An investigation as to the cause of beam asymmetry in a compact gas discharge ion source: A focus on beam-wall interaction

Abstract

Helium beams in a compact gas discharge source lose their transverse symmetry when the extracting electrode is biased at high potentials. Further, this condition gives rise to excessive formation of electrons within the vicinity of the beam line of propagation. A plausible explanation for these abnormalities is explained via the effects of the source extractor's lens property on the beams' physical configuration. The optical relation of the extractor shows that when the extraction voltage (Ve) exceeds half the discharge voltage (Vd), its focal length extends backward pass the discharge region's emitting orifice. As a consequence, beam divergence is increased akin to light beams expanding when the source is positioned between a negative lens and its principal focus. Numerical simulations of the beams' envelopes at different discharge and biasing conditions provide further proof of the theory's validity. When Ve>Vd/2, the construct shows exiting beams to have waists greater than the diameter of the drift tube suggesting increased interaction between beam edge and the tube's interior walls resulting in secondary electron emissions. The presence of electrons inexorably leads to charge neutralization thus creating asymmetric beams downstream. Mass spectroscopic detection of O- ions most likely from surface oxides, and increased electron densities obtained by way of Langmuir measurements are phenomenological evidences to this effect. This work intends to establish the explicit causality relation between secondary electron emission and the formation of asymmetric beams in miniaturized ion sources.