# Presentation: 943950

## 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

## Ultimate Vacuum

- Steady decrease interrupted by gauge limitations 1920-1950
- Bayard-Alpert gauge introduced in 1950
- Plateau ~1x10-14 Torr for nearly 3 decades again

## 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
- 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)

- CERN aerial view

## Jefferson Lab

- CEBAF: Nuclear physics electron accelerator laboratory and               Free Electron Laser (FEL)
- User community of 2000+ physicists
- GaAs Photoelectron gun            (100 kV, 200 μA, 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

## Photocathode Lifetime

- Quantum Efficiency (yield) of GaAs photocathode decays
- Lifetime of inversely proportional to vacuum conditions
- Residual gas ionized
- Ion backbombardment damages
- Crystal structure
- Surface chemistry
- Lifetime very good: ~200 Coulombs,     85% polarization
- Future applications: higher currents
- Electron/ion colliders: >1 mA polarized
- Novel light sources: 100 mA unpolarized
- Electron cooling applications: 1 A+, unpolarized
- RF photoguns – GaAs photocathodes

- Laser spots

- >15.0E-11 Torr

- ~5.0E-11 Torr

- ~2.5E-11 Torr

## 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

## Hydrogen reduction through heating

- Calder & Lewin 1967 calculate time and temperature to reduce stainless steel outgassing

- Fick’s law governs diffusion of hydrogen from bulk metal
- Initial concentration
- Time
- Temperature
- Wall thickness
- Surface recombination

## Outgassing Rates for SS

- Q = 10-12 – 10-16 mbar·L/s·cm2

- Y. Ishikawa, V. Nemanič Vacuum 69 (2003) 501

- } M. Bernardini Virgo, JVSTA 1998

- } P. Marin Virgo, Vacuum1998

- }H. Hseuh Brookhaven JVSTA 1998

- }G. Messer, 1977

- } V. Nemanič thin walls JVSTA 1999

- } V. Nemanič JVSTA 2000

- Other exceptional outgassing rates (in Torr·L/s·cm2)
- 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 Torr·L/s·cm2

## JLab Preparation

- 304 SS vacuum chambers
- Untreated
- Electropolished and vacuum fired 900°C      4 hours
- Baking
- 30 hours, 250°C
- Unfired chamber    1x10-12 Torr·L/s·cm2  ~13 bakes
- Vacuum fired chamber 8.9x10-13 Torr·L/s·cm2   3 bakes

- 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**

## XHV surface coatings

- 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

- P.He, H.C.Hseuh, M.Mapes, R.Todd, N.Hilleret
- Outgassing for SNS ring material with and without TiN coatings

- K. Saito et al
- JVSTA 13 (1995) 556

- TiN

- TiN

- 10-16

- 10-12

- 10-13

- 10-14

- 10-15

- 10-11

- (mbar·L/s·cm2)

## SiO2 Coatings

- 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

- JLab: Y. Prilepskiy, G.R.Myneni, P.A. Adderley, M.L.Stutzman

- SiO2 coatings
- Thin: 3 um
- Thick: 6-12 um

## Cr2O3 Surface passivation

- K.R. Kim et al Proceedings of APAC 2004 Gyeongju, Korea

- 304L Surface passivation
- Vacuum fire 450°C, 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)

## Distributed beamline pumping

- Beamlines coated with getter material (Ti/Zr/V)
- activated through bakeout temperature ~200°C
- No conductance limitation
- Reduces beam induced pressure rise

- Distributed Ion pump: Y.Li et al., JVSTA 15 (1997) 2493.

- RIKEN from SAES literature

## JLab’s 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

- EDS analysis of getter coating composition

- 25% Ti
- 50% V
- 25% Zr

## Load Locked Electron Gun

- NEG pipe

- NEG coated high voltage chamber

- NEG coated HV chamber
- Vacuum measured: ~1.2x10-11 Torr
- Lifetime doubled
- 5-10 mA, 100 keV electron beam

## 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, N2
- Don’t pump methane, noble gasses
- Question about pump speed at base pressure
- Ti Sublimation
- Cryo pumps
- Turbo pumps – cascaded pumps

## Ion pump limitations

- 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

- Varian

- Maruo Audi, 45th IUVSTA

## JLab UHV ion pump current monitoring

- 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

- Current vs. Pressure

- ~10-10 Torr
- Full Scale
- Discharge event in beamline

- Current (10-10A)

## Base pressure in CEBAF guns

- Why isn’t our chamber pressure as low as calculated?
- Is outgassing much higher?
- Is pump speed much lower?
- Are we unable to measure lower pressures?
- First measured outgassing rate from chamber
- 1x10-12 Torr·L/s·cm2
- Typical value for baked 304SS

- Measured and predicted pressure for
- 304 SS chambers and ST707 SAES
- getter modules

- CEBAF guns

- Test chambers

- Getter Surface area (m2)

- Log Pressure (Torr)

- -12

- -10

- -11

## 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?

## Alternate analysis of pump speed measurement

- Q=S*P
- Plot Q instead of S vs. P
- Linear fit indicates constant pump speed throughout range
- Discrepancy:
- Problem with throughput vs. pump speed at low pressures?
- Problem with accurately measuring low pressures?

- M. Stutzman *et al.* submitted to NIM 2006

## 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

## X-ray limit

- filament

- +V

- collector

- +

- Bayard-Alpert gauge 1950’s led to UHV measurements
- smaller collector
- modulation techniques
- Extractor gauge geometry reduces measurement limits to ~XHV range
- Improved Helmer gauge, Watanabe gauges optimize geometry

- Ionized gas molecules collected, proportional to gas pressure
- Electrons strike grid, generate x-rays
- X-rays striking collector photoemit
- Collector current is sum of ionized gas and photoemitted electrons

## Extractor Gauge X-ray limits

- Fumio Watanabe JVSTA 9 (1991).

- JLab 2006

| 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 | >2 x 10-12 |

## Extractor gauge comparison

- Three extractor gauges
- Factor of 8 difference in readings
- Identical ports
- Symmetric positions
- Multiple degas cycles

- Divergence in pressure readings below 5x10-11 Torr

## Electron Stimulated Desorption

- ESD ions
- Have energy higher than gas phase
- Energy discrimination
- ESD neutrals
- Same energy as gas phase
- Hotter grid: less adsorbed gas
- Electron bombardment
- More outgassing
- Resistive heating
- ESD and outgassing decoupled
- Watanabe heated grid gauges: total pressure and residual gas analyzer
- BeCu walls
- Low emissivity
- High thermal conductivity
- Cold cathode
- Decouple grid temperature from filament

- Ref: Fumio Watanabe
- JVSTA 17 (1999) 3467,
- JVSTA 20 (2002) 1222.

## 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_

## 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.
- 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.

## 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
- 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

## 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

- 10-14

- Ultimate Vacuum (Torr)

- 10-10

- 10-8

- 10-6

- 10-12