Interplay between collective and collisional processes in critical velocity ionization

Abstract

Numerical simulations of critical ionization velocity (CIV) discharges have been performed, taking into account several collisional processes. The simulations are one‐dimensional with density variation in the direction of plasma‐neutral relative velocity. The heating of electrons by wave‐particle interactions results in an extended electron distribution with “hot tail” formation, which enables excitation and ionization of neutrals to occur. The tail formation is observed to persist with a maximum energy less than twice the beam energy. The collisional processes included in this simulation are ground state ionization, excitation and ionization of metastable states, ion‐neutral charge exchange, and electron‐neutral elastic collisions. It is found that near the critical velocity, metastable states participate strongly in ionization, while at high velocities the metastable effect becomes relatively unimportant. Electron‐neutral elastic collisions, within the approximations used in this model, appear to be unimportant. Charge exchange is found to be crucial in sustaining the discharge when the beam velocity is low but is unimportant when the beam velocity is high. Detailed results are shown for the case of neon, while threshold behavior is investigated for helium, xenon, and barium as well. The density dependence of ionization in neon and barium is investigated. Although we examine the effect of line excitation on thresholds for CIV in all species, xenon and barium are especially of interest because both species have been used or proposed to be used in space CIV experiments. We report that line excitation strongly drains energy for barium beams in space. As a result, despite a low ionization potential, the threshold velocity for critical ionization of barium is raised substantially. This suggests a reason why barium beam ejections, the most common method for investigating CIV in space, have so far achieved inconclusive results. We conclude that low ionization potential may not be the best criterion for choosing the optimal beams in space experiments on CIV.