Report from WG1 DC guns. John Lewellen Nobuyuki Nishimori

Transcription

1 Report from WG1 DC guns John Lewellen Nobuyuki Nishimori

2 DC gun status tt talks tlk

3 Discussions i on technological lchallenges hll Apology for missing electrode shape for small emittance. At least 30 minutes would be necessary for a subject.

4 Outline for a joint paper writing Title DC gun technological challenges Abstract, Introduction [N. Nishimori (JAEA)] A comprehensive overview of current status and challenges for > 500 kv DC guns for ERLs HV power supplies and SF6 [B. Dunham (Corenll)] list ofcurrent status ripple (< 10 4 ) 100 ma

5 Outline Insulator [L. Jones (Daresbury), B. Dunham(Cornell), oneof of JAEA/KEK] List of current status of conical, inverted, segmented insulators Braze joint on large, (14 and 16) inch CF flanges X rays and secondary electron emission i for conical and inverted insulator Cleaning will bechallenging for segmented HV processing [C. Hernandez Garcia (JLab)] Krypton processing, 10 3 Pa (JLab, Daresbury, Cornell) Monitor (vacuum, current, radiation by PMT) With small gaps (movable anode) by M. Poelker

6 Vacuum [M. Yamamoto (KEK)] Outline List of vacuum chambers Outgassing of the material (SUS316,SUS304, SUS304 Ti ) Heat treatment of stainless steels [C. D. Park et al.] Chemical treatment of titanium [H. Kurisu et al.] Ion pump as source of outgassing? By M. Poelker Field Emission from electrodes [M. Poelker (JLab)] SUS304, SUS316, Nb, Mo, Ti List of electrode configuration Small electrodes (inverted guns, JLab) Cleaning (HPR, dry ice cleaning?)

7 Basic information of Nagoya 200kV gun 1. Chamber area (vacuum side): Metal part ( ~1.7 17m 2 ), Ceramic part ( ~0.6 06m 2 ) 2. Chamber material: SUS, Titanium, Copper, Aluminum, M. Yamamoto, KEK 3. Surface finishing & condition: CP/EP (standard EP), Vacuum/air fire ( nothing ) Coating ( nothing ) 4. Main vacuum pump & pumping speed: Ion pump ( 400 L/s), NEG ( 850 L/s), other pump (nothing) NEG pump activating condition (400C,~3hours,~10-3 Pa) 5. In situ baking temperature and duration: 200 C, 100~150 hours, (~10 times baking up to now) 6. Ultimate vacuum: 2x10-9 Pa, (5.6x10-10 Pa with additional ~3800 L/s NEG)

8 Outline Load locked design [B. Militsyn (Daresbury)] List of load locks Ion back bombardment bombardment and cathode degradation [J. Grames (JLab)] Anode biasing i exp. (JLab) New ideas and research development [I. Bazarov New ideas and research development [I. Bazarov (Cornell)]

9 Preparations facilities for the III-V photocathodes B. Militsyn, Daresbury Installation/ Institution Design Preloading treatment Preactivation treatment Activation procedure Results with bulk GaAs Rejuvenation procedure Vacuum conditions Remarks Cornell University Two chambers chemical etching (H 2 SO 4 ) and anodizing Heating 550 C for 2 hrs Cs-NF 3 Yo-Yo 10-15% at 532 nm Heat cleaning PC mbar LC mbar STFC/DL Three HCl etching Heat cleaning Cs-O 2 /NF 3 Yo- 15% at 635 nm Atomic PC mbar chambers (6 (not at 450 C Yo hydrogen LC mbar samples in implemented cleaning HCC mbar carousel) yet) JAEA Tow HCl etching Heat cleanin at Cs-O Yo-Yo 7-10 % at 780 nm Atomic AC Pa chambers 500 C 1 hr (?) hydrogen LC cleaning (optional) KEK (Nagoya) Tow HCl etching Heat cleanin at Cs-O Yo-Yo 7-10 % at 780 nm Atomic AC Pa, LC chambers 500 C 1 hr (?) hydrogen cleaning (optional) Pa TJNAF/CEBAF Four chambers inclusive suitcase and bakable adapter Heating 550 C Cs-NF3 Yo-Yo 20% at 532 nm Heat cleaning PC mbar Mask for 2 hrs LC high mbar activation B.L. Militsyn, ERL 09 Workshop, Cornel University, Ithaca, USA, 8-12 June /

10 Photocathode Lifetime at electrostatic center at 2mA M. Poelker, JLab

11 Summary slide for each talk tlk

12 CeB 6 Thermionic Electron Gun for Single pass Free electron electron Lasers (K. Togawa/RIKEN) 500 kv Pulsed Electron Gun CBC CeB 6 Cathode Cathode Specifications Single crystal CeB 6 (φ3 mm, ~1500 deg C) Beam Voltage 500 kv, (ripple < 10 4 ) Peak Current Pulse Width Repetition Rate Normalized Emittance 1 A ~2 μs (FWHM) (1 ns part is cut out by deflector.) 60 pps 0.6π mm mrad (rms, 90% core) Transverse Phase Space *Technologies from electron microscopes and high voltage klystron devices are used. *Stable and low emittance. *Long lifetime ~20,000h. *Strong against surface contamination. *Do not need extremely high vacuum.(<3x10 7 Pa) *Delivering stable beams for user operation at the SCSS test accelerator. *Be used for the XFEL at SPring 8.

13 B. Dunham, Cornell Cornell DC Gun Cathode Preparation and Load Lock -750 kv 16.5 inch flange Insulator In operations for the past 750 kv, 100 ma HVPS several years for Drive Laser cathode/emittance studies and as a source for the ERL injector HV operation limited to 250kV due to insulator problems new, improved insulator just received and ready to try Laser input Electron beam June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University

14 B. Dunham, Cornell The gun system was used initially with a heavily populated p diagnostic beamline to allow complete phase space characterization at 77pC/bunch. Many publications, in particular, excellent agreement between simulations and measurements DC currents up to 20mA The gun was moved and jointed to the injector cryomodule, and has performed reliably. 4mA (CW@1300MHz) A new insulator material that has a bulk conductivity, which is first tried at Daresbury Alternative design options are still being pursued as backups June 8-12, Ithaca, NY Energy Recovery Linac Workshop Cornell University

15 CEBAF/ILC Inverted Gun Present Ceramic Exposed to field emission Large area Expensive (~$50k) Medical x-ray technology e New Ceramic Compact ~$5k neg modules New design Move away from conventional insulator used on most GaAs photoguns today expensive, months to build, prone to damage from field emission.

16 Single Crystal Niobium: Capable of operation at higher voltage and gradient 140 Buffer chemical polish (BCP) much 120 easier than diamond-paste-polishd 100 Field Emission Current (p pa) BCP Niobium vs Stainless Steel niobium 304 SS 304 SS # Voltage (kv) Conventional geometry: cathode electrode mounted on metal support structure Replace conventional ceramic insulator with Inverted insulator: no SF6 and no HV breakdown outside chamber Work of Ken Surles-Law, Jefferson Lab

17 Summary Jlab DC gun C. Hernandez-Garcia, JLab Delivered record 7000 Coulombs from single GaAs wafer and 900 hours of CW beam at over 5 ma Photocathode lifetime 550 Coulombs or 50 hours at 5 ma CW Photocathode suffered surface damage multiple times, but recovered each time with heat clean and NEA reactivation Achieved 485kV after high voltage conditioning, but operating at 350kV after ceramic punch through at 485 kv Very successful high voltage conditioning using Kr gas at 1E 5 Torr, conditioned up to 375 kv with Kr. Implemented motorized cathode retractor, decreasing FEL down time for QE replenishing from 3.5 hours to 0.5 hours. Need to further study surface charge limit from GaAs Jlab FEL developing inverted insulator gun concept with load locked system, expect high voltage testing by January 2010

18 ERL Photoinjector status summary L. Jones, Daresbury Photoinjector output characterised using dedicated diagnostic beamline: - Described in Y.M. Saveliev et al.; Proc. EPAC 08, ; MOPC ; MOPC063 Problems encountered with field emission, beam halo, HV breakdown, poor quantum efficiency and lifetime during commissioning. Much experience gained. Successful use of Kr processing in HV conditioning to counter field emission. Application of modulated lightsource plus improved vacuum techniques yielded greatly increased QE and dark lifetime approaching 900 hours. Repeated failure of photoinjector ceramic has delayed programme and limited operating voltage. Now have a butt-joint unit from CPI. Investigating other options. Issues with laser cooling and corrosion => Power drop. Plan to re-install diagnostic beamline in parallel with an external cathode preperation system (collaboration with A.S.Terekhov, Instiute of Semiconductor Physics). 5 4 Amplitude [arb] Photocurrent Pressure 15% Q.E. at 635 nm 0

19 JAEA/KEK gun status N. Nishimori, JAEA 500kV-10mA HV power supply and SF6 tank Segmented insulator with guard rings Preparation chamber HV test will be performed soon. 1