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

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

Compact ERL (test facility) Principal parameters Conceptual design report: KEK Report 2007-7/ JAEA-Research 2008-032 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

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

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)

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

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 ? 1143.5 mm 101.6 f dummy cup instead of cathode electrode Mar. 2, 2010 SLAC

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

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.

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

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

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

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

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

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

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

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

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

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