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So how likely is a molecule to fly into a pump-trap? That depends on how big the pipe is leading to the pump.
Conductance:
Molecules will fly through only as fast as the tube allows. A long/narrow pipe restricts that flow more than a wide/short pipe. That rate of flow is called the tube's conductance; the wide/short pipe has the higher conductance of the two.
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When we want to get to really low pressures (which, at SLAC, is most of the time) we use layered tiers of pumping (Fun Fact: for similar reasons we have to use layered tiers of pressure gauges as well)
An example pumping sequence goes like this:
A vacuum chamber filled with air at room pressure is to be pumped down into really low pressures:
Turn on the first pump, it pulls air and the pressure drops, through viscous flow and a bit into molecular flow, he pressure drop slows and then stops. The pump can't remove molecules any faster than they're leaking into the chamber. Turn on the second pump. It could probably viscous flow as it starts up, but as it's blades spin faster and faster (think jet engine)
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for more details on this, see the confluence page dedicated to vacuum pumps. and for that matter, the one on gauges too.
Pressure, revisited: this time it's chambers.
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