Combining NEG pumps and an XHV BNNT cryopump

Transcription

1 Combining NEG pumps and an XHV BNNT cryopump Marcy L. Stutzman, Anahí Segovia Miranda, Veronica Over, Philip Adderley, Matt Poelker Thomas Jefferson National Accelerator Facility Newport News, VA 23601, USA

2 Thomas Jefferson National Accelerator Facility US Department of Energy, 12 GeV electrons, recirculating linear accelerator Up to 90% polarization from DC photoemission source Electron currents to 200μA beam (CW) to four experimental halls Source Hall D (0.012 TeV) Halls A, B, C Polarized source 2

3 -100 to -350 kv DC Photoemission Source cathode electrode anode (ground) light in electrons out 14 layers GaAsP Strained Superlattice Photocathode GaAs GaAs Polished high voltage electrodes Strained superlattice GaAs/GaAsP photocathode Surface preparation: Cs, NF 3 Electron beam polarization ~90% Residual gasses ionized, limit operational lifetime Base pressure approaching XHV P < 1x10-10 Pa 3

4 Vacuum research to extend lifetime Add NEG array to original ion pumped system C. K. Sinclair et al., "A High Average Current Polarized Electron Source with Long Cathode Operational Lifetime", Phys. Rev. ST Accel. Beams 10, (2007) Outgassing reduction M.A. Mamun, et al., "Effect of heat treatments and coatings on the outgassing rate of stainless steel chambers, Journal of Vacuum Science & Technology A 32, (2014) NEG coating large diameter chamber M. L. Stutzman et al., "Nonevaporable getter coating chambers for extreme high vacuum," J. Vac. Sci. Technol. A 36, (2018) Incorporating additional XHV cryo-pumping in the system? Leybold XHV cryopump Boron Nitride Nanotubes (BNNT) as cryosorber Marcy Stutzman, Roy Whitney and Kevin Jordan Nano-materials for adhesive-free adsorbers for bakable extreme high vacuum cryopump surfaces Patent US B2 4

5 Cryopumps typically not bakable XHV Cryopumping Adhesives for charcoal cryosorption surface Mechanics of closed-cycle helium refrigerators Solutions other places: Charcoal with Bakable adhesives (Silversolder or Thermoguss) KEK: 2K helium flow Our research Leybold bakable cryopump LN 2 chill circuit to keep charcoal, adhesive, mechanics cold during vacuum bakeout Valve outgassing mitigates pump effect Alternatives to Charcoal not requiring adhesive? Boron nitride nanomaterial as cryosorber material 5

6 Freestanding boron nitride nanotubes Chemically inert Very porous structure Thermal Conductivity 3000 W/mK vs. Cu = 400 W/mK, Investigate potential as bakeable cryosorber Fibril BNNT material 1 g BNNT 10 g CNT (similar scale) 6

7 Boron Nitride NanoTubes Freestanding cryosorber Retaining wire tack welded - no adhesive 1g = 300 m 2 surface area Total 4.4g BNNT, ~1000 m 2 Novel Cryopump: BNNT Attachment on both sides of fins Cryopump base temperature ~12.5 K US Patent issued October

8 Pressure (Torr) Cryopump Temperature (K) BNNT Cryopump performance: First and Second builds First build 1g BNNT Copper retaining mesh Base temp 15K heat load due to mesh Ion pump + BNNT CP Second Build 4g BNNT Tack welded wires Base temperature 12.5K Ion pump L/s NEG + BNNT CP 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 1.E-11 1.E-12 4g BNNT 1g BNNT T Hours

9 Chamber Pressure (Torr) Pump Speed (L/s) BNNT Cryopump speed and capacity, 4g BNNT 9E-11 8E-11 7E-11 6E-11 5E-11 4E-11 3E-11 2E-11 1E Pump speed setup hydrogen gas conductance limiting orifice pressure ~10-11 Torr Pump capacity S = S 1/e capacity ~0.03 TorrL Commercial cryopump: Very open porous structure High pump speed Low pump capacity Gas dose (TorrL) 0 9

10 Pressure (Torr) BNNT cryopumping 1.E-07 1.E-08 1.E-09 1.E-10 Add BNNT CP to NEG/ion pump chamber 1.E-11 1.E-12 1.E-13 1g 4g 4g + NEG array Pwarm Pcold??? Base pressure ~10-12 Torr before cooling cryopump 10

11 System setup for BNNT Cryopump + NEG pump system Heat treated stainless steel Outgassing rate ~5x10-13 TorrL/s/cm 2 NEG pump array 10 WP950 ST707 modules Ion pump Gamma XHV/SEM 40 L/s Extractor and 3BG gauges BNNT Cryopump 4g BNNT cryosorber Remove displacer for bake Bake system to 250 C, 30 hours with turbopump cart Turn on gauges, Measure pressure* Ion pump NEG array Gauges NEG array BNNT Cryopump 11

12 Gauges for XHV Extractor gauge x-ray limit reduced through geometry Extractor Watanabe 3BG (Bent Belt Beam) gauge 230 deflector BeCu housing JVSTA 28, 486 (2010) 3BG 12

13 Extractor current (fa) x-ray induced current of gauges Collector current (fa) Extractor Gauge Bent Belt Beam Gauge Signal: background 30: x-ray background 10 5 x-ray background 0 Signal: background 1: Repeller voltage (V) Deflector voltage (V) Other background effects: ESD, gauge heating/pumping

14 Pressure (Torr) 7.E-12 6.E-12 5.E-12 4.E-12 3.E-12 2.E-12 1.E-12 0.E E E-12 NEG+IP+CP Warm Results Before x-ray subtraction after 5.1E-13 Extractor - x-ray limit 3BG 3.3E-12 NEG+CP cold BNNT outgassing low No valve P~3x10-12 Torr BNNT warm Cryopump reduces pressure x-ray limit 1.2x10-12 Torr dominates extractor gauge reading 3BG readings still have good signal:background, negligible x-ray effect 14

15 Recorded pressure (Torr) x-ray limit measurements with BNNT cryopump warm and cold 8E-12 7E-12 6E-12 5E-12 4E-12 3E-12 2E-12 1E-12 Repeated x-ray limit measurements, cryopump warm x-ray limit measurement, cryopump cold averaged x-ray limit Voltage applied to reflector (V) Are we at, or beyond extractor measurement limitation? 15

16 Extractor Pressure (Torr) Extractor vs. 3BG calibration issues 1E-09 1E-10 1E-11 1E-12 1E-13 1E-13 1E-12 1E-11 1E-10 1E-09 3BG Pressure (Torr) Good gauge agreement >10-11 Torr Diverge by 1x10-12 Torr x-ray limit of Torr subtracted from extractor 3BG has good signal to noise, never reads below Torr Limitations? Electrometer resolution Other background in 3BG? 16

17 Next steps and conclusions CP cold CP warm Preliminary Molflow simulations to model outgassing rate and pump speed / sticking coefficient Warm: outgassing Cold: sticking coefficient Potential for XHV cryopumping Low capacity, but XHV applications include high current polarized electron sources erhic 5e-13 5e-11 Ionization gauges at XHV are complicated at pressure 1000x below calibration points Looking for a paradigm shift in XHV pressure measurement (NIST?) 17

18 Acknowledgements U.S. DOE Contract No. DE-AC05-06OR23177 and with funding from the DOE R&D for Next Generation Nuclear Physics Accelerator Facilities Funding Opportunity Number: DE-FOA Jefferson Lab Center for Injectors and Sources Staff: Matt Poelker, Joe Grames, Don Bullard, Marcy Stutzman, John Hansknecht, Shukui Zhang, Carlos Hernandez Garcia, Philip Adderley, Riad Suleiman, Md. Abdullah Mamun (post-doc). Students: Yan Wang, Gabriel Palacios Serrano, Sajini Wijethunga, Joshua Yoskowitz, Anahi Miranda Segovia, Veronica Over 18

19 Backup 19

20 BNNT properties TEM Purity Residual Walls Tube Length Surface Area 40 to 50% by mass hbn flakes and micro-droplets of elemental boron, by TEM 1 to 5 walls, mostly 2 or 3 walled tubes up to 200 microns by SEM up to 300 m 2 /g by BET Bundles single through bundles of 5 tubes, TEM Impurities: B and hbn / pdf Band Gap Strength in air Thermal oxidation resistance Thermal Conductivity 5.7 ev (semiconducting) EELS spectroscopy 800 C (CNT is 400 C) Stable to at least 920 C in air 3000 W/mK (Cu = 400 W/mK, CNT 60-40,000 W/mK) 20

21 NEG array + Room Temperature Cryopump Stainless steel outgassing rate 5x10-13 TorrLs -1 cm -2 NEG pumps fully activated, WP950 ST 707, 525 L/s each Outgassing rate of fins adjusted to reach measured pressure of 5.2e-12 Torr on 3BG BNNT outgassing rate 5e-12 Torrl/s/cm^2 effective 21