Tests for High Voltage & Vacuum of the 2 nd 500 kv DC Gun at KEK

Transcription

1 Tests for High Voltage & Vacuum of the 2 nd 500 kv DC Gun at KEK OLAV 3 rd April 2014 Masahiro Yamamoto KEK

2 2 nd DC-gun Development Team T. Miyajima, Y. Honda, T. Uchiyama, M. Kobayashi, Y. Saito, M. Yamamoto High Energy Accelerator Research Organization (KEK) N. Nishimori, R. Nagai, R. Hajima Japan Atomic Energy Agency (JAEA) M. Kuriki Hiroshima University M. Kuwahara Nagoya University H. Yoshida National Institute of Advanced Industrial Science and Technology (AIST) H. Kurisu Yamaguchi University 2

3 Outline Introduction Electron gun system design & fabrication Evaluation -Total outgassing rate of DC-gun system - Pumping speed of 4K bakeable cryopump High voltage test Vacuum test of the 2 nd gun system - DC-gun system with main pumps - Photocathode preparation system Summary 3

4 Outline Introduction Electron gun system design & fabrication Evaluation -Total outgassing rate of DC-gun system - Pumping speed of 4K bakeable cryopump High voltage test Vacuum test of the 2 nd gun system - DC-gun system with main pumps - Photocathode preparation system Summary 4

5 ERL Light Source Project PEARL at KEK Photon Factory ERL Advanced Research Laboratory Parameters for DC-Gun 6 (7) GeV λ rf /2 path-length changer Beam energy Beam Current Bunch Charge Repetition Rate Norm. Emittance GeV ERL 3 GeV ma pc 1.3 GHz mm mrad XFEL-O mode 6-7 GeV 20 µa 20 pc 1 MHz 0.2 mm mrad PPPW2010 BNL 5

6 Proof of key technology at the Compact-ERL The commissioning started from Apr :Injector section Dec :Recirculation return loop beam dump Main linac superconducting cavities Injector superconducting cavities Electron gun tasks: 1.3 GHz CW beams Long cathode lifetime Low emittance (ε n.rms :0.1 ~1 mm.mrad) High avarage current (>10 ma) 500kV DC-gun Mar. 2014: Energy recovery operation with 6.5 ua CW beam PPPW2010 BNL 6

7 Special points of GaAs-type photocathode High Q.E. (0.1~10%) High beam current (I peak ~ 1A/mm 2 ) Cold electrons (k B T 0.1 ev) Polarized electron (P>85%) ps~ns temporal response Negative electron affinity (NEA) surface is absolutely necessary to make these effects. Reference: I. V. Bazarov et al., (2008) Reference: Y. Honda et al., (2013) 7

8 Weakness of GaAs-type photocathode In order to protect NEA surface... Need good vacuum - very low pressure of H 2 O, CO 2 is required. - Decrease ion back-bombardment in high current beam (>ma) operation. (cathode life 1000 Pa) Good vacuum is indispensable for GaAs-type electron source. Lifetime (sec) Reference: D. Durek et al., (1999) Pressure (mbar) Reference: C. K. Sinclair et al., (2007) 8

9 The 1 st and 2 nd 500kV DC Guns 1 st DC-Gun & preparation system C-W HVPS Resistor 4.0m Developed at JAEA from 2007 ~ Installed compact-erl at Oct

10 2 nd DC-Gun & preparation system The 1 st and 2 nd 500kV DC Guns Developed at KEK from 2009 ~ 10

11 Outline Introduction Electron gun system design & fabrication Evaluation -Total outgassing rate of DC-gun system - Pumping speed of 4K bakeable cryopump High voltage test Vacuum test of the 2 nd gun system - DC-gun system with main pumps - Photocathode preparation system Summary 11

12 2 nd 500kV DC-gun design High voltage insulation in vacuum Insulator protection structure Special insulator material 500kV terminal Ultra low outgassing material Titanium chamber & electrodes High performance vacuum pumps 4K bakeable cryopump Massive NEG pumps ( 1~2x10 4 L/s) e - beam Goal Ultimate pressure : ~1x10-10 Pa (during the gun operation) Anode (ground) Cathode (-500kV) 12

13 Fabrication of titanium gun chamber Material Chamber body: *pure Titanium (JIS-2) Flanges: *pure Titanium (JIS-3) (Hv>180) *6Al4V-Titanium alloy (Hv>330) All base materials for flanges are forged. Chemical polishing & Cleaning process 1. Degreasing wash. 2. Chemical polishing 3. Pure water rinse (HPR) 4. Nitric acid treatment 5. Pure water rinse (HPR) & wiping 6. Ultra pure water rinse (clean room) 7. Dry by N 2 blowing & package (clean room) 13

14 Material choice of insulator Pure almina (Al 2 O 3 ) AM997 purity: >99.7% grain size: ~3 um A480S purity: >99.7% grain size: ~3 um Almina based compound TA010 Al 2 O 3 ~ 90% Remainder: secret.. note: optimized to reduce creeping discharge. temperature ( o C) Samples are evaluated by thermal desorption spectroscopy. TA010 is the most promising material for our DC-gun. 14

15 Structure of segmented insulator Base ceramic: TA010 (Kyocera) Metalize: active-metal soldering Flange: pure titanium (JIS-3) inside(vacuum) Ti sleeve Brazing connection outside(sf6) Ceramic Kover Welding connection Ti flange 15

16 2 nd 500kV DC-gun assembly All in-vacuum electrodes are made of pure titanium. All in-vacuum electrodes have no welding joint. All items were polished by buffing & chemical polishing, and washed by ultra pure water. 16

17 Outline Introduction Electron gun system design & fabrication Evaluation -Total outgassing rate of DC-gun system - Pumping speed of 4K bakeable cryopump High voltage test Vacuum test of the 2 nd gun system - DC-gun system with main pumps - Photocathode preparation system Summary 17

18 Outgassing rate measurement Kovar ring Guard ring electrode (2.02 m 2 ) IG(EXG) Ceramic insulator Segmented insulator tubes 8.78x10-2 m 3, 1.26 m 2 TMP 1000 l/s SRG QMS (WATMASS) Ti chamber m m 2 ICF253 GV 150~200 o C baking for 100 hours. All components except main pump (NEGs, cryopump) were installed. S.P. 18

19 Outgassing rate measurement Outgassing rate was measured by rate of rise method using SRG. Total outgassing rate 8.1x10-11 Pa m 3 /s (H 2 equivalent) 19

20 bakeable 150~200 o C 4K bakable cryopump 4K bakable cryopump was developed by ULVAC Cryogenics Inc. Cryo-chamber is also made of titanium. 20

21 Pumping speed measurement (Method) Effective Pumping speed Relative sensitivity factor: α Base pressure: P 0 P C P f Key elements for measurement Standard conductance element Extractor gauge Capacitance diaphragm gauge The standard conductance element was calibrated at AIST. C=3.03x10-10 [Pa m 3 /s] (T 0 =T=300K, for nitrogen) 21

22 Pumping speed measurement (Result) Cryopump was operated after ~150 o C baking for 100 hours. High pumping speed was obtained for CH 4, N 2, CO, CO 2 under XHV condition. Ultimate pressure was limited by adsorption equilibrium of adsorbent for hydrogen. 22

23 Outline Introduction Electron gun system design & fabrication Evaluation -Total outgassing rate of DC-gun system - Pumping speed of 4K bakeable cryopump High voltage test Vacuum test of the 2 nd gun system - DC-gun system with main pumps - Photocathode preparation system Summary 23

24 High voltage test setup 2 nd 500kV DC-Gun 600 kv HVPS Resistor 3.3m HV conditioning was carried out without NEGs & bakeable cryopump. (the chamber was evacuated by 1000 L/sTMP only.) 24

25 Outline of HVPS connection A 1 A 2 6GΩ Oil-impregnation C-W HVPS 100MΩ Protection resistor SF6 vessel A L A 3 A 1 :HVPS voltage monitor (V mon = A 1 *6GΩ) A 2 :Total current monitor A 3 :Current of divide resistors A L :Load (beam) current (A 2 -A 1 -A 3 ) 500M x 10 = 5GΩ Divide resistors anode V DC :Anode current (dark current) monitor 1MΩ V DC cathode Gun chamber 25

26 Electric Field 500 kv Field Map Cathode center: 6.9 MV/m Cathode electrode: *Outer surface: 11.0 MV/m (max.) *Edge of wehnelt: 11.9 MV/m (max.) Anode electrode: 8.8 MV/m (max.) 26

27 HV conditioning history Base pressure: 1.0x10-8 Pa. SF 6 pressure: +0.2 MPa. All trips involved vacuum trip and almost detected radiation. Trip voltage slope to be gentle around 500 kv. 27

28 Safety interlock Monitors * Vacuum (trigger level: >5x10-7 Pa) * Radiation (trigger level: >20 usv/h) * Anode current (trigger level: >500 na) HVPS Controller * Over Current (Total Current max.: 300uA) * Current Continuous mode > 1s (Load Current setting: 20uA) * Other troubles Total Trips 351 times 314 times 37 times Voltage Vacuum Anode current Current trigger level Vacuum trigger level ~90% trips happened between anodecathode gap. Anode current monitor was helpful to reduce X-ray dose because of its quick response. 28

29 480 kv test 2 hours Vacuum Voltage 2x10-8 Pa The test was carried out after HV conditioning of 350 trips. HV down didn t happen during 2 hours operation. Some small vacuum trips still remained. Anode current level: 1nA Anode current Anode current was lower than 1nA normally. Dependence of base pressure on applied voltage was observed. 29

30 Outline Introduction Electron gun system design & fabrication Evaluation -Total outgassing rate of DC-gun system - Pumping speed of 4K bakeable cryopump High voltage test Vacuum test of the 2 nd gun system - DC-gun system with main pumps - Photocathode preparation system Summary 30

31 Main pump installation NEG:400 L/s (H 2 ) x 24 = 9600 L/s (catalog value) 4K bakable cryopump ~800 L/s (Ar), ~1000 L/s (N 2 ), ~1200 L/s (CH 4 ) (measured values@1x10-9 Pa) 31

32 1 st baking with main pumps C,D E F Cryo start G B A NEG activation Ultimate pressure ~ 6x10-10 Pa 32

33 RGA spectrum (1) w/o EM A: bake beginning P: 3.1x10-6 Pa w/o EM B: after ~30 hours P: 3.6x10-5 Pa with EM C: NEG activation beginning ~200 o C P: 2.4x10-6 Pa 33

34 RGA spectrum (2) w/o EM D: NEG activation beginning ~300 o C P: 1.0x10-3 Pa (RGA) w/o EM E: NEG activation ~430 o C after 2.5h P: 3.5x10-4 Pa (RGA) 34

35 RGA spectrum (3) F: after ~150 hours baking end. P: 3.2x10-9 Pa with EM with EM G: after ~180 hours (working cryopump) P: 6.5x10-10 Pa Main residual gas components are m/z = 16, 17, 28, 44 Not clean vacuum..? ESD from RGA..? 35

36 Cathode preparation system 36

37 Cathode preparation system (2) Short dead time for exchange cathode during gun operation. Vacuum Suitcase (Option) Cathode Storage Cleaning chamber Storage chamber Activation chamber Gun chamber Activate & storage 6~9 cathodes per day. Cs & O 2 NEA Activation Atomic H Heating Cathode Cleaning Cathode-container Loading Gate valve 37

38 Cathode preparation system (3) Cleaning chamber Activation chamber Storage chamber All chambers are made of SS304L and polished by EP. All functions are equipped. Heaters & Cs sources are wired by degassed kapton insulated wires. Baking condition: ~200 o C ~100 hours Cleaning chamber: 8.0x10-8 Pa (100 L/s IP) Activation chamber: 5.6x10-9 Pa (200 L/s IP, 400 L/s x2 NEGs) Storage chamber: 1.0x10-9 Pa (100 L/s IP, 400 L/s x4 NEGs) Vacuum is ready for cathode activation. First cathode activation test will be started at April

39 Summary The 2 nd DC-gun is almost constructed. The total out gassing rate of overall dc gun system was suppressed to Q ~8.1x10-11 [Pa m 3 /s]. The pumping speed of the 4K bakeable cryopump was measured. - ~800 L/s (Ar), ~1000 L/s (N 2 ), ~1200 L/s (CH 4 Pa. HV conditioning was carried out and reached over 500 kv. First vacuum test of 2 nd DC-gun with main pump was done. - Ultimate pressure ~6x10-10 Pa (preliminary) - CO, CO2, CH4 rich vacuum Simultaneous triplet cathode preparation system is constructed.

40 Acknowledgments Komiyama Electron Corp. (K. Watanabe, T. Nishidono et al.), Kyocera Corp. (K. Iwamoto, H. Yoshizumi et al.), ULVAC Cryogenics Inc. (H. Kobayashi, H. Yamakawa et al.) Nichicon Corp. (T. Terada, H. Yoshimoto et al.), for fabrication of key components. S. Nagahashi and T. Obina for their contributions to build HVPS control and safety interlock system. H. Iijima for help to assemble of all vacuum systems. 40

41 Backup slides 41

42 Low outgassing material & treatment Air-baked stainless steel (SS316L) 400 o C air-bake in ~100 hours 150 o C in situ baking Outgassing rate ~ 2x10-11 Pa m/s (*~1.6mm thickness) C.D. Park et. al., J.Vac.Sci.Technol. A Pure titanium & titanium alloy (KS100) CP treatment Outgassing rate <10-12 Pa m/s (150 o C in situ baking) H.Kurisu et. al., J.Vac.Sci.Technol. A21 L % BeCu 400 o C vacuum fire in 3 days Outgassing rate < Pa m/s F.Watanabe. J.Vac.Soc.Jpn Vol.49, No

43 Standard Conductance Element Special features The element was made of sintered stainless steel with a pore size of less than 1 µm. Molecular flow is realized at a pressure of < 10 4 Pa. Gas flow Q [Pa m 3 /s] (P f >> P) In this experiment, C=3.03x10-10 [Pa m 3 /s] (T 0 =T=300K, for nitrogen) Advantages Conductance is easily compensated for gas species by the molecular mass. Conductance is constant against changing in the upstream pressure of < 10 4 Pa. Dependence of flow rate on the temperature is small in RT. The calibrated element is commercially available from AIST. 43

44 Special RGA to evaluate UHV ~ XHV Ultra low outgassing RGA (WATMASS) Ultra low outgassing RGA based on using BeCu material was developed by Dr. F. Watanabe. The detail report has been published in J. Vac. Sci. and Technol. A 13, (1995) 44

45 Integrated Q 45