Pressure Considerations Associated with Ion Sampling from Glow Discharges

A discussion is presented of the influence of ion-neutral collisions downstream from an orifice through which ions from a glow discharge are being extracted. The effective pressure on the low pressure side of the orifice is treated as the sum of a static pressure (determined from pumping speed and orifice conductance considerations) and a localized beam pressure which is due to the neutral species effusing through the orifice with the ions. It is shown that in most experimental situations the beam pressure has a much greater influence on a collimated ion beam than does the static pressure. Some experimental results which support this conclusion are presented.