Cathode potential of RGA and outgassing from vacuum wall around the ion source

Abstract

A residual gas analyzer (RGA), where a negative cathode potential referred to the ground potential at its ion source is used, has a potential problem giving large outgassing compared with one that has a positive cathode potential. This is because that the potential configuration causes emitted electrons to stimulate gas desorption on a potentially grounded vacuum wall around the ion source. We measured the dependence of the outgassing from a vacuum wall on the cathode potential. It was observed that outgassing by the electron stimulation increased around the point that the cathode potential dropped below the ground potential. Cross sections of electron stimulated desorption of H 2 gas on a stainless steel surface were also measured. There are commercially available RGAs operated under the condition that a negative potential is used in the ion source and an electron repeller is equipped to confine emitted electrons in the ion source. The measured outgassing rates of such RGAs, however, were found to be remarkably large and not suitable for XHV measurement compared with the other types. This is probably because a considerable fraction of electrons escape from the repeller through a hole or a gap for the feedthroughs. This problem was confirmed by both the behavior of electron trajectories around the source in computer simulations and experimentally obtained outgassing properties depending on electrical configurations.