1. JAEA/KEK DC gun for ERLs
N. Nishimori, R. Nagai, R. Hajima
Japan Atomic Energy Agency　(JAEA)
M. Yamamoto, T. Miyajima, T. Muto, Y. Honda
KEK
H. Iijima, M. Kuriki
Hiroshima University
M. Kuwahara, S. Okumi, T. Nakanishi
Nagoya Univiersity
Mar. 2, 2010
SLAC

2. Outline Introduction (Compact ERL, a 500 kV DC gun)
Recent results: High voltage testing of segmented ceramics
HV processing up to 550 kV
500 kV for eight hours without any discharge
Preparation of electrodes and NEG pumps
Summary
Mar. 2, 2010
SLAC

3. Compact ERL (test facility)
Principal parameters
Conceptual design report: KEK Report / JAEA-Research
Beam energy
35-60 MeV
Beam current mA
Normalized emittance en = e/(gb)
1 mm·mrad (77 pC/bunch)
0.1 mm·mrad (7.7 pC/bunch)
Energy spread （rms）
< 3 10-4
Bunch length (rms)
1 – 3 ps (non compress.)
100 fs (bunch compression) *
Mar. 2, 2010
SLAC
* With some emittance growth due to CSR

4. Development of a 500 kV photocathode DC gun at JAEA
HV terminal
low emittance: <=1 mm-mrad (normalized)
beam current: >=10 mA
segmented
insulator
High DC voltage >= 500kV
CockCroft Walton power supply
Segmented insulator with guard rings
High voltage testing
Electrodes and vacuum
Cathode and anode electrodes
Low outgassing material (titanium)
NEG pumps
support rod
gun chamber
cathode
anode
NEG pumps
electron beam
Mar. 2, 2010
SLAC

5. Field emission from support rod
1200
Field emission from support rod
1000
Employed a segmented insulator to mitigate field emission problem.
uniform electric field
means to attach rings which guard ceramics against field emission.
segmented insulator
800
600
6.8 MV/m
height (mm)
guard rings
400
guard rings
8.3 MV/m
field emission
support rod
200
support rod
14.3 MV/m
support rod nose
beam axis
gun chamber
-200
ceramic
200
400
600
Mar. 2, 2010
SLAC
radius (mm)

6. 500kV DC gun SF6 tank 550kV Cockcroft Walton power supply
segmented insulator
SF6 tank
400mm in diam.
730mm
1 m
in diam.
3.8 m
gun chamber made of titanium
Mar. 2, 2010
SLAC

7. High voltage testing with a support rod
Applied 550 kV to the insulator without a support rod in July 2009.
N. Nishimori et al., Proc. of FEL2009, tupc17.
support rod
Can we apply 500kV to the insulator with a support rod ? mm
101.6 f
dummy cup instead of cathode electrode
Mar. 2, 2010
SLAC

8. HV processing 550kV 4kV/hour Vacuum pump: 1000L/s-TMP
190℃ baking for 8 hours
start processing at 3x10-8[Pa]
one hundred hours to reach 550 kV
quarter hour for each 1kV step from 250 kV to 500 kV
slower processing above 500 kV
Note: The waiting time for vacuum recovery is not included in the integrated time.
R. Nagai et al., “High-voltage testing of a 500-kV dc photocathode electron gun”, to be published in RSI.
Mar. 2, 2010
SLAC

9. HV application after processing
outer guard rings and registers
RDIV=5.75[GΩ]
C.W. + insulator
RCW=6.27[GΩ]
C.W. only
voltage divider:
500MW/segment x 10=5GW
Radiation level is within the background.
No clear evidence for dark current
ROUT=0.1[GΩ]
Rout
RCW
RDIV
C.W.
Mar. 2, 2010
SLAC
R. Nagai et al., to be published in RSI.

10. Dark current estimation
Thank you Pavel Evtushenko at JLab!
without support rod
corona discharge
with support rod
Mar. 2, 2010
SLAC

11. HV processing technique
C.V. (constant voltage)
C.C. (constant current)
C.V.
C.C.
C.V.
0.3s
164mA
163mA
discharge
charging up
1mA
3GW
0.6nF
C.W.
1mA
output register of 100 MW to limit abrupt current drawn from HV in case of discharge
set current limit at V/R+1mA , which helps to stop discharge when average discharge current > 1mA.
Interlock system at vacuum >5x10-6[Pa] and radiation > 3μSv/h
Mar. 2, 2010
SLAC

12. Stable operation at 500 kV for 8 hours
C.W.:510kV
Rdiv：5GΩ
Rout：0.1GΩ
No indication of discharge, local heating due to dark current
R. Nagai et al., to be published in RSI.
Mar. 2, 2010
SLAC

13. Insulator 500kV gun thermioinc gun Max voltage 500 kV 250 kV
Number of segments 10 8
Segment length
65 mm
45 mm
Ceramics
A99P (Al2O3 99.8%)
Shinagawa Fine Ceramics
Kyocera
brazing, welding
Hitachi Haramachi Electronics
silver brazing
silver brazing
SUS flange
Kovar (Fe-Ni-Co) ring plated with Ni
Ceramic
SUS ring
Mar. 2, 2010
SLAC
TIG welding

14. Cathode electrode and NEG pumps
R251(gun chamber)
R198(NEG)
R107(NEG)
electron beam
Φ197(flange)
Mar. 2, 2010
SLAC

15. Cathode electrode: POISSON calculation
6.75 MV/m
cathode
R=67
100mm
anode
forward
10.32 MV/m
NEG
backward
10.51MV/m
64.5mm
NEG
Mar. 2, 2010
SLAC

16. Cathode electrode head (Ti alloy) body (pure Ti) tail (Ti alloy)
f164
head
(Ti alloy)
body
(pure Ti)
tail
(Ti alloy)
1201
spacer
(pure Ti)
support rod
(pure Ti)
Mar. 2, 2010
SLAC

17. Vacuum：pump shield against HV
cathode
cathode
2000L/s NEG
400L/s NEG
Mar. 2, 2010
SLAC

18. RGA spectrum at preparation chamber
2.35x10-9 Pa (RGA on)
5x10-10 Pa (off) H2
CANON/ANNELVA:
M-201 QA-TDM
SEM 1200V
3s/amu
H2O
CH4
NH3 CO N2
CO2
species
Partial pressure[Pa] H2
1x10-9
CH4
1.4x10-13
NH3
2.3x10-13
H2O
1.0x10-12
CO/N2
4.2x10-12
CO2
1.6x10-12
Hydrogen: 99.3 % of total pressure
CP Ti chamber has low outgassing of heavy molecules.
Mar. 2, 2010
SLAC

19. Summary 500kV gun status to do list
HV test: 550kV conditioning, 500kV for 8 hours without any discharge
cathode electrode: maximum field <10.5 MV/m, 6.7 MV/m on cathode
NEG pumps: to be installed in gun chamber
vacuum: 1x10-9 [Pa] (H2 corrected), 99.3 % H2 in preparation chamber
to do list
HV test with electrodes and NEG pumps
improvement of photocathode preparation system
connection between gun and preparation chambers
beam generation
Mar. 2, 2010
SLAC