Status of 500-kV DC gun at JAEA N. Nishimori, R. Nagai, R. Hajima Japan Atomic Energy Agency (JAEA) M. Yamamoto, T. Miyajima, Y. Honda KEK H. Iijima, M. Kuriki Hiroshima University M. Kuwahara, S. Okumi, T. Nakanishi Nagoya Univiersity

Outline Introduction (Compact ERL, a 500 kV DC gun) High voltage conditioning of segmented insulator with a stem electrode  510kV for 8 hours after removing field emission site with current conditioning High voltage conditioning with cathode electrode in place  Conditioning up to 526 kV by removing field emission site with wiping  Demonstration of 440kV for 8 hours (present status)  Plan of gas conditioning to remove field emission site Beam generation at 300 kV Summary and Outlook 2Mar. 6, 2012N. Nishimori, Jefferson Lab, Newport News, VA

3 Compact ERL (test facility) Mar. 6, 2012N. Nishimori, Jefferson Lab, Newport News, VA Energy35/245 MeV Current10mA Emittance 0.1 – 1 mm ・ mrad E gun (Installation by Oct Beam generation by Mar. 2013) superconducting accelerator supercavity LCS gamma-ray exp. hall

electron beam Mar. 6, N. Nishimori, Jefferson Lab, Newport News, VA HV terminal segmented insulator stem electrode cathode A 500 kV photocathode DC gun at JAEA NEG pumps 18,000 l/s anode gun chamber

Segmented insulator ceramic stem electrode field emission stem electrode guard rings 6.8 MV/m 8.3 MV/m MV/m radius (mm) height (mm) Mar. 6, 20125N. Nishimori, Jefferson Lab, Newport News, VA stem electrode nose Can guard ceramics against field emission generated from stem electrode. Cannot diminish field emitter itself. Reliable way to remove field emitter still needs to be developed. segmented insulator gun chamber beam axis guard rings

500kV DC gun at JAEA 400mm in diam. 730mm 550kV Cockcroft Walton power supply 3.8 m segmented insulator SF 6 tank gun chamber made of titanium Mar. 6, 20126N. Nishimori, Jefferson Lab, Newport News, VA 1 m in diam.

Mar. 6, 2012 High voltage testing with a stem electrode stem electrode dummy cup instead of cathode electrode mm 7N. Nishimori, Jefferson Lab, Newport News, VA 

HV conditioning Mar. 6, 20128N. Nishimori, Jefferson Lab, Newport News, VA 4kV/hour R. Nagai et al., “High-voltage testing of a 500-kV dc photocathode electron gun”, RSI (2010). 550kV 190 ℃ baking for 8 hours start conditioning at 3x10 -8 [Pa] Vacuum pump: 1000L/s-TMP Why slow conditioning ? Because of radiation increase above 520kV

HV(kV) time(hrs.) Radiation(  Sv/h) at 500kV rapid increase and slow decrease every discharge Conditioning with stem electrode (Oct. ’09) Mar. 6, 2012N. Nishimori, Jefferson Lab, Newport News, VA9 1Sv=100rem

HV(kV) time(hrs.) Radiation(  Sv/h) Conditioning with stem electrode (Oct. ’09) decrease of radiationat 500kV at 550kV Mar. 6, 2012N. Nishimori, Jefferson Lab, Newport News, VA10

HV(kV) time(hrs.) Radiation(  Sv/h) demonstration of 510 kV for eight hours radiation similar to B.G. level Conditioning with stem electrode (Oct. ’09) Mar. 6, 2012N. Nishimori, Jefferson Lab, Newport News, VA11

Cathode electrode: POISSON calculation 10.3 MV/m cathode anode 100mm Mar. 6, N. Nishimori, Jefferson Lab, Newport News, VA 6.7 MV/m NEG 9.1 MV/m R=250

Mar. 6, N. Nishimori, Jefferson Lab, Newport News, VA HV(kV) time(hrs.) Vacuum(Pa) Radiation(cpm) High voltage Vacuum GM survey meter 1x10 -9 Pa Conditioning with cathode electrode (Jun. ’11)

HV(kV) time(hrs.) Vacuum(Pa) Radiation(cpm) HV(kV) Radiation(cpm) 510kV 350kV radiation spot Mar. 6, 2012N. Nishimori, Jefferson Lab, Newport News, VA14

N. Nishimori, Jefferson Lab, Newport News, VAMar. 6, HV(kV) time(hrs.) Vacuum(Pa) Radiation(cpm) Vacuum (NEG reactivated without baking) GM survey meter HV 1x10 -8 Pa Go up to 445kV within 1 hour Conditioning after wiping cathode (Jun. ’11)

HV(kV) time(hrs.) Vacuum(Pa) Radiation(cpm) found another local radiation from survey 3x10 -9 Pa 526kV Mar. 6, 2012N. Nishimori, Jefferson Lab, Newport News, VA16 radiation spot HV(kV) Radiation(  Sv/h) Sv=100rem

Possible local radiation sources found 17Mar. 6, 2012N. Nishimori, Jefferson Lab, Newport News, VA NEG 400L/s NEG holder Polishing powder found in holes of NEG holder parts

HV(kV) time(hrs.) Radiation(  Sv/h) Still suffered from field emission generated from cathode electrode Status after removal of dust source (Feb. ’12) 400 (8MV/m) (10MV/m) radiation(  Sv/h) > 8hours without discharge at 440kV time(hrs.) hours at 480kV 2hours at 460kV HV radiation time(hrs.) HV(kV) N 2 equivalent pressure (Pa) 8x Pa HV pressure Mar. 6, 2012N. Nishimori, Jefferson Lab, Newport News, VA18 HV (kV) radiation spot GM survey meter 1Sv=100rem

Gas conditioning with segmented insulator stem electrode field emission 2.6 MV/m on backside of ring 8.3 MV/m on stem electrode Mar. 6, N. Nishimori, Jefferson Lab, Newport News, VA Pa, 10mA would produce the same amount of ion with Pa, 10nA Ion back-bombardment would be ~10 -6 of field emission current for Pa He gas. No field emission from stem electrode, but some emission from backside of guard rings. Still be careful about damage on ceramics by field emission. Measurement of HV power supply current with high accuracy. HVPS current with gas should be equal to that without gas. guard rings 2.6MV/m

I-V curve of HV power supply without gas Mar. 6, N. Nishimori, Jefferson Lab, Newport News, VA 0500 HV (kV) HV power supply current (  )  I (  ) I[  A]=0.3312xHV[kV]-0.29 HVPS current deviation from fitting

Gun configuration Mar. 6, 2012 N. Nishimori, Jefferson Lab, Newport News, VA kV terminal cathode segmented insulator preparation chamber loading chamber anode 1.5m transfer rod 1m transfer rod HV chamber e beam

Mar. 6, 2012 N. Nishimori, Jefferson Lab, Newport News, VA Downstream beam line 22 solenoid Electron gun Window, Laser shutter Beam dump Beam profile monitor Bending magnet lase r 1.2 m from anode to bending magnet 0.7 m from bending magnet to beam profile monitor

Mar. 6, 2012 N. Nishimori, Jefferson Lab, Newport News, VA 300 keV beam generation 23 Beam profile on screen placed 1.9m downstream from anode. dump, laser power 1.4  QE=2.5% 5.7  dump at maximum due to radiation from beam dump High-Elow-E top bottom diaphragm:4 22 High-Elow-E top bottom diaphragm:8

Summary and Outlook 500kV gun status Scheduled to be installed by Oct HV test with a stem electrode: 510kV (500kV) for 8 hours without any discharge after removal of FE site by current conditioning for 50 hours HV test with a cathode electrode: 440kV (430kV) for 8 hours without any discharge. FE site cannot be removed by current conditioning so far. Vacuum: 8x [Pa] N 2 equivalent in HV chamber with HV on Beam generation at 300kV Outlook Repeated wiping with NEG reactivation might be a solution to 500kV Gas conditioning with monitoring HVPS current carefully Beam generation at > 400kV Mar. 6, N. Nishimori, Jefferson Lab, Newport News, VA