RF GUN
1T. Sugimura for Injector Linac Group
2010/02/16@ KEKB Review

CONTENTS
 Current status
 RF guns in some projects
 Cathode material, Q.E. and Laser power Emittance evaluation
 Strategy (Summary) Schedule2

CURRENT STATUS“SuperKEKB nano-beam scheme” requires smallemittance beam.
• The electron beam emittance should be less than 20[mm mrad] and beam charge should be no less than 5[nC/Bunch] at the injection point.
• We have decided to develop a photo-cathode RF gunfor the electron beam.
• At the moment, we have two thermionic guns anddon’t have a firsthand knowledge for an RF gun at all.
• We have to boot up an RF Gun including laser systemfrom scratch to finish within a few years.
•3

COMPARISON OF RF GUNSATF-KEK*1TESLA-DESY*2

LCLS-SLAC*3

RFS-Band

L-BandS-BandCathode

Cs2Te

Cs2Te

Cu

Laser power

3 μJ/bunch

50 μJ/bunch

500 μJ/bunch

Charge

1.6 nC/bunch

8 nC/bunch

1 nC/bunch

Laser
Wavelength

266 nm262 nm

255 nm

Life8 Months>1000 Hours? Years

Q.E.

14％ → 1%

1％

3×10-5NormalizedEmittance

1.3 mm mrad

15 mm mrad

1 mm mrad*1 “Improvement of an S-band RF gun with a Cs2Te photo cathode for the KEK-ATF”, N. Terunuma et al., NIM A 613 (2010)*2 “THE RF-GUN BASED INJECTOR FOR THE TESLA TEST FACILITY LINAC”, S. Schreiber, Proc. of the 6th EPAC
*3 “COMMISSIONING RESULTS OF THE LCLS INJECTOR”, David H. Dowell et al. , Proc. of FEL 2007

4

CATHODE MATERIAL Our linac operates 7000 hour/year for 10 straightmonths. Taking such circumstances into account, longlife Cu is a suitable material as a cathode.
 The Q.E. of Cu is about 2 orders of magnitude lessthan that of Cs2Te.
The work function of Cu is 4.6eV and it is much higher thanthat of Cs2Te(3.6eV).

 How to achieve much higher Q.E. for Cu?

5

PHOTO-ELECTRON CURRENT DENSITY , I2
I = α A T f ( δ )α：transition probability by Photon absorption
 A：4πmek2/h3
 T：Temperature δ：h（ν-ν0）/ｋT hν：Photon energy hν0：Work function（include schottky effect）
“Electron gun”, Ohsawa, OHOʼ90, in Japanese6SCHOTTKY EFFECT
Schottky effect（eV）

0.60.50.4

Spring8
BNL type

0.3
0.20.1

Desy0.0050

100

150200250

Electric field gradient（MV/ｍ）
7F(Δ) @T=300K
16000
14000

ｆ（δ）(A.U.)

12000

Wavelength (nm)

260

10000

205
193

8000
6000
4000
2000

X 8.4
X 66

02

2.5

3

3.5

4

Work func0on (eV)

Cu+170MV/m

4.55Cu8

HOW TO ACHIEVE MUCH HIGHER Q.E.?
 Laser Energy with wavelength of about 260nm isalmost equivalent to Work function of Cu.
 Laser with wavelength of 205nm increases Q.E.
by nearly two orders of magnitude.
 Electric field of 170 MV/m reduce a work function
by 0.5 eV.

Shorter wavelength laser and higher electric fieldare essentials.

9EXPERIMENTAL RESULT＠CERN
The QE rose by about two orders of magnitude by shorten
the wavelength of laser from 266 nm to 193 nm.

2.5E‐04

2 orders of magnitude

Q.E.

2.0E‐04

1.5E‐04

1.0E‐04Cu

5.0E‐05

0.0E+00
150200

250

300

350

400

Wavelength (nm)“Photocathodes tested in the dc gun of the CERN”, 1994
E. Chevallay, J. Durand, S. Hutchins, G. Suberlucq, M. Wurgel

10Q.E. COMPARISON
1.0E+00

1.0E‐01
1.0E‐02

Q.E.

1.0E‐03
1.0E‐04

RF processing
IonizedcontrolledetchingCu

1.0E‐05

Cu(ICE)
Cs2Te

1.0E‐06
1.0E‐07
1.0E‐08
1.0E‐09
150200250300

Wavelength (nm)E.Chevallay et al., Proc. Of workshop on High Intensity Electron Source, 1993
Nuclear Instruments and Methods in Physics Research A 340 (1994) 146

350

40011LASER ENERGY ON CU CATHODE FOR 5NC

 Assume the Q.E. on the previous page and at LCLS.

10.57330827

Laser energy （ｍJ）

100.808823529

10.160297927

0.193661972
Cu0.1

0.06916499

Cu (ICE)
Cu @LCLS

0.021373057
0.01
180200220240

Wavelength (nm)26028012

EMITTANCE The required normalized emittance at theinjection point is ２０ mm・mrad, hence theemittance just after RF Gun should be less than
２０ mm・mrad.
 Kwang-Je KIM showed an analytical way tocalculate emittance in RF Guns and Christian
Travier reduced the equations to practical form.

“RF AND SPACE‐CHARGE EFFECTS IN LASER‐DRIVEN RF ELECTRON GUNS”, Kwang‐Je KIMNuclear Instruments and Methods in Physics Research A275 (1989) 201‐218“An introduc0on to photo‐injector design”, Chrisaan Travier
Nuclear Instruments and Methods in Physics Research A 340 (1994) 26‐3913RF GUN CAVITIESSPring-8 type

HIGH POWER TEST OF THE SPring-8
PHOTOCATHODE RF GUN, T. Taniuchi,
et.al., Proceedings of the 25th Linear
Accelerator Meeting in JapanBNL type

DESY type

FIRST RESULTS OF THE CAESIUMTELLURIDE PHOTO-CATHODE FOR ATF
RF-GUN, N. Terunuma,et.al

RF gun design for the TESLA VUV Free
Electron Laser, B. Dwersteg, et. Al., NIM A393 (1997) 93-95

14.

Val

Unit

Comment

Spring-8 type

BNL type

Desy type

E0MV/ｍPeak electric field
gradient

170

120

35fMHｚfrequency

2856

2856

1300

Number of cell (n+0.5)011nσｘmm

rms transverse bunchsize

112σbps

rms bunch length in time

101010QｎC

total bunch charge

555λL

nm

Laser Wavelength

205205205

ΦeVWork function ofCathode

4.6

4.6

4.6

15

CALCULATED EMITTANCE
Unit

Desytype

formulaα46.7＊E0/f

Dimensionless parameter

2.8

2.0

1.3

γ１＋１４６．８＊(n+0.5)*E0/f１／β

5.4

10.3

6.9σΔγ2.9e-4*E0*σｂ

rms Energy Spread

0.5

0.3

0.1

ΔU/U

2e-4*f*σｂ/(n+0.5)%

rms relative Energy Spread

11.4

3.8

1.7

σｘ'

E0＊σｘ/γ

mrad

rms angular divergence

31.7

11.7

10.1

εrf

2.73e-11*E0*f^2*σｘ^2*σb^2mm mrad

rf contribution to normalized rms emittance

3.8

2.7

0.6

I

399*Q/σbA

beam current

199.5

199.5

199.5

εsc

3.76e3*Q/E0/(2σｘ＋σｂ)

mm mradspace charge contribution to normalized rmsemittance

9.2

13.1

38.4

Eschottky

SQRT(e*E0/4/Pi/ε０）eV

Schottky effect

0.5

0.4

0.2

thermal contribution to normalized rms emittance

1.9

1.9

3.6

Total rms normalized emittance

101339εth

σｘ*SQRT（(hc/λL‐Φ＋Escｈｔ
mm mradｔky）／ｍｃ＾２））εtot

SQRT(σｒｆ＾２＋σｓｃ＾２＋σｔｈ＾２）

mm mradComment

Spring-8
BNL typetype

Val

16

STRATEGY FOR SUPERKEKB RF GUN
 RF: S band (2856MHz)
 Cathode material : Cu
 Laser Wavelength: ~205 nm
 Laser Power : > 0.1 mJ @ cathode Cavity shape: half cell cavity
Mechanical engineering center at KEK have fabricatedmany BNL-type guns and had good background infabrication, so a BNL-type cavity is the second candidate.

17SCHEDULE
JFY20
JFY2010
JFY2011
JFY2012
JFY201309
1
2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3

Clean Room @ RF Gun
test standLaser bed
Laser procurement
Laser commisioning

RF GUN Cavity design
Cavity fabrication

KLY,WG, radiation shield,,,
GUN setup＆processing
GUN commissioning
Acc section install
beam commissioning@test
stand

Move in A1 GUN room
Gun commissioning→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→→
→

→→
→→
→→→→

18
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