Some aspects determining the electrical strength of the accelerating gap in a fore-vacuum-pressure plasma-cathode electron beam source utilizing helium and argon as the working gas were explored. Penetration of plasma into the accelerating gap is the cause of breakdown. We find that gap breakdown occurs at larger emission hole diameter and at higher plasma density in helium than in argon. The greater stability of the plasma boundary in helium is explained by the observed greater anode potential drop and consequently greater sheath thickness.
Burdovitsin et al. (Thu,) studied this question.