An Analysis of Accumulator Ring Pressure Date Page: 3 of 9
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1). Data from Gauges Mounted on the Chamber
The expected reading of any of these ion gauges relies upon a
knowledge of the chamber geometry, the pumping speed(s) and the
chamber's specific outgassing rate. While the Accumulator Ring's
geometry and pumping speeds are well-known, reliable specific
outgassing rates are, in general, notoriously difficult to assign.
Data from the LEAR and AA Vacuum Group for unbaked UHV-treated
stainless steel indicate specific outgassing rates between 2 x
10 9 and 7 x 10 12 T9 sec 'cm2 depending upon the length of time
for which the system is pumped. 3
Data on outgassing rates of UHV-treated stainless steel which
is additionally baked in situ at low temperature are more
confusing and appear to be less reliable. The results appear to
depend upon a large number of parameters - including the
time-integrated history o the material subjected to the low
temperature in situ bake. () For UHV-treated stainless steel
chambers baked in situ for 24-36 hours at 150 -300 C, recommended
specific outgassing rates cover the range from 4 x 10-12 to 1 x
10-13 T. sec~'cm-2. The "most popular" recommendation seems to be
a value of 1 x 10-12 T sec~1cm 2 for UHV-treated systems baked to
300 C for 24 hours or more.
In view of (and in spite of) these uncertainties, we have
elected to use the outgassing rates shown in Table 1.
Table 1. Assumed Outgassing Rates for
Accumulator Ring Vacuum Chamber
Temperature at Which Specific Outgassing Rate
The Chamber was Baked ( C) (TL sec 1cm 2)
100 2.0 x 10-12
150 1.5 x 10-12
200 1.0 x 10-12
The choices shown in Table 1 are close to the values
recommended in the literature and they accommodate the fact that
different portions of the Accumulator Ring's vacuum chamber have
been baked at different temperatures.
The data of Table 1, together with a pumping speed of 2000
i.sec' for the TSP's, is sufficient to calculate the expected
pressure at each ion gauge in the non-tank areas. The results,
shown in Table 2, indicate:
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Gormley, M. An Analysis of Accumulator Ring Pressure Date, report, January 9, 1986; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc934801/m1/3/: accessed January 20, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.