RF‐Gun for SuperKEKB
SuperKEKB review @ 05 Mar, 2013
Mitsuhiro Yoshida

SuperKEKB upgrade
for low emiGance electron beam
High charge low emiGance isrequired for SuperKEKB.

KEKB obtained(e+ / e-)SuperKEKB
required (e+ / e-)

Beam energy

3.5 GeV / 8.0 GeV

4.0 GeV / 7.0 GeV

Bunch chargee- e+ / e10  1.0 nC / 1.0 nC

e-  e+ / e10  4.0 nC / 5.0 nC

Beam emittance(γε)[1σ]2100 µm / 300 µm

6 µm / 20 µm

5 nC 10 mm‐mrad electron beam generated by RF gun.
+ 10mm‐mrad emiGance preservaNon is required.
DAW RF‐Gun was installed at Dec 2012
Quasi –travelling side couple RF‐Gun will be installed soon
Preliminary test at 3‐2due to earthquake

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 emiGance diluNon• 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 Nme)=> Ir5Ce has very long life >me and QE>10‐4 @266nm• Laser : Stable laser with temporal manipulaNon– LD pumped laser medium => Nd / Yb doped– Temporal manipulaNon => Yb doped=> Minimum energy spread• RF‐Gun– Design of RF‐Gun cavity• Disk‐And‐Washer (DAW)• Quasi travelling wave side couple– Cathode– Laser– Test stand and schedule

RF‐Gun for 5 nC20

• Space charge is dominant.
• Stable operaNon 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 psemittance(RF)̲x@5nC&90MV/m

18emittance(total)̲x@5nC&90MV/m

14
12108Total

64Space charge
RF200

1020

3040

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

50

DAW (Disk and Washer) type RF‐GunElectric ﬁeld focus

Large coupling

Strong focussing ﬁeld
Curved surface cathodefor low emiGance.

CathodeLow electric ﬁeld

EmiGance
6 mm‐mrad（5 nC）

Beam size (σ)
1.2 mmBunch length (σ)
8 psecFabricaNon of DAW RF‐Gun

Adjust with spacer
on cathode rod

ReﬂecNon raNo

G = 0.119

Coupling

β = 1.27

Q factor

Q0 = 6007.3

Loaded Q

QL = 2646.4

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 andaccelerated gradient than DAW.

2D Designing of the quasi traveling wave side couple RF gun
OpNmize 1st cell by
mulN‐dimensionalparameters

Maximum surface
E‐ﬁeld is 120 MV/m

time=2.48098e-009

EmiGance 6 mm‐mradat 5 nC

Energy [MeV]

11.7
11.6
11.5
11.4

Energy : 11.5MeV

11.3

728
GPT729

730

z

731

732733

Quasi‐traveling wave side couple
Field calculaNon

CathodeCoupling caviNes

Acc. caviNes

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 (0.2 mm)

Beam Size• 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 Ir5Ce
JusNn Jimenez, A SystemaNc Cathode Study , Ph.D Thesis, Monterery, California

Low WorkfuncNon (2.8 eV)and enough QE (10‐4)
at room temperature.
• InacNve in air• Recover by heaNng orlaser cleaning•

Best choicefor SuperKEKB 5 nClong >me opera>onPhysica Scripta. Vol. T71, 39‐45, 1997.
Cathodes for Electron Guns
G. I. Kuznetsov

Condi>on
HV = 16kV
Vacuum ; 5.8×10‐6 Pa
Cleaning Nme ; 10 min

Quantum eﬃciency improvement
by Laser cleaning

QE

1500
1200

LaMz

900

LaB6
SEM‐EDX

Oxigen
OKsum

Counts1800

Carbon
LaMa

BKa

2100

CKsum OKa

2400

CKa

2700

10‐4LaLa

Ir5Ce Cathode600

00.01

1.51

2.01

2.51

3.01

3.514.01

4.51

keV

Ir5Ce
Carbon

8000

CKa

4000
3000
2000

0.50

1.00

1.50

IrMr

5000

IrMz

6000

CeMa
OKsumFKa

7000

CeLa

10000

IrMa

SEM‐EDX

11000
9000

Ir5Ce@266nm

1.01

12000

LaB6@266nm
LaB6@532nm

0.51

CKsum OKa

×２０300Counts

10‐5Lower energy density
is enough to acNvate Ir5Ce

1000
00.00

10‐60

2.00

2.50

3.00

3.50

4.00

4.50

keV

1000
2000
3000
4000
Energy density of cleaning laser(μJ/mm2)

Non acNvaNon
QE = 5.00×10‐6×20Laser cleaning
Max QE = 1.00×10‐45000

No oxidizaNon is observed[SUPER‐KEKB e Linac]
Laser Power ; 233μJ/pulse（λ=266nm）
Target value ; 5nC

5.00

LifeNme measurement（LaB6 / Ir5Ce）
10QE‐4IrCe
LaB610‐505101520253035

Life>me (hour)

Condi&on
Con>nuous pulse laser 200μJ@266nm)
Irrada>on => 2.5 nC emission

404550• RF‐Gun– Design of RF‐Gun cavity– Cathode– Laser• Nd:YVO4 / Nd:YAG Solid state laser• Yb ﬁber laser– Test stand and scheduleNd‐doped

Laser medium and its eﬀciency
τ〜200μs, 40%○ 4-state laser is easy to operate.
○ High power pump LD is available.
○ Large crystal is available
× Pulse width is determined by SESAM.
(Gaussian)Pump(808nm)808nm
200μs Nd:YVO41064nm 40%

532nm 20%SHG

FHG

Nd:YAG
Nd/Cr:YAG
Nd:YLF○ Wide bandwidth => pulse shaping
τ〜900μs, 40%○ Long fluorescent time => High power
976nm
Yb‐glass 1040nm 40%○ Fiber laser => StablePump
Yb:YAG○ Small state difference => High efficiency
SHG(976nm)× ASE
Yb:KGW× Absorption

800nmTi:Sapphire○ Very wide bandwidth
TW laser is based on Ti‐Sapphire○ High breakdown threshold× Low cross secNon
× Short ﬂuorescent Nme => Q‐switched laser is required for pumping

40%SHG

520nm 20%

Wavelength
Fluorescent Nme
Spectral width
Fourier minimum
pulse width
Wavelength
Spectal width
Quantum eﬃciency

260nm 10% 208nm

FHG

400nm 20%

Material

Fluorescence

Pump(808nm)

τ〜3μs, 40%

532nm
10ns

AbsorpNon

Ti‐doped

τ=200μs, 40%

808nm
1064nm40%
200μs
Nd:YAGSHG

5HG 3%

Nd or Yb solid state laseris required for stable operaNon

Yb‐dopedPump

266nm 10% 213nm

5HG 3%266nm 10% 200nm

THG

FHG
Ti‐Sapphire

0.5%

Energy spread reducNon using temporal manipulaNon
315nC

Gaussian
5nC 10nC

2.520nC

σE/E (%)

21.5

1

Square

5nC
10nC
15nC
20nC

0.5

005

10 5nC electron
15FWHM (ps)15nC202530Primary beam for positron producNon

Nd based laser system• Nd:YVO4 oscillator + Nd:YAG mulN‐pass ampliﬁer

30 ps (10 mm)

Yb‐ﬁber & Yb solid state laser developmentYb Fiber Main-Amplifier
70W

LDPumpλ/4 λ/2

Yb Fiber Pre-Amplifier

Solid-state Amplifier Yb:KGW

2.5µJ


100Hz

LD200W

10W

Pump Laser

3mJ

Frequency doubling LBO
517nm

BBO

Yb Fiber Oscillator
LD~30ps
0.5mJ
Spectral Range
Adjustment

1035nmWDM

Prism Pair

Intensity [a.u.]

Spectrum of Seed (52MHz)

980

990

1000

1010

1020 1030 1040
Wavelength [nm]

1050

1060

1070

1080

Oscillator & pre‐ampliﬁer are already working.

259nm

Yb disk Laser
Yb:YAG disk
10 % doped
2mm thickness

Yb disk laser

35030% eﬃciency was achievedat room temperature Yb:YAG300Eout (mJ)

250
200
1501005000200400600800
Epump (mJ)

1000

940nm LD (2.4 kW / module)

RegeneraNve ampliﬁer using Yb disk laser

1200

1400

• RF‐Gun– Design of RF‐Gun cavity– Cathode– Laser– Test stand and schedule• 3‐2 RF‐Gun for preliminary test & PF injec>on• A‐1 RF‐Gun

3‐2 RF‐Gun

3‐2 Laser hut

3‐2 RF‐Gun (2011/10)

・52MHz Oscillator
・DPSS Module
・AO + EO x 2 pulse picker

Cathode LaB6 => Ir5Ce （2012/03)
LaB6Ir5Ce

Laser injecNon with angle （2012/05)

5nC was achieved !

• 4 mJ @ 266nm => 1.5 mJ on cathodeQ‐scan=> 14.1 mm‐mradfor 1nC(Preliminary)A‐1 RF gun‐ DAW RF‐Gun was installed=> Quasi‐travelling wave side couple RF‐Gun will be installed soon.
‐ Yb based laser system is under test

Exist DC gun(Positron beam primary)

SH buncher

Pre buncher,Buncher
DAW installed
2 m Acc.

A1 sector

Summary• RF‐Gun cavity– 5nC Demonstra>on was done using DAW‐type RF‐gun.
– Quasi travelling wave side couple structure :FabricaNon is almost done.
– Ageing process was ﬁnished for less than only one week.

• Cathode– Room temperature 6mmφ Ir5Ce cathode has enough QE.
– Laser cleaning & laser injecNon angle is eﬀecNve.

• Laser & control– Nd based laser system : 3‐2 RF‐Gun– Yb based laser system : A‐1 RF‐Gun• Yb‐ﬁber :Precise RF synchronizaNon• Yb‐disk ampliﬁer:
High power output• Temporal manipulaNon To be developed.

– Stability / Control:

under test