Generation and Acceleration ofLow-Emittance, High-Current
Electron Beams for SuperKEKB
M. Yoshida, N. Iida, S. Kazama, T. Natsui, Y. Ogawa, S. Ohsawa,
H. Sugimoto, L. Zang, X. Zhou,
High Energy Accelerator Research Organization
D. Sato, Tokyo Institute of Technology

SuperKEKB Upgrade and RF gun development
KEKB obtained(e+ / e-)SuperKEKB required(e+ / e-)Energy3.5 GeV / 8.0 GeV

4.0 GeV / 7.0 GeV

Chargee-  e+ / e10  1.0 nC / 1.0 nCe-  e+ / e10  4.0 nC / 5.0 nC

Emittance[mm-mrad]2100 / 300ee+J-linacPF
2.5~3.0 GeV

HER
7 GeV

6 / 20SuperKEKB
LER
4 GeV
AR
6.5 GeV

5 nC 10 mm-mrad electron beam generated by RF gun.
+ 10mm-mrad emittance preservation is required.
Quasi –travelling side couple
RF-Gun with Yb laser system.
(2013)

Preliminary test using DAW
RF-Gun with Nd laser system
due to earthquake. (2011)

B sector A sector A-1 RF gun

J arc

3-2 RF gun

C sector

1 sector

2 sector

3 sector

4 sector

5 sector

RF-Gun development strategy for SuperKEKB• Cavity : Strong electric field focusing structure– Disk And Washer (DAW)=> 3-2, A-1(test)– Quasi Traveling Wave Side Couple=> A-1=> Reduce beam divergence and projected emittance dilution• Cathode : Long term stable cathode– Middle QE (QE=10-4～10-3 @266nm)– Solid material (no thin film) => Metal composite cathode=> Started from LaB6 (short life time)=> Ir5Ce has very long life time and QE>10-4 @266nm• Laser : Stable laser with temporal manipulation– LD pumped laser medium => Nd / Yb doped– Temporal manipulation => Yb doped=> Minimum energy spread• RF-Gun– Design of RF-Gun cavity• Quasi travelling wave side couple– Cathode– Laser– Test stand and schedule

RF-Gun for 5 nC
20

• Space charge is dominant.
• Stable operation is required.
– Lower electric field : < 120MV/m• Focusing field must be required.

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

16

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

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

18

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

1412108Total

64Space chargeRF20010

20

30

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

4050

Electric focusing field by narrow gap
Closed gap makes focus field
Side coupled cavity is one candidate (or DAW / ACS / CDS …)

Accelerating field
加速電場

Concentrated field
has focusing effectThis structure
has long drift
spaceThis structure has focusing field.
Long drift space is problem.

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

Quasi traveling wave sidecouple structure

Quasi traveling wave side couple has stronger focusing fieldQuasi traveling wave side couple RF gun
This RF gun has total of seven acceleration cavities. These are divided into two
standing wave structure of 3 and 4 side coupled cavities respectively.
543rxyCathode cell
To avoid beam defocusing, emittance
growth and field concentration, a lot ofparameters were searched for design.

2100

100

200300GPTcathodeMaximum E-field atsurface: 120 MV/m

Maximum E-field at surface: 100 MV/m

Emittance: 5.5 mm-mrad @ 5 nC
This RF gun can generate 10 nC beam

400500z600

700

8009001000

Beam tracking simulation result
5 nC

10 nC

Emittance 5.5 mm-mrad

Gun Exit
Size 0.4 mm

time=2.48098e-009

Energy [MeV]

11.7
11.6

Energy spread
0.6%

11.5

Bunch
shape

11.4
11.3

728
GPT

729

730

z731

732733

RF-Gun comparison
Quasi traveling wave side couple RF gun(100 MV/m, 6mm-mrad, 13.5 MeV)

DAW-type RF gun(90 MV/m, 5 mm-mrad, 3.2 MeV)
BNL-type RF gun(120 MV/m, 11.0 mm-mrad, 5.5 MeV)

BNL (3.8 mm)
DAW (1.7 mm)
QTW (0.2mm)

Beam SizeCavity design042

3.1

Field calculation
Frequency GHz

3.0k : 3.0 %
2.9

2.8

2.7042Phase shift cell radcathodebeam

coupling cavities

acceleratingcavity

No reflection to klystronMechanical design and manufacturing• RF-Gun– Design of RF-Gun cavity– Cathode• Advantage of LaB6
• Measurement equipment of quantum efficiency• Laser cleaning & Heat treatment– Laser– Test stand and schedule

Cathode : Advantage of LaB6 or Ir5CeJustin Jimenez, A Systematic Cathode Study , Ph.D Thesis, Monterery, California• Low Workfunction (2.8 eV)and enough QE (10-4)
at room temperature.
• Inactive in air• Recover by heating orlaser cleaning

Best choicefor SuperKEKB 5 nClong time operation

Physica Scripta. Vol. T71, 39-45,
1997.
Cathodes for Electron Guns
G. I. Kuznetsov

900

LaB6
SEM-EDX

Oxigen
OKsum

1200

LaMz

CKa

1500

LaMa

BKa

2100

CarbonCKsum OKa

2400

1800
CountsQuantum efficiency improvement
by Laser cleaning

2700

LaLa

Ir5Ce Cathode600
300
00.01

1.01

1.51

2.01

SEM-EDX

IrMa

10000
9000

2000

CKa

4000

0.50

1.00

1.50

IrMr

5000

IrMz

6000

CeMa
OKsum

FKa

7000

CKsum OKaCounts

8000

3000

3.01

3.51

4.01

4.51

keV

12000
11000

2.51

Ir5CeCarbon
No oxidization
is observed
CeLa

Lower energy density
is enough to activate Ir5Ce

0.51

1000
00.00

×２０
QE lifetime

2.00

2.50

3.00

3.50

4.00

4.50

5.00

keV

QE Enhancement of IrCe cathode• RF-Gun– Design of RF-Gun cavity– Cathode– Yb Laser for spatial & temporal manipulation.
– Test stand and scheduleEnergy spread reduction using temporal manipulation
Energy spread of 0.1% is required for SuperKEKB synchrotron injection.

315nC

t

5nC10nC

2.520nC

Gaussianσ E/E (%)

21.51tSquare 5nC
10nC

0.515nC
20nC
00510 5nC electron
15FWHM (ps)2015nC

2530

Primary beam for positron productionProperties of laser medium

Nd laser system for 3-2 RF-Gun

τ～200μs, 40%

Nd-doped○ 4-state laser is easy to operate.
SHG(532nm)
Nd:YVO4○ High power pump LD is available. LD Pump40%(808nm)○ Large crystal is available
Nd:YAG
FHG(266nm)
× Pulse width is determined by SESAM.
808nm
20%
1064nm(Gaussian)
5HG(213nm)
3%

Yb-doped○ Wide bandwidth => pulse shaping
τ～900μs, 40%○ Long fluorescent time => High power
SHG(520nm) 40%
Yb-glass○ Fiber laser oscillator => Stable
LD PumpFHG(260nm) 20%○ Small state difference
Yb:YAG(941/976nm)
5HG(208nm) 3%
× ASE
Yb:BOYS
941/976nm
1040nm
× Absorption

Ti-dopedPump τ=200μs, 40%

τ～3μs, 40%40%

Pump
Nd:YASHG(808nm) 808nm
G1064nm

Ti:Sapphire

Fluorescence

800nm
532nm
○ Very wide bandwidth○ High breakdown threshold
TW laser is based on Ti-Sapphire× Low cross section
× Short fluorescent time => Q-switched laser is required for pumping

Absorption

Best for RF-Gun

Material

Nd:YAG

Yb:YAG

Ti:Sapphire

Wavelength

1064nm1030nm

660-1100nm

Fluorescent time

230ms

960ms

3.2ms

Spectral width

0.67nm

9.5nm

440nm

Fourier minimum
Pulse width
Wavelength

2.48ps

165fs

2.59fs807.5nm

941nm

488nm

Spectral width

1.5nm

21nm

200nm

Quantum efficiency76%

91%

55%

SHG(400nm) 40%
THG(266nm) 20%
FHG(200nm) 10%Ti:Sapphire laser system.

Yb fiber & thin disk hybrid laser system

258 nmQE = 10-4515 nm

A few mJ @ 258nm, 50Hz is required.

Yb Fiber Laser

WDM

LDYb Fiber←←

Transmission
grating pairThin-disk multi-pass amplifier• 0.5 mmt Yb:YAG thin-disk• 3-stage 4-6 multi-pass amplifier
Yb:YAGLDLD5
3
1

6
4
2

SHG+FHG

A few mJ @ 258nm

• RF-Gun– Design of RF-Gun cavity– Cathode– Laser– Test stand and schedule• 3-2 RF-Gun for preliminary test & PF injection• A-1 RF-Gun

A-1 RF gun- Quasi-travelling wave side couple RF-Gun- Yb based laser system

Existing DC-Gun & pre-bunche
Installed RF-Gun

Chicane for bunch compression
30ps => 10ps

Installed RF gun
CathodeLaser port

A-1 RF gun results 5.6 nC bunch charge
was observed.

RF gunA1 sector at KEK linac

beam size measurement for Q-scan

Q-scan emittance measurementxy32.7 ±3.1 mm-mrad10.7 ±1.4 mm-mrad

Emittance preservation

Emittance preservationJ-ARC R56

Emittance growth after 500 m

Initial offset

Hardware for emittance preservation• Alignment– Continuous monitor (HLS, Wire)+ Active mover– Beam based alignment(Higher mode measurement)• Temporal manipulation– Laser pulse shaping– Bunch compression• Beam diagnostics for offset injection– RF Deflector

Developed by SLAC

Preliminary test for higher order transverse wakefieldfrom accelerating structure.
Accelerating structure

BPM

TM12 : 8.728 GHz
Dipole mode
DBM

Synchronized RF
８～12 GHz(Base RF: 10.38MHz)

90 degHybridIQ

Magic T• RF-Gun cavitySummary– Quasi travelling wave side couple structure.

• Cathode– Room temperature Ir5Ce cathode has enough QE.
– Laser cleaning & laser injection angle is effective.
– R&D for the QE improvement.

• Laser & control– Yb based laser system : A-1 RF-Gun• Yb-fiber :Precise RF synchronization.
• Yb-disk amplifier:High power output.
• Temporal manipulation Under experiment.

– Stability / Control:

Improved but not enough.

• RF gun comissioning– 5.6 nC bunch charge was generated by this RF gun.

• Emittance Preservation– Alignment / Bunch compression / Monitor etc.