Chapter 11
Injector Linac

394

11.1

Introduction

The electron–positron injector linac at KEK has delivered electrons and positrons forparticle physics and photon science experiments since 1982. It was originally constructed for Photon Factory as a 2.5-GeV electron linac beginning in 1978 and was commissioned in 1982. A positron generator was added for the TRISTAN electron/positron
collider project, and was operated until 1995. Then, the linac was upgraded for the
KEKB asymmetric-energy collider project with energy enforcement up to 8 GeV [1, 2].
Since 2010, KEKB has been further rejuvenated for the SuperKEKB collider project.
This project aims at a 40-fold increase in luminosity over the previous KEKB project, inorder to increase our understanding beyond the standard model of elementary particle
physics [3] after the KEKB project’s decade of successful operation. The SuperKEKB
asymmetric-energy electron–positron collider, with its extremely high luminosity, requires injection beams with high current and low emittance in the transverse andlongitudinal directions [4]. It should also perform simultaneous top-up injections into
four storage rings and a DR by pulse-to-pulse modulations (PPMs) to avoid interfere
between three facilities: SuperKEKB, Photon Factory (PF), and PF Advanced Ring
(PF-AR).
DC-gun 10 nC for e+

B

A

RF-gun 4 nC for e–fiber & thin disk laser
J-arc
IrCe cathode,
1.5 GeV
Primary e– for e+ QTWSC cavity Bunch
3.2 GeV, 10nC x 2
Compression
Bunch
Compression

C

e+
1 New
2
CapturePF
2.5 GeV
0.3 nC

1.1 GeV e+damping ring

LER e+
Energy4.0 GeV
Compression 4 nC x 2

EnergyCompression

3

with FC

4

5

PF-AR
6.5 GeV
0.3 nC

HER
7.0 GeV
4 nC x 2

Figure 11.1: Overview of 600-m electron/positron injector linac with 60 accelerating
units.
The 600-m injector linac is composed of 60 high-power accelerating units followed
by a beam switch yard. The overview of the linac is shown in Fig. 11.1, and the photoof A and B sectors is shown in Fig. 11.2 as a typical injector section. The injector
should meet the requirements of the SuperKEKB rings, with a small aperture at theinteraction region, doubled stored beam currents, and short expected lifetimes. Lowemittance, high-current electrons are delivered by employing a photo-cathode RF gun.
High-current primary electrons for positron production are generated by a thermionicgun, and then high-current positrons are produced using a flux concentrator (FC) andlarge-aperture accelerating structures (LASs), which are then damped to low emittancethrough a DR. Design parameters of the injection beams are listed in Table 11.1.
395

Figure 11.2: A typical part of the injector linac at the A and B sectors.
Beam
Beam energy
Normalized emittance γεx /yEnergy spread
Bunch charge
No. of bunches/pulse
Repetition rate

Positron
4.0
100/150.16
42Electron
7.007
GeV
40/20
µm
0.07
%
4
nC
2
50
Hz

Table 11.1: Injection beam parameters.

A low-emittance electron beam source and its transport are indispensable to realizing SuperKEKB’s nano-beam scheme for higher collision rates. Although DR isemployed to reduce positron emittance, cost and space restrictions make the same solution infeasible for electrons. Thus, we have developed a photo-cathode high-current
RF gun. Generating a high-charge electron bunch (up to 4 nC) is a challenging target [5].
The positron beam should also be enhanced to a dual-bunch 4 nC beam in stages,
because SuperKEKB has the doubled stored beam current and a much shorter beam
lifetime of several minutes. A high-charge positron bunch is generated with a conventional 14-mm thick tungsten target, and is captured by employing an FC and LASs
with velocity bunching, followed by a series of solenoid and focusing magnets [6]. As
the generated beam emittance is large, it is damped by employing a DR at 1.1 GeV.
If a beam bunch accelerated in the structure is offset from the center, the generated
396

wakefield induces a transverse force to the tail, and the projected emittance can become
very large. Suppressing this effect requires mechanical alignment of the quadrupole
magnets and accelerating structures. The beam orbit should be stabilized by finding a
low-emittance condition empirically [7]. A beam position read-out system of precisionless than 10 µm is also needed to support the method [8].
Devices and monitors in the injector linac are operated via event-based, global,
and synchronized controls to inject beams with different properties into four separate
storage rings simultaneously [9]. A single injector linac would behave as four independent virtual accelerators (VAs) with hundreds of independent parameters modulated
pulse-by-pulse at 50 Hz (Fig. 11.3) [10]. A new beam transport line for PF-AR direct injection prevents interference between the PF-AR and SuperKEKB HER, whichshared the same beam transport line in the KEKB project [11].
Event-based
Control System
Every
20 ms
F.B
e− GunARC

F.B

PF Injectione– (2.5GeV, 0.2nC)

F.B
e− GunARC

Damping ring

SuperKEKB-LER Injection

e–

F.B

(3.5GeV, 10nC)

F.B

e+ Targete+ (4GeV, 4nC)

e− GunARC

SuperKEKB-HER Injection

F.B

e– (7GeV, 4nC)
e− Gun

ARC

F.B

F.B

PF-AR Injectione– (6.5GeV, 0.2nC)

Figure 11.3: Single linac behaves as four virtual accelerators to inject their beams into
four separate storage rings.
In the following sections the details of the injector design are described.

397

11.2

Electron Beam Injection

The electron beams with a charge of 4 nC and a normalized emittance of less than 10
mm mrad are expected to be generated in the photocathode RF gun for injector linacof SuperKEKB HER injection. To high-current, low-emittance beams generation, thelaser source with mJ pulse energy, center wavelength of 260 nm and a pulse width of 20
ps are required at the A-1 unit (Fig. 11.4). Further its pulse shape should be reshaped
to rectangle structure to obtain the required energy spread of 0.1 %.
The thermionic electron gun is rearranged to the upstairs for the 10 nC primaryelectron beam for the positron generation as shown in Fig. 11.5.

Figure 11.4: Layout of RF-gun and thermionic electron gun.

For positron generation

70 MeV
Acc.tube

Prebuncher SHB2 SHB1

Buncher

30 MeV
ChicaneAcc.tubeQTW RF gun

10 ps

CDS RF gun

Bunch 20 ps Energy slope
compression
2 nC (Phase-II)
Pulsed magnet
4 nC (Phase-III)

Normalinjection

Thermionic
DC gunPrimary laser(Yb:Fiber + Nd:YAG)
Secondary laser(Yb:YAG)

Figure 11.5: Layout of injector part.

11.2.1High charge and low emittance electron beam

The acceptable normalized emittance for the SuperKEKB injection is 20 mm mrad.
To achieve this emittance with the initial emittance of 10 mm mrad from RF-gun, theemittance dilution of 10 mm mrad is acceptable for the entire linac.
398

The advanced RF-Gun, which adopts the quasi-traveling wave annular coupledcavity, provides the high charge and low emittance beam for SuperKEKB. The optimum
bunch length at RF-Gun for the low emittance electron beam is around 20 ps against
the space charge effect. The longitudinal bunch shape was controlled by the laser
system to obtain the required energy spread. After the RF-Gun, it is compressed to
10 ps using a chicane section.
The transverse wakefield becomes the main source of the emittance dilution for thehigh bunch charge of 4 nC after the beam orbit is well corrected in the LINAC beam
optics. And the energy spread is determined by the longitudinal wakefield strength.
The smaller alignment tolerance and the shorter bunch length can reduce the transverse
wakefield strength. However, the shorter bunch length causes the stronger longitudinal
wakefield.
Figure 11.6 shows the analytical calculation of the projected emittance in several
cases of the bunch length and the alignment error. The bunch lengthes of 4 ps and 10 ps
are required for the alignment error of 0.3 mm and 0.1 mm respectively from Fig. 11.6.
Since our normal bunch length is 10 ps, we are working to reach the alignment errorof 0.1 mm for each local section which consists of 8 units. However, it seems very
hard to keep this small alignment error in our old support structure and the frequent
earthquake. Thus we have to consider the 4 ps option for the alignment error of 0.3
mm.

Figure 11.6: Analytical calculation of the projected emittance in several cases of thebunch length and the alignment error.

399

Further, the initial offset can compensate the transverse wakefield to recover theemittance dilution. The initial offset is taken as the two steering magnets with 90 degree
phase difference of the beta function for each axis. Figure 11.7 shows the emittance
optimization using the initial offset scan. It is realized using the RF deflector to observe
the longitudinally sliced bunch shape.

Figure 11.7: Initial offset to optimise the projected emittance.

The shorter bunch can suppress the emittance dilution due to the transverse wakefield. However, it causes a stronger longitudinal wakefield. The RF phase becomesdeeper and deeper corresponding to the longitudinal wakefield strength to suppress
the energy spread. Thus the bunch length was chosen by those balances.
The initial bunch length of 20 ps generated by the photocathode RF-Gun is compressed to 10 ps using the chicane at the low energy section. And the J-ARC sectionis placed at the energy of 1.5 GeV. This J-ARC optics was changed to the achromaticcondition from the original isochronous design to obtain certain R56 to compress thebunch as shown in Fig. 11.8.

Figure 11.8: Achromatic design of J-ARC section.

400

We chose the positive R56 of 0.3, since negative R56 makes low energy tail and it
consumes more RF slope to recover the energy spread against the longitudinal wakefield. Figure 11.9 shows the measured R56 of J-ARC using the RF phase variationof C-1 unit after J-ARC corresponding to the energy knob of B-5 and B-6 unit. This
inclination leads the R56 of 0.23. It is slightly different from the designed value due tothe fudge factor of the quadrupole magnets. However, the measured value is enoughto determine the RF voltage for the designed bunch compression factor. One RF unit
(B-7) with zero crossing RF phase is used to make the energy slope for the bunch compression. The bunch compression is performed only for the SuperKEKB HER injection
since the timing of this RF unit for the energy slope is thrown away to the standby
timing for the positron injection and PF injection. To compress the bunch length from10 ps to 4 ps, the RF voltage becomes 60 MV from eq. 1. Thus the RF power of B-7
unit reduced to 8 MW.
σz − σz0 = R56 δ,

δ=

∆p
p

V1unit × 2π ×=V1unit
p

p
λ 1−

=

σz0
σz

2πR56

σz
λ,

!
.

Figure 11.9: Measured R56 of J-ARC using the RF phase variation.

401

Figure 11.10 shows the beam tracking simulation of the transverse emittance growthwith the alignment error of 0.3 mm with and without the bunch compression in the
J-ARC [12]. The compressed bunch length of 4 ps leads to suppress the emittance
growth.

Figure 11.10: Beam tracking simulation with and without bunch compression in the
J-ARC.

11.2.2Longitudinal wakefield

The required energy spread of 0.1 % was achieved for the bunch charge of 1 nC at
the previous KEKB operation. However, the required bunch charge becomes 4 nC for
SuperKEKB and it induces the stronger wakefield. Further, the stronger longitudinal
wakefield induced by the shorter bunch causes the larger energy spread [13]. Fortunately, the energy spread at the optimum RF phase dramatically decreases using theuniform charge density bunch as shown in Fig. 11.11.
As described before, the shorter bunch length suppresses the emittance dilution
due to the transverse wakefield. However, it causes a larger energy spread due to thelongitudinal wakefield. Also, the shorter bunch length requires the deeper RF phase
to compensate the longitudinal wakefield and it causes a lower accelerating gradient.
Thus the uniform charge distribution must be required for any bunch length toreach the energy spread of 0.1 % at 4 nC operation. The longitudinal bunch shape iscontrolled by the laser system using the pulse stacking method or the chirped pulse
amplification method.

402

3.0

90
80

2.5
70

2.0

60

Minimum energy spreadfor gaussian bunch shape

50

1.5
40

Optimum phasefor gaussian bunch shape
1.030

Minimum energy spreadfor square bunch shape

0.5

20
10

0.0

0
0

5

10

15

20

25

30

Bunch length (FWHM) (ps)Figure 11.11: Minimum energy spread and optimum phase.

403

Phase (degree)

Energy spread (%)

Optimum phasefor square bunch shape11.3

Accelerating Structure

One of the most basic missions of the accelerating structure is to accelerate the twobunches of electrons or positrons of 4-5 nC, being separated by 96 ns. The final electronenergy for HER is 7 GeV, while the positron energy for DR is 1.1 GeV and that forLER 4 GeV. To meet this requirement, the accelerating structures are distributed asshown in Fig. 11.12 and listed in Table 11.2. The basic configuration of an accelerating
unit is exactly the same as that of KEKB [14], having four accelerating structures being
fed by a klystron with energy gain doubled by SLED.

Figure 11.12: Linac schematic of acceleration unit configuration.

11.3.1

Electrical design

All the accelerator structures are the 2m-long, disk-loaded structures operated at 2π/3mode with pseudo-constant gradient in accelerating field by making the beam-hole
aperture varying linearly from the fist disk to the last. These structures are categorized
into three types: PF (photon factory) type [14], KEKB type [14] and LAS (LargeAperture-S-band) type [15].
The primary mission of the PF and KEKB types is the beam acceleration, while themission of the LAS type includes the capture and bunching of positrons. The relevant
design parameters are listed in Table 11.3. The electrical design of the KEKB typeis basically the same as that of the PF type, except for the structure of coupler. The
length of the coupler gap of PF type is the same as the wave guide height of WR-284.
On the other hand, the lengths of all the regular cells are 34.99 mm and the cells arecomposed of disks of 5 mm in thickness and cylinders of 29.99 mm in length.
Ten LAS-type structures have been introduced for the SuperKEKB. The basic
design features are 1: large beam-hole aperture for large transverse acceptance, 2:
shorter filling time, at least by half than the nominal accelerating structure of the PF
and KEKB types, 3: smaller outer diameter than the inner diameter of the solenoid
magnet for the LAS-type structure to be inserted, 4: the structure length is close404

SectorUnit(s)No. ofunits

Structure
type

SLED

No. of structures
per unit

A

AT
GR A1
A2 - A4
B1 - B8
C1 - C5
C6 - C8
11 - 1315
16 - 1718
21 - 23
24
26 - 28
DN (ECS)
DS (BCS)32 - 38
41 - 48
51 - 5455
56 - 57
61 (ECS)

1
1
3
8
5
3
3
1
2
1
3
1
3
1
1
7
8
4
1
2
1

KEKB
KEKB
KEKB
KEKB
KEKB
PF
PF
LAS
LAS
PF
PF
PF
PF
KEKB
KEKB
PF
PF
PF
KEKB
PF
PF

N
N
Y
Y
Y
Y
Y
Y a)
Y
Y
Y
Y
Y
N
N
Y
Y
Y
Y
Y
Y

2
1
4
4
4
4
4
2
4
4
4
3+1 b)
4
2
1
4
4
4
4
4
2

B
C
1

2

D
3
4
5

6

a) System is with SLED, but it is operated without SLED as of 2018.
b) The fourth accelerating structure is 1 m long.
Table 11.2: Location of acceleration units.

405

to 2 m trying long to minimize the perturbation against solenoid-field uniformity by
the wave-guide input and output but limited by the realistic handling size, and 5: ashigh acceleration efficiency as possible. The first feature is the natural demand forlarge acceptance. The second one stems from the operational experience of the 2mlong capture cavity used in the initial stage of KEKB, which suffered from frequent
breakdowns and it was replaced by the shorter structures (1m long) [14]. This problem
happens especially in the accelerating structures immersed in solenoidal field just after
the target area. Since the configuration is the same for the LAS case, the short filling
time is one of the most important requirement. The short filling time of LAS-type
structure, 180 ns, less than a half of those of the PF/KEKB-type ones, is realized
naturally from the large beam-hole aperture with associated high group velocity.
The minimum aperture diameter was set at 30 mm from positron beam dynamics.
The disk thickness was chosen following that of PF/KEKB type. There remained achoice of taking the higher phase advance per cell to keep the reasonable gain withlarge aperture. We investigated the designs of 2 m accelerator structures with the
3π/4 and 4π/5 phase advances comparing to that of the 2π/3 one. Even taking thesmall acceleration gain by 7% (20%) with the 3π/4 (4π/5) phase advance design, theassociated increase of filling time becomes as much as 30% (80%), and we finally chosethe phase advance of 2π/3 by giving priority to short filling time. We kept roughly
the nominal length of 2 m for LAS but actually added 3 more cells with respect to
the PF/KEK type to increase the effective acceleration length. In addition, the LAS
input coupler has two irises opened symmetrically to the J-type waveguide as shownin Fig. 11.13 and canceling the dipole field component. In order to be inserted into
the solenoid magnet with an inner diameter of 200 mm, the output coupler has twocompact ports with the flanges mounted as close to the beam axis as possible, as shownin the same Fig. 11.13.

11.3.2Mechanical design

There are five types of accelerating structures, being categorized mechanically as shownin Fig. 11.14. Firstly, there are two types of PF-type accelerating structures, both withthe same electrical design but one, called “4-m structure” shown in Fig. 11.14a, whichis 4 m long by mechanically connecting two electrically 2-m structures, and the other,
the stand-alone 2-m long structure shown in Fig. 11.14b.
The beam pipe flanges of all the structures are EVAC NW40, except for the 4-m
structure, where the upstream flange is MKH-type, while the downstream one EVAC
NW40. The structures are evacuated from their beam pipes or the input and output
waveguides.
406

Structure typePF-type

KEKB-typeNew structure

LAS-type

←

←

57+2

←
←
29.99
←
←
0
1959.442072.43

←
←
←
1.9
←
13
←
2066.43

←
←
←
2.4
←
0
2064.41
2191.01

1936.43

1928.49

2037.68←

23 - 19.73 (CG)

←
←
←
←
←
←

15238
61.5 - 68.3
1.17
0.563
0.33
7.99

31.86 - 29.97(0.033mm step)
13600
46.07 - 48.44
3.86
0.179
0.118
5.19

Number of cells NC
54+2(regular+coupler)
Phase advance per cell
2π/3Regular cell gap length D [mm]29.99
Coupler cell gap length DC [mm]34
Maximum Ep / Eacc
2.1
Regular cell disk thickness t [mm]5Cell wall corner radius [mm]0a)Active structure length [mm]1967.46
Beam hole flange-to-flange length
2066.447 b)[mm]
Distance between wave guides
1928.447[mm]
Tapering of beam hole24.875 - 19.7
aperture diameter (2a) [mm] (0.075mm step) c)
Average intrinsic Q value Q0
13700
Shunt impedance Rs [M Ω/m]
57.3 - 58.3
Group velocity vg /c [%]1.37 - 1.13
Filling time Tf [µs]0.462 - 0.558
Attenuation parameter τ
0.302 - 0.368
Energy gain without SLED
7.00 - 7.58√
[M eV /structure/ M W ]Energy gain ratio with SLED d)
2
Wave guide coupling aperture
SingleWave guide iris lip
Sharp edge
Care against HOM
Linear taper in 2a
Wave guide flange type
MKH

←
←
←
←
SLAC

←
2.4
←
Dual
Rounded (r1)Rounded2a variation for CG Linear taper in 2a
MKH
SLAC

a) Active length is calculated as (λ/3) ∗ NC + (DC − D) ∗ 2, where λ is free space wave length and otherparameters are given in the Table.
b) Details are given in the following section, “Mechanical design”.
c) Linear tapering in beam-hole aperture is implemented. In addition, 5 types (A, B, C, D, E) of
tapering are introduced to interleave among these types.
d) Klystron RF pulse is 4µs in width and its phase is flipped at 3µs, making the compressed pulse of 1µs
by using SLED.

Table 11.3: Accelerator structure parameters.

407

Figure 11.13: Input and output couplers of LAS.

The distance from the upstream flange to the downstream flange of the 4-m structure (a) is 4250.3mm, while those of others are listed in Table 11.3. Both of thestructures, (a) and (b), are equipped with the diaphragms attached at both ends of theelectrically 2-m structure, as shown in Fig. 11.15, where the distance between the twodiaphragms is 2028.447 mm. The beam pipe flanges of 2-m structure (b) are NW40
type and the flange face is separated from the diaphragm by 19 mm. It is to be notedthat the 4-m structure can be disassembled at the TiG joints of such diaphragms andcan be converted to the 2-m structures (b) by adding the attachments converting from
diaphragm to NW40.

11.3.3Fabrication technology

The assembly technology applied for production of PF/KEK-type accelerating structures is based on the electroplating process over high-precision machined disks andcylinders. On the contrary, that of the LAS type is based on the high temperature
brazing. These are different in surface qualities, such as out gassing feature, surface
crystal appearance and mechanical hardness, etc. Accelerating structures which arebeing made from now on for the SuperKEKB will also be made by brazing technique,because because there is no company with a fabricating technique by electroplating inJapan.
The fabrication method of the PF/KEKB type is the following. The regular cells
were made by the ultra-fine machined with diamond tool. These were then stacked andaxially pressed by several tons of axial force while electroplating their outside by several
millimeters. The coupler with a waveguide and a flange was fabricated by brazing andfinally assembled to the electroplated regular cells with EBW (electron beam welding)408

(a) PF-type 4-m accelerating structure.

(b) PF-type 2-m accelerating structure.

(c) KEKB-type 2-m accelerating structure.

(d) LAS-type accelerating structure.

(e) 2-m accelerating structure under development.

Figure 11.14: Rough sketches of mechanically five types of accelerating structures forSuperKEKB.

method. It is to be noted that the regular-cell section is not high-temperature treated,
resulting in keeping the surface hardness as of machining, though the coupler regions
are high-temperature treated. The cooling water is introduced through two inlet ports
mounted on input coupler body, flows around the regular section which is covered bystainless-steel jacket to make a water channel, and finally runs away through two outlet
ports connected to output coupler body.
In contrast, the LAS-type structure was assembled by high-temperature vacuum
brazing, meaning that the body was well annealed at higher than 800 degrees Celsius
and the copper material becomes soft. Their couplers were machined by ultra-precision
lathe, precision milled and then assembled with waveguide flanges using gold brazing.
These are assembled with regular section by silver brazing. The cell frequencies weredimple-tuned after brazing. The water channels are integrated in each cell body andthey are vacuum sealed from accelerating cell inside by brazing in the assembly stage.

409

Figure 11.15: End part of the PF-type structures comprising of coupler, diaphragmand NW40 flange.

11.3.4High power performance

PF/KEKB-type accelerating structures
The standard unit configuration is the same as that of KEKB and is shown in Fig. 11.16 [14].
Here, the 40 MW klystron output power is compressed by SLED and fed to four accelerating structures. This makes 20 MV/m acceleration.
From the KEKB operation period, many PF-type accelerator structures have been
suffering from frequent breakdowns and heavy field emission so that such units haveoperated in reduced power, as shown in Fig. 11.17. The PF-type structures are mostly
distributed downstream of the J-ARC (as shown in Fig. 11.12) so that we suspect that
more electrons from the beam halo have been impinging into these structures.
At the beginning of KEKB, a few KEKB-type structures were tested up to 40
MV/m in order to prove the feasibility of the structures. They reached the nominal
field of 20 MV/m in typically 100 - 200 hours at 50 Hz operation and the field emission
is a few µA with decreasing feature as operation time [16]. However, the PF-type
structures have been in operation for more than 30 years, comparing to the KEKBtype structures for 20 years, we speculate that the longer year operation and possible
beam hitting might have caused the deterioration in many of the PF-type structures.
LAS-type accelerating structures
The three units, 15, 16 and 17, are comprised of the LAS-type structures. The unit
15, with two structures, are immersed in the solenoid field and driven by an RF unit

410

Figure 11.16: Schematic of unit accelerator.

with SLED. This unit is now being operated with rectangular pulses of 500 ns width.
The unit 16 has four structures immersed in the solenoid field connecting to that of
the unit 15. The four structures in the unit 17 are outside of the solenoid channel andin turn each structure is surrounded by three Q magnets.
All these LAS-type structures suffer from the huge gas burst especially when themagnet field ON and/or the SLED operation ON. The gas burst is sensitive to themagnitude of the magnetic fields. Also it frequently appears when SLED ON and wassensitive to the low power region before or even after the compressed output period.
The effect of RF conditioning at a certain parameter works but does not continue longand the gas burst comes back easily. This means that the conditioning memory does notsurvive over days or weeks needed for stable operation. Moreover, the SLED operation
411

Figure 11.17: Operating voltage gain per RF unit as of spring 2018.

at even very small power causes the gas burst and sometimes including the vacuum
breakdowns. One of the most annoying features against continuous linac operation isthat once such gas burst happens, the operation of the unit should be stopped until thevacuum level around 10-6 Pa recovers and the RF power becomes back again, typicallytaking an hour or more.
There is the evidence of breakdown at the first LAS-type structure behind target
in the unit 15, appeared only the upstream one. It shows the RF pulse shapes different
than that of the usual vacuum breakdowns developed in a few tens of ns, which appear
in many high gradient accelerating structures. The transmitted wave showed the linear
decrease in RF voltage over 500 ns pulse, while the reflection was increased linearlyover the same time. This feature is shown in Fig. 11.18.
The mechanisms of the gas burst and vacuum breakdown, should be identified andthe problem must be cured for the stable operation. For the time being, the gradientof the LAS-type structures are limited and the recovery protocol from these problems
is improved.

11.3.5

Water leakage issue

The water leakage from the couplers of PF-type structures has become more and more
frequent in these years [17]. The schematic of the responsible area is shown in Fig. 11.19.
The EBW was applied to connect the coupler to the main body composed of the412

Figure 11.18: RF pulse shapes under vacuum breakdown at the first LAS structure inunit 15.

regular cells. This EBW is so tight that in some cases the cylinder next to the couplersuffered from the surface damage. Moreover, the cooling water runs with touching
this welding line. If water leaks crossing this line and faces the vacuum, we should
immediately drop the unit from the operation and the water channel is evacuated until
the structure is lined off. The problem happened twice until now but we should notethat more structures might actually suffer from it but not yet visible in vacuum gauge.
Practically the more frequent water leakages happened toward outside at mostly the
input coupler side. It is speculated that the route of the leak runs through brazing
joint, touching the cooling water. This leakage is temporally cured from outside by
a glue called “liquid seal”. Since this problem comes from the intrinsic mechanicaldesign of the coupler, the tight EBW/braze joint in the cylindrically non-symmetricstructure, we should admit the rate of the leakage be accelerated. The only practicalsolution is to replace it with a new accelerating structure. It is to be noted that theEBW position is much far from the coupler in KEKB-type structure and it relaxes the
problem. In addition, the KEKB-type structures are in operation for much shorter
period than the PF-type period, so that the KEKB-type structures will be free fromthe problem during the SuperKEKB operation.

11.3.6

How to cope with the deterioration

The most robust way to cope with the above mentioned problems is to replace the badly
behaved structures by new manufactured structures. They are mechanically designed
413

Figure 11.19: Cross section of coupler area of PF-type accelerating structure.

the same as that of PF-type 2-m structure and the detail is described in the following
section.
Since the production takes years and needs non-negligible amount of budget, weshould minimize the needed amounts of the new structures by rescuing some damaged
structures. When the damage occurs in the 4-m PF type structure, the structure canbe converted into two 2-m structures as described above. We can drop the bad one andrescue the well-behaved one in the 4-m structure. Since such converted 2-m structures
are mechanically the same as the PF-type 2-m ones, Fig. 11.14b, they can be used as
the independent 2-m structures.

11.3.7Development for fabrication of new accelerating structure

The realistic and sure solution to obtain the stable acceleration of the linac free fromthe above-mentioned problems through the SuperKEKB period is to make the newaccelerating structures and replace the deteriorated PF-type structures with them.
We have started the development of the first prototype shown in Fig. 11.20 [18]. The
relevant parameters are listed in Table 11.3.
The coupler cell and a half of the next cell is precision machined in a single body.
The regular cells are rounded in their walls to increase their intrinsic Q values andthe shunt impedance, resulting in a higher acceleration gain by 10% than that of414

PF/KEKB-type one. The beam-hole apertures were designed to make the structure
constant-gradient and make HOM (higher-order-mode) frequencies naturally spread sothat the method for curing the multi-bunch (2 bunch) wake field instabilities by the
HOMs is the same as that of PF/KEKB type. The iris connecting the coupler to the
waveguide is precision milled to be all rounded, without any sharp edge. The crosssectional shape of the beam-hole iris was designed in the ellipsoidal shape to reducethe Ep/Eacc (the ratio of surface electric field to the accelerating field) as small as
possible. Thus by avoiding the local high surface field in both electric and magnetic
fields, the more stable performance under high gradient operation is expected. The
structure is going to be assembled in vacuum brazing furnace and dimple-tuned finally.

Figure 11.20: Accelerating structure under development.

415

11.4

RF source

After approximately 10 years of operation of the KEKB, the KEKB injector linac has
been upgraded for use in the SuperKEKB. To archive 40-times higher luminosity by
the nano-beam scheme compared with the KEKB, the electron/positron beam intensities and their emittances required for the linac must be upgraded. To satisfy theserequirements at the linac, a photo-cathode RF gun is installed for the electron injection
to the high energy ring (HER) and a damping ring (DR) is introduced for the positron
injection to the low energy ring (LER).
Furthermore, the KEKB injector linac is used as a multi-purpose injector not only
for the KEKB HER/LER, but also for the photon factory (PF), and the photon factory advanced ring for pulse X-rays (PF-AR). During the KEKB operation period,
simultaneous top-up injection into the three storage rings with different energy (HER,
LER, and PF) was established. In order to realize this operation, the beam injection
parameters to each ring, such as the RF phase, must be switched pulse by pulse witha 50-Hz repetition rate of the linac using an event-based system.
To satisfy the requirements of the SuperKEKB and synchronize with the eventbased system for top-up injection, various upgrades have been conducted from the RF
system of the KEKB injector linac. These upgrades consists of the developments ofcompact klystron pulse modulator using a switching power supply and various lowlevel RF (LLRF) components, such as LLRF control unit and RF monitor unit, theinstallation of new RF sources (photo-cathode RF gun, energy compensation systemat the DR, and bunch compensation system for the DR), and the reconfiguration of
main drive line. Figure 11.21 shows the diagram of the upgraded RF system for theSuperKEKB injector linac. The entire rf system consists of a main drive system, eight
sub-booster systems with two stand-alone rf drivers for the sub-harmonic buncher(SHB), and 60 high-power pulse klystrons with modulators.

11.4.1

Klystron pulse modulator

The rf system requires 60 high-power klystrons, each capable of 50 MW, 4.0 µs pulsed
power at a repetition rate of 50 Hz [19]. The specification of the klystron pulse modulator is summarized in Table 11.4. Fourteen pulse modulators were replaced with acompact modulator to make space for new equipment in klystron gallery. The pulse
modulator is a conventional line-type modulator with a charging power supply, a Pulse
Forming Network (PFN), a thyratron switch tube, and a pulse transformer as shownin Fig. 11.22 [1]. In the conventional modulator, a LC resonant charging power supply
was used to charge a PFN. The compact modulator was replaced it with a switching416

Figure 11.21: Diagram of the upgraded RF system for the SuperKEKB.

power supply, which reduces the modulator size by one-third of the conventional one.
Figure 11.23 shows an overall view of the compact modulator with the klystron assembly and the SLED cavity. The switching power supply was developed by Nihon
Koshuha Co., Ltd in collaboration with KEK [20]. The power supply is designed to beable to deliver 34 kJ/s with a maximum output voltage of 43 kV. The power supply isbased on an IGBT-switched 20 KHz inverter. It has a voltage regulation of less than
0.3% peak-to-peak and is fed with a three-phase ac voltage of 420 V. Table 11.5 shows
the specification of the switching power supply.

Figure 11.22: Simplified schematic diagram of the klystron modulator.

417

Parameter

Value

Klystron
Output powerrf pulse width
Efficiency
Perveance
Beam voltage
Repetition rate
Pulse transfirmer
Step-up ratio
Primary voltage
Primary current
PFN(2 par. PFN)
Impedance
No. of cells
Cell inductance
Cell capacitance
Total capacitance
Pulse width
Thyratron
Anode voltage
Anode current

46 MW
4.0 µs45%
2.1 µA/V 3/2
298 kV
50 pps1:13.522.0 kV4.6 kA
4.6 Ω
40
1.31 µH
15.5 nF
0.62 µF
5.7 µs
44 kV
4.6 kA

Table 11.4: Specification of the klystron modulator.

418

Figure 11.23: Overall view of the compact modulator with the klystron assembly andthe SLED cavity.

11.4.2

Drive system

The main drive system is equipped with a frequency multiplier/divider and newly installed two-frequency (571 MHz and 2856 MHz) phase shifters as shown in Fig. 11.21 [21].
The master frequency of the SuperKEKB injector linac is 571.2 MHz. The signal isgenerated with a signal generator (Keysight; E8663D-HY2), which is synchronized with
the SuperKEKB master oscillator via a 10-MHz reference signal. In the main drive
system, frequency multiplier/divider generates a 2856-MHz signal for the linac fundamental frequency, 571.2 MHz for SHB2, 114.24 MHz for SHB1 and clock signal for the
event-based control system, 10.385 MHz for the beam trigger signal, and 508.8 MHz
Parameter

Value

Maximum output voltage
43 kV
Average capacitor charging power
34 kJ/s
Voltage regulation< 0.3% peak-to-peak
Switching frequency
20 kHzInput voltage
420 V, 3 Phase, 50 Hz AC
Cooling
Water, 5 L/min.
Size
480 mm(W), 760 mm(D), 680 mm(H)
Table 11.5: Specification of the switching power supply.

419

for the monitor signal to the SuperKEKB ring. The frequency multiplier/divider was
manufactured to have a low noise performance to minimize any RF phase jitter. The
block diagram of the frequency multiplier/divider is shown in Fig. 11.24 and the measured phase noise of each frequency is listed in Table 11.6. The phase drift of the signals(114.24 MHz/571.2 MHz/2856 MHz/508.9 MHz) output from the main drive system ismeasured with a sampling oscilloscope (Tektronix; TDS8000) and the measured data
is recorded to the EPICS archiver.
SignalPhase noise (jitter)

SignalPhase noise (jitter)
114.24 MHz 3.2 mdeg. (78 fs)
571.2 MHz
10.4 mdeg. (51 fs)
2856 MHz54.7 mdeg. (53 fs)
508.8 MHz
10.4 mdeg. (57 fs)
Table 11.6: Phase noise and jitter integrated from 10 Hz to 1 MHz of the outputs ofthe frequency multiplier/divider.

For laser oscillation of the RF gun, a supply of clock signals of 10.38 MHz, 114
MHz, and 2856 MHz are required from the main drive system. In the KEKB injector
linac, the switching of the HER/LER beam mode and the injection phase adjusting
was controlled with the phase shifter located in the KEKB ring, as shown in Fig. 11.24.
However, the laser oscillation could not be maintained by the signal phase modulation
caused by the fast ( 50 Hz) beam mode switching.
To establish the compatibility between the laser oscillation and the fast beam modeswitching, the 571-MHz phase shifter was developed and the main drive system wasreconfigured as shown in Fig. 11.24. The 571-MHz phase shifter consists of two I/Q
modulators located in series and an event-receiver (EVR), and its phase modulationis controlled with the event-based system. The first I/Q modulator adjusts the HER
injection phase with the phase modulation speed ( 1 degree/ms) that the laser oscillation is maintained. The second I/Q modulator is used for the injection phase switching
between the HER and LER and for adjusting the LER injection phase.
The positron beam emitted from the DR is accelerated with RF sources locatedfrom sector 3 to sector 5, as shown in Fig. 11.21. However, the operating frequency of
the DR and the HER/LER (508.9 MHz) is not an integer relation with the operating
frequency of the linac (571.2 MHz). Owing to this relation, the positron injection to the
LER is limited by a certain condition. To perform the positron injection to the LER
420

Figure 11.24: Configuration of the main drive system for the (a) KEKB injector linac,(b) SuperKEKB injector linac.

effectively, the 2856-MHz phase shifter was developed based on the LLRF control unit
mentioned latter to change the phase during the positron injection mode. The output
signal is converted into an optical signal with an E/O convertor and transmitted to
sectors 3, 4, and 5.
These components of the main drive system are installed into a thermostatic chamber and placed under a temperature control of 28 ± 0.03◦ C. The thermostatic chamberis composed of two Peltier devices with an endothermic quantity of 250 W and a heaterfor precise temperature control. By adopting a Peltier component instead of a compressor, the RF components located inside the thermostatic chamber are not affectedby mechanical vibrations and a long operational life can be expected.
As shown in Fig. 11.21, 2856 MHz signal is distributed into eight sectors (A-C
and 1-5). In each sector, 60-kW sub-booster klystron supplies power to four or eighthigh-power klystrons as shown in Fig. 11.25 (left). However, for special purposes suchas ones in the electron bunching section including electron gun, positron target andfocusing section, and energy spread adjustment, one high-power klystron is directly
driven with one 600-W solid-state amplifier as shown in Fig. 11.25 (right).
Owing to the recent technological advances and cost reduction in solid-state amplifiers, the cost of one 60-kW klystron and that of eight 600-W solid-state amplifiers necessary for constructing one sub-booster system is balanced. Therefore, 60-kW
klystrons are expected to be obsoleted in the future and replaced with new sub-booster421

Figure 11.25: Klystron drive-line from a sub-booster klystron (left), and independent
drive-line (right).

systems of 600-W solid-state amplifiers, as shown in Fig. 11.26.

11.4.3

LLRF control and RF monitor units

In simultaneous injection at the KEKB linac, the phases of RF pulse are controlled at
50 Hz repetition. To satisfy the requirement for the simultaneous injection, two typeof the digital LLRF unit have been developed [22, 23]. One is LLRF control unit tocontrol the phase of the RF pulse at 50 Hz repetition and the other is RF monitor unit
to measure the amplitude and phase of RF pulse in each beam mode. These digital
LLRF units consist of a FPGA board with an analog-to-digital converter (ADC) /digital-to-analog converter (DAC) board (six 14-bit ADC: AD9254, six 14-bit DAC:
AD9744 for LLRF control unit and ten ADC, two DAC for RF monitor unit) connected
via FMC connector, I/Q modulator/demodulator, and Armadillo CPU board with an
LCD touch panel as shown in Fig. 11.27. The sampling clock of the FPGA, ADC and
DAC is 114.24 MHz, which is the same frequency synchronized with the event-based
system to cancel the sampling jitter. The RF pulse timing and phase inversion timing
for SLED are supplied from the external signal generated with the event system and therecognition of a beam mode and the setting value of RF phase for LLRF control unit arereceived directly with the event receiver (EVR) implemented in the FPGA board via
optical fibre link. The correction for the nonlinearity of I/Q modulator/demodulator
is performed by calculation inside the FPGA using I/Q calibration table stored in
the compact flash memory on the FPGA board. The specifications of the RF pulse
422Figure 11.26: Diagram of the sub-booster system in the KEKB injector linac (up) andplanned for Sthe SuperKEKB injector linac (down).

modulator are shown in Table 11.7. The typical result of the amplitude and phaseaccuracy of the RF output is shown in Fig. 11.28. The amplitude error is 0.02% rms
and the phase error is 0.04deg rms in an amplitude setting of 100%.

Figure 11.27: Block diagram of the LLRF control unit and RF monitor unit.

11.4.4

Application for stable operation

The I/Q component data of waveform measured in the digital LLRF unit are sent to the
EPICS-IOC server at the 50Hz repetition rate through a Gigabit Ethernet implemented
on the FPGA board. The EPICS-IOC server converts from the data of I/Q component
to the data of amplitude and phase and sorts the amplitude and phase for every beammode. The amplitude and phase waveform are monitored with waveform viewer in423Frequency
RF output level
Linearity of amplitude
RF pulse rise time
Phase setting-range
Linearity of phase2856 MHz+10 dBm (100%)
0.3% rms and max < ±0.5%< 35 ns (0 - 90%)
0 - 400◦
0.3◦ rms and max < ±0.5◦

Table 11.7: Specifications of RF Pulse Modulator.

Figure 11.28: Amplitude error (left) and phase error (right) of RF output.

real time and the sampled data of amplitude and phase from waveform are stored in
EPICS archiver.
Figure 11.29 (left) shows an SLED output signal with the waveform viewer. The
upper waveform is the amplitude and the lower waveform is the phase. The user canchoose the monitor signal and the beam mode by a pull-down menu. Figure 11.29(right) shows the time trend of archived phase data and the chosen beam modes are
KEKB e− (KBE) and KEKB e+ (KBE) and no-injection (NIM).
The waveform data is accumulated on the ring memory of the EPICS-IOC server.
When the operation of high-power RF source is stopped by a safety interlock, the wave
form data on the ring memory are archived to confirm the phenomenon and investigate
the cause. Figure 11.30 displays the last and the next to the last event before a highpower RF source suspends due to over VSWR limit. It is recognized that sudden
increase in the backward power of the SLED (SLEDPB) occurred at the last event.

424

Figure 11.29: Display of waveform viewer (left) and archiver (right).

Figure 11.30: Archived waveform of the last and the next to the last event.

425

11.5

MagnetIn this section, the magnets introduced in the upgrade of the injector linac for SuperKEKB
are described and the parameter tables are shown.

11.5.124 degree beam merger line

The two pre-injectors in double-deck layout as shown in Fig. 11.31 are used for SuperKEKB
beam injection. The A1 pre-injector with a photo-cathode RF gun and a magnetic
bunching section in the lower deck is used for an electron injection to HER. The AT
pre-injector with a thermionic gun and an RF bunching section in the upper deck isused for an injection of the primary electron beam for positron production. It is usedalso for the injections to the Photon Factory ring and to the PF-AR rings. The beams
from the AT pre-injector pass through the 24-degree beam merger line into the main
beam line in the lower deck.

Figure 11.31: Double deck pre-injectors of SuperKEKB injector linac.
For the pulse-by-pluse switching of the beams from the AT and A1 pre-injectors,
two pulsed bending magnets are used in the merger line as shown in Fig. 11.32. The
beam line of a pair of the bending magnets and three quadrupole magnets interleaved
between the them is designed to be achromatic and isochronous in the first order beam
optics. For an independent beam optical matching and beam orbit tuning for the beams
from the two pre-injectors, two pairs of the focusing and defocusing quadrupoles andthe two pairs of the horizontal and vertical steering magnets were installed. A pair of
the quadrupoles which is not shown in Fig. 11.32 was installed three meters downstream
from the first pair.
The design parameters of the magnets are shown in the tables; the bending magnets
are type AT J in Table 11.8, the quadrupole magnets are type A1 M in Table 11.9 andthe steering magnet are type A1 M in Table 11.12. The pulsed current operation testsof the bending magnets were performed to investigate temperature rises in the stainlessvacuum chamber and the coils of the magnet. It revealed that the temperature rise426

Figure 11.32: 24-degree beam merger line.
in the chamber was tolerable with a water cooling, however that in the air-cooled coil
exceeded 70 degC at a condition of full pulse repetition of 50 Hz [24]. The bending
magnets are planned to be replaced by those with sufficient cooling capacity.

11.5.2

Full pulse-magnet equipped beam line in Sector-3, 4, 5

In the upgrade for SuperKEKB, all the quadrupoles and the beam-steerings were replaced from DC magnets to pulsed magnets in the Sector-3, 4, 5 which is the region
after the DR till the end of the linac. It is for the independent tuning of the beam
optical matching and of the beam orbit for four beam-modes of different beam energies
and beam parameters (7.0-GeV electron for HER, 4.0-GeV positron for LER, 2.5-GeV
electron for PF and 6.5-GeV electron for PF-AR). The pulsed magnets were installed
as a set of a focusing quadrupole, a horizontal steering, a vertical steering and a defocusing quadrupole magnets as shown in Fig. 11.33. This layout was determined tomitigate the required field gradient for the quads keeping the configuration as a periodic doublet focusing system. The parameters of the quads of type 32 4 used here areshown in Table 11.9 and those of the steerings of type 32 4 are in Table 11.12.
The 13 sets of the pulsed magnets were deployed in the Sector-3, 4, 5 in intervalsof 19.2 m typically as shown in Fig. 11.34.

11.5.3

Positron focusing beam line in Sector-1, 2

In accordance with the upgrade of the positron captures section, the quadrupole focusing system was upgraded in the accelerator-units from #1-6 to #2-3 as shown in
Fig. 11.35. In the unit #1-6 after the solenoidal focusing system of the positron capture427

Figure 11.33: A set of pulsed quadrupole and steering magnets.

Figure 11.34: Layout of the pulsed magnet sets in Sector-3, 4, 5.
section, the 11 quads are used for a beam-optical matching to the subsequent FODO
focusing system. The beam size of the positrons become large for the matching, thequads of type 16 41 in Table 11.10 and the steerings of type 16 5 in Table 11.12 were
designed to have the beam-duct aperture diameter of 60 mm. In the units #1-7 and#1-8, most of the quads were installed outside of the accelerating structures, thus thequads of type 17 11 were design to have the bore diameter of 140 mm, sufficient for
the LAS type structures. The quads of type 17 14 in Table 11.10 placed in between
the accelerating structures have the bore diameter of 44 mm. The quads of type 21 11
have the bore diameter of 120 mm, sufficient for the ordinary S-band structures in theunits #2-1, #2-2 and #2-3. While the quads in the Sector-3, 4, 5 are pulsed magnets,
those in the Sector-1, 2 are DC magnets. Thus, the beam-optical settings are almost
optimized for the positron beam, however it should be compatible with the high-energy
injection electron beam in the common magnet fields. Though the quad settings are428

common to the different beam modes, the beam orbits are independently tunable withthe pulsed steering magnets. The steering magnets of type 16 5, type 17 2, type 12 2
and type 13 2 as shown in Tables 11.11 and 11.12 are used in Sector-1, 2.

Figure 11.35: Layout of the FODO magnets in Sector-1, 2.

429

Bending magnets
type AT J
type 16 C1
pole gap [mm] @center
20
76
pole width [mm]40230
pole length [mm]
154.9
180
yoke outer diameter [mm]200 × 200
540 × 465
coil wire dimension
2 × 6 × 2pair
2×92
coil wire cross section [mm ]2418max. current [A]30060max. field strength [T]
0.67
0.34
coil-wire turns/coil
18
175
coil length along beam line [mm]250
239
current type (DC/pulsed)
pulse
DC
pole type (block/laminated)laminatedblockcoil resistance [Ω] (2coils)@50degC
0.016
0.371
coil inductance [mH] (2coils)
1.00
0.35
interlock (temper.) normal state
open
interlock (water) normal state
open
cooling (water/air)
air
water
cooling water flow rate [L/min]
2.8
effective field length [mm]
175.3
253.3
Table 11.8: Specifications of Bending magnets.

430

Quadrupole (pulsed)
type A1 M
type 32 4
bore diameter [mm]3920pole width [mm]6040pole length [mm]100200
yoke outer diameter [mm]360260
coil wire dimension
2 × 9 × 4pairs 2 × 14 × 2pairs
coil wire cross section [mm2 ]72
56
max. current [A]330
330
max. field gradient [T/m]20*
60*
coil-wire turns/coil
10
8
coil length along beam line [mm]190
303
current type (DC/pulsed)
pulsedpulsedpole type (block/laminated)laminatedlaminatedcoil resistance [mΩ] (4coils)@50degC
5.4
8.0
coil inductance [mH] (4coils)
0.80
1.0interlock (temper.) normal state
close
open
cooling (water/air)air
aireffective field length [mm]
122.4
212.4
Table 11.9: Specifications of Pulsed Quadrupole magnets.

431

Quadrupole (DC)type 16 41 type 17 11 type 17 14 type 21 11
bore diameter [mm]
68
140
44
120
pole width [mm]
80
80
80
80
pole length [mm]
160
300
160
300
yoke outer diameter [mm]
432
552
432
552
coil wire dimension
6.4×6.4 -φ4 6.4×6.4 -φ4 6.4×6.4 -φ4 6.4×6.4 -φ42coil wire cross section [mm ]
28.4
28.4
28.4
28.4
max. current [A]
80
100
80
80
max. field gradient [T/m]
9.0
7.3
17
8.1
coil-wire turns/coil
55
156
47
156
coil length along beam line [mm]
280
530
280
530
current type (DC/pulsed)
DC
DC
DC
DC
pole type (block/laminated)laminatedlaminatedlaminatedlaminatedcoil resistance [mΩ] (4coils)@50degC
101
459
86
457
coil inductance [mH] (4coils)
35
272
32
316
interlock (temper.) normal state
open
open
open
open
interlock (water) normal state
open
open
open
open
cooling (water/air)
water
water
water
water
cooling water flow rate [L/min]
1.5
5.6
1.7
5.6
effective field length [mm]
180.8
334.4
173.7
327.8
Table 11.10: Specifications of DC Quadrupole magnets.

432

Steering (High inductance type)
type R0 0pole gap [mm] @center
44
pole width [mm]70pole length [mm]100
yoke outer diameter [mm]210
coil wire dimension
1×4
coil wire cross section [mm2 ]4max. current [A]10max. field strength [mT]
70.1
coil-wire turns/coil
123
coil length along beam line [mm]185
current type (DC/pulsed)
pulsedpole type (block/laminated)laminatedcoil resistance [Ω] (2coils)
0.6 @45degC
coil inductance [mH] (2coils)
30.00
interlock (temper.) normal stateclose
effective field length [mm]141.5
comment

type 12 235
80
150
300 × 265
2×5
10
10
77
108
254
pulsedlaminated
0.25 @25degC
38
close
190.7
reuse

type 13 2
35
150
150
330 × 280
1×6
5.8
10
71
100
234
pulsedlaminated
0.53 @25degC
57
close
192.3
reuse

Table 11.11: Specifications of Steering magnets (High inductance type).

433

Steering (Low inductance type)
type A1 M
pole gap [mm] @center
39
pole width [mm]80pole length [mm]50
yoke outer diameter [mm]220 × 219
coil wire dimension
1×5
2
coil wire cross section [mm ]5max. current [A]10max. field strength [mT]
22.6
coil-wire turns/coil
36
coil length along beam line [mm]120
current type (DC/pulsed)
pulsedpole type (block/laminated)laminatedcoil resistance [Ω] (2coils)@50degC
0.12
coil inductance [mH] (2coils)
2.0interlock (temper.) normal stateclose
effective field length [mm]91.8

type 16 5
72
140
150
340 × 312
2×5
10
10
9.0
26
228
pulsedlaminated
0.075 @35deg
2.4
open
211.5

type 17 2
3980
80
220 × 219
1×5
5
10
22.8
36
148
pulsedlaminated
0.133
2.6
open
120.4

Table 11.12: Specifications of Steering magnets (Low inductance type).

434

type 32 4
20
60
80
180
1×5
5
10
44.3
36
148
pulsedlaminated
0.12
2.9
open
107.4

11.6

Pulsed Magnet

The KEK injector linac [2] has delivered electrons and positrons for particle physics andphoton science experiments for more than 30 years. Figure 11.36 shows electron andpositron accelerator complex in the KEK Tsukuba site. There are four storage rings, i.e.
two rings for light source, PF and PF-AR, and two rings for electron positron collider,SuperKEKB [3] HER and LER. In addition to them, positron damping ring havebeen in operation since February 2018. All of the rings require full energy injection,
2.5 GeV for PF, 6.5 GeV for PF-AR, 4 GeV for SuperKEKB LER and 7 GeV for
SuperKEKB HER as shown in the Fig. 11.36. To satisfy the requirement, 64 pulsed
magnets were installed in 2017 and 32 more are planned to be installed in 2018. Using
these magnets, magnetic field can be changed shot by shot in 20 ms and was optimized
for each destination.

Figure 11.36: A schematic view of the electron and positron accelerator complex in the
KEK.
There are 8 (A-C, 1-5) sectors in the linac. Each sector is about 80 m long and thetotal length of the linac is about 650 m. Figure 11.37 schematically shows beam energy
along the beam line. To minimize the installation of the pulsed magnets, acceleration
scheme is carefully adjusted to minimize the length of the beam line where the beamwith different energy pass through.
The beam for the SuperKEKB HER, it is accelerated in all of the sectors. The
beam for the PF, it is accelerated in sector A to 2 and decelerated in sector 3 to 5.
The beam for the SuperKEKB LER, electrons are accelerated in sector A to 1 andgenerate positrons at the target located in the middle of the sector 1. Positrons, shownin dashed line, are accelerated rest of the sectors. Since the beam energy is the same435

Figure 11.37: Beam energy along the beam line. Solid line shows electron beam dashedline shows positron beam respectively.
in the sector A to C and first half of the sector 1 regardless of the destination, it is notnecessary to use pulsed magnets. On the other hand, in sector 3 to 5 shaded by light
blue in the Fig. 11.37, beam with very different energy pass through, magnets in these
sectors were completely replaced with pulsed magnets. In sector 2 and a part of sector
1, beam energy of the electron and positron is very different. Magnetic field optimized
for the low energy positron beam affect the orbit of the high energy electron beam
very little. In these section, DC magnets with compatible setting for both electron andpositron beam are used except for several pulsed steering magnets at the moment.

11.6.1

Magnet and support

Several types of magnets were installed in the beam lines as summarized in the Table 11.13. Appearance of these magnets are shown in Fig. 11.38. In sector 3 to 5,
a common unit shown in Fig. 11.39, which consists of two quad magnets (PM 32 4),a horizontal steering magnet (PX 32 4), a vertical steering magnet (PY 32 4) and abeam position monitor located on a motor controlled movable support, was installedat 13 points. Other types of steering magnets were installed in sector 1 and sector
2. Two quad magnets (PM 32 4) were installed in sector 1 just before the positron
production target to focus the electron beam.

436

Type

L
(1 kHz)R

PX 16 5
PY 16 5
PX 17 2
PY 17 2
PX 32 4
PX 32 4PM 32 4

2.4 mH
2.4 mH
2.6 mH
2.6 mH
2.9 mH
2.9 mH
1.0 mH

71 mohm
71 mohm
127 mohm
126 mohm
115 mohm
115 mohm
8 mohmMax. Magnetic
current
field
40 A
40 A
40 A
40 A
40 A
40 A
330 A

1040 AT
1040 AT
1440 AT
1440 AT
1440 AT
1440 AT
60 T/mgap

Installed
Number

72 mm
72 mm
39 mm
39 mm
20 mm
20 mm
φ 20 mm

1
1
4
4
13
13
28

Table 11.13: Several parameters of the installed pulsed magnets.

Figure 11.38: Appearance of installed pulsed magnets.

11.6.2

Pulsed power supply

Concept and overview
There are many requirements for the power supply to truly realize the required flexible
operation. In addition, since the required number of the power supply is not a few,
unit cost, installation space, energy efficiency are crucially important.
437

Figure 11.39: The upper photo shows old DC magnets before replacement. The lower
photo shows newly installed unit.
Trigger rate of our linac is 50 Hz, but injection pattern is not fixed. The destinationof the beam is delivered by the EVENT timing system via optical fiber just after theprevious pulse. To interpret the information and change setting within 20 ms, fast andintelligent control system is necessary. To realize these requirements with minimum
risks and cost, we decide to develop new power supplies by ourselves following theconcepts bellows.
• realize high efficiency with energy recovery• separate power circuit and control system• use commercially available components as much as possible• compatible with existent EVENT timing system• compatible with existent EPICS control system [25]Figure 11.40 shows a block diagram of the pulsed power supply system [26]. One
standard unit which covers 4 quad and 4 steering magnets, shown in Fig. 11.41, consist438

of a PXI express unit for fast control, DC power supplies, pulse drivers and NI cRIO
unit for interlock and slow data acquisition.
There is one computer to control and monitor all of the 64 DC power supplies (DCpower supply control server). A CSS archiver and A Network attached storage are used
to log various data not only output current of the power supplies but temperature of the
IGBT in the pulse driver, status of the DC power supplies and so on. EPCIS channel
access protocol is mainly used for the communication among computers. LXI ver 1.3
protocol is used to control the DC power supplies and NI network shared variable isused to send fast waveform data.
As for the event timing system, dedicated optical fiber is directly connected to themodule. Most of the software is written by LabVIEW 2016 (32 bit) with NI DSC
module and running on Windows 8.1.
CSS archiverNAS

DC power supply
control server

DC power supply

PXI express(fast control)

pules driver

event signal
magnetEPICS CALXI ver. 1.3
NI network shared variablecRIO(interlock and data logging)

operator´s machineLAN

standard unit

Figure 11.40: Block diagram of the power supply control and monitor system.

Pulse driver for quad magnet
Design parameters of the pulse driver for the quadrupole magnet is summarized inTable 11.14. To achieve these values, a novel switching power supply with energy
recovery circuit was developed439

Figure 11.41: A photo of the standard unit for 4 quad and 4 steering magnets.
parametervalue

max current
330 A
max voltage
230 Vstability
0.1%
cooling
water cooled
power consumption
1500 W
repetition
50 Hz
Table 11.14: Design parameters of the pulse driver for quad magnet.
Figures 11.42 and 11.43 show an appearance and a conceptual circuit diagram ofthe pulse driver respectively. The circuit consists of two IGBTs (IGBT 1 and IGBT2), two capacitors (C HV and C LV), two DC power supply (VH and VL) and several
diodes. After the trigger signal arrives, first, both of the IGBTs turn on and current
provided from both of the capacitors pass the load through red lines shown in the upper
figure in Fig. 11.43. When the current reach the set value, IGBT 1 turn off and the gate
voltage of the IGBT 2 is controlled to keep constant output current. Finally, after the
beam pass through the magnet, IGBT 2 turn off. Stored energy in the load (magnet)
is recovered to the capacitor (C HV) through the red line shown in the bottom figure
440

Figure 11.42: A photo of the pulse driver for quad magnet.
Fig. 11.43.
By combination of such energy recovery mechanism and water cooling system, theoutsize dimension of the pulse driver is as small as EIA 3U in height. Output current ismonitored and fed back by two independent DCCTs with hall sensor (LEM LF 1005-S)
which are calibrated by a more precise external DCCT (Hioki 9709).
Pulse driver for steering magnet
Design parameters of the pulse driver for steering magnet is summarized in Table 11.15.
Compared with the driver for quad magnet, output power is very small but bipolar
output is required. To meet with this condition, power operational amplifier (Apex
PA12 [27]) was used for the key device. Although such kind of amplifier is low efficiency
compared to switching power supply, the output power is very small compared to thequad magnet above and the low efficiency affect the total power consumption very
little.
Figure 11.44 shows an appearance of the pulse driver for the steering magnet. Air
cooling was adopted for the simplicity and cost. Usually steering magnets are used
in a pair of horizontal and vertical magnets, there are two circuit in one box of JIS
2U height (99 mm). Output current is monitored and fed back by two independent
precision resister of 1 ohm which are calibrated by more precise external DCCT (Hioki
6862).

441

㻻㻺㻵㻳㻮㼀㻌㻝

㼘㼛㼍㼐㻵㼂㻸㻌㻔㻟㻜㼂㻕

㻵㻳㻮㼀㻌㻞㻯㼋㻸㼂㻯㼋㻴㼂㼂㻴㻌㻔㻞㻟㻜㼂㻕

㻻㻺

㼠

㻻㻲㻲
㻵㻳㻮㼀㻌㻝

㼘㼛㼍㼐㻵㼂㻸㻌㻔㻟㻜㼂㻕

㻵㻳㻮㼀㻌㻞㻯㼋㻸㼂㻯㼋㻴㼂㼂㻴㻌㻔㻞㻟㻜㼂㻕

㻻㻺

㼠

㻻㻲㻲
㻵㻳㻮㼀㻌㻝

㼘㼛㼍㼐㻵㻵㻳㻮㼀㻌㻞

㻯㼋㻸㼂㼂㻸㻌㻔㻟㻜㼂㻕

㻯㼋㻴㼂㼂㻴㻌㻔㻞㻟㻜㼂㻕㻻㻲㻲㼠

Figure 11.43: Conceptual circuit diagram of the pulse driver for quad magnet. Red allows shows the current path. Right side figures shows current profiles at corresponding
states.
parametervalue

max current± 10 A
max voltage± 40 Vstability
0.01%
cooling
air cooled
power consumption 750 W
repetition
50 Hz
Table 11.15: Design parameters of the pulsed driver for steering magnet.

442

Figure 11.44: A photo of the pulse driver for steering magnet.
Control and monitor system
For the fast control, PXI express unit plays major role. The unit shown in Fig. 11.45
consists of a PC and a PXI express chassis (NI PXIe-1082) with four cards, a controller
board (NI PXIe-8381) to connect to the PC, an event receiver board (EVR) (MRF PXIEVR-230), a DAC board (NI PXI-6733), and a ADC board (NI PXIe-6356), which can
set and monitor output current up to 8 pulsed power supply independently in 16 bit
resolution with 1 MSa/s sampling or update rate. They are placed in the same rackand connected by a metal cable.
For the slow data acquisition and interlock system, NI compactRIO (NI cRIO-9063)
unit is used. One unit consists of a 32 ch 250 kSa/s/ch ADC module (NI 9205), a 32
ch TTL digital I/O modules (NI 9403) and two 4 ch relay output modules (NI 9482).
It covers 4ch of the pulsed power supply, namely for one PXI express unit (8 ch), twocRIO units are needed. To realize robust interlock system, cRIO is working not ina scan engine mode but in a FPGA mode. On the upper layer, EPICS CA server isrunning on the NI RT Linux. Not only the PVs for the ADC and DIO data, but thePVs for the high and low threshold, average number to control data acquisition are
also prepared which can be set and read using EPICS CA protocol.
Trigger system
An EVR board in PXI form factor is used to integrate the timing system to thelinac timing system. In our linac, event timing system by Micro-Research Finland Oy(MRF) is adopted. The EVR receive not only the timing information but also themode number and shot ID. The mode number is used to determine the next output
setting. The shot ID is used to tag the monitored information. The trigger timing is443

NAS Qnap TS-431XU-2G

operator´s machineLAN

EVR MRF PXI-EVR-230

DAC NI PXI-6733

PC Windows 8.1
LabVIEW 2016 (32 bit)

ADC NI PXIe-6356

PCIe
Gen2 x8

Controller NI PXIe-8381

chassis NI PXIe-1082

Figure 11.45: Components of the fast control system.
precisely (8.77 ns step) adjusted 3 ms before the beam arrival. Since the rising andfalling time of the magnet current is in the order of ms, pules shape of the power supply
does not important as long as the reproducibility is assured.

11.6.3

Performance evaluation

Efficiency
Power consumption of the pulsed power supply for quad magnet was calculated andmeasured by a power meter (Hioki, PW3335). Blue bars in Fig. 11.46 shows calculated
power consumption from measured resistance and inductance of the magnet and cables
when one magnet was operated at 300 A and 50 Hz without energy recovery. About
15% of energy is lost by ohmic heating of the coil and cable but 85% are stored in
the inductance of the coil. It is in principle possible to recover the latter part of theenergy to capacitors. Green bar in Fig. 11.46 shows measured power consumption in
total including DC power supply. The red allow corresponds to recovered energy. Theenergy recovery efficiency compared with stored energy in inductance and total energy444are as high as 80.1% and 68.5% respectively.

Figure 11.46: Energy balance of pulsed power supply for quadrupole magnet.

Stability
Figure 11.47 shows a stability measurement result of pulsed power supply of quadmagnet for 24 hours. Horizontal axis shows the measured output current and verticalaxis shows the number of entry. Bin width is 0.2 mA Output current was set to alternate
between 0 A and 166 A at 50 Hz. Measured mean value and standard deviation of
the output current were 165.946 A and 14.578 mA respectively which corresponds to
88 ppm (0.0088%) relative stability at this current. This is much smaller than design
value (see Table 11.14).

11.6.4

Operation

After four months installation work from May, comprehensive test was done for amonth in September in 2017. In November, simultaneous injection using the newly
installed pulsed magnets were demonstrated successfully. Since the SuperKEKB phasetwo operation start from March in 2018 and light source rings restart in May, real
simultaneous injection operation started in the middle of May and ended July in 2018
for summer shutdown. During entire period, there was no sever problem and the beam
was delivered to all of the four rings stably.
445

Figure 11.47: A histogram of the stability measurement of pulsed power supply forquad magnet.
Large number of DC magnets was replaced with new pulsed magnets in 2017 forthe pulse-to-pulse beam modulation for 4 storage rings. Two kinds of pulsed power
supplies for quad and steering magnet respectively were developed. Installation andcommissioning of the system were successfull and they are now working in the KEK
electron positron injector linac without any severe trouble for about 1 year.

446

11.7

Alignment

All of the hard wares in the beamline were re-aligned after the big earthquake in March
2011. This was done in the level needed for injecting into PF rings, but the requirement
for the suppression of the emittance growth in injecting to the SuperKEKB main ringis tighter than KEKB so that the alignment should be improved to practically realize
the regime of 0.1 mm in sigma.

11.7.1Requirement for SuperKEKB

The linac is configured as J-shaped as shown in Fig. 11.48. Therefore, the alignmentof the linac consists of those for the two straight lines and one 180-degree turnaround,starting from the gun and ending at the exit point at “E2”.
The bunch charge injecting into the main rings has increased by 4-5 times comparedto the KEKB with the emittance of an order of magnitude low. One of the mostconcerns to suppress the emittance growth of such a high-charge bunch in the linac is
the single-bunch wakefield driven by the accelerator structures. From the emittance
growth simulations assuming random misalignment of the accelerating structures, wefound that the local alignment tolerance should be 0.1 mm in sigma and the global one
0.3mm [28]. The same tolerance is applied for the alignment of the quadrupoles andother hard wares unless specifically required by other reasons. Most of the BPM’s aremounted in the quadrupole magnet bore. The tolerances for the horizontal and verticalmisalignments are set the same because the tolerable emittance growth in both planes
differs only by a factor 2.
In the above requirement, we should define the words “local” and “global” to specify
the practical requirement. Since the emittance growth becomes large when the misalignment pattern along the beam direction coincides with the betatron wavelength,
the tolerance requirement should be defined taking this feature into account. The typical betatron wavelength for electron beam is about 25m for sectors A to B, 50m fromsectors 1 to 2, and 100 m from sectors 3 to 5. Those for positron vary from 7 m at
the beginning just after target to 20m level at the transfer line to DR, while that fromsector 3 to 5 is 50 m. We define the region of the local alignment tolerance to be withina fraction of the wavelength. On the other hand, it is natural that the global alignment
tolerance is applied in the full span of each straight line.

447

11.7.2

Measurement tools

There are two key devices for linac alignment, a commercially available laser tracker
and the laser QPD (quadrant-type Si photo-diode) system [29].
The introduction of the laser tracker, AT401 of Leica, is the most important change
from the era of KEKB to SuperKEKB. It is used for the measurement over 10-20
m, covering one to a few girders. The measurement precision over 10 m is 0.1 mm,
15 + 6×L (microns) based on the specification of the device, where L is the distancein meter. Connecting multiple data sets can be used for the evaluation over the longer
distance but not very long range because many-time connection integrates random andsystematic errors. Rather than connection, the girder alignment over a medium range
up to the betatron wavelength, such as 50 m in both directions giving 100 m in total,
can be measured in one setting (without connection), though the bigger error in thelong distance in mind, but it is to be noted that the linearly increasing error gives thesame angle error even in a long distance.
Another important device is the Laser QPD system. The sensors and the vacuum
chamber are the same as those of KEKB, but the laser passage was improved byproperly shaping the beam with a big profile needed for passing over 500 m and feedingback the laser beam position at the endpoint by adjusting the laser pointing angle.
This system gives the straight reference line rather straightforwardly, but we knowthat the QPD sensors suffer from the gain loss due to the irradiation by beam related
radiation. Actually, 1/3 of the sensors used for KEKB were replaced by new ones.
If the sensitivities of the four quadrants are the same, the calculated position is kept
but we do not know confidently about the variation of radiation damage among four
quadrants. Therefore, we use this system intensively for the initial alignment in theearly stage. Note that we can measure the sensitivity as needed by tilting the laser beam
so that this system can evaluate the relative motion precisely on-line and even after
beam operation years. However, the actual center evaluation, not only the sensitivity,
is needed for the alignment. Therefore, when we need any reference of a long-range
straight line, we need to use this system with confirming the QPD sensor characteristics
beforehand.

11.7.3

Reference frame

The coordinate system for the alignment of the linac is defined as Fig. 11.48. The
nominal coordinate is defined as x-axis the right-ward direction seen sitting on thebeam, y-axis upward and z-axis forming the right-handed Cartesian coordinate system.
Note that the z-axis is in the negative beam direction. There are three regions to be448

connected to make the J-shaped linac. These are the two straight lines, one 120 m line
(AB line) from A1 gun to B sector end and another 500 m line (C5 line) from C sector
to 5 sector connected at a turn-around called J-ARC. This configuration was formed
for KEKB and we use it for the SuperKEKB.
We adopted the mutual relationship among these three regions as was to keepaway from any large modification. The related measurement results and the adopted
reference frame definition are listed in Table 11.16 and shown in Fig. 11.49. The startand endpoints of the longest straight line from sector C to 5 are the end of J-ARC(BM 06 D) and the reference point (E2) connecting the linac to the BT line for mainrings. We tried to set the J-ARC plane to sit in the same plane with C5 line and AB line.
However, the measured inclination angle of C5 line at J-ARC is 69 microrad from thehorizontal surface, which is 19 microrad more than that of the AB line. Note that this50 microrad is the comparable inclination angle to the geometrical one, 40 microrad,
determined by the earth round surface between the horizontal surface at J-ARC andthe 500-m straight line connecting the J-ARC and E2 point. As the practical value
for the SuperKEKB design, we chose this inclination amount (50 microrad), taking
the benefit that we do not need to change the level of the gun area. On the other
hand, the C5 line was defined as the straight line connecting two points, BM 06 D
and E2. It means that we accepted the difference in the inclination angle of C5 line
between measured and design. However, the measurement error may be larger than
this difference, because the measurement device is the laser tracker, which cannot
accurately measure over a long distance and limited by several tens of microradians,
and the measurement distance was over a limited length, only between C1 to C5 torepresent the C5 line. The 500-m C5 line can only be evaluated using the laser QPD
system but was not in full operation as of the reference frame definition in 2013. Now we
can check the validity of the straightness by laser QPD system and the data measured
in late 2017/early 2018 indicates that the line between C1 to C6 is only 2 microrad
different w.r.t. the C5 whole line. However, even now, we cannot evaluate the mutual
inclination difference between AB and C5 lines precisely with only a laser tracker sothat it is not meaningful to discuss it further. We let the C5 line as is only if any needto adjust the absolute inclination angle comes up.
In summary, the inclination angle of the J-ARC was set the same as that of the ABline, while that of the C5 line is about 19 microrad downward toward E2 point. Even
if this difference in the inclination at the J-ARC is real, it gives only 10 mm differenceat the end of linac after passing 500 m so that this difference at the end is not the big
issue.
The opening angle in the horizontal plane between AB and C5 lines was measured449

as 114 microrad less than 180 degrees and we set the J-ARC rotation to symmetricallyshare the mismatch between the tangent of J-ARC and the straight lines, 57 microradat each end, resulting in the rotation angle 180.007 degrees, as shown in Fig. 11.49.
The LTR and RTL lines are made referring to the G4’ point near the down endof the unit #2–8 girder and the upstream points along the linac in the region over 20
meters or so. These reference points and angles were set by measuring the laser QPD
in the C5 line mentioned above.

Figure 11.48: Linac configuration and the coordinate system for alignment presentation. It consists of the two straight lines, the first AB line and the second C5 line, andthe 180 degree ARC, called J-ARC, connecting two straight lines. Typical coordinate
system at each representative point in the linac is shown in the figure.

11.7.4

Nominal hardware setting mechanism and new features
for SuperKEKB

The configuration of the accelerating structures sitting on a unit girder of 8.5 m is
the same as KEKB and shown in Fig. 11.50. Sub-support planes (green plates in the
figure) are precision set by the laser tracker with respect to the two reference points(red dots in the figure) at both ends of the girder shown. The introduction of thesereference points is one of the new features for the SuperKEKB. These points are wellaligned to the laser QPD system. The structures are mechanically set at 300 mm above
the plate and at 120 mm from the reference bar precisely set on the plane.
Another new feature is the direct measurement of the accelerator structure positionby the laser tracker w.r.t. the reference points of the girder. Typical pictures for thereflector setting are shown in Fig. 11.51 (left), where the reflector is precisely mounted
on top of the coupler body of the accelerating structure. Many old quadrupole magnets,
450

(a) Vertical inclination.

(b) Horizontal plane.

Figure 11.49: Relationship among straight lines and turn around. Figure (a) shows
the vertical configuration, while (b) shows the horizontal configuration.

used for KEKB, sit on the independent girders which are aligned w.r.t. the adjacentaccelerator unit girders by extending the arms and positioned by pin mechanism. The
same new feature for the alignment of these magnets is that a precision base for setting
the tracker reflector is mounted on each magnet yoke as shown in Fig. 11.51 (right).
Then we can measure the magnet position directly by using the tracker w.r.t. anyreference points nearby.
All the pulse magnets in sectors 3 to 5 are set on newly developed girders. The
girder position can be measured by the laser tracker by using the reference points
attached to the near-by girders. Another new feature on this girder is the integrated
active alignment mechanism with the movers in horizontal, vertical and rotation along
the beamline.
The hard wares in the new positron generation system are aligned by using the laser
tracker. The hard wares in the electron generation area called AT for thermal electron
gun system and A1 for RF gun system are all aligned with using a laser tracker. The
reference line here is that defined with aligning unit girders in two sectors, A and B.
451

Relation

Value

Comment

Measured and set vertical inclining
50 microrad
Angle w.r.t. the horizontal
angle of J-ARC *surface at J-ARC
Measured and set vertical inclining
50 microrad
Measured over B4-B8 unit w.r.t.
angle of A-B sector line *the horizontal surface at J-ARC
Measured vertical inclining
69 microrad
Measured over C1-C6 unit w.r.t.
angle of C5 sector line *the horizontal surface at J-ARC
Measured and set distance
15.015 mfrom BM 01 U to BM 06 D
of ARC start and endpoints
Measured and set opening angle
2π − 114 microrad Two lines are symmetrically setof two straight lines, AB and C5
w.r.t. J-ARC symmetry axis
* Values with respect to the level surface at J-ARC start/endpoints, measured as of
October 2013 using laser tracker.

Table 11.16: Measurement/set results on reference-plane-related geometries.

The hard wares in the positron production area with target, flux concentrator andbeam spoiler followed by solenoid magnet channel are also aligned with using the laser
tracker with referring to the straight line defined by the unit girder alignment over along distance.

11.7.5

Procedure for hardware alignment

The basic procedure of the alignment of the hard wares is in three steps. The first step,
initial alignment of the girders, is performed by using the laser QPD system, where

Figure 11.50: Unit girder equipped with laser QPD at each end with an arm forextending alignment fiducial to the place where the tracker laser beam can access.

452

Figure 11.51: Direct position measurement point with laser tracker for all relevant hardwares.

the laser pass can define a straight line. The reading error is as small as 0.03 mm, but
the systematic error is speculated to be an order of 0.1 mm or more. The girders forthe quadrupole magnets from KEKB stage are aligned on a support structure restingon the adjacent accelerator unit girders with using mechanical pins or bars. On thecontrary, the new pulse magnet (quadrupole magnets and steering ones) supports are
independent on but aligned w.r.t. the adjacent accelerator girders by using a laser
tracker.
In the second step, or frequently earlier than the girder alignment process, thebeamline hard wares are set on the girder, mechanically referring to the reference bar
and plate sitting precisely w.r.t. the girder reference points. This mechanical alignment
error in this process is assumed ±0.05 mm. The positions of the reference bar andreference plane are measured using laser tracker w.r.t. reference points attached to
the girder as shown in Fig. 11.50. In addition, the hardware positions can directly bemeasured and confirmed and corrected if needed. We checked the stability of the hardwares on the girder. Most of the cases, the variation of the position w.r.t. the girderreference points is within 0.05mm. Therefore, we do not realign the hard wares unlessnoticing any suspect of movement on the girder.
In the third step, the hard wares are directly measured among each other, in addition to measure w.r.t. the reference points on the girder, with using the laser tracker.
This is the process to estimate the hardware position information over a short distancein a most-precise manner, though not referred to the girder reference points.

453

11.7.6

New beam-line hard wares for SuperKEKB

In the alignment viewpoint, there are three most significant differences of SuperKEKB
from KEKB. One is a double-deck electron source area with a thermionic gun and two
photo-RF guns, the second a new positron generator at different position in linac than
KEKB, and the third an introduction of positron DR.
The thermionic gun is elevated by 750 mm above the nominal beamline at 1200
mm above the floor. The two RF-guns are located at the nominal level, one in straight
position along beamline and the other injected by 45 degrees from aside. These doubledeck lines merge after acceleration by an angle of 24 degrees. The alignment of thehard wares in this area should be better than others especially due to the low beamenergy. The alignment of this area can be measured by using the laser tracker so that
we set the alignment tolerance of this area to be 0.1 mm, based on the capability ofthe measurement accuracy at a distance of 10 m spreading over 20 m scale.
For the positron production, heavy positron production hard wares were moved
towards upstream by 40 m from the old (KEKB) location at the unit #2–1 to the newlocation (SuperKEKB) downstream of the 13 unit. By the end of January 2016, theradiation shield for this new positron production system, one the heaviest hard wares( 40 tons) in the linac was completed and set at unit 15, spreading over 5 meters. In
the same area, heavy hard wares, solenoid magnets with the weight of the order of 30
tons in total, spread over 15m from the unit 15 to 16, sitting on the aluminum girders.
Two new beam transport lines, to (LTR) and back (RTL) from DR, were createdconnected to the linac at a space between sectors 2 and 3. The old positron production
area, unit 21, was now converted to the standard acceleration unit, but the weight did
not change much because the radiation shield was integrated to the tunnel ceiling andwas kept. Therefore, we do not expect any big change in this area due to the heavy
load change on the floor. In turn, many heavy large-bore quadrupole magnets weredensely located from unit 17 to 21. Due to the construction of LTR and RTL, the area
between the units 28 and 32 was used for placing bending magnets and chicanes.

11.7.7

Refining alignment

The basic alignment information is obtained during any maintenance period by the laser
QPD system. One of the typical recent measurement results is shown in Fig. 11.52.
The calibration factors for each QPD sensor are assumed to be the same as thoseobtained one year ago. We should admit that the calibration factors are subject tothe beam radiation and others and may vary. We assume from various evidence andexperiences that the error is well below 1 mm but not as small as 0.1 mm level.
454

Figure 11.52: Recent laser QPD measurement result along C5 line.

The above is the result of the initial alignment and we do not adjust each girderalignment to zero the values. As the next stage, the girder alignment over a medium
range is measured by a laser tracker. An example of the measurement in summer 2018is shown in Fig. 11.53. Here each measurement spans over several tens to 100 m range
and the overlapped regions are used for connection to estimate the whole feature. If
we subtract smooth sinusoidal variation with the wavelength longer than 200m, muchlarger than the betatron wavelength at any place, the residual deviation is 0.5mm insigma. Taking this residual information into account, we can improve the smoothnessof the girders within much less than 1 mm over 100 m range by adjusting such area as
those shown in the figure indicated by unit numbers. This local misalignment correction
on the girder position is proceeded by the laser tracker measurement with a precisionof ±0.1 mm over 20 30 m range.
On top of this improvement of the girder alignment, we can use the direct measurement of the hardware positions among each other to further improve the smoothness
in a short distance over 10-20 meter range with a precision of ±0.1 mm, limited by the
specification of the laser tracker device.
The beam based position measurement of quadrupole magnets can be applied foralmost all of them. Assuming the mechanical precision of the magnet w.r.t. themagnetic center, we deduce the alignment information of the magnet. It will give us
more precise alignment information among quadrupole magnets and the result can beused for improvement of the magnet alignment. We can also deduce the reference point
position attached to the girder with respect to the quadrupole magnet measurement.
In principle, it will serve to improve the girder alignment.
Such information of the direct alignment information and the indirect one basedon the beam should be integrated and used in an optimized manner until reaching the455goal.

11.7.8Floor movement and compensation

Once any golden alignment is obtained, the rest of the work is to keep it. The underneath base blocks, where all of the linac hard wares sit, extend over 80 m or so inbeam direction and are supported with many piles of longer than 30m reaching to thefirm soil. However, the very surface of the floor on which most hardware girders sit
has only 150 mm thick concrete plate which is paved on the underneath base. Due tothis feature, we suspect the very surface plate to move rather easily. Moreover, the underneath blocks are loosely connected for stress release purpose at the expansion joints
and we know such joint areas are subject to the big mutual movement of a fractionof a millimeter. Considering this situation, we should design the way how to keep thealignment of the hard wares sitting on such a moving floor.
We measured the floor movement by using the laser QPD system for long period [30].
Figure 11.54 shows the overlay plot of all of the measurements in vertical direction over
two years from May 2016 to June 2018. The vertical lines show the position of the
expansion joints. The range of the vertical movement near the expansion joint is morethan those of the area away from the joints. This feature in the vertical direction
shown in Fig. 11.54 is more evident than that of the horizontal direction. From thismeasurement, we see the big motions of a fraction of a mm near the expansion joints
which show a yearly cyclic feature, in contrast to the relatively quiet area away from the
expansion joints by more than 10 m, as shown in Fig. 11.55. Since the daily movement
is about 50 microns and the evolution speed over a week or month is at most of theorder of 0.1 mm per week, a slow feedback, such as once a day, is enough to compensate
the motion.
Thus, we obtained a gross feature of the floor movement. Then the most criticalissue is the practical definition of the reference for the position feedback but we should
admit that any practical method to define such feedback point is yet to be realized.
What we have in mind now is the following. We can use online monitoring information,such as the laser QPD system with active insertion mechanism, the beam position
monitor and the stretched wire/water leveling system. For the first two we have alreadythe prototype hard wares but we need to develop the practical feedback protocol.
For the last one, wire/water, the technology should be robust at least in a medium
long range, 100 m or so, but we need some time to implement into the linac system.
Assuming that we obtain such information well, we can feed back the girder positions
toward the golden position found at some earlier time.

456

11.7.9

Active moverOnce we define the reference position as mentioned in the preceding section, we movethe girders. We have developed the mover for the pulse magnet girder and installed in
the sectors 3 to 5. The specification of the mover resolution is 0.01 mm and the actual
positioning capability was found better than a micron level. We are also developing
unit accelerating structure girder.

11.7.10

Resistivity against earthquake

Most of the injector linac for SuperKEKB suffered from the big Earthquake in March
2011. Linac is aligned in north-south direction and the long-lasted vibration made theaccelerator structure girders swing in a large amplitude. Since the relation between
the spring constant of the girder support and the weight load on the girder differs
in many places, the resonantly excited swing frequency differs from a girder to thenext. It resulted in a pi-mode oscillation between some girders and many of the hardwares in between suffer from damage and falling. Taking this problem in mind, we
have installed the mechanism of fixing or limiting big movement on the unit girders.
Actually, the girder center position is grabbed by a mechanism firmly fixed on the floor.
The movers mentioned above should be stiff enough to replace this mechanism when
actively controlled.

457

(a) Horizontal direction.

(b) Vertical direction.

Figure 11.53: Eye-connected ensemble of long-span laser-tracker measurements in (a)horizontal direction and (b) vertical direction. It is to be noted that in these plots thelong-range bending feature over 100 m or more should not be understood as actual
because it may come from the systematic error in measurement, due to the device andin the environment, etc

458

Figure 11.54: Ensemble of twenty data sets taken during two years, between May 2016
and June2018, measured by the laser QPD system.

Figure 11.55: Vertical position over two years measured by laser QPD system. Moremovement with yearly-cyclic feature is seen in bluish points near expansion joints (ajoint between unit 38 and 41 and another between 48 and 51) comparing to the reddish
points in calm area away from joints.

459

11.8

Electron source

In order to realize different injection demands, one thermionic gun and two RF guns areused from SuperKEKB phase I, as listed in Table 11.17 [31]. The thermionic gun (ATGun) is adopted for generated electron beams for Photon Factory ring (PF), Photon
Factory Advanced Ring (PF-AR), as well as for positron beam generation. Meanwhile,
because the SuperKEKB project requires the electron beam for High Energy Ring
(HER) with high charge and low emittance, the RF gun (A1-Gun) is adopted as theelectron source for generating high-quality electron beam. To allow for smooth operation of SuperKEKB, a secondary RF gun (AS-Gun) is installed in 2017 as a backup
for the main RF gun. All of these three guns are installed in the A sector of linac, as
shown in Fig. 11.56.
Beaminjection

Electron beamsourceElectron charge
at gun (nC)

Beam energy
(GeV)

HER
LER a)PF
PF-AR

RF gunThermionic gun
Thermionic gun
Thermionic gun

5 b)
10
0.3
0.3

7
4
2.5
6.5

a) For positron beam generation.
b) Phase III requirement for electron beam generated by RF gun.
Table 11.17: Electron beam generation source for different injection mode.

Figure 11.56: Linac schematic from electron beam source to beam transport (BT) part.

For the purpose of reserving the low emittance of the electron beam which is generated by the RF gun, the RF gun is installed in the main beamline. Accordingly, thethermionic gun, sub-harmonic buncher (SHB), pre-buncher (PB), buncher and accelerator structures are raised to the second floor (750 mm higher than the central beamlineof the first floor). As shown in Fig. 11.57 a 24◦ line with 3 quadrupole magnets andtwo bending magnets are used for combining the electron beam, which is generated by
the thermionic gun, into the main beamline at the first floor.
460

Figure 11.57: Layout of the thermionic gun, RF gun and the combination part at linac.

The layout of two the RF guns is illustrated in Fig. 11.58. The main RF gun(A1-Gun), quasi-traveling wave structure gun (QTW), is selected as main RF gun
for SuperKEKB commissioning. It is installed in main beamline for generating 5 nC
electron beam with low emittance. Laser impinges into the QTW RF gun with anangle of 60◦ . The secondary RF gun (AS-Gun), cut disk structure RF gun (CDS),
is installed with a 90◦ angle with the beamline. Two bending magnets are applied toconverge the electron into the beamline for acceleration. Due to the 90◦ angle, the laser
beam injects into the CDS RF gun normally. A deflector, operated in HEM11 mode,
will be added to measure the slice emittance of the electron beam in the pre-injector
part between the two bending magnets [32]. Both of these two RF guns have the ability
to generate more than 5 nC electron beam with less than 20 µm emittance [33].
For reducing the injection time into SuperKEKB rings, especially for the LER it isbeing inevitable to increase positron intensity as the stored beam increases, 50 Hz twobunch beams are required for both thermionic gun and RF gun.

11.8.1

Thermionic Gun (AT-Gun)

As to SuperKEKB commissioning, the main mission of thermionic gun is to generated
10 nC electron beam for the generation of positron beam (about 7 nC electron charge at
positron target positon). The schematic layout of thermionic gun and pre-injector partis shown in Fig. 11.59, and the parameters are listed in Table 11.18 and Table 11.19.
The triode-type thermionic gun is under operation at 200 kV accelerating voltage.
It is connected by a 300 mm long ceramic insulator with ICF-253 flanges. The cathodeis a commercial EIMAC Y-796 product of 2 cm2 . 20 nC maximum charge can begenerated with a pulse width of less than 1 ns. A high speed grid pulser is employed
for such a high current which has a superb pulse feature of small time jitters (σ = 4 ps)
as well as short pulse lengths. An electric analogue time delay module is also used that461

Figure 11.58: Layout of two RF guns at linac A1 unit.

continuously varies the gun trigger timing over a 3 ns range. This function is adopted
for a digit feedback system to stabilize the beam timing. The grid pulser also controls
the beam repetition rate with external trigger signals, as well as the beam intensity bychanging the output voltage [1].
As listed in Table 11.19, The SHB frequencies are chosen from subharmonics near
100 and 500 MHz with the condition that each frequency should be a multiple of thelinac/ring common frequency of 10.385 MHz which is a 5 × 5 × 11th sub-harmonic of2856 MHz, resulting in the 25th and 5th subharmonics. The first SHB operating at
114.24 MHz (25th subharmonic) and the second SHB at 571.2 MHz (5th subharmonic)
are both a standing-wave type re-entrant cavity with a tuning knob. The dimensions
are given in Fig. 11.60 (a) and (b). The rfs are fed by corresponding 10-kW solid-state
amplifiers through coaxial cables.
As shown in Fig. 11.61, the prebuncher and the main buncher are both travelingwave type disk-loaded waveguides in which the phase shift is 2π/3 per cell. Since thegun beam is to be velocity-modulated at around β = 0.7, all of the cell lengths are
βλ/3 in the prebuncher, where β is the velocity in units of the light velocity, and λis the free-space wavelength of the accelerating RF (10.5 cm). In the main buncher,however, the cell length varies from βλ/3 to λ/3 during the first 6 cells, because the gun

462

Figure 11.59: Schematic layout of pre-injector part at linac A1 unit.

Element

Repetition or
frequency

PeriodVoltage or
power

Grid pulserGun

1-50 Hz
50 Hz-

450-800 V
200 kV

25th SHB1 (standing wave)
5th SHB2 (standing wave)
Pre-buncher (traveling wave)
Buncher (traveling wave)
Accelerating sections (traveling wave)
Common frequency of SuperKEKB

114 MHz571 MHz2856 MHz
2856 MHz
2856 MHz
10.385 MHz

8.75 ns
11 kW
1.75 ns
7 kW
350 ns1 MW
350 ns23 MW
350 ns
12M W × 2
96.289 ns-

Table 11.18: Parameters for pre-injector part.

beam has to be accelerated to form a tight bunch as well as be modulated in velocity.
As shown in the figure, the KEK prebuncher and the main buncher are assembled
into one united body in order to reduce the distance between them and to suppress
the divergence due to a strong space-charge force. The number of cells is four in the
prebuncher; although that in the buncher is 33, bunching occurs through only the firstfew cells and the β of the beam reaches almost unity.
For stable operation, the bunch timing or phase with respect to the accelerating RF
wave must be fixed as well as the bunch shape. If the status of the gun and the bunching
section would become unstable, the bunch timing and shape would be changed. The
tolerances for the devices are severe in the case of a high-intensity beam of 10 nC, as
are shown in Table 11.20. The stability in the operation meets the requirements owing
463

Beam current
Grid pulse
Bias voltage
Grid/cathode distanceHeater power
Cathode temperature
Acceleration voltage
Gun pulse duration
Beam duration
Beam time jitter
Residual gas pressure
ML magnetic field

0 15 A-300 -700 V+300 +700 V
180 µm40 W
950◦ C
200 kV
4 µs
1 ns
8 ps
1 × 10−7 Pa
900 G

Table 11.19: Parameters for thermionic gun.

to digital feedback loops. The tolerances on the devices came from the goal of keeping
the beam transmission rate above 90% at the target.
Beam timing
Gun high voltage
SHB1 phase
SHB2 phase
Buncher phase±45ps±0.38%
±1.1◦
±1.3◦
±1.7◦

Table 11.20: Tolerance to stabilize a high current electron beam.

11.8.2

RF Gun (A1-Gun and AS-Gun)

The main mission of RF gun is to generate required 5 nC electron beam with lower
than 20µm normalized emittance for SuperKEKB HER. As shown in Fig. 11.58, two
RF guns are employed. The main RF gun (A1-Gun) is installed on the beamline forgenerating high charge electron beam with low emittance. The secondary RF gun (ASGun) is perpendicular to the beamline. It acts as a backup for smooth commissioningof SuperKEKB.
As shown in Fig. 11.62, the main RF gun is a quasi-traveling wave structure (QTW).
Two side coupled cavities are staggered for achieving two standing wave fields. As
464

Figure 11.60: Configuration of SHB1 and SHB2.

Figure 11.61: Configuration of prebuncher and buncher.

shown in Fig. 11.63, these two standing wave side coupled cavities are independent
electromagnetically. By feeding RF power with π/2 phase difference, the acceleration
field inside the RF gun is similar to a traveling wave for electron beam acceleration.
Because it is possible to design the two side coupled cavities coaxially, a quasi-travelingwave is available. The quasi-traveling wave can realize beam acceleration efficiently andstrong focusing [34].
The QTW RF gun has seven acceleration cavities. These are divided into two
standing wave structure of 3 and 4 side coupled cavities respectively. There is no
coupling between two adjacent cavities on the main axis of RF gun. Figure 11.64 shows
the simulated electric filed in the RF gun. The maximum electric filed strength in thefirst cavity (cathode cell) and regular cells are 120 MV/m and 100 MV/m separately.
The strong focus filed in the first cell is very helpful to realize low emittance because465

Figure 11.62: Configuration of QTW RF gun.

the beam size and emittance with low energy in cathode cell initially is affected by
space charge easily. Meanwhile, the beam tracking simulation for 5 nC result is shownin Fig. 11.65. At the exit position of QTW RF gun (z = 250 mm), the beam emittanceis 5.5 mm-mrad with a beam size of 0.4 mm. It is also obvious that the beam size
becomes smaller gradually inside RF to the strong focusing electric field. The beamenergy will achieve 11.5 MeV under 20 MW RF input. And the energy spread is
0.6%. In addition, the QTW RF gun has the ability to generate 10 nC electron beam
with1.2 mm beam size, 10 mm-mrad emittance and 1.0% energy spread. All of theseparameters fulfill the requirements for linac of SuperKEKB.
The secondary RF gun (AS-Gun) is simple a cut disk structure (CDS) with large
coupling value for improving RF conditioning efficiency, as shown in Fig. 11.66. Low
RF power (7 MW) is enough for acceleration with low risk of discharge. In addition,
the choke configuration is helpful to solve the breakdown problem for the photocathoderod. As shown in Fig. 11.67, the electric filed and magnetic field are weak around thephotocathode rod thanks to the choke configuration, it is good for keeping high vacuum
level inside the RF gun.
The beam tracking simulation for 5 nC electron beam results are shown in Fig. 11.68.
All the results show the CDS gun can generate qualified electron beam for SuperKEKB.
In addition, different from the QTW RF gun, the CDS gun has bigger aperture (8 mm),and the laser beam can impinges into the photocathode with an angle of 0◦ or 60◦ .

466

Figure 11.63: Quasi-traveling wave generated by the side coupled cavities.

Figure 11.64: Designed RF gun cavities.

11.8.3

PhotocathodeFor SuperKEKB smooth operation with long term, the RF gun photocathode is critical.
It must possess high quantum efficiency to generate high electron charge to fulfill the
5 nC charge requirement. Meanwhile, the lifetime must longer than 1 year under 106 vacuum environment. Therefore, iridium cerium metallic compound is selected as
the photocathode for RF gun. Iridium cerium has a low work function of 2.57 eV at
1300 K and a high melting point (1900 ◦ C 2500 ◦ C). Moreover, the iridium cerium
compound possesses long life time thanks to its good resistance to poisoning and ion
bombardment. The iridium cerium photocathode with a diameter of 8 mm is shownin Fig. 11.69, as well the quantum efficiency map of the Ir7 Ce2 photocathode which isused in SuperKEKB RF gun. The highest quantum efficiency is about 1.49 × 10−4 [35].
In order to enhance the quantum efficiency during the SuperKEKB operation time,
an electron beam heating type photocathode plug is design, as shown in Fig. 11.70.
The photocathode is bombarded by an accelerated electron beam as a thermal source

467

Figure 11.65: Beam tracking simulation result of emittance, beam size and energy
spread for 5 nC and 10 nC electron beam.

which is emitted behind the photocathode. This heating method can be used for alonger term than the other heating methods without any maintenance since it is notnecessary to heat the filament with a high temperature. 1000 ◦ C heating temperature
can be realized during SuperKEKB commissioning. In addition, the new photocathodeplug is designed with a choke structure for RF shielding, as shown in Fig. 11.71.

468

Figure 11.66: Configuration of CDS RF gun.

Figure 11.67: Distribution of the electric field and magnetic field near photocathode
rod.

469

Figure 11.68: Beam tracking simulation result of emittance, beam energy, beam sizeand energy spread for 5 nC electron beam.

Figure 11.69: Ir7 Ce2 photocathode and the measured quantum efficiency map.

470

Figure 11.70: Electron beam heating type photocathode plug for SuperKEKB RF gun.

Figure 11.71: RF shielding simulation result for electron beam heating type photocathode plug choke structure.

471

11.8.4

Laser source

The electron beams with a charge of several nC and a normalized emittance of lessthan 10 µm are expected to be generated in the photocathode RF gun for injector linacof SuperKEKB accelerator project. To high-current, low-emittance beams generation,
the laser source with mJ pulse energy, center wavelength of 259 nm and a pulse widthof 20 ps are needed at the A-1 unit (Fig. 11.4). Especially, the pulse shape should be
reshaped to rectangle structure. Also, the high reliability and remote control system
is required for long-term operation.

11.8.5Laser typeFor high peak power ultra-short UV pulse generation, the most common approach is toemploy the nonlinear processes of solid-state Ti:Sapphire lasers. A Ti:sapphire laser isusually pumped with another laser with 532 nm, for which frequency-doubled Nd:YAG
lasers are used. However, the low energy-extraction efficiency and heavy maintain limit
the application for long-term running.
In contrast, laser diodes (LD) pump laser structure offers high efficiency for reduction of the laser price and maintenance costs. Especially, the Yb-doped fiber laser fiber
laser has attracted attention as one of the promising practical alternatives to usual
solid-state lasers. The stability of the fiber laser is evidently higher than the othersolid lasers with simple cooling conditions because of high energy-extraction efficiency
of fiber. But, because of the core diameter size limitation, the fiber structure is difficult realized the mJ level amplification. Then from µJ to mJ energy amplification,
the Yb:YAG or Nd:YAG solid states amplifier with LD pump should be employed.
Yb:YAG crystal has large absorption bandwidth to reduce thermal management
requirements for diode lasers, a longer upper-state lifetime, three to four times lower
thermal loading per unit pump power, which is more suitable for diode-pumping forhigh power diode-pumped lasers. Compared with the Yb:YAG crystal, commonly used
Nd:YAG rods with high optical homogeneity, high damage threshold, consistent performance and high processing accuracy. Although the pulse shaping of Nd:YAG is difficult
to be adjusted because of the narrow spectrum gain, the structure of the Nd:YAG is
simple, that means reliable convenience and less maintenance. The Emission wavelength of Yb:YAG and Nd:YAG are around 1030 nm and 1064 nm with the bandwidthof 3.0 ns and 0.5 ns, which contained in the spectrum of Yb-doped fiber oscillator.

472

11.8.6

Optical design for ground laser (Yb fiber + Nd:YAG)

The on the ground laser system consists Yb-doped fiber oscillator, Yb-doped fiber
amplifiers and Nd:YAG rod laser amplifiers. The detail of laser part that shows in
Fig. 11.72.

Figure 11.72: Layout of on the ground laser system.

Fiber oscillator
The laser system starts with 114 MHz mode lock Yb-doped fiber oscillators [36]. Threedifferent structure Yb-doped fiber oscillators are employed, two of them are commercial
products (Menlo system), and the other one is homemade cavity with the all normal
dispersion (ANDi) structure.
As the Fig. 11.73, the ANDi type oscillator is developed with a spectral filter inside
the ring fiber cavity. By increasing the nonlinear phase shift accumulated by the pulse
and inserting a spectral filter in the cavity, self-amplitude modulation via spectral
filtering is enhanced. The pulse duration and chirp decrease in the spectral filter.
One MEMS optics switch is adopted to select oscillator for the following amplifier
stages (Fig. 11.74).

473

Figure 11.73: ANDi type Oscillator.

Yb-doped fiber amplifier
The nJ level seed pulse with center wavelength of 1064 nm is selected by MEMS opticalswitch. A transmission grating pair stretcher was employed to expend pulse to 10 ps
and select the spectrum width of 0.5 nm with the centre wavelength of 1064 nm. The
weak pulses reduced repetition rate to 10.38 MHz by a semiconductor optical amplifier(SOA) pulse picker, which was driven by a 5 ns 100 mV electrical pulse at 10.38 MHz
amplified through a high power RF amplifier. To compensate the energy loss of thegratings and SOA, two stages Yb-doped single mode fiber amplifier was employed [36](Fig. 11.75). Because the amplified spontaneous emission (ASE) easy to be amplified
with the weak seed pulse by strong pump power, an additional transmission grating
was used to separate and cut the noise by a slit. After fiber amplifier, an Electro-optic
module (E.O.) adopted as a pulse picker is used to pick out the 1-25 Hz signal from asequence of pulses of 10.38 MHz.

474

Figure 11.74: MEMS optics switch.

Nd:YAG amplifier partIn order to guarantee the injection and commissioning reliably, two beam lines wereprepared. At the end of the fiber part, the signal was separate to two equal parts (firstbeam line and second beam line) by a polarizer (Fig. 11.72).
The Nd:YAG lines which has 5 stages Nd:YAG rod amplifiers is proposed to provide
the stable laser pulses with high quality. 2×79 mm Nd:YAG rod crystal are used in thefirst and second stages for high amplification efficiency [37, 38]. A Pockels cell pulse
picker is insert to switch the single/double bunch mode. After the Pockels cell, thethird and fourth stages amplify the pulse energy up to the 4.0 mJ by using of 4×93 mm
Nd:YAG rod crystal. The firth stage (Φ6 mm) is built to pursue higher pulse energy.
The final pulse energy is up to 8.0 mJ by five Nd:YAG amplifier stages. A beta barium475

Figure 11.75: Fiber amplifier system.

borate (BBO) crystal is used to second harmonic generation (SHG) stage. The pulse
energy of SHG beam is more than 3.8 mJ in both sides.
Transporting line
There is about 12 m distance from the laser clean room to the underground RF gun.
The transport line is focus to reduce the energy loss and jitter in the air. Therefore,vacuum tube was built to isolate the air turbulence between the laser room and RF
box [38]. Two laser beams are converged by a thin film polarizer and transported to
RF gun box by one transporting line. Inside the RF gun box, the laser beams are
separated again by another polarizer. The transport rate can be up to 80%. UV pulses
were generated by BBO frequency-doubling stages. 500 µJ are achieved by the twolaser lines to RF gun. The optics system is shown in Fig. 11.76.
The QTWSC RF gun could be irradiated by each laser sources with 60 degree, andanother laser source works with the backup line. Furthermore, it is optional to selecttwo laser injection with both sides. A delay line is inserted into the second laser line toadjust the optical path for realizing two laser synchronous injection into RF gun. Also,a flip mirror is placed in the first laser line for changing the transmission direction to
the 90 degree CDS RF gun.

476

Figure 11.76: Transporting line.

11.8.7

Optical design for Underground laser (Yb fiber + Yb:YAG)For Yb:YAG crystal amplifier, the broadband gain spectrum is expect. To avoid thenonlinear effect, the chirped pulse amplification (CPA) method has emerged as a usefultechnique for the generation of high energy pulses. Therefore, the signal pulses should
be firstly stretched to longer pulse width, and compressed to 20 ps after amplification.
As the Fig. 11.77, the pulse that generated by Yb-doped fiber amplifier is separated
to two parts. One part of the signal is transport to underground laser room by a 15m
long PM fiber. Pulse width is adjusted to 1 ns in a chirped volume Bragg grating(CVBG) stretcher. After several fiber amplifier stages and Yb:YAG thin disk amplifier
stage, the mJ pulses is reshaped to 20 ps in a CVBG compressor.
CVBG
In CPA systems where stretching is provided by a CVBG, the same device with reverse
incidence can also be used for compression (Fig. 11.78).
The reflection band of the compressor CVBG (OptiGrate) is centered at 1030 nm
and exposes a width of 3 nm. The antireflection (AR)-coated, rectangular shaped
CVBG (aperture: 10 × 8mm2 ) with a length of 60 mm exposes a stretching factor of
600 ps/nm. Signal pulses are stretched to 820 ps and the measured reflectivity amounts
to 90%.

477

Figure 11.77: Layout of Underground laser system.

Figure 11.78: Layout of CVBG stretcher.

The Yb:YAG thin-disk amplifier
For high energy amplification, the thin-disk laser optimizes heat removal, which makes
possible the high energy pulse amplifier [39]. Because the thickness of the gain crystal is
only 1 mm, to increase the gain, multi-pass or regenerative amplify structure is neededto added the round trips. As the Fig. 11.79, a multi-pass amplifier inset into a cavity
was designed. The inject signal beam amplified 10-15 times pass through the Yb:YAG
thin-disk crystal with single loop. After the two loops, the amplified pulse went out
the cavity by the reflection of polarizer.

478

Figure 11.79: Multipass amplifier.

11.8.8Synchronization system

The frequency of the seed pulse signal that generated by fiber oscillator is locked
to 114.24 MHz with the cavity length control. Firstly, the phase shifter controller
system is performed to synchronize the laser pulses with both 114.24 and 2856 MHz
RF frequency. Then, frequency of the laser signal is reduce to 10.38 MHz by the SOA
section, which is also synchronized with 10.38 MHz of RF frequency.
After fiber amplifier system, two E.O. pulse picker are employed to pick the laser
signal to low frequency (1-25 Hz, double bunch). And a Pockels cell is employed toswitch the single/double bunch signal. Both of the sections are synchronized with the
Event generator (EVG) signal that produced by the event timing system.
For final amplification, 10 pump modules are used in the Nd:YAG amplifier section.
All the modules is also synchronized with the EVG signal. For the thermal stability,
the frequency of pump is fix to 25 Hz. The detail is shown in Fig. 11.80.

11.8.9

Monitoring and remote control system

Since the electron beam with high stability and quality are required in commissioning,
the status of laser system and RF cavity are monitored and recoded.
In oscillator, the stepping motor and piezo that adjust the frequency in the cavity
is controlled to synchronize RF frequency signal. Status monitors and I/Q monitor
are employed for detect the mode-lock and synchronization. When the RF frequency
is drift, the laser frequency can be auto-adjusted immediately.
The temperature of the laser room and RF gun cavity and all the current of thepower supply are monitored and recorded. Furthermore, the pulse output power of the479

Figure 11.80: Layout of trigger system.

every stages and the pulse shaping of the every pulse pickers are also monitored byphoto diode detectors. The long term reliability of laser are recorded, and any unusualchanges will be alarmed by monitoring system.
The optics system inside RF gun box is set near the electron gun. A beam profile
monitor and laser energy meter are set in the two laser lines to oversee the laser status.
The real-time data can be confirmed on internet anytime. For optimize the laser sourcein the commissioning study, lots of adjustment unit were insert, such as telescope lenses
distance, BBO crystal angle, X/Y stage and rotation stage. All of these units areadjusted by fully remote control. Thus, we can get the best condition to generatehighest electron charge during beam study. Figure 11.81 shows the monitoring andremote control system panel by Windows-PC.

11.8.10

Pulse shaping in time domain

In order to get low energy spread in a strong space charge effect, laser pulse withrectangular temporal shape is required. Temporal shaping scheme by pulse stacking
technology is developed for Nd laser system. The pulse is directly stacked up using

480

Figure 11.81: Monitoring and remote control panel by windows.

birefringent effect of a crystal array (Fig. 11.82). Output laser pulse with temporal
rectangular shape is restricted from the input Gaussian shape pulse.

Figure 11.82: Pulse stacking technology.

481

11.9

Postron Source

11.9.1

Layout of positron source in KEK linac

The positron source of the KEK electron/positron injector linac experienced two major
upgrades, at first in the upgrade from Photon Factory/TRISTAN injector to KEKB
injector and recently in the upgrade to SuperKEKB injector. At the time of TRISTAN
injector, it had a 400-m long 2.5-GeV main linac and a 70-m long 0.25-GeV linac
dedicated to positron production. The main linac had five sectors of accelerator eachhaving eight accelerator-units and the positron production linac had six acceleratorunits. The first three units in the positron linac accelerated a primary electron beam
up to 0.25 GeV to irradiate a positron production target, the fourth unit was used for
positron capture and the remaining two units accelerated positrons up to 0.25 GeV.
The positron beam was deflected 30 degrees toward the main linac and merged in the
middle of the Sector-1 (at the unit #1-5).
In the first upgrade to KEKB [14], the positron beam intensity was required toincrease by 20 times (0.032 → 0.64 nC/pulse) to achieve high luminosity. The positronsource (the positron production target, the pulsed solenoid coil and the DC solenoids
in the positron capture section) were relocated to the Sector-2 of the main linac (at theunit #2-1) to increase primary electron beam energy from 0.25 GeV to 4.0 GeV. It isbecause the produced positron intensity is proportional to the primary beam energy.
The position #2-1 was selected because it has heavy concrete in the ceiling between
the beam line tunnel and the klystron gallery. It was advantageous in view of shielding
against radiations from the target. For the injection to the KEKB electron storage
ring (HER) at 8.0 GeV and to the positron ring (LER) at 3.5 GeV, the injector linac
was reformed in J-shape by reusing the facilities in the positron linac and by extending
the building. Three new sectors (called as Sector-A, B, C) were added. The 8.0 GeVelectron beam energy was achieved with the 55 accelerator units which have typical 0.16
GeV energy gain for HER injection with sufficient energy margin. With the position of
the positron source at #2-1, the 26 accelerator units were allocated for an accelerationof the primary electron beam up to 4.0 GeV and the remaining 29 units were used for
positron acceleration up to 3.5 GeV with sufficient energy margin for LER injection.
In the recent upgrade to SuperKEKB, the beam injection energy was changed to
7.0 GeV for HER and 4.0 GeV for LER. Since the positron beam emittance for LER
injection is required to be two orders of magnitude smaller compared with the KEKB,
a positron damping ring (DR) is introduced beside the linac building. The positionof the extraction and re-injection of the positrons is determined to the region between
the Sector-2 and the Sector-3 because it has a beam switch-yard (SY2) convenient
482

for construction of the beam transport line to the DR. The beam energy of the DR
was determined to 1.1 GeV in balance of the radiation damping time and the ringcompactness. Details of the DR is described elsewhere. The position of the returnfrom DR at #3-2 was chosen since the 22 accelerator units are secured for accelerating
positrons up to 4.0 GeV. However, if the positron source is remained at #2-1, we have
only six units for positron acceleration except for the capture section before the DR
and it is not sufficient for 1.1 GeV injection. Thus, the relocation of the positronsource became necessary. We decided to relocate the positron source to #1-5 where wehave an available wide area for the equipments around the target, originally used forthe beam merging from the positron linac in the TRISTAN period [40]. The primarybeam energy is 3.3 GeV there with the beam energy 1.5 GeV at the J-arc section.
If definitely needed, the J-arc beam energy can be increased up to 1.7 GeV, so somedesign parameters were optimized at 3.5 GeV primary electron beam energy. We haveten units for the positron acceleration up to 1.1 GeV before the DR with sufficientenergy margin. However due to the selection of this location, it is required to build aheavy radiation shield structure inside the tunnel. Details of the shield is described in
section 11.9.6.

11.9.2

Positron intensity upgrade for SuperKEKB

Figure 11.83: SuperKEKB positron source
In the upgrade from KEKB to SuperKEKB, the positron beam intensity was required to increase by four times (1 nC/bunch → 4 nC/bunch). Because it was difficult483

to increase primary beam energy or intensity from a viewpoint of the cost, we took acourse for increasing the capture efficiency of positrons produced at the target. At the
KEKB positron source, a air-core pulsed solenoid coil (peak field 2.3 T, field length
65 mm FWHM) was used for transverse phase-space matching for positrons emerging
from the target. This short high-peak field worked as a Quarter-Wave Transformer
whose positron-energy acceptance is narrow. For SuperKEKB, we introduce a flux
concentrator (FC) type of pulsed solenoid (3.5 T) and DC bridge coils (1.0 T) which
generate a slowly varying field from a 4.5 T peak field to a long solenoid field (0.4 ∼ 0.5T) generated by DC solenoids as shown in Fig. 11.84. Details of the FC are describedin section 11.9.4.

Figure 11.84: Field distribution by FC, bridge coils and DC solenoid
Focusing with this kind of field profile called as ”Adiabatic Matching” has the broad
positron-energy acceptance. It results in the larger longitudinal phase-space acceptancefor positrons.
The positron capture section is a pre-accelerator to capture positrons from the
FC by the solenoidal field and accelerate them before passing to regular accelerator units. Transverse phase-space acceptance of the capture section is determinedby the solenoidal field strength and the aperture of the accelerating structures inside
the solenoids (U ∝ Ba2 ; U:acceptance, B: field stregnth, a:aperture radius). For theSuperKEKB capture section, large-aperture S-band accelerating structures (called as
LAS-type structures) are introduced. They have the aperture diameters (30 mm),larger than those (typically ∼20 mm) of the regular S-band structures. Details of the
LAS structures are described in Section 11.3. While the KEKB capture section usedfour accelerating structures (two 1m-long and two 2m-long structures) whose total ac484

celeration length is 6 m, the SuperKEKB capture section uses six LAS structures (2min each length) whose total acceleration length is 12 m. The positron beam energy
from the capture section is raised from 70 MeV to 120 MeV. It improves the positronenergy-spread and the beam transmission efficiency to the subsequent accelerator unit(#1-7). The field distribution of the solenoids covering the capture section is shownin Fig. 11.86. The field dips are caused by solenoid gaps for the waveguides of theaccelerating structures. The capture section is composed of two accelerator units. In
the first unit (#1-5), the klystron feeds an RF power of flat pulse shape directly into
the two accelerating structures in the capture section. In the second unit (#1-6), theRF power from the klystron is compressed by a SLED cavity and is fed into the four
structures. The unit #1-5 also has a SLED cavity, but it is detuned and not usedbecause RF breakdowns happened in the structures.

Figure 11.85: SuperKEKB positron capture section layout
The RF phases of the accelerating structures in the capture section are set so as
to decelerate positrons from the FC. The simulation study reveals that the positrons
captured by the decelerating phase contains smaller numbers of particles in the energy
distribution tail, which results in less beam background at the DR injection [41]. In thecase of acceleration capture, the longitudinal distribution of the positrons in a bunchdoes not change and the particles in the energy tail remains the same. On the other
hand in the deceleration capture, since the positrons are decelerated, their longitudinalvelocity becomes slow and starts to slip from the velocity of the microwave in the
structure. As the phase slip continues, the positrons finally reaches to the acceleratingphase and the phase slip stops. In this phase slipping process, compression in thelongitudinal phase space occurs and the positrons contains less energy tail particles.
Particle tracking simulation studies were performed to evaluate the expected positronyield, that is the ratio between the positron beam intensity and the primary electron
beam intensity. Positron production in the target was simulated with the GEANT4[42], magnetic field distribution by the FC, the bridge coils and the DC solenoids were485

Figure 11.86: Field distribution by Bridge coils and DC solenoids (FC field not included)
evaluated with the CST EM STUDIO [43], the particle motion in the capture section
was simulated with the GPT code [44] and the beam transmission was evaluated with
the SAD code [45]. Figure 11.87 shows the expected beam size and the positron yieldfrom the exit of the capture section till the end of the Sector-2 for the case of theparameters satisfying the specification (the primary electron beam energy 3.5 GeV,
the FC current 12 kA, the bridge coil current 600 A, the DC solenoid current 650 A,
the acceleration field in the unit #1-5 14 MV/m, that in the unit #1-6 12 MV/m, thedeceleration capture and the offset positions of the target and the FC taken into account). The positron yield at the end of Sector-2 (that is the entrance of the transportline to the DR) is 0.39, which corresponds to a positron beam intensity 3.9 nC/bunch
for a primary electron intensity 10 nC/bunch. The positron yield dependences upon
the parameters of the capture section were also estimated with the simulations. The
positron yield at the exit of the capture section was evaluated with the GPT code invarious cases of the FC and the bridge coil currents as shown in Fig. 11.88.

11.9.3

Positron production target and beam passage hole

The positron production target is made of tungsten because of its large cross sectionof the electron-positron pair creation and its high melting temperature. The length of
the target is 14 mm (4.0 X0 radiation lengths). The tungsten is attached in a copper
support by hot isostatic pressing. A cooling pipe is wound around the copper support
with a radius of 20 mm as shown in Fig. 11.89.

486

Figure 11.87: Beta-functions, dispersions and beam size of positrons in design optics
and positron yield
Even the stationary heat is removed by the cooling water, due to repetitive energy
deposition in the target by the intense primary electron beam, the target could be
potentially damaged by fatigue breakdown. To avoid the target destruction, the peakenergy deposition density (PEDD) should be well below the critical limit (35 J/g)
based on the SLC operation experience [46]. For the case of 3.5-GeV primary electron
beam with a charge of 20 nC/pulse (10 nC/bunch × 2 bunches), a spot size (σx , σy )of primary electron on the target entrance should be larger than 0.7 mm with a factor
two of the safety margin. To avoid the spot size to become too small, a beam spoiler
made of an aluminum-oxide screen (0.14 mm, 0.002 X0 ) and aluminum foil (0.25 mm,
0.028 X0 ) is installed three meters upstream from the target to enlarge the spot sizeby multiple scattering [47]. The spoiler is removable and is used in the situation where
the PEDD approaches to the critical limit.
In the KEKB period, pulse-by-pulse switching of electron injection to HER andpositron injection to LER has begun. To achieve the switching, a small hole was madein the copper support beside the tungsten target. Electron beam orbit is switched bypulse steering magnets making an electron beam for HER to pass through the hole,while the primary electron beam for positron production strikes on the target. In the
KEKB positron source, the tungsten target was placed on the central axis of the beam
487

Figure 11.88: Positron Yield dependence on Capture section Parametersline, while the hole had a few mm offset from the axis. For SuperKEKB, the injection
electron beam generated with a photo-cathode RF-gun has low emittance and the holeis required to be on the axis to avoid an emittance growth by the orbit excursion in
the accelerating structures. This makes a strict constraint on the layout of the target,
the hole and the FC. Because the aperture of the FC at the entrance is only sevenmm in diameter, the target and the hole should be contained within the aperture. At
first the diameter of the hole is determined to be 2.0 mm assuming an rms radius of
the spot size of the injection electron beam is 0.5 mm. Then, the diameter of thetungsten is determined to be 4.0 mm keeping a 0.5 mm thickness copper layer between
the tungsten and the hole, to avoid direct exposure of the target surface to the hole.
As a result, the center of the target has 3.5 mm offset and the axis of the FC aperturehas 2.0 mm offset from the central axis of the beam line as shown in Fig. 11.90.

11.9.4Flux Concentrator

FC has been used for positron focusing at BINP [48], SLAC [49], IHEP [50] and otherfacilities. The design of the FC for SuperKEKB is basically the same as those of SLAC
and IHEP. They have a slight difference. While the FC of the SLAC operated at a pulse
repetition of 60 Hz, the FC of the IHEP operated at 50 Hz has slight different dimensionof the central conductor copper block to make the resonant vibration frequency to beaway from 50 Hz. The pulse repetition of the SuperKEKB linac is 50 Hz and thedimension of the FC is the same as that of IHEP.
488

Figure 11.89: Target of SuperKEKB positron source/,9<-$&7*9'#!"# $%&';,90(,)&$1?5&@A

E77$'F&#%,+7%+,$#;*9'$,%(,-$%(&)(*$%&'#*'$6*9<!"#$%!&
56"-%!%#
7+/&-!"#$%!&'(()%!
*+,&--

$8
/$(0
2*'$

96:$7%9*6&$1
3'+4&
5*67$6%,(%*,;+'#$&=>.+/&-01&'(()%!

8'9%56"-%!%#.+/&-2+/&-01&"3%#!4#%;+'#$&@A

Figure 11.90: Target, FC offset layout
The FC is made of a copper cylinder block of 108 mm in diameter and 100 mm inlength and it has a conical hole inside along the axis whose hole diameter are 7 mm at
the entrance and 52 mm at the exit as shown in Fig. 11.91. The cylinder has a spiralslit processed by discharging wire-cut at KEK and formed as spiral coil of 12 turns as
described in reference [51].
A copper pipe is brazed around the cylinder along the spiral and serves as a pipe
feeding a pulsed current and as a water cooling pipe. When a pulsed current of 12
kA at peak in half-sine shape of 8 µs is introduced to the conducting pipe, an eddy
current flows in the inner surface of the FC. The magnetic flux is concentrated inside
the conical hole and forms a strong solenoidal field (3.5 T at peak). The positrons
emerged from the target are focused by this solenoidal field.
Each turn of the copper spiral is isolated by 0.2 mm gap of the slit. At both ends
489

Figure 11.91: SuperKEKB FC dimensionof the spiral slit, it runs in radial direction. Because of the radial slit, the magnetic
field has transverse component and axially non-symmetric. This transverse magnetic
field component gives a ‘transverse kick to the positrons.
As described in section 11.9.3, the target has 3.5 mm offset and the FC has 2.0 mm
offset from the beam line axis. These offsets degrade the positron yield, however it canbe mitigated by utilizing the transverse kick. With a simulation study, the orientationof the radial slit was determined to optimize the yield [52].
The front surface of the FC is placed 2 mm apart from the exit of the target to
avoid discharging between them. Due to an eddy current in the copper part of target
support induced by a time-varying magnetic flux from the FC, magnetic field by the490

Figure 11.92: SuperKEKB FC photo
FC is almost zero at the target. The positrons feel only DC field by the bridge coils at
the target and they feel strong field after entering in the FC as shown in Fig. 11.84.
The design of the pulse modulator for the FC is based on the klystron modulator of
the KEK linac, except that the pulse-forming-network for flat output pulse is replacedwith a capacitor bank for a single half-sine pulse and two sets of the thyratron switch
and the capacitor bank are used [53, 54]. The specification of the modulator is describedin table 11.22. The inductance of the modulator and the power transmission line could
be comparable to that of the FC (1 µH) and should be minimized to reduce induced
high-voltage. The ten parallel coaxial cables (18 m long) are used in the transmission
line from the modulator to the linac tunnel [55]. In the tunnel the cables are connected
to copper three-layer parallel plates (2 m long, 250 mm width, 35 mm plates distance).
The plates are connected to copper two-layer parallel plates (0.54 m long, 75 mm width,
35 mm plates distance). At the connection to the FC, the copper plates with fork plugs
are used for quick disconnection.
The copper plate transmission line is used to make the terminal of the coaxial cables
distant from the high-radiation area because a non-radiation-resistant material is usedin the terminal. A CR snubber circuit (2.5 Ω, 100 nF) is connected in the middle of the491

LengthOuter diameter
Inner diameter (min.)Inner diameter (max.)
Peak current
Peak field
Inductance

100 mm
108 mm
7 mm
52 mm
12 kA
3.5 T
1.0 µH

Table 11.21: Specifications of SuperKEKB FC
copper plate line to compensate for the pulse wave form deformation by a reflection
due to mismatching to the load.
Peak current
Pulse width (half-sine)
Load (FC) inductance
Total inductance
Capacitance
Charging voltage
Pulse Repetition rate

12 kA
8 µs
1 µH
∼ 3.35 µH
1.4 µF
17 kV
50 Hz

Table 11.22: Specifications of FC modulator

11.9.5

Quick exchange system of FC and target

The residual radiation dose from the target will be much increased because the timeintegrated target irradiation beam power is higher at SuperKEKB operation than thosefor KEKB. An excessive exposure dose of workers in the exchange of the target, theFC and the accelerating structure in the capture section is a serious problem. The FC
assembly and the exchange mechanism are designed to minimize the exposure dose.
The assembly is composed of the FC, the target, the vacuum chamber, the bridge
coils and the return yokes. The assembly can be exchanged in short time without
dismantling the component. It is advantageous because the the bridge coils and thereturn yokes surrounding the target works as a shield and the radiation dose outside
the assembly is relatively low. In the connecting part of the power cables of the bridge
coils, the water cooling pipe of them, the signal cables for interlocks and the signal
cables of thermal sensor to the assembly, the multi-coupling plates of Staubli company
is adopted. They can connect and disconnect these cable and pipe connections in a492

single action. The power feed line to the FC is connected by copper plates with forkplugs. These plates can be disconnected in a short time. The FC assembly has wheels
and mounted on rails. It is easy to move the assembly to the position for lifting up by
a crane as shown in Fig. 11.93 (left)

Figure 11.93: Quick exchange mechanism of FC and target
For the quick exchange work and minimizing the exposure dose, jigs for the exchange
and a trailer with a radiation shield are prepared. For a case of exchange of the FCbase which contains only the endplate, the return-yoke head, the target and the FC,
jigs with shield walls are also prepared as shown in Fig. 11.93 (right). The entrance
part of the first accelerating structure in the capture section is also highly radioactive
after the beam operation. Jigs with a radiation shield for exchanging the structure isalso prepared.

11.9.6

Radiation shield structure

As describe in section 11.9.1, the ceiling (2.5 m thickness) between the klystron gallery
and the tunnel at #1-5 where the SuperKEKB positron source is installed, is madeof normal concrete unlike the heavy concrete at #2-1 for the KEKB positron source.
493

Simulation studies using the MARS code [56] showed that an additional shield iron1.25 m width, 500 mm thickness, 6.0 m length was required to keep the radiation doseat the klystron gallery below the limit 20 µSv/h of the radiation controlled area forthe 1250 nA full-specification primary beam intensity. A support structure by 11 ironpipe pillars sufficient for this weight and additional supports for suppressing vibrations
were designed and installed in the tunnel [57, 58]. In the first stage, shield irons were
installed in 2.0 m width, 100 mm thickness, 6.0 m length in the first layer, in 1.25 m
width, 100 mm thickness, 6.0 m length in the second layer. The shield was composedof pieces whose weights are less than the load limit of a crane, 1000 kg.

Figure 11.94: Improvement of Radiation dose by increasing shield thickness
The shield thickness was increased step by step in 4.0 m length region and theradiation doses were measured in each step as shown in Fig. 11.94. It was shownthat the dose with 400 mm thickness at 200 nA beam intensity was 2.2 µSv/h. whichcorresponds to 14 µSv/h at 1250 nA. The thickness 400 mm is sufficient for the region
straight above the target. However, to make the radiation dose outside the linac
building to be below the limit 0.2 µSv/h, an additional shield of 2.0 m width, 400
mm thickness, 4.0 m length (including the first-stage layers) and a side wall (0.8 m
height, 100 mm thickness, 4.0 m length) were installed as shown in Fig. 11.95. The
resonant vibration frequencies of the shield structure were calculated to be sufficiently
above the frequency of earthquake vibration by simulation studies and confirmed bymeasurements.

11.9.7

Beam separator and secondary-electron stopper

A beam from the capture section contains not only the positrons but comparable
amount of secondary-electrons produced in the target. These electrons prevent mea494

Figure 11.95: Radiation shield structure
surements of the positron beam position and charge due to cancellation of the signals
in the beam position monitors. They are accelerated up to 1.1 GeV, and lost at theentrance of the beam transport line to the DR and make beam line components radioactive unnecessarily. To eliminate the secondary electrons, a beam separating chicane
with four bending magnets and a beam stopper were installed after the exit of thecapture section as shown in Fig. 11.96. The orbits of the positrons and the secondary
electron (∼ 120 MeV) are separated approximately 70 mm in the chicane. On thepath of the secondary electrons, a beam stopper block made of tungsten-copper alloy(tungsten 70%, copper 30%) of 76 mm thickness (∼ 15.0 X0 ) is inserted. The block isretractable using a pulsed motor which is radiation tolerant (Empire Magnetics company). The position of the block is adjusted not to prevent the electron beams for
HER injection.

Figure 11.96: Beam separator and Secondary-electron stopper495

11.10Machine operation and safety

11.10.1

Overview

The basic function of the linac safety system is to ensure that the injector linac is
operated only when a pre-determined set of personnel and machine safety conditions aremet [59, 60]. The required safety conditions are symbolically summarized in Fig. 11.97.

Figure 11.97: Safety conditions to be met before beam operation of the injector linac
is allowed.

The whole set of beam generating equipment for the linac is housed in an underground tunnel which is designated as radiation controlled area. The tunnel is built withshielding capability and containment measures to keep the radiation in non-controlledareas below the levels specified by the KEK radiation safety regulation. Beam operation is allowed only when no personnel is present there (“All personal keys returned”)
and all the entry and exit doors for the underground tunnel are closed (“All tunnel
doors closed”).
High power klystrons which produce the microwave power required for operating thelinac are installed at the surface level (called, “klystron gallery”) of the linac building.
Since the klystrons also produce a certain amount of X-rays during operation, all
the entry points to the klystron gallery are closed during operation, so that only theregistered radiation workers, if any, can be present in the klystron gallery (“All gallery
doors closed”).
496Since the linac situates at the upstream end of other electron/positron accelerators
at the Tsukuba campus of KEK, all the downstream accelerators have to positively
give their beam permission (called “downstream READY”) signals to the injector linac(“All downstream accelerators READY”), before the linac starts beam operation. It
may mean that the downstream accelerator actually intends to accept the beam fromthe linac and it is ready for that. It may also mean that the downstream accelerator
intends to allow personnel into their accelerator tunnels for maintenance and otheractivities and it has completed a prescribed set of preparatory arrangement, such as
beam stoppers being “in” and a designated set of major beam bending magnets being“off” in the beam transport line etc, so that it knows that it can guarantee that the
linac beam does not affect the safety of any personnel and equipment there.
The linac safety system is also linked with environmental radiation monitors at the
ground level in the perimeter of the linac system. Reports of radiation levels fromthese monitors have to stay below the limits set forth by the internal rule of KEK andnational regulation (“Environmental radiation monitors OK”). Signals from a selected
set of beam current monitors along the linac beam lines are also fed to the safetysystem. The beam currents (product of beam charge and repetition rate) have to staywithin the range (“Beam charge monitors OK”), as authorized by Nuclear Regulation
Authority. If the accumulated beam current approaches the authorized limit, the linac
safety system issues a warning and stops beam operation before exceeding the limit.
Additionally, the linac safety system is linked with certain status signals from thelinac beamline equipment, such as those from vacuum gate valves and large bending
magnets, so that the beams do not inadvertently hit vacuum system components to
produce unintended radiation or radio-activation.
With all other conditions met, the safety system requires that an operator uses a
special key called “beam key” on the console and physically turns it to activate the
Linac READY status (“Linac READY key turned at console”). Incorporation of thisactive human action eliminates the possibility of accidental automatic start-up of beam
operation.
The linac safety system ensures that if any of the conditions above are violated, thebeam operation of the linac immediately stops. If for any reason one of the emergency
stop buttons are pressed by an operator in the operation room or by personnel accidentally trapped in the tunnel, the linac beam operation also immediately stops. In thefollowing, some key points in implementation of this linac safety system are described.

11.10.2

System layout

Figure 11.98 shows a sketch of the linac safety system.
497

Figure 11.98: Schematic layout of the linac safety system.

A programmable logic controller (PLC) manufactured by Yokogawa Co. serves
as the main CPU for managing the safety logic (main safety PLC). The main PLC
communicates with two sub PLCs, and these three safety PLCs are responsible forcollecting the safety-related signals directly from the linac equipment via metal cables
and for performing logical evaluations of the combined status of the injector linac.
The communications among these PLCs are made through dedicated digital I/O
lines, without relying on the local area network which are used by the rest of the control
system of the injector linac.
Table 11.23 shows the destinations and numbers of linac interlock signals.
The linac safety system communicates with downstream accelerators and receives
downstream READY status signals from them, too. For instance, the SuperKEKB has
its own safety PLC, and its status information is fed to the linac main safety PLC aslisted, among others, in Table 11.23.
The status information of the linac safety system is made available to the linac
control system through additional PLCs (two communication gateway PLCs). The
communication gateway PLCs are connected to the main or sub safety PLCs via PLC
links, and are also connected to the local area network (LAN) of the linac control
system. This arrangement allows to display the status of the safety system at operators’
terminals along with the rest of hardware status of the linac in a unified graphics userinterface, as shown in Fig. 11.99. Yet the use of dedicated safety PLC links ensurescontinued availability and operation of the safety system, even during possible failuresof the control system LAN.
At this moment the injector linac can be operated either from the linac control

498

Destination of signal

Number of I/Os

Personnel
protection

20 / 08/0
0 / 1050 / 172/1
17 / 36
10 / 036 / 015 / 0
4/2
4/5
6/5
4/6
1/0
1/0
1/0
1/0
1/0
1/0

X
X
X
X
X
X
X
X

Doors
Emergency switches
Beam trigger system
High power RF PS
Electron guns
Operation console
Charge monitors
Vacuum gate valves
Vacuum gauges
PF-AR
PF-ring
Damping Ring
SuperKEKB
Radiation monitors
Focus coil and e+ target interlock
Bending magnet status
Sector B beam dump water flow
Tunnel clear
Maintenance mode

Machineprotection

X
X
X
X

X
X
X
X
X
X
X
X
X
X

Table 11.23: Destination and number of linac interlock signals at the central CPU.

room located near Sector 1 of the linac, or from the SuperKEKB control room. For
this reason another PLC is prepared for remote-controlling the linac safety systemfrom the SuperKEKB control room, and it is linked with the linac main PLC (seeFig. 11.98).

11.10.3

Operator console

As described in the discussion on Fig. 11.97, an operator must turn the “beam key” on
the console to bring the linac safety system into the Linac READY status when otherrequirements are met. As shown in Fig. 11.100 this console is prepared as a physicalpanel, separated from the graphical user interface of the computer control system of
the linac. It can be manipulated only by hand in the linac control room.

499

Figure 11.99: Typical display of the status window for the linac safety system.

Figure 11.100: Image of the latest control panel. The beam key is used to switch thesafety status from OFF, RF OK and Linac READY.

11.10.4

Personnel safety management

To account for personnel who enter the underground linac tunnel, personal keys (Fig. 11.101)
are used at the injector linac. When a radiation worker wishes to enter the linac tunnel
while the injector linac is not in beam operation, each individual worker must first use
her registered card key on the key bank to pull out a personal key. This action is
automatically logged by the safety system. The access door to the underground tunnel
can be opened with this personal key. After completing the work in the tunnel, theworker must return the personal key to the key bank. Otherwise, the system assumes
that the worker still could be in the tunnel, and “All personnel keys returned” status500

is not made up. The “All personnel keys returned” status is realized only when all the
personal keys are physically returned to the key bank, and only then, with all otherrequired condition met, the Linac READY status can be realized.

Figure 11.101: Personal keys used for entrance to the linac tunnel and the key bank.

11.10.5

Transitionary period during linac upgrade

In the course of preparation toward SuperKEKB, the injector linac had to go throughimmense renovation work both in the underground tunnel and in the klystron gallery
while electron beams had to be kept available for operation of PF and PF-AR. To copewith this situation in the period between September 2010 and May 2017, the linac
tunnel was treated as two distinct radiation generating equipment rooms, upstream anddownstream sections, partitioned by a shield wall introduced at the downstream endof sector 2. The upstream section included sectors A through 2, while the downstream
section consisted of sectors 3 through 5 and the beam switchyard.
In the downstream section a temporary thermionic electron gun was installed at
the upstream end of the sector 3, and electron beams generated there were accelerated
up to 4 GeV by using klystrons within the downstream section. The beam energy of 4
501

GeV was sufficient for the PF which operates at 2.5GeV and acceptable to the PF-AR
which could accelerate the injected 4GeV beam up to 6.5GeV for user operation. The
shield wall separating the upstream and downstream section was built thick enough so
as to allow presence of personnel in the upstream section while the downstream section
was in beam operation for PF and PF-AR. This arrangement allowed construction of
new RF gun at the upstream end of sector A, excavation of additional new tunnels
for beam transport lines to/from the damping ring, and other work while the PF and
PF-AR operation is supported.
During this transitionary period the door at the shield wall between the upstream
and downstream sections incorporated interlock sensors which communicate their status with the safety system. Also, two sets of key banks were used so as to deal withhuman access control to upstream and downstream sections of the injector linac separately. However, in the summer maintenance period of 2017, when beam transportlines with the new damping ring were mostly completed near the end of sector 2, thepartitioning of upstream and downstream sections was abolished. The partitioning
shield wall and the temporary thermionic gun at sector 3 were removed then, and since
the Fall of 2017 the entire linac tunnel has been treated, again, as a single radiation
generating equipment room in terms of radiation safety.

11.10.6

Operation of electron guns

As described earlier in this design report, the present electron source for the injector
linac includes a thermionic gun and RF guns. In case of the thermionic gun, the “DChigh voltage (DC-HV) applied to the gun electrodes” can be reasonably equated to
the “beam on” status in the language of the safety system. Thus, the thermionic gun
DC-HV-on was allowed only after the linac safety status turns to “Linac READY”.
In case of the RF gun, since it has no DC-HV, per se, “RF power applied to thegun assembly” is equated to the “beam on” status in terms of the safety system (seeFig. 11.102).
In addition, a “Laser shutter” is incorporated in the laser beam path and a gate
valve is introduced on the beamline as shown in Fig. 11.103. The gate valve has to
be closed during human access to the tunnel during maintenance so as to guarantee
human safety. The safety system incorporates a logic where it demands both “Linac
READY” and “gate valve open” before the RF source for the RF gun can be applied.

502

Figure 11.102: A sketch of RF source and laser beam used to generate beams with an
RF gun.

Figure 11.103: Requirements for operating the RF gun.

11.10.7

Latest status

The linac safety system, compatible with operation of SuperKEKB together with thedamping ring, PF and PF-AR has been in active service since Fall, 2017. It has beenreliably functioning to ensure the safe operation of the linac and to control accesshuman entries to the linac tunnels for maintenance activities.

503

Bibliography[1] I. Abe et al., “The KEKB Injector Linac”, Nucl. Instrum. Meth. A 499, 2003, p.167,
doi:10.1016/S0168-9002(02)01787-4 11.1, 11.4.1, 11.8.1[2] M. Akemoto et al., “The KEKB Injector Linac”, Prog. Theor. Exp. Phys. 2013, 2013,
03A002, doi:10.1093/ptep/ptt011 11.1, 11.6
[3] Y. Ohnishi et al., “Accelerator Design at SuperKEKB”, Prog. Theor. Exp. Phys. 2013,2013, 03A011, doi:10.1093/ptep/pts083 11.1, 11.6
[4] M. Satoh et al., “Commissioning Status of SuperKEKB Injector Linac” in Proceedings of IPAC2016, Busan, Korea, 2016, THPOY027, p.4152, doi:10.18429/
JACoW-IPAC2016-THPOY027 11.1
[5] M. Yoshida, N. Iida, S. Kazama, T. Natsui, Y. Ogawa, S. Ohsawa, H. Sugimoto, L. Zang,
X. Zhou, D. Sato, “Generation and Acceleration of Low-emittance, High-current Electron Beams for SuperKEKB”, in Proceedings of LINAC2014, Geneva, Switzerland, 2014,
MOIOB03, p.21, https://jacow.org/LINAC2014/papers/moiob03.pdf 11.1
[6] T. Kamitani et al., “SuperKEKB Positron Source Construction Status”, in Proceedings of IPAC2014, Dresden, Germany, 2014, MOPRI004, p.579, doi:10.18429/
JACoW-IPAC2014-MOPRI004 11.1
[7] Y. Seimiya, Y. Enomoto, K. Furukawa, T. Higo, T. Kamitani, F. Miyahara, Y. Ohnishi,
M. Satoh, T. Suwada, M. Tanaka, “Emittance Growth by Misalignments and Jittersin SuperKEKB Injector Linac”, in Proceedings of IPAC2016, Busan, Korea, 2016, THPOR040, p.3871, doi:10.18429/JACoW-IPAC2016-THPOR040 11.1
[8] F. Miyahara, K. Furukawa, M. Satoh, T. Suwada, R. Ichimiya, H. Saotome, “Highposition resolution BPM readout system with calibration pulse generators for KEK
e+/e− linac”, in Proceedings of IBIC2015, Melbourne, Australia, 2015, TUPB023, p.369,
doi:10.18429/JACoW-IBIC2015-TUPB023 11.1
[9] K. Furukawa, M. Satoh, T. Suwada, T.T. Nakamura, “Pulse-to-pulse Beam Modulation and Event-based Beam Feedback Systems at KEKB Linac”, in Proceedings of504

IPAC2010, Kyoto, Japan, 2010, TUOCMH01, p.1271, https://jacow.org/ipac10/
papers/tuocmh01.pdf 11.1
[10] K. Furukawa et al., “Upgrade of KEK Electron/positron Linac Control System for theBoth SuperKEKB and Light Sources”, in Proceedings of ICALEPCS2017, Barcelona,
Spain, 2017, THMPL02, p.1257, doi:10.18429/JACoW-ICALEPCS2017-THMPL02 11.1
[11] N. Higashi et al., “Construction and Commissioning of Direct Beam Transport Linefor PF-AR”, in Proceedings of IPAC2017, Copenhagen, Denmark, 2017, WEPAB044,
p.2678, doi:10.18429/JACoW-IPAC2017-WEPAB044 11.1
[12] H. Sugimoto, M. Satoh, M. Yoshida, “Design study on KEK injector linac upgrade forhigh-current and low-emittance beams”, in Proceedings of IPAC2012, New Orleans,
USA, 2012, TUPPC021, p.1206, https://jacow.org/IPAC2013/papers/tupfi004.
pdf 11.2.1[13] M. Yoshida, N. Iida, T. Natsui, Y. Ogawa, S. Ohsawa, H. Sugimoto, L. Zang, X.
Zhou, “Longitudinal manipulation to obtain and keep the low emittance and highcharge electron beam for SuperKEKB injector”, in Proceedings of IPAC2013, Shanghai,
China, 2013, WEPME061, p.1337, https://jacow.org/IPAC2013/papers/tupfi004.
pdf 11.2.2[14] I. Sato et al., “Design Report on PF Injector Linac Upgrade for KEKB”, KEK Report
95–18, 1996, https://lib-extopc.kek.jp/preprints/PDF/1995/9524/9524018.pdf
11.3, 11.3.1, 11.3.4, 11.9.1[15] S. Matsumoto, T. Higo, K. Kakihara, T. Kamitani, M. Tanaka, “Large-Aperture
Travelling-wave Accelerator Structure for Positron Capture of SuperKEKB Injector
Linac”, in Proceedings of IPAC2014, Dresden, Germany, 2014, THPRI047, p.3872,
doi:10.18429/JACoW-IPAC2014-THPRI047 11.3.1[16] S. Yamaguchi, Y. Igarashi, K. Kakihara, A. Enomoto, “High power test of the S-band
Accelerator Guide for the KEKB Injector Linac”, in Proceedings of APAC98, Tsukuba,
Japan, 1998, 4D023, p.136, https://jacow.org/a98/APAC98/4D023.PDF 11.3.4[17] T. Higo, Y. Enomoto, H. Ego, S. Ushimoto, “Inspection of inner surface of S-band
accelerating structures of KEK electron-positron injector linac used for more than 30
years”, in Proceedings of PASJ2018, Nagaoka, Japan, 2018, WEP055, p.445, https://www.pasj.jp/web_publish/pasj2018/proceedings/PDF/WEP0/WEP055.pdf 11.3.5[18] A. Saji et al., “Manufacturing of S-band Accelerating Structure”, in Proceedingsof PASJ2018, Nagaoka, Japan, 2018, THP048, p.918, https://www.pasj.jp/web_
publish/pasj2018/proceedings/PDF/THP0/THP048.pdf 11.3.7

505

[19] M. Akemoto, S. Fukuda, H. Honma, M. Kawamura, H. Nakajima, T. Natsui, T. Shidara,“Present Status of the Klystron Modulator for the KEK Electron-Positron Injector
Linac”, in Proceedings of PASJ2018, Nagaoka, Japan, 2018, WEP060, p.476, https://www.pasj.jp/web_publish/pasj2018/proceedings/PDF/WEP0/WEP060.pdf 11.4.1[20] M. Kawamura et al., “Present Status of Capacitor-Charging Power Supplies for the
Klystron Modulators in SuperKEKB Injector Linac”, in Proceedings of LINAC2018,
Beijing, China, 2018, THPO095, p.898, https://jacow.org/linac2018/papers/
thpo095.pdf 11.4.1[21] T. Matsumoto et al., “Low-Level RF System for the SuperKEKB Injector Linac”,
in Proceedings of IPAC2018, Vancouver, Canada, 2018, WEPAK017, p.2131, doi:
10.18429/JACoW-IPAC2018-WEPAK017 11.4.2[22] T. Miura, M. Akemoto, D. Arakawa, H. Katagiri, T. Matsumoto, F. Qiu, Y.
Yano, N. Liu, “LLRF Control Unit for SuperKEKB Injector Linac”, in Proceedingsof the IPAC2018, Vancouver, Canada, 2018, WEPAK018, p.2134, doi:10.18429/
JACoW-IPAC2018-WEPAK018 11.4.3[23] H. Katagiri, M. Akemoto, D. Arakawa, T. Matsumoto, T. Miura, F. Qiu, Y. Yano, “RFMonitor System for SuperKEKB Injector Linac”, in Proceedings of the IPAC2018, Vancouver, Canada, 2018, WEPAK016. p.2128, doi:10.18429/JACoW-IPAC2018-WEPAK016
11.4.3[24] K. Kakihara et al., “Experimental study of stainless steel chamber for a pulsed bending magnet of the KEK e−/e+ injector linac”, in Proceedings of the PASJ2018, Nagaoka, Japan, 2018, THP118, p.1185, https://www.pasj.jp/web_publish/pasj2018/
proceedings/PDF/THP1/THP118.pdf 11.5.1[25] A. Akiyama, K. Furukawa, E. Kadokura, M. Kurashina, K. Mikawa, T. Nakamura, J.
Odagiri, M. Satoh, T. Suwada, “Accelerator Control System at KEKB and the Linac”,
Prog. Theor. Exp. Phys., 2013, 2013, 03A008, doi:10.1093/ptep/pts081 11.6.2[26] Y. Enomoto, K. Furukawa, T. Natsui, M. Satoh, H. Saotome, “A New Pulse
Magnet Control System in the KEK Electron Positron LINAC”, in Proceedingsof IPAC2018, Vancouver, Canada, 2018, WEPAK014, p.2121, doi:10.18429/
JACoW-IPAC2018-WEPAK014 11.6.2[27] Apex Microtechnology, https://www.apexanalog.com/products/pa12.html 11.6.2[28] S. Kazama, M. Satoh, H. Sugimoto, M. Yoshida, Y. Ogawa, “Emittance Preservation in SuperKEKB Injector”, in Proceedings of IPAC2015, Richmond, USA, 2015,
MOPWA053, p.239, doi:10.18429/JACoW-IPAC2015-MOPWA053 11.7.1

506

[29] T. Suwada, “High Stabilization of 500-m-Long Laser-Based Fiducial Line for HighPrecision Laser-Based Alignment”, J. the Particle Accelerator Society of Japan, 10(4),2013, p.226, https://www.pasj.jp/kaishi/cgi-bin/kasokuki.cgi?articles/10/
p226-237.pdf 11.7.2[30] M. Tanaka et al., “Measurement of Long-Period Floor Movement in the KEK
Injector Linac Tunnel”, in Proceedings of PASJ2018, Nagaoka, Japan, 2018,
WEP136, p.740, https://www.pasj.jp/web_publish/pasj2018/proceedings/PDF/
WEP1/WEP136.pdf 11.7.8[31] M. Yoshida, N.Iida, T. Natsui, M.Satoh, Y. Ogawa, L. Zang, “SuperKEKB injector
upgrade for high charge and low emittance electron beam”, in Proceedings of IPAC2012,
New Orleans, USA, 2012, TUPPD035, p.1482, https://jacow.org/IPAC2012/papers/
tuppd035.pdf 11.8
[32] A. Inoue, T. Natsui, M. Yoshida, “Cut disk structure type RF-deflector for slice emittance measurement for RF-gun at SuperKEKB”, Energy Procedia, 131, 2017, p.334,
doi:10.1016/j.egypro.2017.09.431 11.8
[33] R. Zhang, X. Zhou, T. Natsui, D. Satoh, M. Yoshida, “Study on stable and highoutput energy laser system for RF-gun at SuperKEKB injector”, in Proceedingsof PASJ2017, Sapporo, Japan, 2017, WEP117, p.1201, https://www.pasj.jp/web_
publish/pasj2017/proceedings/PDF/WEP1/WEP117.pdf 11.8
[34] T. Natsui, M. Yoshida, X. Zhou, R. Zhang, Y. Ogawa, “Commissioning ofRF gun for SuperKEKB”, in Proceedings of PASJ2015, Tsuruga, Japan, 2015,
WEP006, p.405, https://www.pasj.jp/web_publish/pasj2015/proceedings/PDF/
WEP0/WEP006.pdf 11.8.2[35] D. Satoh, R. Zhang, X. Zhou, M. Nishida, T. Natsui, M. Yoshida, K. Furukawa,
H. Sugawara, “Improvement of photoemission properties of iridium cerium compoundfor SuperKEKB injector linac,” in Proceedings of PASJ2017, Sapporo, Japan, 2017,
WEP106, p.1165, https://www.pasj.jp/web_publish/pasj2017/proceedings/PDF/
WEP1/WEP106.pdf 11.8.3[36] X. Zhou, T. Natsui, M. Yoshida, R. Zhang, Y. Ogawa, T. Shibuya, “Neodymium andYtterbium Hybrid Solid Laser of RF Gun for SuperKEKB”, in Proceedings of IPAC2016,
Busan, Korea, 2016, THPMY041, p.3748, doi:10.18429/JACoW-IPAC2016-THPMY041
11.8.6, 11.8.6[37] X. Zhou, T. Natsui, M. Yoshida, R. Zhang, Y. Ogawa, “Developing an Yb/Nd Doped
Hybrid Solid Laser of RF Gun for SuperKEKB Phase II Commissioning”, in Proceedings of IPAC2017, Copenhagen, Denmark, 2017, THPVA047, p.4540, doi:10.18429/
JACoW-IPAC2017-THPVA047 11.8.6

507

[38] X. Zhou, M. Yoshida, R. Zhang, Y. Ogawa, “The Developing of the Beam Injection
Section with Laser Source and S-Band Electron RF Gun for SuperkEKB Project”, inProceedings of LINAC2018, Beijing, China, 2018, MOPO007, p.50, doi:10.18429/
JACoW-LINAC2018-MOPO007 11.8.6, 11.8.6[39] X. Zhou, T. Natsui, M. Yoshida, R. Zhang, Y. Ogawa, “25 Hz, Sub-mJ Ytterbium Laser
Source of RF Gun for SuperKEKB Linac”, in Proceedings of IPAC2015, Richmond,
USA, 2015, WEPMA044, p.2862, doi:10.18429/JACoW-IPAC2015-WEPMA044 11.8.7[40] T. Kamitani et al., “Positron Injector Linac Upgrade for SuperKEKB”, in Proceedings of LINAC2012, Tel-Aviv, Israel. 2012, MOPLB02, p.141, https://jacow.org/
LINAC2012/papers/moplb02.pdf 11.9.1[41] N. Iida, H. Ikeda, T. Kamitani, M. Kikuchi, K. Oide, D. Zhou, “Beam Dynamics in Positron Injector Systems for Next Generation B-Factories”, in Proceedingsof IPAC2011, San Sebastian, Spain, 2011, THYA01, p.2857, https://jacow.org/
IPAC2011/papers/thya01.pdf 11.9.2[42] S. Agostinelli et al., “Geant4 – a simulation toolkit”, Nucl. Instrum. Meth. A 506, 2003,
p.250, doi:10.1016/S0168-9002(03)01368-8, http://geant4.web.cern.ch 11.9.2[43] “CST EM STUDIO”, Dassault Systems, https://www.cst.com/products/cstems
11.9.2[44] “General Particle Tracer (GPT) code”, Pulsar Physics, http://www.pulsar.nl/gpt/
M.J. de Loos et al., “General Particle Tracer: A new 3D code for accelerator andbeamline design”, in Proceedings of EPAC1996, Sitges, Spain, 1996, THP001G, p.1241,
https://jacow.org/e96/PAPERS/THPG/THP001G.PDF 11.9.2[45] “SAD: Strategic Accelerator Design”, http://acc-physics.kek.jp/SAD/references.
html 11.9.2[46] V.K. Bharadwaj, Y.K. Batygin, J.C. Sheppard et al., “Analysis of Beam-Induced Damage to the SLC Positron Production Target”, in Proceedings of PAC2001, Chicago, USA,
2001, WPAH019, p.2123, https://jacow.org/p01/PAPERS/WPAH019.PDF 11.9.3[47] L. Zang, K. Kakihara, T. Kamitani, K. Mikawa, F. Miyahara, T. Suwada, “Design,
Manufacture and Operation of the Beam Spoiler for Positron Target Protection”, inProceedings of IPAC2014, Dresden, Germany, 2014, MOPRI002, p.573, doi:10.18429/
JACoW-IPAC2014-MOPRI002 11.9.3[48] F.A. Emanov et al., “Feeding BINP Colliders with the new VEPP-5 injection complex”,
in Proceedings of RuPAC2016, St. Petersburg, Russia, 2016, WEXMH01, p.56, doi:
10.18429/JACoW-RuPAC2016-WEXMH01 11.9.4

508

[49] A.V. Kulikov, S.D. Ecklund, E.M. Reuter, “SLC Positron Source Pulsed Flux Concentrator”, in Proceedings of PAC1991, San Francisco, USA, 1991, p.2005, doi:
10.1109/PAC.1991.164851 11.9.4[50] S.H. Wang et al., “Injector Linac Upgrade for the BEPCII Project”, in Proceedingsof LINAC2004, Luebeck, Germany, 2004, MOP34, p.111, https://jacow.org/l04/
PAPERS/MOP34.PDF 11.9.4[51] Y. Enomoto et al., “Development of Flux Concentrator for SuperKEKB”, in Proceedings of PASJ2016, Chiba, Japan, 2016, MOP063, p.520, https://www.pasj.jp/web_
publish/pasj2016/proceedings/PDF/MOP0/MOP063.pdf 11.9.4[52] L. Zang et al., “Positron Yield Optimization by Adjusting the Components Offset andOrientation”, in Proceedings of IPAC2014, Dresden, Germany, 2014, MOPRI003 p.576,
doi:10.18429/JACoW-IPAC2014-MOPRI003 11.9.4[53] M. Akemoto, S. Fukuda, H. Honma, T. Kamitani, S. Michizono, H. Nakajima, T.
Shidara, “Development of Flux Concentrator Modulator for SuperKEKB”, in Proceedings of PASJ2013, Nagoya, Japan, 2013, SUP057, p.1015, https://www.pasj.jp/web_
publish/pasj10/proceedings/PDF/SUP0/SUP057.pdf 11.9.4[54] M. Akemoto, Y. Enomoto, T. Kamitani, M. Kawamura, H. Nakajima, S. Fukuda, K.
Yokoyama, “Present Status of Flux Concentrator Modulator for SuperKEKB”, in Proceedings of PASJ2017, Sapporo, Japan, 2017, TUP067, p.504, https://www.pasj.jp/
web_publish/pasj2017/proceedings/PDF/TUP0/TUP067.pdf 11.9.4[55] T. Kamitani et al., “Present Status of the Positron Source Upgrade for SuperKEKB”, inProceedings of PASJ2015, Tsuruga, Japan, 2015, THP044, p.1064, https://www.pasj.
jp/web_publish/pasj2015/proceedings/PDF/THP0/THP044.pdf 11.9.4[56] N.V. Mokhov et al., “Recent Enhancements to the MARS15 Code”,
FERMILAB-Conf-04/053-AD, 2004, http://lss.fnal.gov/archive/2004/conf/
fermilab-conf-04-053-ad.pdf 11.9.6[57] S. Matsumoto, H. Iwase, K. Kakihara, T. Kamitani, T. Sanami, T. Higo, H. Yamaoka,“Radiation Shield Structure for the Positron Target Region of KEKB Injector Linac”,
in Proceedings of PASJ2015, Tsuruga, Japan, 2015, THP135, p.1403, https://www.
pasj.jp/web_publish/pasj2015/proceedings/PDF/THP1/THP135.pdf 11.9.6[58] S. Matsumoto et al., “Radiation Shield for the Positron Target of KEKB Injector Linac”,
in Proceedings of PASJ2016, Chiba, Japan, 2016, TUP133, p.1288, https://www.pasj.
jp/web_publish/pasj2016/proceedings/PDF/TUP1/TUP133.pdf 11.9.6

509

[59] A. Shirakawa, H. Honma, Y. Ogawa, “Upgrade of Safety Interlock System of e+ /e−
Linac for SuperKEKB Project”, in Proceedings of IPAC2013, Shanghai, China, 2013,
THPEA007, p.3161, https://jacow.org/IPAC2013/papers/thpea007.pdf 11.10.1[60] I. Satake, H. Honma, A. Shirakawa, N. Toge, “Improvement of Personnel and MachineProtection System in SuperKEKB Injector Linac”, in Proceedings of IPAC2018, Vancouver, Canada, 2018, WEPAL002, p.2140, doi:10.18429/JACoW-IPAC2018-WEPAL002
11.10.1510