N. Nishimori, R. Nagai, R. Hajima Japan Atomic Energy Agency (JAEA) M. Yamamoto, T. Miyajima, Y. Honda KEK H. Iijima, M. Kuriki Hiroshima University

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1 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 1

2 Outline Introduction (Compact ERL, a 500 kv DC gun) High voltage testing of segmented ceramics with a stem electrode HV processing up to 550 kv 500 kv for eight hours without any discharge High voltage processing with electrode in place HV processing up to 526 kv Local radiation problem Beam generation at 300 kv Summary and Outlook 2

3 Compact ERL (test facility) Energy Current Emittance 35/245 MeV 10mA mm mrad E gun exp. hall LCS gamma-ray supercavity 3

4 A 500 kv photocathode DC gun at JAEA HV terminal segmented insulator stem electrode gun chamber cathode anode NEG pumps electron beam 4

5 Field emission from stem electrode Employed a segmented insulator to mitigate field emission problem. uniform electric field means to attach rings which guard ceramics against field emission. guard rings field emission stem electrode ceramic height (mm) beam axis 6.8 MV/m guard rings 8.3 MV/m stem electrode 14.3 MV/m stem electrode nose radius (mm) 5 segmented insulator gun chamber

6 500kV DC gun at JAEA 550kV Cockcroft Walton power supply SF 6 tank segmented insulator 400mm in diam. 1 m in diam. 730mm 3.8 m gun chamber made of titanium 6

7 High voltage testing with a stem electrode stem electrode mm dummy cup instead of cathode electrode 7

8 HV processing 190 baking for 8 hours start processing at 3x10 8 [Pa] Vacuum pump: 1000L/s TMP applied voltage [-kv] kV/hour 550kV one hundred hours to reach 550 kv quarter hour for each 1kV step from 250 kv to 500 kv slower processing above 500 kv time [hrs.] R. Nagai et al., High voltage testing of a 500 kv dc photocathode electron gun, RSI (2010). 8

9 Stable operation at 500 kv for 8 hours applied Voltage [-kv] x current [ A] C.W.:510kV R div :5GΩ R out :0.1GΩ 500kV@insulator pressure [Pa] 4x10-8 3x10-8 2x10-8 1x radiation [ Sv/h] No indication of discharge, local heating due to dark current time [hrs.] R. Nagai et al., RSI (2010). 9

10 Cathode electrode: POISSON calculation 6.7 MV/m cathode 100mm anode R= MV/m 10.3 MV/m 10 NEG

11 Installation of electrodes anode cathode 11

12 Installation of NEG pumps 400L/s NEG To feedthrough 8,000 l/s around electrodes 10,000 l/s downstream of cathode 12

13 Processing setup Interlock system: >1x10-6 Pa >3 Sv/h NaI/GM detector: 0.2m downstream of anode Time constant 3s 13

14 600 High voltage 400 HV(kV) Vacuum Vacuum(Pa) Radiation(cpm) GM survey meter time(hrs.)

15 High voltage 526kV HV(kV) Vacuum Vacuum(Pa) Radiation(cpm) GM survey meter time(hrs.)

16 Local radiation 526kV (Aug. 2011) 466kV (Feb. 2011) 510kV (Jul. 2011) 16

17 What causes local radiation? dust? NEG Local radiation Does not exist at fist Appears suddenly after a discharge Disappears by hand wiping dust inside HV chamber Generated from max. E field Levitation of dust? Observed upward radiation only Fall from NEG, mesh or holder? 17

18 Possible source of dust 400L/s NEG holder Polishing powder found in holes of NEG holder parts 18

19 History of HV processing Stable operation at 500kV Goal: 550kV High voltage (kv) Installation of NEG, electrodes earthquake Local radiation Electron beam generation at 300kV 100 with a stem electrode with NEG, electrodes 0 09 Jul. 10 Jan. Jul. 11 Jan. Jul. 12 Jan. time 19

20 Electron gun solenoid Downstream beam line Beam profile monitor Bending magnet laser Window, Laser shutter Beam dump 1.2 m from anode to bending magnet 0.7 m from bending magnet to beam profile monitor 20

21 300 kev beam generation top top low-e High-E low-e High-E 2 bottom bottom diaphragm:4 diaphragm:8 Beam profile on screen placed 1.9m downstream from anode. 15nA@beam dump, laser power 1.4 W@532nm, QE=2.5% 5.7 A@beam dump at maximum due to radiation from beam dump 21

22 Summary and Outlook 500kV gun status HV test with a stem electrode: 550kV processing, 500kV for 8 hours without any discharge Electrodes: installed. Maximum field <10.3 MV/m, 6.7 MV/m on cathode NEG pumps: Installed in gun chamber Vacuum: <1x10 9 [Pa] in HV chamber HV test with electrodes in place: 526kV processing Beam generation at 300kV Outlook HV processing up to 550kV by solving local radiation problem 22