Experiment on the Interaction between a Plasma and a Neutral Gas

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Abstract
In an experiment where a high velocity plasma stream penetrates a stationary neutral gas cloud in a magnetic field, it is found that the interaction between the plasma and the gas is far more violent than what can be expected from simple atomic interaction theories. The parameters are such that the character of the interaction is collisionless. The observations demonstrate that the plasma is very efficiently retarded as it penetrates into the gas cloud and that the electron energy distribution changes drastically towards higher energies, at least for a part of the population, resulting in partial ionization of the gas. In order to account for the observations it is necessary to assume some kind of collective interaction between the plasma and the neutral gas. The results are analyzed from the point of view of Alfvén's critical velocity hypothesis.