Introduction of Linac and RF Gun Drive

Laser System for SuperKEKB

The Meeting of Accelerator Laboratory 2021.10.19

Rui Zhang (ACCL 5, Injector Group)CONTENTS• General introduction of Linac injector for 4 rings• Development of RF gun drive laser system for SuperKEKB• Summary and outlook

2-35

2-27

GENERAL INTRODUCTION of LINAC INJECTOR
General Layout of Linac Injector

3-27

GENERAL INTRODUCTION of LINAC INJECTOR
Electron Source PartTop view

RF gun (GR_A1)

・e- study: 1 - 3 nC (KBE for HER injection)

Thermionic DC e- gun (GU_AT)
Pulse bends in 24o line:
Pulse to pulse beam switchingfor RF e- gun/thermionic e- gun・e+ production e-: 13 nC (KBP for LER injection)・e- study: 1 nC (JBE)
・PF injection: 0.1 - 0.3 nC (QFE)
・PF-AR injection: 0.1 - 0.3 nC (ARE)Side view

4-27

GENERAL INTRODUCTION of LINAC INJECTOR
RF Gun at Sector A1 of Linac

0o RF Gun••••

Primary RF gun
QTWSC cavity
Two laser beams 60º
oblique incidence
Ir7Ce2 photocathodeSide view
M. Yoshida, X.Y. Zhou, R. Zhang

5-27

GENERAL INTRODUCTION of LINAC INJECTOR
Electron Beams Merge Part

Thermionic DC e- gun (GU_AT)
2 subharmonic bunchers (114 MHz, 571MHz) and 2 bunchers• e- for e+ production (LER): 13 nC• e- study: 1 nC
• PF injection: 0.1 - 0.3 nC• PF-AR injection: 0.1 - 0.3 nC

0o RF Gun••

Primary RF gun
e- for HER injection: 2 nC

90o RF Gun••

Spare RF gun
Under testing

6-27

GENERAL INTRODUCTION of LINAC INJECTOR
Positron Target and Capture Section

Tungsten Target & Flux Concentrator
Y. Enomoto, et.al7-27

GENERAL INTRODUCTION of LINAC INJECTOR
Positron Target and Capture Section

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)

Y. Enomoto, LCG, 2020.07

8-27

General Introduction of Linac Injector
Beam Injection Pattern Generation for 4-Ring Simultaneous Top-up Injection

KBE: SuperKEKB e- (HER)
KBP: SuperKEKB e+ (LER)
QFE: PF
ARE: PF-AR

M. Satoh, et.al•

Selectable beam repetition rate according to the demand from each ring•

Adjustable priority for different case
9-27

GENERAL INTRODUCTION of LINAC INJECTOR
Smooth 4-Ring Simultaneous Top-up Injection

M. Satoh, KEKB ARC, 2021.09

10-27

DEVELOPMENT of RF GUN DRIVE LASER SYSTEM
Neodymium-doped YAG (Nd:YAG) and Ytterbium-doped YAG (Yb:YAG) for 1 μm Laser

Nd:YAG Laser

Yb:YAG Laser• 4 energy level laser with high quantum
defect (24%)• Short fluorescent time (230 μs)• Narrow bandwidth (0.67 nm)• Wide laser pulse (~15 ps for A1 laser)
𝜏𝐹𝑊𝐻𝑀 = 0.44/∆𝜐 (Bandwidth limit)• Used in phase II and III• Quasi 3 energy level laser with low
quantum defect (8.7%)• Long fluorescent time (960 μs)• Wide bandwidth (9.5 nm)• Narrow laser pulse (~2 ps for A1 laser)• Used in phase I• Novel Yb:YAG rod laser system underconstruction

11-27

LONG-TERM OPERATION PLAN of LASER SYSTEM for RF GUN
Yb:YAG Disk Laser and Nd:YAG Rod Laser

Yb:YAG thin disk laser

Nd:YAG rod laser

Yb:YAG disklaser head

Nd:YAG rod length:80~95 mmNd:YAG rod laser amplifier module

Yb:YAG disk thickness: 1 mm•••••Good thermal management and modern laser design
Necessary multi-pass for amplification
Used in Phase I
Poor stability and complex adjustment
Not suitable for accelerator long term operation•••••

Mediocre thermal management
Single or double-pass for efficient amplification
Used in Phase II and III
Good stability
Simple adjustment and fast recovery during trouble
case
12-27

INTROCUTION of CURRENT LASER SYSTEM

Upstream

Yb-Fiber and Nd:YAG Hybrid Laser System→→→→→→→

→

13-27

→→

Downstream→

INTROCUTION of CURRENT LASER SYSTEM
Yb-Fiber and Nd:YAG Hybrid Laser System

Laser fromfiber stage

1st Nd:YAG rod
amplifier stage①
2nd stage②

3rd stage③⑤5th stage④4th stage

To BBO forgreen laser

14-27

INTROCUTION of CURRENT LASER SYSTEM
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 RoomLT

Thermionic Gun

Tunnel A1 Part

24o Line

0o RF Gun
A1 Chicane

15-27

INTROCUTION of CURRENT LASER SYSTEM
Yb-Fiber and Nd:YAG Hybrid Laser System

16-27

INTROCUTION of CURRENT LASER SYSTEM
Yb-Fiber and Nd:YAG Hybrid Laser System

17-27

INTROCUTION of CURRENT LASER SYSTEM
Yb-Fiber and Nd:YAG Hybrid Laser System

Simple illustration for 2 lasers incidence
A1 ground laser hut

Tunnel optics table
90o RF gun

2nd line

BBO

Flip mirror

Delay line

HWPPolarizer

1st line
HWP Polarizer HWP

11 m long transporting line

Laser with vertical polarization,

BBO

e- beam0o RF gunBBO

laser with horizontal polarization, HWP: half wave plate
18-27INTROCUTION of CURRENT LASER SYSTEM
DOE for Laser Spatial Distribution Reshaping• DOE: Diffractive Optical Element for high quality e- beam
• Principle: Diffraction optics by lens and micro-configuration• Desired intensity distribution can be realized (phase coding)

6mm• World’s first application of DOE in UV laser part

3mm

19-27

INTROCUTION of CURRENT LASER SYSTEM
DOE for Laser Spatial Distribution Reshaping• 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 (LA6mm SA3mm) for

low emittance e- generation and avoiding RF gun discharge

R. Zhang, LCG Meeting, 2020.07

1st laser before DOE

2nd laser before DOE

1st laser after DOE

2nd laser after DOE

20-27

INTROCUTION of CURRENT LASER SYSTEM
Laser Pointing Stability Fluctuation in A1 Laser Ground Laser Hut• Daily temperature drift in A1 ground laser hut due to Klystron gallery and outside environment• Long laser transporting line from ground to tunnel• Laser pointing stability affects stability of electron beam from RF gun

21-27

INTROCUTION of CURRENT LASER SYSTEM
Beam Position Monitor and Feedback System

LaserTransportingLine (LT)A1 Ground Laser Hut
Wide range Piezomirror Mount
Green Lasers

Klystron A1 Gallery

1st Laser Line
2nd Laser Line

Position monitor

GS_A1 RoomTunnel A1 Part

24o Line

0o RF Gun
A1 Chicane

R. Zhang, Linac Technology Meeting, 2020.10.06

22-27

INTROCUTION of CURRENT LASER SYSTEM
Beam Position Monitor and Feedback System

Laser position feedback system

Laser pointing stability at virtual photocathode

Measured in 2019.06
without DOE & beamposition sensorH 2σ: 48.04±0.51
V 2σ: 46.08±0.69

Measured in 2021.06
with DOE & beam
position sensor
H 2σ: 24.30±3.06
V 2σ: 10.08±0.46

R. Zhang, Linac Technology Meeting, 2020.10.06

23-27

INTROCUTION of CURRENT LASER SYSTEM
Photocathode for RF Gun

Quantum Efficiency of PhotocathodeR. Zhang, Linac Technology Meeting, 2020.12.22•

QE Ranking:
KEK Ir7Ce2 >KOBELCO Ir2Ce >KOBELCO Ir7Ce2 >KEK Ir2Ce•

Discharge Probability
KEK Ir7Ce2 <KOBELCO Ir2Ce•

Adoption of KEK Ir7Ce2 from 2021ab•

Achievable bigger size

24-27

INTROCUTION of CURRENT LASER SYSTEM
Laser Status Monitoring Information Page

https://www-linac.kek.jp/ope/opelog2/opelog.html

25-27

INTROCUTION of CURRENT LASER SYSTEM
2 nC Low Emittance Electron Beam Generation

Wire scanner @ B-sector(2021.10.11)• With DOE and position
feedback system• Two lasers incidence• 2 nC from RF gun• 𝛾𝜀𝑥 = 8.574 ± 1.131 𝑚𝑚 ∙ 𝑚𝑟𝑎𝑑
• 𝛾𝜀𝑦 = 8.890 ± 1.215 𝑚𝑚 ∙ 𝑚𝑟𝑎𝑑
• The best record so far

Two lasers incidence at 2021c (2021.10.11)• 2 nC after RF gun for HER beam (by partial energy of two lasers)

Single laser incidence at 2021c (2021.10.18)• 1st laser energy: 250 uJ → Peak charge: 1.4 nC• 2nd laser energy: 250 uJ → Peak charge: 1.5 nC

26-27

SUMMARY & OUTLOOK• Summary• Linac injector operates well for successful 4-ring simultaneous top-up injection• Stable operation of Yb-fiber/Nd:YAG laser provides continuous SuperKEKB HERinjection• Outlook• Current Yb-fiber/Nd:YAG laser will be upgraded during 2021 winter maintenancefor higher energy• Novel Yb-fiber/Yb:YAG laser system is under construction (as one part of LinacUpgrade Project)

THANKS27-27