Effects of collisional processes on the Critical Velocity Hypothesis

Abstract

Plasma production in a gas release from a body moving in low Earth orbit may occur via Alfven's critical ionization velocity (CIV) process. The effects of the collisional processes of charge exchange and elastic collisions of neutrals with ions and electrons are examined by numerical simulations of a plasma with a beam of fast neutral molecules. The rate of electron heating via plasma interactions increases as the fraction of ions in the fast beam increases. When the initial beam fraction is 0.005, increasing the neutral number density nN (for nN < 1012 cm−3) increases the number of excitations and ionizations by electrons. This increased electron heating results from ambient charge exchange collisions increasing the fraction of beam ions. When the initial beam fraction is 0.5, the number of ionizations and excitations is shown to go through a maximum as nN increases. The initial increase is the result of the shorter collision times for the fast electrons, and the later decrease is the result of elastic collisions that disrupt the electron heating. In the higher density regime, the lower energy process of excitation is favored over ionization. Resonant charge exchange is an important process in maintaining the energy of the fast ion beam, and it lowers the threshold velocity for the CIV process.