The 27th KEKB Accelerator Review
Committee/Injector / RF-Gun and electron-beam
2024.03.25
Mitsuhiro Yoshida ( INJ-Group of Injector Linac )11

Low emittance and high charge photocathodee-Line)gun
QTW RF gun or operation (0rfdeg.

RF e- gunPhotocathode(metal
Ir7Ce2 cathode : Infinite lifetimeand no pollution

Laser system• Photocathode:
7Ce2
CDS RF Ir
gunfor backup (90 deg. Line)• Cavity:
QTWSC (Quasi Travelling Wave Side Couple)– Strong focusing electric fieldshort drift space
Secondary RF gun
for test and backup2Recent Achievement

33

DOE for reshaping of laser spatial distribution
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.

New DOE for large area was installed at Jan, 2024.

world first DOE application in UV laser

Bunch charge
increased

R. Zhang 4

A1 Nd:YAG Laser amplifier system
Amp1

Amp2

Amp3

Amp4

Amp5

1st Line

Φ2mm LD

Φ2mm LD

Φ4mm LD

Φ4mm LD

Φ8mm VCSEL

2nd Line

Φ2mm LD

Φ2mm VCSELΦ4mm VCSEL

Φ6mm VCSEL

Φ10mm VCSEL

Laser Amplifier

Power supply

Rated Output

Setup
Voltage(V)1stagePWX750MHF

0～230V、10A、750W

66

2stagePWX750MHF

0～230V、10A、750W

67

3stagePWX1500MH

0～230V、20A、1500W

68.5

4stagePWX1500MH

0～230V、20A、1500W

68.5

5stagePWX1500MH

0～230V、20A、1500W

69

1stagePWX750MLF

0～80V、28A、750W

66

2stagePWX750MLF

0～80V、28A、750W

74

3stagePWX750MHF

0～230V、10A、750W

72

4stagePWX750MHF

0～230V、10A、750W

85

5stagePWX1500H

0～650V、7A、1500W

233

1st Line

2nd Line

1st Charge vs laser power

6nC

SP_61_H 4nC achievement / emittance 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

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
505B
Wire Scanner 測定セクター

BTSector
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
Horizontal Emittance

0B
5
Wire Scanner 測定セクター

BTSector
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
Vertical Emittance6

Operation history (charge and laser power)

2nC charge

2nC charge at 50 % laser power

Charge decreasement is occured due to RF phase drift and window transmission efficiency.
=> Precise phase measurement system of RF-laser and new RF-Gun are under development.
7

Emittance history for 3-month8

Upgraded part of RF-Gun• Improvement of IrCe quantum efficiency• Laser upgrade• Replacement of laser module• Flattening of temporal shaping• Larger area DOE to fully cover cathode• Chiccane slit
• Beam commissioning• Semi-automated dispersion correction

9

Cathode : Ir7Ce2

KEK house made IrCe cathode has best quantum efficiency

Charge vs laser power from RF-Gun
4.5nC by only 1st line laser

4.7nC by only 2nd line laser

Laser System for RF Gun
Yb-Fiber and Nd:YAG Hybrid Laser SystemUpstream

Menlo Orange 1 (1030 &1064 nm)

FiberOscillators (114 MHz)

Menlo Orange 2 (1030 & 1064 nm)

Fiber
Amplifiers (10 MHz)

Semiconductor Optics
Yb Single Mode Fiber
Yb Single Mode Fiber
Amplifier (SOA)
Amplifier 1→ 114 MHz → 10 MHz → Amplifier 2

Nd:YAG Rod Laser
Amplifiers (0.1-25 Hz)→ MEMS Switch

Home made ANDi type 1064 nm

3 In 1 Out

EO Pulse Picker→ 1st Nd:YAG Laser Line with 5 Stages Rod Amplifiers (High power 5 stage)

Beam splitter 50:50

2nd Nd:YAG Laser Line with 5 Stages Rod Amplifiers (High power 5th→ stage) with delay line

10 MHz → 1- 50 Hz
Double bunchthRF Gun

Beam Polarizer
Combination for Two→ Laser Lines

532 nm → 266 nm → DOE

Beam Polarizer
Division for Two Laser
BBO in 2nd Laser Linelines→ 532 nm → 266 nm → DOE→YCOB in 2nd Laser
Line 1064 nm → 532 nm

BBO in 1st Laser LineGround Laser Hut→ Tunnel RF Gun Box→

Transporting Line for
Two Lasers (11 m long)1064 nm → 532 nm→

Downstream

YCOB in 1st Laser Line

Wavelength Conversion& Spatial Reshaping

Yb Single Mode Fiber→ Amplifier 3Injection From TwoWindowsLaser System for RF Gun
Yb-Fiber and Nd:YAG Hybrid Laser System

Laser
TransportingLine (LT)A1 Ground Laser Hut
Green Lasers

Klystron A1 Gallery

1st Laser Line
2nd Laser Line

GS_A1 RoomLTTunnel A1 Part

24o Line

0o RF Gun
A1 Chicane

Thermionic Gun

Laser System for RF Gun
Two Laser Beams Injection for e- Beam Generation

Laser System for RF Gun
Two Laser Beams Injection Mode for Better Beam Quality

Simple illustration for 2 lasers incidence (out of ratio)
A1 ground laser hut

Tunnel optics table
90o RF gun

2nd line

BBO

60o injection
HWP

Flip mirror

Delay line

Polarizer

1st line
HWP Polarizer HWP

11 m long transporting line

Laser with vertical polarization,

BBOBBOe- beam

0o RF gun60o injection

laser with horizontal polarization, HWP: half wave plate

Laser System for RF Gun
Spatial Reshaping for Lower Emittance by DOE• Spatial flat top distribution achieved by Diffractive Optical Element (DOE)for high quality e- beam generation• Principle: Diffraction optics by lens and micro-configuration• Desired intensity distribution can be realized (phase coding)

8mm• World’s first application of DOE for UV laser

4mm

Laser System for RF Gun
Spatial Reshaping for Lower Emittance• Application DOE in 1st laser line from 2020c and in 2nd laser line from 2021c• Elliptical flat-top spatial distribution on the surface of photocathode (LA-8mm SA-4mm) forlower emittance e- generation and less discharge

1st laser before DOE

2nd laser before DOE

1st laser after DOE

2nd laser after DOE

Laser System for RF Gun
Better Laser Pointing Stability for Stable and Long-term Operation

Laser position feedback system

Laser pointing stability at virtual photocathodeMeasured in 2019.06
without DOE & laser
position feedback
H 2σ: 48.04±0.51 μm
V 2σ: 46.08±0.69 μm

Measured in 2021.06
with DOE & laser
position feedback
H 2σ: 24.30±3.06 μm
V 2σ: 10.08±0.46 μm

Improvement plan ofelectron beam

19

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]20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

21

New Quasi-Travelling Wave Side Couple RF-Gun[Current RF-Gun issue]- Laser window life time
- Dischargement at choke structure- Dark current- Energy slope- Focusing magnet

will be installed in next summeremittance～ 7mm・mrad22

BTe accelerating structure installation• Bunch compression to reduce short range transverse wakefield→ Longitudinal wakefield causes large energy spread (>5ps @ 2nC)
• Harmful fine structure due to longitudinal wakefield=> BT-ECS is effective for lower energy spread and smoothing• Energy jitter reduction• Additional voltage for 6S resonance and multi-bunch operation.
• 3m(longer) acceelerating structure are transferred from HarimaBTe-ECS (FY2024) to improve injection efficiency

ECS1 was chosen due to building difficulty of ECS2

24

BT-ECS at BT1
4 x 3m accelerating structure

Low loss
circular waveguide

Combine two klystron outputs

The Single-stage Pulse Stacking Birefringent Filter② Pulse double stacking.
45𝑜𝜃n =∙ 2𝑛 − 1
𝑁YVO4

𝑙𝑐𝑟𝑦𝑠𝑡𝑎𝑙 = 19 𝑚𝑚
𝜃1 = 45𝑜

Polarizer 1
Crystal 1① Stretchedpulse width to18 ps.

Polarizer 2

Fiber
Output

Fiber
Input

Grating pairStretcher

Fiber Amplifier

Menlo Oscillator

Fiber Amplifier

OSA Pulse Picker③ Compensate theenergy loss.

Fiber Amplifier& Noise cut off

EO Pulse PickerSetup diagram⚫ The new fiber amplifier compensate the energy loss of the pulsestacking.
⚫ After the modification, there is no impact on main laser
amplification efficiency and electron generation rate.
⚫ Due to the optical path expansion, it is necessary to adjust thephase delay of the SOA pulse picker and EO pulse picker.
⚫ For double bunch amplification, the efficiency of the secondpulse is reduced.

Grating pair
Stretcher

Pulsestacking

Fiber
AmplifierLaser line before
Laser line inset the stacking
Laser Line now

Intensity [a.u.]

Auto-correlation of A1 laser
without YVO4With 19mm YVO421 ps
24 ps

-50

-40

Slit-30-20-100
Delay [ps]1020304050

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 (fully covered 8 mm cathode area) worked fine without any significant trouble.
• High bunch charge e- was demonstrated. Achieved 6 nC from e- gun and 4 nC at the linac end.
• New piezo mirror feedforward system improve beam stability.
• Better QE IrCe composite cathode was developed and under testing.
==== To Do ====• Gradual decrease of bunch charge due to laser window deterioration : new QTWSC RF-Gun will be installed.
• Beam stability should be improved : Laser frequency comb in vacuum line(Zhou)• 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.
=> Best temporal manipulation• Temporal manipulation system for laser system.
for better emittance and lower energy spread28