JAEA/KEK DC gun for ERLs

N. Nishimori, R. Nagai, R. HajimaJapan Atomic Energy Agency 　 (JAEA) M. Yamamoto, T. Miyajima, T. Muto, Y. Honda KEK

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 mA

Normalized emittance n = /()

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 ringsHigh voltage testing

Electrodes and vacuumCathode 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 rings6.8 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

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

curr

en

t [A

]voltage divider:500M/segment x 10=5G

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

pre

ssure

[P

a] ra

dia

tion

[Sv/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

[-k

V] C

urrent [A

]

Time [s]

HV processing technique

Mar. 2, 2010 11SLAC

output register of 100 M to limit abrupt current drawn from HV in case of discharge set current limit at V/R+1A , which helps to stop discharge when average discharge current > 1A. Interlock system at vacuum >5x10-6[Pa] and radiation > 3μSv/h

0.6nF

3G

C.V. C.V.

C.W.

C.C.

1Adisc

harg

e

char

ging

up

0.3s

C.V. (constant voltage)C.C. (constant current)

163A

1A

164A

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

pre

ssure

[P

a] ra

dia

tion

[Sv/h

]

time [hrs.]

0

100

200

300

400

500

600

0

50

100

150

200

250

300a

pp

lied

Vo

ltage

[-k

V]

curre

nt [

A]

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

backward

6.75 MV/m

forw

ard

NEG

Cathode electrode

Mar. 2, 2010 16SLAC

120

1164

head(Ti alloy)

tail(Ti alloy)

body(pure Ti)

spacer(pure Ti)

support rod(pure Ti)

Vacuum ： pump shield against HV

Mar. 2, 2010 17SLAC

2000L/s NEG 400L/s NEG

cathode

cathode

1x10-14

1x10-13

1x10-12

1x10-11

1x10-10

0 10 20 30 40 50

curr

en

t(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

H2OCH4

NH3

CON2

CO2

2.35x10-9 Pa (RGA on)5x10-10 Pa (off)

Summary

500kV gun statusHV test: 550kV conditioning, 500kV for 8 hours without any dischargecathode electrode: maximum field <10.5 MV/m, 6.7 MV/m on cathodeNEG pumps: to be installed in gun chambervacuum: 1x10-9 [Pa] (H2 corrected), 99.3 % H2 in preparation chamber

to do listHV test with electrodes and NEG pumpsimprovement of photocathode preparation systemconnection between gun and preparation chambersbeam generation

Mar. 2, 2010 19SLAC