RF GUN, LASER & ELECTRONBEAM COMMISSIONING
The 23rd KEKB Accelerator Review Committee
Rui Zhang 2019.07.09
Accelerator Laboratory Division V, Injector G

SuperKEKBREQUIREMENTS
Electron
HER 7 GeV

Positron
LER 4 GeV

Normalized Emittance

40 / 20 [μm]100 / 15 [μm]Energy Spread

0.07%

0.16%

Bunch Charge at Injection
Point

4.0 nC

4.0 nC

Stable electron beam with high charge and low emittance isrequired for SuperKEKB

SuperKEKB RF Gun
Requirements• 4 nC electron charge generation• 10 μm emittance preservation• Long term operation
Operating at 4 nC• Less space charge effect• Longer pulse: 20-30 ps• Necessary strong focusing field• Preserves the emittance• Stable long time operation• Lower electric field: < 100 MV/m

Side coupler or Disk and washer is preferredRF gun cavity: strong focusing electric field• Disk and washer (DAW): tested in 3-2• Quasi travelling wave side coupler (QTWSC): A1 primary RF gun• Cut disk structure (CDS): A1 secondary RF gun

Photocathode: long life time
• Medium QE with long life time: 10-4~10-3 @266nm > 1 year
• Metal composite: Ir5CeLaser: simple, stable with temporal and spatial manipulation• Laser active material pumped by laser diode– Yb doped fiber & Neodymium (Nd) doped laser crystal: A1 ground laser hut– Yb doped fiber & Ytterbium (Yb) doped laser crystal: A1 underground laser hut• Temporal and spatial reshaping for minimum energy spread and emittance04-17

05-17

Ir7Ce2 photocathode & QE map

Novel type cathode plug in this stagee-x (mm)• QE > 10-4 in off-line measurement (10-6 Pa)• QE = 10-5 in operation condition becauseof vacuum level (10-5 Pa)• Electron beam heating: the main cathode is bombardedby an accelerated electron beam
• Successful heating over 1000 ℃• QE increased 3 times after cleaning
06-17

4 months
0 fault
operation!

4 months
0 fault
operation!

Layout of Nd:YAG rod amplifier
Input @1064 nm (<1 μJ)
1st stage

3rd stage

2nd stage

4 m x 0.8 m
Optical path: 22 m

5th stage

Pockels cell

4th stage

Output(10 mJ)

09-17

A1 ground laser hut

Tunnel RF gun box

2nd line

90o RF gun

BBO

Delay line

HWPFlip mirror
Polarizer

1st line
11 m longtransporting line

HWP Polarizer HWP

Laser with vertical polarization,BBOe-0o RF gun

BBO

Irradiated by two lasers

laser with horizontal polarization, HWP: half wave plate

10-17

1st laser
2nd laser

1.3 nC

Optics layout in RF gun box

11-17

12-17

SP_A1_G Charge>4 nC

2.8 nC2856 MHz phase

B-sector Wire
Scanner
2019.06.27
1.2 nC @ SP_A1_G
4-wire ABCD:
γεx=12.655 μm,
γεy=11.225 μm

14-17

15-17

Laser temporal reshaping for low emittance, low energy spread, and injection noise suppression
I t1

t2

• Coherent pulse stacking technology by use of the birefringent effect of crystal• Demonstrated in Nd laser system (Supported by Dr. Honda in 2018, R4.2)• Due to the narrower spectrum width of the current Nd laser system (wider
temporal width), the real rectangular shape is impossible (Fourier transform limit)t

I

Laser temporal distribution

x’ t 1Et2
x

Phase spacet
Energy spread2019 Q1

Nd Laser SystemYb Laser SystemI

2019 Q2tt

Nd laser system: 17 psYb laser system: 2.5 ps• Yb laser system is the best and only candidate for low energy spread and low
emittance electron beam generation in Phase III
2019 Q3

2019 Q4

2020 Q1

2020 Q2

2020 Q3

2020 Q4

Primary laser for RF gun and HER injection
Laser & monitoring system preparation with stacking

2021 Q1

2021 Q2

2021 Q3

2021 Q4

Backup laser system for SuperKEKB
e- beam test

For RF gun and HER injection

16-17

17-17

15-20

16-20Input laser pulse
with temporal
Gaussian shape(FWHM~10 ps)

Pulse stacking setup made by a crystal array(birefringent effect of crystal)

Output laser pulsewith temporal
rectangular shape(FWHM~40 ps)

Two stages stacking system made by YVO4 crystals
Input Nd:YLF mode lock laser
pulse with 9.4 ps FWHM width* Supported by Dr. Yosuke Honda

Achieved laser pulse by stacking

17-20Simulation results: elliptical beam pattern

6 mm

3 mm

18-20

19-20MTBF (Mean Time Between Failure、平均故障間隔) MTTR (Mean Time To Repair、平均修理時間)
稼働率＝MTBF ÷ (MTBF+MTTR)Yb:fiber/Nd:YAG laser in Phase II
Oscillator 1

Oscillator 2Oscillator 3

Nd:YAG
1st line
Yb:fiber
AmplifierTransportingline
Nd:YAG
2nd line

RF gunDelay line

12-26

Improvement of e- Stability in Phase III
Stable e-  Improved the laser stability by water cooling for optics table
Two days laser operation history

Laser output

1st Amp2nd Amp3rd Amp4th Amp
5th Amp Time

19-26

For High Quality eLaser Spatial Reshapingx’ EmittanceI

Laser Temporal Reshaping
I t1t2x’ t1xr

Linearity

Laser spatial distribution

Phase spacet
Laser spatial distributionI• Laser beam shaper for flat-top• For Nd and Yb laser• Under purchasing

t2xPhase space
Itt

Nd:YAG laser

Yb:YAG laser• Pulse stacking by birefringent crystals• Demonstrated in Nd laser• Will be used in Yb:YAG laser for real flat
24-26

• F: 1 m (Focal length of F lens)• Ф: 6 mm (on target surface)• Off-axis: 6 mm (distance between optical axis andcenter of target spot)• β≈1o• Simulation result• peak to valley modulations on the top hat: ±10%~15%• Transmitting rate> 80%

RF-Gun for 5 nC20

• Space charge is dominant.
• Stable operation is required.
– Lower electric field : < 100MV/m• Focusing field must be required.

emittance(SC)_x@5nC&90MV/m

16Emittance (mm mrad)– Longer pulse length : 20 - 30 ps

emittance(RF)_x@5nC&90MV/m

18

emittance(total)_x@5nC&90MV/m

1412108Total

64Space charge
RF200102030

40

Tlaser (ps)– Solenoid focus causes the emittance growth.
– Electric field focus preserve the emittance.
Epaxial coupled cavity : BNL
Annular coupled cavity : Disk and washer / Side couple

50Required laser pulse energy
Current laser energy(500μJ)
1.00E-07

Charge( C )Charge(QE=1e-5)@266nm
Charge(QE=1e-4)@266nm
Charge(QE=1e-3)@266nm

Ce2Te

QE=10-3

QE=10-4

Offline measurement
S-polarization
Offline measurement
P-polarization

1.00E-08

QE=10-55nC

1.00E-09

1.00E-10
1.00E-06

ATF

Inclined injection

1.00E-05

1.00E-04

Laser power ( J )Normal injection (old 3-2)

1.00E-03

1.00E-02

Pill-box cavity

Annular coupled cavity with nose

Accelerating field

Accelerating field
加速電場ビーム
Beam

Beam

FieldEz, Er/r

Ez, Er/r=> 集束力
Field
Focusing fieldz

Accelerating field

Focusing
field
Given forcez

Focusing
field

Accelerating fieldEz, Er/rEz, Er/r

Accelerating fieldz

Accelerating field

Given forcez

Focusingfield37

Design of a quasi traveling wave side couple RF gun
Normal side couple structure

Quasi traveling wave sidecouple structureLong drift space is problem.

Quasi traveling wave side couple has stronger focusing andaccelerated gradient than DAW.

Electron beam (EB) heating type
cathode plug
【Design of EB heating type cathode plug】

EB heating is …
a main cathode is bombarded by anaccelerated electron beam as a thermal
source behind the cathode.

e-【 RF shielding structure 】
S-parameter (dB)0-20-40-60
Reflection
Transmission-80-100
22.5
33.5
Frequency (GHz)439

Heating Test
【Cathode plug】【Experimental Setup & Results】
Radiation
Thermometers

W wire (0.7φ)

Heater Current (A)Heater Voltage (V)Intermediate electrode(V)High Voltage (kV)Beam current (mA)
Beam Power (W)Surface Temperature(degC)

16
3.38
-25
2.3
2352.91029

Heating a photocathode over 1000℃ byEB heating method was succeeded.
40

YCOB CRYSTAL at LINAC

From Prof. YQ Zheng’s Slide, the 1st KEK and SICCAS
nonlinear optics forum, Tsukuba, May 28, 2018

Phase II Nd laser module