Transcript A Comparison of Outgassing Measurements For Three Vacuum Chamber Materials P. Adderley, M.

A Comparison of Outgassing Measurements
For Three Vacuum Chamber Materials
P. Adderley, M. Baylac, J. Clark, T. Day, J. Grames, J. Hansknecht, G. Myneni, M. Poelker, P.M. Rutt †, C.K. Sinclair*, M.L. Stutzman
Thomas Jefferson National Accelerator Facility 12000 Jefferson Ave., Newport News, VA 23606 USA. Operated by SURA for the DOE
Chambers
Abstract
Method Pros and Cons
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In order to understand the limitations of the existing gun vacuum
chamber, and examine possible new chamber materials, we
have undertaken a study of the outgassing rate for chambers of
three different materials: 304 stainless steel (the present
chamber material), 316L stainless steel, and aluminum.
304 SS
Outgassing rates for each chamber were measured using two
independent techniques. For one technique, the test chamber is
separated from a vacuum pump by a well-defined conductancelimiting aperture. The outgassing rate of the test chamber can
be determined by measuring the pressure drop across the
orifice. The other technique measures the rate of rise of vacuum
pressure within the test chamber using a spinning rotor gauge
over an extended period of time. After experimenting with a
variety of vacuum gauges, procedures and chamber
configurations, good agreement was obtained between the two
techniques. Outgassing rates for each material will be presented
and details of the measurements will be described.
Spinning rotor gauge does not affect vacuum
Spinning rotor gauge cannot read at pressures < 1e-6
Method takes many days to complete
Room temperature cannot change during measurement
316L SS
Aluminum
Outgassing Rate
Methods
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9.65e-13
1.26e-12
9.2e-13
Orifice Method
Quick measurement taking ~12 hours to equilibrate
Data
Extractor gauges are sources of gas at low pressures and
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pump at higher pressure (crossover ~1e Torr)
Test Chamber
Rate of Rise with Spinning Rotor Gauge
for Three Vacuum Chamber Materials
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Pressure (Torr)
Orifice
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Test and pumping chambers are evacuated and baked at
250°C for 30 hours.
Chamber vacuum allowed to equilibrate for ~12 hours
following bakeout.
Pumping for test chamber is now through orifice.
At steady state,
LeakRate   P  C
.
with C the conductance of the orifice
Outgassing rate, Q, is defined as leak rate divided by
surface area, so:
Q
C  P
SA
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Evacuate test chamber as with orifice method
Eliminate all pumping
Track rising pressure using spinning rotor gauge for
times ~70 hours
Calculate outgassing rate
Q
P V

T SA
1.0E-04
8.0E-05
6.0E-05
316L SS
304 SS
6061 Al
4.0E-05
2.0E-05
0.0E+00
0
20
40
60
80
100
120
Time (hours)
Extractors Orifice Spinning Rotor
Conclusions
System Schematic
Rate of Rise
†Now
304 Stainless
Steel
316L Stainless
Steel
Aluminum
6061-T6§
Rate of Rise
Method
1.05e-12
(68 hours)
1.15e-12
(71 hours)
1.05e-12
(70 hours)
1.2E-04
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Gun Vacuum Chamber
Orifice Method
(Torr·l/s·cm )
Rate of Rise Method
Jefferson Laboratory operates a nuclear physics accelerator
delivering polarized electron beams with energies up to 6 GeV to
three experimental halls. The polarized electrons are produced
from photoemission cathodes in 100 kV electron guns. The
vacuum requirements for these guns are very demanding, since
the photocathode lifetime is limited by ion backbombardment
from the ionization of the residual gas in the cathode-anode gap.
Production Photoguns
Measured Outgassing Rates
Pump
Chamber
Three vacuum chambers have been constructed to
determine relative outgassing rates for the different
materials.
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All test chambers exhibit quite similar outgassing rates.
Surface finish effects and chamber history (number of
bakes, vacuum firing and electropolishing) have been
different for each and these effects need to be quantified.
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We have seen good agreement between the two methods
of calculating outgassing rate of the test chambers.
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Future studies include a measurement of outgassing rates
vs. coatings such as an external Titanium Nitirde layer.
Test Vacuum
Chamber
0.082” Orifice
at Helix Technology, [email protected] *Now at Cornell University, [email protected], § Aluminum Chamber by Atlas Technologies, www.atlasbimetal.com
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