Extreme High Vacuum: The Need, Production, and Measurement

Extreme High Vacuum: The Need, Production, and Measurement

Extreme High Vacuum: The Need, Production, and Measurement Marcy Stutzman, Philip Adderley, Matt Poelker Thomas Jefferson National Accelerator Facility (Jefferson Lab) Polarized Electron Gun Group Newport News, Virginia Run by JSA for the US DOE What is XHV Extreme High Vacuum P < 1x10-10 Pa = 1x10-12 mbar = 7.5x10-13 Torr Baked, metal systems, low outgassing, coatings to reduce outgassing Combinations of pumping Ion, Getter, Cryo, Titanium Sublimation, Turbo Measurement: Ionization gauges Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd P.A. Redhead Vacuum 53 (1999) 137 Ultimate Vacuum Bayard-Alpert gauge introduced in 1950 Plateau ~1x10-14 Torr for nearly 3 decades again Ultimate Vacuum (Torr)

Steady decrease interrupted by gauge limitations 1920-1950 10-6 10-8 10-10 10-12 10-14 Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Who needs XHV Storage Rings CERN ISR: beam lifetimes > 10 hours, pressure < 1x10-12 Torr Vacuum in interaction region in the 10-14 Torr range Large Detector Systems KATRIN (later in this session) Surface Science applications

CERN aerial view Alkali metals on surfaces surface contaminates within ~1 hour Surface X-ray diffraction at synchrotrons, He scattering low signal, long collection times Dynamical surface analysis High current polarized photo-electron guns Surface Imaging (LEED) Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Jefferson Lab CEBAF: Nuclear physics electron accelerator laboratory and Free Electron Laser (FEL) User community of 2000+ physicists GaAs Photoelectron gun (100 kV, 200 A, 85% polarization)

delivers beam simultaneously to three experimental halls Nuclear physics gun on up to 310 days/year, 24 hours/day CEBAF pressures ~1.2x10-11 Torr Guns pumped with combination of NEG and ion pumps FEL gun operates 350 kV, 9 mA unpolarized electron gun Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Photocathode Lifetime Laser spots Quantum Efficiency (yield) of GaAs photocathode decays Lifetime of inversely proportional to vacuum conditions Residual gas ionized Ion backbombardment damages Crystal structure Surface chemistry ~2.5E-11 Torr ~5.0E-11 Torr

>15.0E-11 Torr Lifetime very good: ~200 Coulombs, polarization Future applications: higher currents 85% Electron/ion colliders: >1 mA polarized Novel light sources: 100 mA unpolarized Electron cooling applications: 1 A+, unpolarized RF photoguns GaAs photocathodes Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Materials and Preparation Low outgassing Stainless Steel Titanium alloys Aluminum OFHC Copper, Cu/Be alloys 300 series austenitic steels (304L, 316L, 316LN) low carbon, 316 series adds Mo for strength

Coatings to reduce outgassing Coatings to add pumping Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Hydrogen reduction through heating Calder & Lewin 1967 calculate time and temperature to reduce stainless steel outgassing Ficks law governs diffusion of hydrogen from bulk metal Initial concentration Time Temperature Wall thickness Surface recombination Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Outgassing Rates for SS Q = 10-12 10-16 mbarL/scm2 Y. Ishikawa, V. Nemani Vacuum 69 (2003) 501 } P. Marin Virgo, Vacuum1998 } M. Bernardini Virgo, JVSTA 1998

}H. Hseuh Brookhaven JVSTA 1998 }G. Messer, 1977 } V. Nemani thin walls JVSTA 1999 } V. Nemani JVSTA 2000 Other exceptional outgassing rates (in Torr L/scm2) BeCu alloy: 4x10-16 F. Watanabe JVSTA 22 (2004) 181, 22(2004) 739. Ti/steel alloy: 7.5x10-15 H. Kurisu et al. JVSTA 21 (2003) L10. JLab: 1x10-12 TorrL/scm2 Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd 304 SS vacuum chambers 2 Untreated Electropolished and vacuum fired 900C 4 hours Outgassing Rate (TorrL/scm ) JLab Preparation Baking 30 hours, 250C Unfired chamber 1x1012 TorrL/scm2 ~13

bakes Vacuum fired chamber 8.9x10-13 TorrL/scm2 3 bakes 5E-12 Untreated: 250C bake EP: 150C bake 4E-12 EP: 250C bake 3E-12 2E-12 1E-12 0 0 5 10 15 Bake number M.L. Stutzman et al. submitted to NIM 2006 Achieve modest outgassing rate for 304SS Lower rates possible with better grade steel Add heat treatment after final welding Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006

rd XHV surface coatings TiN P.He, H.C.Hseuh, M.Mapes, R.Todd, N.Hilleret Outgassing for SNS ring material with and without TiN coatings -11 (mbarL/scm2) TiN, SiO2, Chromium oxide Diffusion barrier for hydrogen Affect surface recombination Can also reduce beam induced pressure rise in storage rings See session VT-WeM 10 TiN 10-12 10-13 10-14 K. Saito et al JVSTA 13 (1995) 556 10-15 10-16 Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE

Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd SiO2 coated 304 SS (Restek prototype) SiO2 coating applied to inside and outside, chemically stripped Accumulation method with spinning rotor gauge Outgassing no better with SiO2 coating Prototype coatings Chemical stripping process Increased surface roughness Outgassing (TorrL/scm2) SiO2 Coatings Outgassing: SiO2 coated 304 SS 2.5E-12 SiO2 coatings Thin: 3 um Thick: 6-12 um 2.0E-12 1.5E-12 1.0E-12

JLab: Y. Prilepskiy, G.R.Myneni, P.A. Adderley, M.L.Stutzman Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Cr2O3 Surface passivation 304L Surface passivation Vacuum fire 450C, 24 hours 1x10-9 Torr O2 partial pressure 5x10-7 Torr total pressure Cr2O3 is one component of air fired, low outgassing materials (VIRGO, LIGO) K.R. Kim et al Proceedings of APAC 2004 Gyeongju, Korea Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Distributed beamline pumping Beamlines coated with getter material (Ti/Zr/V) activated through bakeout temperature

~200C No conductance limitation Reduces beam induced pressure rise RIKEN from SAES literature ESRF insertion device 20 mm 30 mm Distributed Ion pump: Y.Li et al., JVSTA 15 (1997) 2493. Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd JLabs NEG coating Ti/Zr/V NEG coating Sputtering without magnetron enhancement Beamline exiting CEBAF electron guns NEG coated since 1999 Enhanced photocathode lifetime: now achieving lifetime ~200 Coulombs High voltage chamber for new load locked gun coated

25% Ti 50% V 25% Zr EDS analysis of getter coating composition Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Load Locked Electron Gun NEG coated HV chamber Vacuum measured: ~1.2x10-11 Torr Lifetime doubled 5-10 mA, 100 keV electron beam NEG pipe NEG coated high voltage chamber NEW OLD Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Pumps for XHV Ion pumps Ion pump performance vs. voltage Ion pump current monitor at UHV pressures Getter coating ion pumps

NEG Great pumping for hydrogen, also pumps CO, N 2 Dont pump methane, noble gasses Question about pump speed at base pressure Ti Sublimation Cryo pumps Turbo pumps cascaded pumps Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Ion pump limitations Varian Ion pump speed decreases at lower pressures Lower nA/Torr at lower pressures Re-emission of gasses Outgassing from pump body Adding NEG pumping to ion pumping decreases hydrogen Pd coated NEG films on inside of ion pumps reduced ultimate pressure to 2-6x10-11 mbar Maruo Audi, 45th IUVSTA Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman

AVS 53 International Symposium 14 November 2006 rd JLab UHV ion pump current monitoring 1E-2 Current vs. Pressure 1E-3 Ion pump current (A) Ion pumps current varies linearly with pressure as low as 1x10-11 Torr Real time monitoring of UHV vacuum Studying optimal voltage for pumping at low pressures 1E-4 1E-5 1E-6 our data 1E-7 manufacturer 1E-8 1E-9 1E-12 1E-11 1E-10

1E-09 1E-08 1E-07 1E-06 1E-05 Extractor Gauge Pressure (Torr) Current (10-10A) ~10-10 Torr Full Scale Discharge event in beamline Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Base pressure in CEBAF guns Why isnt our chamber pressure as low as calculated? First measured outgassing rate from chamber 1x10-12 TorrL/scm2 Typical value for baked

304SS 1E-10 -10 Pressure (Torr) Log Pressure (Torr) Is outgassing much higher? Is pump speed much lower? Are we unable to measure lower pressures? Measured and predicted pressure for 304 SS chambers and ST707 SAES getter modules Test chambers measured -11 1E-11 CEBAF guns predicted -12 1E-12 0 0.2 0.4

0.6 0.8 1 1.2 GetterSurface Surface Area (m ) Getter area (m2) 2 Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Pump speed measurements Measured pump speed vs. pressure from base pressure of chamber to 2x10-10 Torr Throughput method conductance limiting orifice RGAs to measure H2 pressure Ultimate pressure method Gas sources: outgassing from walls and gauge Measure with extractor gauge

Found very good pump speed at higher pressures 500 L/s with bakeout 1150 L/s activated (430 L/s quoted) Found drop in pump speed as function of pressure: WHY? 1400 1200 Speed per module (L/s) 1000 resistive activation 800 600 400 passive activation 200 0 0 5.0E-11 1.0E-10 1.5E-10 2.0E-10

Pressure (Torr) ' ' C * Porf Porf ( Pmain Pmain ) Qwall Qgauge S Pmain Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Alternate analysis of pump speed measurement Problem with throughput vs. pump speed at low pressures? Problem with accurately measuring low pressures? 2.5E-07 Throughput (TorrL/s) Q=S*P Plot Q instead of S vs. P

Linear fit indicates constant pump speed throughout range Discrepancy: 2.0E-07 Slope = Pump Speed = 1150 L/s 1.5E-07 1.0E-07 Throughput 5.0E-08 Ultimate 0 0 1.0E-10 2.0E-10 Pressure (Torr) M. Stutzman et al. submitted to NIM 2006 Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd XHV Pressure Measurement Ionization Gauges Hot Cathode: Extractor, Improved Helmer, Axtran,

Modulated BA, spectroscopy and bent beam gauges Cold Cathode: Magnetron, inverted magnetron, double inverted magnetron Laser ionization gauges X-ray limits Electron stimulated desorption limits Gauge outgassing Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd X-ray limit Ionized gas molecules collected, proportional to gas pressure +V Electrons strike grid, generate x-rays X-rays striking collector photoemit + filament collector A Collector current is sum of ionized gas and photoemitted electrons Bayard-Alpert gauge 1950s led to UHV measurements smaller collector

modulation techniques Extractor gauge geometry reduces measurement limits to ~XHV range Improved Helmer gauge, Watanabe gauges optimize geometry Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Extractor Gauge X-ray limits Extractor collector current vs. voltage 1E-11 Collector Current (A) Ext A Gun 2 1E-12 Gun 3 1E-13 1E-14 1E-15 0 100 200 300 400

500 600 Voltage (V) JLab 2006 Fumio Watanabe JVSTA 9 (1991). Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Gauge X-ray Limit (Torr) Watanabe A 2.1 x 10-12 Watanabe B 1.6 x 10-12 Watanabe C 1.9 x 10-12 JLab A 0.63 x 10-12 JLab Gun 2 >2 x 10-12 JLab Gun 3

Stutzman >2Marcy x 10-12 rd AVS 53 International Symposium 14 November 2006 Extractor gauge comparison Ext K Ext A Ext D 1E-11 8E-12 6E-12 4E-12 2E-12 0 0 24 48 72 96 Hours Three extractor gauges Factor of 8 difference in readings Identical ports Symmetric positions Multiple degas cycles E x tra c to r D(Torr) Ext. D Log Pressure

3 Extractors vs. time 1.2E-11 Pressure (Torr N2) -9 1E-09 1.4E-11 Extractor A vs. Extractor D 1E-10 -10 1E-11 -11 1E-12 -12 -13 -13 1E-13 1E-13 1E-12 1E-11 1E-10 -11 -10 -12 Extractor A Ext. A Log Pressure (Torr) -9

1E-09 Divergence in pressure readings below 5x10-11 Torr Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Electron Stimulated Desorption +V ESD ions Have energy higher than gas phase Energy discrimination ESD neutrals + Same energy as gas phase + Hotter grid: less adsorbed gas o Electron bombardment More outgassing filament

Resistive heating ESD and outgassing decoupled collector A Watanabe heated grid gauges: total pressure and residual gas analyzer BeCu walls Ref: Fumio Watanabe JVSTA 17 (1999) 3467, JVSTA 20 (2002) 1222. Low emissivity High thermal conductivity Cold cathode Decouple grid temperature from filament Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Gauge solutions Extractor commercially available X-ray limits can be in the 10-13 Torr range (barely XHV) Reasonable residual current caused by ESD due to geometry Work needed to ensure accuracy over time, between gauges Improved Helmer gauge used at CERN Frequent pressure measurements in 10-14 Torr range quoted

Watanabe proposes heated filament gauges Separate ESD, outgassing problems Laser ionization gauge Ionize gas with powerful laser, count ions: direct gauge of low pressures Calibration techniques Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Calibration Techniques Careful calibration needed for measurements below 5x10 -11 Torr Cross calibration with transfer standards Dynamic or static expansion methods Relatively complex systems Not common in gauge user laboratories Reported XHV pressure measurements should make note of the calibration method C. Meinke and G. Reich JVST 6 (1967) 356. A. Berman and J.K. Fremerey JVSTA 5 (1987) 2436. W. Jitschin et al. JVSTA 10 (1992) 3344. S. Ichimura et al. Vacuum 53 (1999) 291. P. Szwemin et al. Vacuum 73 (2004) 249. Thomas Jefferson National Accelerator Facility

Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Future work at JLab Get best available material Polish, vacuum fire after welding Optimize and calibrate extractor gauges, or Replace extractor gauges with better XHV gauge UHV ion pump supplies NEW OLD Optimize voltage, geometry for pressure Investigate NEG coatings in ion pumps Use cathode lifetime as a relative gauge Gauge exchange / cross calibration at different facilities Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd Future of XHV Gauging issues are coming along, but still an art, calibration critical Materials exist many different recipes to get very good outgassing rates NEG, TiN coatings becoming widespread

Pumping technologies existing technologies can achieve XHV room for improvement and study When XHV becomes routine, high current electron guns, surface science, accelerators, semiconductor industry, and others will benefit Ultimate Vacuum (Torr) 10-6 10-8 10-10 10-12 10-14 Thomas Jefferson National Accelerator Facility Operated by JSA for the US DOE Marcy Stutzman AVS 53 International Symposium 14 November 2006 rd

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