Vacuum System of ERL High Voltage DC Gun at Cornell Xianghong Liu Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE) Cornell University, Ithaca, NY 14853, USA 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 1
Outline Introduction First Gun Second Gun Photocathode Pursuing extreme high vacuum General procedure of preparation and assembly Summary 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 2
The Energy Recovery Linac Project at Cornell Cornell Electron Storage Ring Tunnel 1. Injector 2. Linac 1 3. 2.5 GeV turn-around 4. Linac 2 5. X-ray beamlines 6. 5 GeV turn-around (CESR) 7. X-ray beamlines 8. Beam dump 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 3
Energy Recovery Linac Without Energy Recovery: 5 GeV * 100 ma = 500 MW of power! With Energy Recovery: 15 MeV * 100 ma = 1.5 MW of power Accelerating bunch Returning bunch 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 4
Main Linac cryomodule for the ERL (MLC) Linac A 344 m with 35 cryomodules Total 64 cryomodules, each: six packages of 7-cell cavity/coupler/tuner a SC magnets/bpms package Linac B 285 m with 29 cryomodules five regular HOMs/two taper HOMs SC magnets & BPMs 7-cell cavity Beamline HOM absorber nominal length: 9.8 m Intermodule unit 4/4/2014 Slides from Ralf Eichhorn 5
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How it looks today Ralf Eichhorn CLASSE 4/4/2014 Cornell University LHeC Workshop 2014, 21.1.2014 8
ERL 7-2 to 7-5 Vertical Test Results 1.8K meas. Design specs 16MV/m, 2.0e10 @1.8K Ralf Eichhorn CLASSE 4/4/2014 Cornell University LHeC Workshop 2014, 21.1.2014 9
Photo-injector prototype (5-15 MeV) The ultimate emittance of the ERL is determined by the injector! 1st Gun 4 1 5 3 2 1 DC Photo-cathode gun 2 kev Beamline w/ Laser ports, buncher, etc. 3 10-cell SC RF cavity Cryo-module 5 600 kw Beam Dump 4 MeV Beamlines w/ full suite of beam instrumentation 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 10
Photoinjector performance 750 kv is not really necessary; 500 kv is enough according to our simulation! We are now moving on to use the injector for LCLS II development 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 11
Photo-cathode DC gun #1 SF 6 Tank 16.5 inch flange -750 kv Insulator HV Supply GaAs Activation Chamber GaAs Cleaning Chamber Load-lock GaAs Cathode Electron beam Laser input 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 12
Vacuum suitcase to transport cathode 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 13
Vacuum system 400 l/s Perkin-Elmer ion pump Massive NEG pumping for H 2 - using 20 modules of WP1650 ST 707. (740 l/s x 20) ~ 15,000 l/s 400 C air firing of vessel and internal components, followed by 160 C bakeout* 5 x 10-12 Torr typical static pressure * Reduction in hydrogen outgassing from stainless steels by a medium-temperature heat treatment J. Vac. Sci. Technol. A 26 (5), 1166 (2008) 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 14
Insulator It has conductive coating on inner surface to prevent charging-up It turned out Field emitted electrons can build up on the insulator and punch through Obvious damage sites can be seen on inside surface Found massive amount of powder on bottom of insulator 12 MV/m at 750kV e - e - -750 kv 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 15
Bulk conductive insulator The problematic insulator was then replaced with a bulk conductive insulator worked well since at up to 400 kv (may work for higher voltage) 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 16
Gun #2 Segmented insulator (with guard rings) Field emitted electrons are stopped by the guard rings Series of HV resistors to assure uniform grading of the HV Larger size flange with wire seal Larger diameter for gun chamber Can install all the pumping elements as Gun #1 First installed an electrically floating anode for diagnosis and mitigating ion back bombardment Replaced with a translatable anode 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 17
Segmented insulator 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 18
Vacuum system at present Two CapaciTorr D3500 NEG pumps and two VacIon 55 ion pumps at present; base pressure 4x10-11 Torr Relaxed on vacuum requirement since using multi-alkali antimonide photocathode 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 19
Cathode field is a crucial figure of merit. Translatable anode allows us to tailor the field. Translatable anode 2-5cm adjustable gap 2 welded bellows Gate valve Anode electrode No vacuum force involved in translation 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 20
Gun #2 assembly (first time) 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 21
Problems During NEG activation, one vacuum window cracked. Processing was rocky: Excess current from power supply (steady state 10s of ua, spikes up to 100uA and beyond), Excess current on resistors (spikes of ~10uA) Current on floating anode (<1uA, excess by definition) Radiation inside lead (up to 10 R/hour) Vacuum (base 1e-10 torr) spikes of 1e-7 or worse. Couldn t go beyond 350kV. Decided to open the gun to investigate Opened bottom flange, purged with N2 gas, counted particles 0.3 um and larger. Saw a few bursts of large counts. Visual inspection didn t find anything suspicious. Quickly sealed and tried to reprocess without success. Made hard decision to reclean and rebuild. Found powder on bottom flange of NEG module assembly Began rebuild in SRF cleanroom facility (always class 10). Stalk showed definite signs of field emission 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 22
A side note: Hollow cathode discharge? Interesting to notice areas of discoloration / bluish coloration Believe being resulted from hollow cathode discharge during gas processing at 1e-5 to 1e-4 Torr Don t think it does any harm to the electrodes 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 23
and the rebuild Hand polished problematic areas of stalk Electropolished all cathodes Did 400 C air bake High pressure water rinse Assembled in class 10 clean room and transport back to Wilson lab Removed all NEG modules Replaced the anode with the translatable one 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 24
It works much better after the rebuild! Reached 470 kv and continuing. High voltage processing 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 25
(R128) New experimental beamline Spec. Dipole F.cup Coll. Slit Def Cav EMS Slits Corr. Mag. Sol Gun Anode 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 26
Why pushing hard on vacuum? GaAs photocathode High QE, fast response, low MTE Negative Electron Affinity (NEA) Requires atomically clean substrate High QE relies on activation by ~monolayer of CsF Sensitive to residual gases, especially oxidant (dark lifetime) Better vacuum = better lifetime Ions generated by the electron beam are accelerated and bombard the cathode; causes decay of QE 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 27
Picture of centered GaAs photocathode To reduce beam halo, we define a much smaller active area (instead of whole wafer). 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 28
Ion back bombardment Off-center cathode Cathode Center Damages -- Ion back bombardment (ions generated downstream of the gun) 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 29
Multialkali photocathode: robust! Before use After use 10s of nm grown on a substrate: bulk! Also suffer from ion back bombardment Therefore most are off-center 60mA run with CsK 2 Sb had ~30 h 1/e lifetime 65mA run with NaKSb had ~66 h 1/e lifetime Ref: Dunham et. al. APL 102, 034105 (2013) 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 30
Pursuing extreme high vacuum P Q S Minimize gas load (outgassing) Maximize pumping speed 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 31
Minimizing outgassing Did 400 C 100 hour air bakeout for all stainless steel parts (which reduces outgassing rate to 2e-14 Torr l s -1 cm -2 for 1.65 mm wall thickness) Reduce area of thick material: total gas load is dominated by thicker parts. Thoughts only: Thin wall (double wall) chamber to take full advantage of 400 C bake Chilled chamber to reduce outgassing rate D(T) e E d kt Diffusion constant Take Ed=14.5 kcal/mol (H in 304 SST) Temperature ( C) Reduction factor 25 1 0 9.4-25 140 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 32
Maximize pumping NEG modules: 15000 l/s for H2 Large ion pump: 400 l/s Looked into cryo-pumping, but gave up due to particulate concern and cost (of bakable cryo-pump and a large all-metal gate valve). 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 33
Parts preparation process (cleaness and particle control) 1. UHV parts standard cleaning 2. Hand polishing (electrodes only); cleaning again 3. Electropolishing (electrodes only), won t touch the polished surface from now on; transported and stored in DI water 4. 400 C-100hr air bakeout (all stainless steel parts: chamber and internal parts) 5. Rinse, ultrasound bath 6. Move to class 10 or 100 clean room 7. High Pressure Rinse all parts with DI water 8. For components that cannot go through HPR, blow with filtered high pressure dry nitrogen and check with particle counter 9. Assembly and leak check Pumping down and venting very slowly to prevent particulate moving around! 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 34
Summary Gun #1 Has been in operation for many years HV processed to over 400 kv; normally operated at 350 kv Base vacuum: 5e-12 Torr (measured by extractor gauge); NEGs haven t been reactivated for many years Delivered up to 75 ma (limited by RF cavities) 1/e lifetime of >60 hours at 65 ma run with alkali cathode Currently being processed for higher voltage Gun #2 First assembled in summer 2012; rebuilt October 2013 HV processed to over 470 kv and continuing Segmented insulator has worked well Base vacuum: 4e-11 Torr (w/o NEG modules or large ion pump) 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 35
Acknowledgements Thanks to The ERL team Especially, Bruce Dunham Karl Smolenski Jared Maxson 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 36
Picture of field emitters (on p22) 4/4/2014 Xianghong Liu, OLAV-IV, Taiwan 37