. Why are the 4 curves in Stage 2 not exactly flat, but has a small but definite
ID: 919667 • Letter: #
Question
. Why are the 4 curves in Stage 2 not exactly flat, but has a small but definite slope of decrease? Select the true statement (only one).
a) In this pressure range, gas flow goes from viscous to the molecular flow regime. Gases flow at much slower rate in molecular flow regime. Thus the turbo pump is much slower. This causes the curve to have a very small rate of decrease.
b) The lower the pressure, the less dense the gas, the fewer molecules the pump can scoop out each cycle. Thus the turbo pump speed is greatly reduced in this pressure regime. This causes the curve to have a very small rate of decrease.
c) The pump throughput is the same as gas load in this stage. Thus the rates of molecules being added to the chamber and removed from the chamber cancel each other out. Thus pressure should no longer decrease. However, our current gas load is caused by outgassing of molecules from chamber surfaces. As the source of outgassing is gradually depleted, Qload gradually decreases. Pmin = Qload/Seff, thus Pmin gradually decreases.
d) The pump is much slower because it is now pumping on lighter molecules such as He or H2. This causes the curve to have a very small rate of decrease.
e) The pump is close to its mechanical limit. This causes the curve to become flatter and flatter.
Explanation / Answer
The correct ption is c) The pump throughput is the same as gas load in this stage. Thus the rates of molecules being added to the chamber and removed from the chamber cancel each other out. Thus pressure should no longer decrease. However, our current gas load is caused by outgassing of molecules from chamber surfaces. As the source of outgassing is gradually depleted, Qload gradually decreases. Pmin = Qload/Seff, thus Pmin gradually decreases
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