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JAEA/KEK DC gun for ERLs
N. Nishimori, R. Nagai, R. HajimaJapan Atomic Energy Agency (JAEA)
M. Yamamoto, T. Miyajima, T. Muto, Y. HondaKEK
H. Iijima, M. KurikiHiroshima University
M. Kuwahara, S. Okumi, T. NakanishiNagoya Univiersity
1Mar. 2, 2010 SLAC
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
2Mar. 2, 2010 SLAC
Beam energy 35-60 MeV Beam current 10 mANormalized emittanceεn = ε/(γβ)
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) *
3
Compact ERL (test facility)
* With some emittance growth due to CSR
Principal parameters Conceptual design report: KEK Report 2007-7/JAEA-Research 2008-032
Mainacceleration
module 2
Merger
Extractor
Chicane Straight section
Electrongun
Solenoid, buncher
Injectorcavity
Arc 1(Bunch compression) Arc 2
Beam dump
Mainacceleration
module 1
Mar. 2, 2010 SLAC
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
electron beamMar. 2, 2010 4
SLAC
HV terminal
segmentedinsulator
support rod
cathode
Development of a 500 kV photocathode DC gun at JAEA
NEG pumps
anode
low emittance: <=1 mm-mrad (normalized)beam current: >=10 mA
gun chamber
Field emission from support rod
ceramicsupport rod
field emission support rod
guard rings
6.8 MV/m
8.3 MV/m
1200
1000
800
600
400
200
0
-200
0 200 400 600
14.3 MV/m
radius (mm)
heig
ht (m
m)
Mar. 2, 2010 5SLAC
support rod nose
Employed a segmented insulator to mitigate field emission problem.
uniform electric fieldmeans to attach rings which guard ceramics against field emission.
segmented insulator
gun chamber
beam axis
guard rings
500kV DC gun
400mm in diam.
730mm
550kV Cockcroft Walton power supply
3.8 m
segmented insulator
SF6 tank
gun chamber made of titanium
Mar. 2, 2010 6SLAC
1 m in diam.
Mar. 2, 2010
High voltage testing with a support rod
support rodCan we apply 500kV to the insulator with a support rod ?
dummy cup instead of cathode electrode
1143
.5 m
m
7SLAC
Applied 550 kV to the insulator without a support rod in July 2009.
101.6 φ
N. Nishimori et al., Proc. of FEL2009, tupc17.
HV processing
0
100
200
300
400
500
600
0 20 40 60 80 100 120
appl
ied
volta
ge [-
kV]
time [hrs.]
Mar. 2, 2010 8SLAC
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.
4kV/hour
R. Nagai et al., “High-voltage testing of a 500-kV dc photocathode electron gun”, to be published in RSI.
550kV
Vacuum pump: 1000L/s-TMP
190 baking for 8 hours start processing at 3x10-8[Pa]
HV application after processing
0
50
100
150
200
curre
nt [µ
A]voltage divider:500MΩ/segment x 10=5GΩ
outer guard rings and registers
RCW=6.27[GΩ]
RDIV=5.75[GΩ]C.W. + insulator
C.W. only
Mar. 2, 2010 9SLAC
0
1x10- 8
2x10- 8
3x10- 8
4x10- 8
5x10- 8
0
0.05
0.1
0.15
0.2
0 100 200 300 400 500 600
pres
sure
[Pa]
radiation [µSv/h]
applied voltage [-kV]
C.W. RDIV
Rout
RCW
ROUT=0.1[GΩ]
Radiation level is within the background.No clear evidence for dark current
R. Nagai et al., to be published in RSI.
Dark current estimation
Mar. 2, 2010 10SLAC
without support rod
with support rod
corona discharge
Thank you Pavel Evtushenko at JLab!
494
495
496
497
498
499
500
150
160
170
180
190
200
210
133.6 133.8 134 134.2 134.4
Vol
tage
[-kV
] Current [µA
]
Time [s]
HV processing technique
Mar. 2, 2010 11SLAC
output register of 100 MΩ to limit abrupt current drawn from HV in case of discharge set current limit at V/R+1µA , which helps to stop discharge when average discharge current > 1µA. Interlock system at vacuum >5x10-6[Pa] and radiation > 3μSv/h
0.6nF
3GΩ
C.V. C.V.
C.W.
C.C.
1µAdisc
harg
e
0.3s
C.V. (constant voltage)C.C. (constant current)
163µA
1µA
164µA
Stable operation at 500 kV for 8 hours
0
1x10- 8
2x10- 8
3x10- 8
4x10- 8
5x10- 8
0
0.05
0.1
0.15
0.2
0 2 4 6 8 10
pres
sure
[Pa]
radiation [µSv/h]
time [hrs.]
0
100
200
300
400
500
600
0
50
100
150
200
250
300ap
plie
d Vo
ltage
[-kV
]
current [µA]
C.W.:510kVRdiv:5GΩRout:0.1GΩ→500kV@insulator
Mar. 2, 2010 12SLAC
R. Nagai et al., to be published in RSI.
No indication of discharge, local heating due to dark current
Insulator
13
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 Kyocera
Kovar (Fe-Ni-Co) ring plated with Ni
Ceramic
silver brazing silver brazingSUS flange
TIG welding
SUS ring
Mar. 2, 2010 SLAC
Cathode electrode and NEG pumps
R251(gun chamber)
Φ197(flange)
R198(NEG)
R107(NEG)
Mar. 2, 2010 14SLAC
electron beam
Cathode electrode: POISSON calculation
10.32 MV/m
cathode
anode
100mmR=67
Mar. 2, 2010 15SLAC
NEG
64.5mm
10.51MV/m
6.75 MV/m
NEG
Cathode electrode
Mar. 2, 2010 16SLAC
120
1φ164
head(Ti alloy)
tail(Ti alloy)
body(pure Ti)
spacer(pure Ti)
support rod(pure Ti)
1x10- 14
1x10- 13
1x10- 12
1x10- 11
1x10- 10
0 10 20 30 40 50
curre
nt(A
)
mass
RGA spectrum at preparation chamber
Mar. 2, 2010 18SLAC
Hydrogen: 99.3 % of total pressureCP Ti chamber has low outgassing of heavy molecules.
CANON/ANNELVA:M-201 QA-TDMSEM 1200V3s/amu
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
H2
H2OCH4NH3
CON2
CO2
2.35x10-9 Pa (RGA on)5x10-10 Pa (off)
Summary
500kV gun status
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 19SLAC