Linac status
Mitsuhiro Yoshida (Acc. Lab. Div. V, KEK)
on behalf of Injector Linac Group and Linac Commissioning Group1

Outline

Injector Linac Layout

・Two electron sources:
RF gun: HER injection
Thermionic DC gun: LER, PF, PF-AR
・Sector 3-5:
All magnets are pulsed magnets.
1.1 GeV e+
Damping Ring
Continuous orbit feedback
・DC magnet settings are same for different
beam mode

60 klystron units
240 accelerating structures (S-band 2-m-long)

8 Sectors (A-C,1-5)
Energy Knob

Thermionic gun
PhotocathodeRF gunJ-ARC
1.5 GeV

B

A

BCS

ECS

C

1

2

3

e+
e+ capture section
target

4

5

PF
ECS

PF-AR

LER

chicaneEnergy Knob

HER

4nC

4nC

HER 7GeV
PF-AR 6.5 or 5 GeV

Acceleration
4.2 GeV

ee+ target

LER 4 GeV e+Stand-byDeceleration

1.1 GeVe+

PF 2.5 GeV

Acceleration

1.5 GeVC
Masanori Satoh (KEK)

1

2

3

4

5
2

Beam energy variation for each beam mode along the beam line after J-ARC

2

e- beam status and issue3

Low emittance photocathode rf e- gunRF e- gunIr7Ce2 cathodee- beam status

Laser system• Photocathode: Ir7Ce2• Cavity: QTWSC (Quasi Travelling Wave Side Couple)– Strong focusing electric fieldshort drift space
Secondary RF gun
for test and backup

M. Yoshida et al.

4

e- beam status

Hybrid laser system for rf e- gun•

Yb doped fiber and Nd:YAG DPSS module Amplifier••••

Output Power in 1st line:ω (1064 nm): 32 mJ
2ω ( 532 nm): 12 mJ
4ω ( 266 nm): 1.3 mJ

1st laser
2nd laser

Laser roomon theground virtualKLY Gallery

cathodediagnosticsLaser line to RFGun
TunnelRF-GunR. Zhang,
X. Zhou et al.

Optics layout in the 5tunnel

Laser transport line

5

DOE for reshaping of laser spatial distribution
2019.06 without DOE & beam position sensor

H 2σ: 48.04±0.51, V 2σ: 46.08±0.69

Pointing stability
can be improved.

e- beam status

2021.06 with DOE & beam position sensor

H 2σ: 24.30±3.06, V 2σ: 10.08±0.46

DOE (diffractive optical element) were installedat 1st /2nd (in summer ‘20/‘21) line laser: Laser beamhomogenizer for low emittance beam with thehigh intensity bunch charge.

DOE is installed in vacuum
chamber filled with Argon gas.

Masanori Satoh (KEK)world first DOE application in UV laser

R. Zhang 6

e- beam statuse- bunch charge history (2020a to 2023)
6

LIiBM:SP_A1_G_1:ISNGL:KBE:10S (from rf gun)2020a,b2020c

2021a,b2021c

2022a,be- Beam charge (nC)52022c (LS1)2023High bunchcharge study4

discharge
3

Bunch charge
feedback

2

1

0
2020/1/1

2020/4/10 2020/7/19 2020/10/27 2021/2/4

2021/5/15 2021/8/23 2021/12/1 2022/3/11 2022/6/19 2022/9/27

2023/1/5

7

SP_61_H 4nC achievement

SP_A1_G 5.2nC, SP_61_H 4 nC reached.
B sector emittance γεx=57.534±19.037), BmagX 2.090, γεy=85.974±20.111,
BmagY 1.104
8

KBE Emittance at each charge condition

SP_A1_G 4.0nC、SP_61_H
3.0nC
SP_A1_G
Charge(nC)

SP_61_H
Charge(nC)

BT EndCharge
(nC)

5.25

3.94

4.0

SP_A1_G 2.0nC、SP_61_H
1.7nC

SP_A1_G 3.0nC、SP_61_H
2.4nC
B-sector

5-sector

BT

EmitX

BmagX

EmitY

BmagY

EmitX

BmagX

EmitY

BmagY

EmitX

BmagX

EmitY

BmagY

-

57.534(Δ19.037)

2.09

85.974(Δ20.111)

1.104

-

-

-

-

-

-

-

-

3.0

3.0

36.897(Δ 7.623)

3.814

36.659(Δ17.921)

1.138

219.399(Δ265.59)

1.86

142.239(Δ 46.19)

1.073

174.842(Δ475.13)

5.214

205.440(Δ56.42)

1.029

3.0

2.4

2.2

-

-

-

-

18.650(Δ 32.39)

2.709

36.270(Δ 6.078)

1.581

34.359(Δ 6.223)

1.03

15.567(Δ 7.388)

2.013

2.0

1.7

1.2

15.723(Δ 4.571)

25.01115.061(Δ 4.469)

2.884

32.800(Δ214.29)

5.05

126.186(Δ 40.76)

1.061

20.144(Δ 3.310)

1.046

17.826(Δ 2.720)

1.054

9

KBE Emittance at each charge condition

Horizontal Emittance

Vertical Emittance

250

250
SP_A1_G 5.25nC (SP_61_H 4nC)

200

200
SP_A1_G 5.25nC (SP_61_H 4nC)

150

SP_A1_G 3nC (SP_61_H 2.4nC)
SP_A1_G 2nC (SP_61_H 1.7nC)

100
50
0

SP_A1_G 3nC (SP_61_H 2.4nC)
SP_A1_G 2nC (SP_61_H 1.7nC)

SP_A1_G 4nC (SP_61_H 3nC)

EmitY

EmitX

150

SP_A1_G 4nC (SP_61_H 3nC)

100
50

5
B
Wire Scanner 測定セクター

BT

Sector

ビームチャージ量ごとの各セクター X 方向エミッタンス比較

0B
5Wire Scanner 測定セクター

BT

Sector

ビームチャージ量ごとの各セクター Y 方向エミッタンス比較

10Issue of rf gun laser window degradatione- beam issue• Long term operation makes rf gun laser windows dirty for both of 1st and 2nd line.
• It decrease the transmittance of laser power through window and bunch charge intensity.
• After replacement of the laser window, the bunch charge intensity is recovered.
• Vacuum ion pump was installed between the laser window and rf gun cavity with theextension vacuum duct for the 1st line laser in this summer maintenance ‘22.

Bunch charge (nC)e- intensity by 1st-line laser

new window
degraded window

1st-line laser incident position [mm]M. Yoshidae- intensity by 2nd-line laser

new window
degraded window

2nd-line laser incident position [mm]11Improvement of laser window degradation with ion pump

e- beam issue• Long term operation for keeping e- bunch charge is very important issue.
• Continuous beam test at e- beam repetition of 22 Hz has been conducted more than 5 days.
• Installed ion pump could help to mitigate the laser window degradation from the experimental
results.
• This test will be continued until the end of this run. Further improvement is also being
considered.
Degradation test of 1st/2nd line laser window

Quantum efficiency

1st laser line (adding ion pump)
2nd laser line

5 daysMasanori Satoh (KEK)

Elapsed time HH:MM:SS

M. Yoshida

12Injection efficiency of 2nd bunch

e- beam issue• HER injection efficiency of e- 2nd bunch is lower than that of 1st bunch and varies gradually.
• In some cases, there is not clear correlation between injection efficiency and orbit of 2nd bunch.
• Low injection efficiency could be also caused by the emittance deterioration.
• Fast kicker for 2nd bunch orbit correction could be effective. Two fast kickers will be installed at the linac end andthe beginning of BT in summer ’23.

Vertical beam position at BT• Prototype kicker was installed at the entrance of J-ARC in this summer ’22. Beam test will be conducted soon.

Ver. position of 2nd bunch at the BT end

Injection efficiency
difference

N. IidaVer. position of 1st bunch at the BT end

Difference between 2nd and 1st bunch
Injection efficiency(Lower 2nd bunch efficiency means negative value)13

BTe-ECS (FY2024) to improve injection efficiency

14

e- beam summary and issue

e- beam issue• Thermionic DC electron gun has worked fine to generate primary electron beam for positron production.
• Photocathode RF-Gun• Laser system and DOE element (5mm beam diameter) has worked fine without any significant trouble.
• High bunch charge e- was demonstrated. Achieved 5 nC from e- gun and 4 nC at the linac end.
==== To Do ====
• DOE with large laser area (fully covered 8 mm cathode area) will be installed until HER injection=> Lower energy spared according to space charge suppression / Stability improvement• New piezo mirror feedforward system improve beam stability.
• Better QE IrCe composite cathode is developed and under testing.
• Gradual decrease of bunch charge due to laser window deterioration : extension vacuum chamber and pump.
• New TW-Gun is under testing for better vacuum in cathode region and lower discharge.
• Beam loss at J-ARC and e+ target location should be minimized : Pulsed magnet• Beam stability should be improved : Laser frequency comb in vacuum line(Zhou) / Pulsed modulator control.

• KBE Beam Issue and study in LS1 period.
• Emittance at linac end and BT1 is almost satisfied while bunch charge (2 nC) is less than final goal (4 nC).
• However, emittance at BT2 is increased due to ISR, CSR, and some other reasons
BT ARC4 realignment / Vacuum chamber / Straight injection line
• Increase of 2nd bunch injection efficiency and improvement of its stability are important issues.
=> Fast Kicker / Long range wakefield suppression• Auto tuning : Dispersion compensation / Fast emittance measurement are under test
• BTe-ECS is planned to install at FY2024 in BT1 or BT2(Fuji-area).

15

e+ beam status and issue

16

e+ beam status

Positron source setup at Sector1
Positron target and capture section

DC solenoid
Large aperture S-band acc. Structures (x10)

W target + flux concentrator (FC)
Y. Enomoto et al.

17

e+ beam status

Positron Capture Section: Flux concentrator, bridge coil, solenoid
Large Aperture S-band acc. structure (LAS)
Bridge Coil (BC)Solenoid

Tungsten target

3.5 GeV e- beam

Flux Concentrator (FC)Cu-alloy (NC50: Cu-Si-Ni)
IFC= 12 kA
Masanori Satoh (KEK)

Y. Enomoto et al.

18

e+ bunch charge history (2020a to 2022c)
2020c

2020a,b2021a,be+ beam status(after e+ target)(before DR) ・Solenoid current (SL_16_)optimization(Linac end) ・beam tuning (primary e- bunch charge ↑)
(BT)
2022c

2021c

2022a,b(LS1)・beam tuning

・Steering, BPM
in Unit 1-5, 1-6
・beam tuning
・New FC
・Solenoid
current↑

・decrease bunch charge(MR request)

19

19

e+ beam issue

e+ beam summary and issue• e+ bunch charge is almost achieved (final target: 4 nC)• 6 nC at BPM “SP_16_5” (1st BPM after e+ target) / 4 nC at LTR (Linac To damping Ring) / 3.5 nC at linac end and BT• e+ production efficiency with the current FC is reached the simulation result (60%).

• Further improvement of positron bunch charge• Increase acc. gradient of first two structures (AC_15_1[2]) at e+ capture section• Increase primary e- bunch charge, beam tuning, FC power supply upgrade, and so on• Auto tuning : Bayesian Optimization approach is now under test by using the beam of injector Linac.
Implementation using GPyOpt Python library (T. Natsui) / In-house developed implementation (G. Mitsuka)• Issue• Emittance at linac end and BT1 (before Arc1) is almost satisfied the final goal.
• However, emittance at BT2 is increased due to ISR, CSR, and some other reasons.
BT ARC4 realignment / Vacuum chamber• Horizontal emittance after DR is larger than design value. Low emittance DR optics will be tested after LS1.

e+ emittance

Measured enx,nxy (3 nC) : 103.5/4.7 mm (at BT1)
Goal: enx,nxy (4 nC) : 100/15 (H/V) mｍ

Y. Enomoto
20Upgrade work during LS1• New accelerating structure• Pulsed Quads• at J-ARC matching section• at Sector1, 2 (e-/e+ compatible optics region)

21

Accelerating structure• Mitigation of accelerating structure failures• Originally designed for 8 MeV/m (PF injector), but used at 20 MeV/m (KEKB upgrade)• Degradation that lead to high field emission rate and discharges• Water leaks, field emission , discharge in waveguide, and so on (29 of 60 units have some problems)• Not only future Y(6S) but even Y(4S) could be suffered
Upgrade during LS1

Damaged iris
Colling water leakage

22New accelerating structures and pulse compressor in beam-acceleration operation• 5-year upgrade plan to fabricate and install new 16 structures (FY2018 – FY2022)• Unit 44 already with four new structures and pulse compressor (SCPC) at the rated 20 MV/m
• Remodeled Unit 44 to be reinforced at an accelerating gradient of 30 MV/m in this summer• New 12 structures completed for replacement of damaged old ones with water leaks or frequent
power-reflections in this summer• New structure: acc. gain ↑7%, surface field ↓20% (reduce breakdown) and 30 MV/m capable• New pulse compressor (SCPC) : RF performance equivalent to the current KEK-SLED, supercompact body, half fabrication-cost

H. Ego
New S-band 2-m-long TW acc. structure

New pulse compressor
Spherical-Cavity Pulse Compressor (SCPC)

Unit remodeled for acc.voltage reinforcement
KEK_SLEDSCPC

KEK_SLEDSCPC

Beam deflection [mm]KL_37 reference

KL_44B
KL_44

KL_44B
KL_44A

KL_44ARF phase [deg]Demonstrated 20 MV/m beam-acceleration
SCPC installed in KL_44B

Old structures x 4

BeforeNew structures x 4

AfterStructures in 44-unitPulsed Quads at J-ARC for optics matchingUpgrade during LS1

e- beam• At the entrance and exit of 180 deg. J-ARC region,a good optics matching is very important to mitigate beamloss and emittance growth.
• Simultaneous matching for both of HER/LER injection
beam requires the pulsed quads.

matching
section• From the simulation result, 4 pulsed quads at both ofentrance and exit of J-ARC are sufficient.

matching
sectionY. Okayasu

e- beam

Bmag yCurrent optics at J-ARC

New optics at J-ARC
with pulsed quads

Bmag xMatching simulatione- beamY. Seimiya 25

Pulsed Quads at J-ARC for e- low beta optics

Upgrade during LS1• Current optics at Sector1, 2• Large emittance e+ beam is accelerated from 0.1 GeV to 1.1 GeV for DR injection.
• Quad settings is optimized for e+ beam.
• For e- beam (3 ～ 4 GeV), focusing force is weak in comparison with optimum parameter. It could cause theemittance growth.

Four DC quads will be replaced by pulsed one.

26

Pulsed Quads at J-ARC for e- low beta optics (cont’d)Upgrade during LS1• After only these four pulsed quads are optimized for the e- beam, the betatronfunction can be decreased.
• Simulation result shows that it can help to decrease the emittance less than half.
Δγε𝑥 ~ 100 um, 𝛽𝑥 = 45.2 m••Δγε𝑥 ~ 40 um, 𝛽𝑥 = 16.3 m

End of LAS cavity
Start of S-band
cavityBunch tail is kicked back byquad, and temporally
emittance is reduced.

e+ target

Sector2 end

Before optimizationachieved smaller
betatron function

e+ target

Sector2 end

After optimization

Y. Seimiya 27

specification
ofsizepulse
quad for Sector-1, 2• requirements on bore
andfield strength are minimized bysimulation while keeping theperformance.
• magnet (size) and ceramic duct
are designed to be replaceablewith existing DC quad and duct.
• current and turns/coil are
optimized to be compatible withthe existing pulse power supply.

parameters

DC quad
17_14
type

new
pulsedquad
17_14

bore diameter [mm]4432field gradient [T/m]

20.9

23.6

max. current [A]

80

300

pole length [mm]

160

160

effective length [mm]

173.8

168.0

B’L [T]

3.63

3.96

nI [A.turn]

3760

2400

turn of coil /pole47

8

inductance [mH]

32.3

0.94

K. Yokoyama

28Linac Fast Kicker• 1st/2nd bunch orbit difference tuning is important because of• Injection efficiency improvement for both of bunches• Suppression of emittance growth with orbit offset in linac• Reduction of beam loss due to orbit difference• Requirements for the linac fast kicker• pulse rise time < bunch interval (96 ns)• sufficient kick angle ~ 0.4 mrad @1.5 GeV
BL = 2.0 x 10-3 T.m

29

Ceramic chamber type Fast Kicker
C. Mitsuda

dipolefield• CCiPM : Ceramics Chamber with integrated Pulsed Magnet• Magnetic field type kicker• This kicker has four parallel coil wires.
• The current configuration described above(parallel and anti-parallel currents) generates
horizontal dipole magnetic field, (vertical beam kick).
30

Pulse power supply for fast kicker• Characteristics ofpulse power supply• SiC FET high-voltage switch(supplied by Nexfi company)• pulse rise time < 96 ns (target value)
to kick only 2nd bunch• peak current 500 A• max voltage 10 kV• precise timing control
for kick angle fine tuning• switch module installed in the tunnel
close to kicker magnet(needs thick radiation shield)

Y. Enomoto, T. Natsui, Y. Okayasu

Achieved pulse waveform:rise time ~ 200 ns
fall time ~ 100 ns
31

Fast Kicker Installation
Y. Okayasu• The first
prototype of ceramic chamber type fast kicker
has been installed in summer of 2022 in J-arc.
e- beame- beame- beam32

Bunch dependent kick test
• Bunch dependent beam kick test was performed with the prototype fast kicker in J-arc (2022.12.26).
• The blue and orange lines show the orbit deviationsof the first and the second bunch of the electron beam
as a function of fast kicker pulse timing.

• Next two kickers will be installed at linac Sector-5 and HER-BT
in summer of 2023 for the operation after LS1

33

Summary•

Simultaneous top up injection operation of 4 storage rings (SuperKEKB HER/LER, PF, PF-AR) + DR has beensuccessfully conducted.

•

e- beam• Laser system has worked fine without any significant trouble.
• DOE was installed also at 2nd laser line in the last summer maintenance, and it has worked fine.
• In the run 2022a/b, bunch charge of 2 nC can be kept with bunch charge feedback.
• 5 nC from gun was demonstrated. Further beam study is on-going during LS1.
• New DOE with large area improve energy spread and emittance until HER injection.
• BTe-ECS is planned to install at FY2024
e+ beam• The new FC is working fine.
• Reached bunch charge of 3.5 nC at BT end (final design 4 nC).

•

•

•

Upgrade work during LS1• Pulsed Quads (x8) at J-ARC for the simultaneous dedicated matching of HER/LER injection beam
• Pulsed Quads (x4) at Sector1, 2 for low beta optics of HER injection beam
• New accelerating structure• Replacement of air conditioners at SectorA, B (in the accelerator tunnel)• Fast kicker for 2nd bunch orbit correction
Issues• Emittance growth at end of BT2 for both of e- and e+ beam (BT report, Injection report)• Low e- injection efficiency of 2nd bunch• Increase the e- bunch charge while keeping small emittance34