Re: How do ion vacuum pumps work?

Dear Taylor:
To answer your questions, first you should know that the pressure at the 
earth's surface, standard pressure, is 760 torr.  A vacuum ion-pump begins 
to function at pressures below 10E(-2)torr.  Therefore another kind of 
pumping must be done first to bring the vacuum chamber down to at least 
this pressure.  When the pressure is lower than this the ion vacuum pump 
works as follows:

A plasma is initiated in the chamber which usually contains one electrode 
at least made up of a reactive metal like titanium.   The gas on the 
chamber is residual air, which contains nitrogen and oxygen. In the plasma 
these ions strike the titanium surface and sputter off titaniun and create 
titanium atoms in the plasma.  The titanium ions neutralize and deposit 
either back on the titanium surface of on the chamber walls or any other 
exposed surfaces. However in striking and sticking to the wall surface the 
titanium atoms trap atoms of oxygen and nitrogen gas onto the surface 
which stick there by chemical attractive forces.

As the pump operates more and more gas molecules are trapped on the 
exposed surfaces by this process so that the pressure in the chamber 
begins to fall until a very low value, where the rate of trapping and rate 
of release of the gas atoms come to equilibrium.  Depending upon the 
gases, the metal used, and the plasma power level very low pressures are 
achieved in this manner.  In the plasma the deposited titanium metal 
surface becomes very chemically reactive and so the gas atoms stick there 
very tightly.  The presence of the metal atoms in the plasma is essential 
for the puming action to take place.

The pressure of 2E(-8) torr is equal to 2E(-8)/760 fraction of one 
atmoshpere.

R. Bersin....