe boundaries of the controlled area, and 0.25 mSv
at the boundaries of the site. However, SPring-8
regulations enforce a dose limit for radiation
workers equal to one third of the legal dose limit(8.3 µSv/h), while at the site boundary (public area)
the total annual dose limit is set at 50 µSv. Actually,
SPring-8 being a multi accelerator facility, themaximum allowance for the SCSS is about 20µSv/y.
Finally the design and operation of the SCSS
safety systems (radiation monitoring system, access
control system, interlock system and data
management) should be consistent the SPring-8
safety system: while the SCSS will be operated
independently from the other SPring-8 accelerators,
its radiation safety systems will be integrated to the
SPring-8 system.

Beam parameters and geometricalconfiguration
Fig. 1 shows a schematic view of the SCSS with
with the closest distance between the loss points,
the beam dump and the site boundary: the linac will
be located along the SPring-8 one kilometer long
beamline, with the undulators and beam dumps
installed in the direction of the SPring-8 storage
ring. As a result, the distances from the electron
beam loss points and the beam dump to the siteboundary are about 180 m and 80 m respectively.
As explained hereafter, such short distances add
extra constraint on the design of the shield(skyshine).

Figure 1 : Schematic view of the SCCS showing theclosest distance between the loss points, the beam dump
and the site boundary (in red).

Table 1: Maximum dose limits set by the law and by SPring-8 regulations
Area
Radiation workers
Boundary of the controlled area (Public)
Boundary of Spring-8 site (Public)

Law
50 mSv/y and 100 mSv/5y
1.3 mSv/3 months
0.25 mSv/3 months

SPring-8 regulation
8.3 µSv/h
50 µSv/yThe SCSS will operate at energies up to 6 GeV insingle bunch mode (1 nC) at 60 Hz. The operation
time is assumed to be the same as SPring-8, that is8000 hours. Once set up for user operation, thelinac is principally a low beam loss machine, withseveral slits to scrap the insufficiently tuned
electrons. Therefore, it is reasonable to assume thatthe point wise beam losses occur at the slits with arate of 1 %.

Figure 2 : Schematic view of a cross section of the SCCS
linac tunnel : the tunnel is 4 meters wide, 3.5 meters high,with the beam at 80 cm from the floor and 2.5 m fromthe wall of the non-controlled area.

Neutrons and gamma-rays due to electron beamlosses were considered in the direction
perpendicular to the electron beam axis, and the
SHIELD 11 code [1] was used with ordinary
concrete characteristics (density 2.1 g/cm3).
Skyshine dose calculations were performed with theStevenson-Thomas formula and the SPring-8
analytical method [2].

Bulk shield
The calculation results show that the annual dose
limit at the site boundary is critical for the shielddesign because of the short distance between the
linac and the site boundary. Moreover, as noted
previously, when considering the total annual doseat the site boundary, the contributions from thedifferent SPring-8 accelerators mean that themargin for the SCSS annual dose is less than 20µSv/y. For the tunnel, the calculations indicate that2 m of ordinary concrete is enough to satisfy thedose criteria for the radiation workers and the dosecriteria at the boundary of the controlled area.
However, to satisfy the annual dose criterion at theboundary of the SPring-8 site, 3 m of concrete or 2
m of concrete plus an additional iron local shield(40 cm) is needed.
As for the beam-dumps (Fig. 4), they will be
installed underground and inclined at 20º. To satisfy
the dose criterion at the boundary of the controlledarea (radiation workers), the thickness of theordinary concrete shield is set to 2 m in the lateral
direction, and 3 m in the forward direction.
However, to satisfy the annual dose criterion at thesite boundary (public) requires 3 m of ordinary
concrete, with an extra 50 cm of iron laterally.

The cross section of the linac tunnel used for
dose evaluations is shown on Fig. 2: the tunnel isassumed to be 4 meters wide, 3.5 meters high, withthe beam at 80 cm from the floor and 2.5 m fromthe wall of the non-controlled area. The beam
dumps, located underground, have a double
structure (graphite and steel) to reduce theproduction of radiation and minimize the thicknessof the shielding material (Fig. 3).

Photon beam shutter and photon absorber

Figure 3 : Cut away view of the beam dump : the innercylinder is made of graphite, the outer one from iron.

In addition to the neutron and gamma photons due
to electron beam losses, the gas bremsstrahlung andassociated photoneutrons must be considered, as
well as the muons emitted forward of the beam
shutter or photon absorber, especially as these are
installed on the beamline axis (the direction of theexperimental hutch). Gas bremsstrahlung has beenthoroughly investigated for third generation

Figure 4 : Schematic view of the beam dump area : the beam dump is set with an inclination of 20º.

synchrotron radiation facilities [3] because of its
high energy and its high directionality, which meanthat it can easily propagate down into the beamline.
However, for the SCSS, unlike storage typefacilities, the intensity of gas bremsstrahlung is
expected to be lower because of the low intensity
current: assuming the same residual gas pressure asthat of the standard SPring-8 straight section (1nTorr), intensity is expected to be five orders ofmagnitude smaller. Still, the maximum dose rate
due to gas bremsstrahlung outside of the shieldtunnel is at least 80 µSv/h, so that a shield such as abeam stopper must be considered.
The high energy gas bremsstrahlung will also
generate photoneutrons through interaction with thehigh Z materials of the beamline components.
However, since in the case of the SCSS theintensity of gas bremsstrahlung is low, the dose dueto the photoneutrons will be, in general, negligibly
small.
Muons are produced and emitted in the forward
direction at the electron beam switch yard,
undulators, slits and the beam dumps. The dose due
to muons is not so high: for example, it is about 1µSv/h at 10m from the source point for a 1% beamloss. However, the relative importance of the dose
due to muons grows up as the shield thickness
increases.

For SCSS, the shield ability that the beam shutteror photon absorber is expected to have is therefore
determined by the neutrons and gamma-rays due tothe electron beam losses. Concretely speaking, abeam shutter with an equivalent shield ability of 1.5
m of iron is required in the case of a 1 % beam loss
seen at a distance of 13 m from the outside of theshield tunnel.
Finally, since synchrotron radiation impartsintense heat to the beam shutter or the photon
absorber, countermeasures should be considered toreduce the heat load. Shielding from the scatteredphotons of synchrotron radiation should also be
considered.

INDUCED ACTIVITY
The induced radioactivity due to irradiation by
the electron or bremsstrahlung beams were
estimated by using the saturation activity of a unit
electron beam loss and the data summarized in the
SPring-8 [2] and IAEA [4] reports. For example, it
was found that the main induced activities are
Nitrogen-13 in air and Beryllium-7 in water.
However, since the SCSS will use an exhaustsystem with long retention time and a closed
cooling water system, additional attention must be
paid to radio isotopes such as Argon-41 and Tritium.
Similarly, the dose due to induced activity could bederived from the saturation activity data and QADCGGP2, a point kernel method code [5] : at 1 mfrom the graphite beam dump, the dose was
evaluated at 83 µSv/h just after the shutdown and
0.43 µSv/h one day after the shutdown.

SAFETY SYSTEMSThe SCSS safety systems, which consist of aradiation monitoring system, an access control
system and an interlock system, are described
conceptually in Fig. 5. All these systems should be
operated consistently under the SPring-8 systems
and all the safety data should be managed in an
integrated fashion. Basically, all parts of the system
which relate directly to human safety are connectedto each other by hardwire. The ones not related tohuman safety can be operated independently.

be checked with off-line liquid scintillation
counters.

Access control system
The controlled area will be accessible through an
auto lock door equipped with an ID card reader:
only the persons having an ID card with the due
authorization will be allowed to enter. The entrance
to the accelerator tunnel will use the same system as
SPring-8: this personal key system, which isconnected to the interlock system, allows to
confirm simultaneously who enters the controlledarea and the accelerator tunnel.

Interlock system
The safety interlock system consists mainly of
PLC (Programmable Logic Controller) and hard
wires which connect the PLCs. In order to keep the
independence, the main interlock must be set up
with two different systems by using PLC andmultipurpose logic circuits. The interlock systemsof the beamlines and the accelerator have to beconstructed independently. The safety interlock
system of the beamline is closed by itself and thestatus is monitored to give the permission to operate
the accelerator. The interlock system of theaccelerator must be designed on the basis of thecentralized operations.

Figure 5 : The SCSS safety systems.

REFERENCES
Radiation monitoring system
The radiation monitoring system consists of area
monitors, which will be installed in and around theradiation controlled areas, and gas monitors which
will be installed in the exhaust systems of theaccelerator tunnel to check the level of radioactivity
concentration. Some of the area monitors will beconnected to the interlock system, to shutdown themachine operation if necessary. Besides, passive
dosimeters will be attached to possible “hot” spot,
to measure the cumulative dose over a week or amonth. Since the SCSS cooling water systems will
run in a closed circuit, the water will normally notdischarge into the general waste lines. Nevertheless,from time to time, the activity of water samples will

[1] SLAC SHIELD11 - an analytical shielding code
maintained at SLAC Current version, 2000a.
[2] Asano Y., and Sasamoto N., “Radiation shielding andsafety analysis for SPring-8” JAERI-Tech 98-009(1998) in Japanese.
[3] Asano Y., “A Study on radiation shielding and safety
analysis for synchrotron radiation beamline”, JAERIResearch 2001-006 (2001).
[4] Swanson P., “Radiological safety aspects of theoperation of electron linear accelerators”, IAEA
Technical Reports series No.188 (1979)[5] Sakamoto Y., and Tanaka S., “QAD-CGGP2 andG33-GP2 : Revised Versions of QAD-CGGP and
G33-GP Codes with Conversion Factors from
Exposure to Ambient and Maximum Dose
Equivalents”, JAERI-M-90-110 (1990)