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.

Simulation of the critical ionization velocity: Effect of using physically correct mass ratios

“…In this scenario, electrons are de-energized by excitational collisions before they heat sufficiently to be capable of ionizing neutrals. Such situations were addressed in the simulations of McNeil et al [1990] and Person et al [1990], as well as by Lai et al [1989] and Newell and Totbert [1985], 'All of the simulations referenced above have used explicit particle-in-cell (PIC) methods. These explicit methods have been similar to the well-known ElectroStatic, 1-dimension (ES1) code and its multidimensional extensions [Birdsall and Langdon, 1985], with a probabilistic collision model included [Birdsall, 1991].…”

Ionospheric Physics

“…Additional HF plasma heating results were described by Isham et al, [1987]; Fejer et al, [1989]; Kuo and Lee [1989]; Thide et al, [1989]; Kuo and Lee [1990]; Milikh [1990]. The critical velocity ionization phenomenon was studied by Lai et al, [1990 a,b]; McNeil et al, [1990]; Person et al, [1990]; Stenbaek‐Nielsen et al, [1990] and others already listed above.…”

Numerical quasi‐linear study of the critical ionization velocity phenomenon

“…As mentioned above, the LH instability, driven by an ion beam produced by the interaction of a neutral gas with the ambient plasma, is considered in almost all of the CIV theories [Galeev, 1981;Formisano et al, 1982;Papadopoulos, 1983Papadopoulos, , 1984Papadopoulos, , 1992Abe and Machida, 1985;Goertz et al, 1985;Goertz, 1986, 1988;Mdibius et al, 1987]. The equations for the particles and waves in a homogeneous infinite medium have been solved in the quasi-linear approximation [Galeev, 1981;Formisano et al, 1982;Papadopoulos, 1983Papadopoulos, , 1984 and have been successfully modeled by particle simulation [McBride et al, 1972;Tanaka and Papadopoulos, 1983;Abe and Machida, 1985;Goertz, 1986, 1988;Goertz et al, 1990;McNeil et al, 1990]. Although there are several observations of frequencies in the LH range, both in laboratory [e.g., Eselevich and Fainshtein, 1986;Chang, 1988;Chang et al, 1989] and in ionospheric releases [e.g., Kelley et al, 1986], there are also measurements [Swenson et al, 1990;Nickenig and Piel, 1987] (see also Brenning [1992]) which are at odds with the wave properties seen in analytic solutions and the simulations mentioned above.…”