A simulation study of the critical ionization velocity process

Abstract

We studied the critical ionization velocity (CIV) process by first investigating a coupled system of equations describing the production of several ion species and electrons by impact ionization, their collisions with neutrals, and the heating of electrons. Analytic relations derived from this were tested with the help of a particle simulation including collisonal processes between neutrals and plasma particles. It was found that resistive heating of electrons plays an important role when the density of the neutrals is high and that electron heating due to lower hybrid waves is significant when the neutral density is low. In both cases we verified the control of the plasma production rate by the ratio of the beam velocity to the critical velocity. Himreel et al., 1976; Axniis, 1978; Mb'bius, 1979; Venkatararnani and Mattoo, 1980;Brenning, 1981], and related phenomena were reported in a rocket experiment [Haerendel, 1982] in relation with the space shuttle [Papadopoulos, 1984; Sasaki et al., 1986] and in some other phenomena in space [Lindeman et al., 1974; Cloutier et al., 1978; Petelski, 1981; Forrnisano et al., 1982; Galeev and Chabibrachmanov, 1983-]. However, the critical ionization velocity (CIV) process itself is not easily explained, since the neutral particles cannot be ionized by collisions with either background ions or electrons when the relative velocity is only slightly larger than V• [Danielsson, 1973].Early attempts to understand this phenomena focused on the interpretation of laboratory experiments and did not specify the basic mechanism underlying the phenomena, although the importance of some anomalous electron heating was recognized. A review of these theoretical considerations is given by Sherman [1973]. Relevance of the lower hybrid wave to the CIV process was postulated and studied by several authors [Raadu, 1978;Galeev, 1981;Abe, 1984;Papadopoulos, 1985]. According to them, it is possible to heat the electrons to energies roughly equal to the kinetic energy of the flowing neutrals. These energized electrons ionize the neutrals, producing new plasma to act as an additional generator of waves and thus forming a positive feedback loop.Investigations of the electron heating in the nonlinear stage of the lower hybrid wave instability were performed by computer simulations [McBride et al., 1972; Tanaka and Papadopoulos, 1983;Abe and Machida, 1985]. Moreover, in order to study the feedback mechanism directly, Machida et al. [1984] In this study we are concerned with the CIV process in both high and low neutral density cases. It is necessary to include several collisional processes between plasma and neutral particles in a high neutral density case in addition to the impact ionization collisions treated previously by several researchers [Raadu, 1978;Galeev, 1981;Papadopoulos, 1985]. In general, elastic collisions between electrons and neutrals occur most frequently when the electron energy is in a range below several tens of electron volts, which is the energy consideration in this work. It shou...

Interplay between collective and collisional processes in critical velocity ionization

“…This is near the 30/to e estimated earlier for VH-For comparison, for the simulation carried out byMachida and Goertz [1986] a similar analysis gives a value of 40/to e for Vc. First, in the lower left panel we observe the rapid onset of electron heating as energy is transferred from the ions to the electrons.…”

“…AND DISCUSSIONAs a necessary prerequisite to the understanding of all results presented here, we examine first the electron temperature and the plasma density as functions of time and the ion and electron distribution functions for a typical run just slightly above the CIV threshold.Figure 4shows these quantities for the model of neon. Although this is about 3 times larger than ours, it should be remembered that direct comparison of our result with theirs is complicated by the fact that our cross section for ionization climbs gradually from the threshold while that ofMachida and Goertz [1986] is represented as a step function. The temperature rises initially to a value of approximately one-third the energy of the beam ions and then drops somewhat and maintains a value of approximately one-fifth the initial beam ion energy.…”

“…Following the usual practice in numerical simulation [Machida and Goertz, 1986], we assume the ion/electron mass ratio M/m to be 100. Following the usual practice in numerical simulation [Machida and Goertz, 1986], we assume the ion/electron mass ratio M/m to be 100.…”

Momentum coupling in the “CRIT II” critical ionization velocity experiment

“…Therefore one would expect a substantial amount of the barium atoms will be ionized by the CIV effect. As reviewed by Torbert [1990], tence of the CIV effect [Fahleson, 1961;Himreel et al, 1977;Axnds, 1978] and a number of theoretical studies [Raadu, 1978;Formisano et al, 1982;Galeev and Chabibrachmanov, 1983;Machida and Goertz, 1986; Mdbius et al, 1987] have shown relatively consistent theories to explain these experiments, the conflicting results yielded from space CIV experiments have posed a question as to the existence of the CIV effect in a free space where it was proposed to have taken place.…”

Simulation study of the ionizing front in the critical ionization velocity phenomenon.

“…2. Whereas the waves and electron heating inside the neutral cloud has been studied extensively before (RAADU, 1978;GALEEV, 1981;MACHIDA et al, 1984;ABE and MACHIDA, 1985;MACHIDA and GOERTZ, 1986) very little theoretical work has been done on the structure and dynamics of the ionizing front. In this paper we report the results of a 2-1/2 D electrostatic code linked with our Plasma and Neutral Interaction Code (PANIC, see MACHIDA and GOERTZ, 1987) which focuses on the understanding of the ionizing front.…”