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The expansion of a collisionless plasma with negative ions into a vacuum is considered. The flow pattern involves a rarefaction shock or a wave with an expanding front, two plateau-like regions with a discontinuity between them and a rarefaction wave. The ranges of the mass and density ratios of negative to positive ions for the existence of the rarefaction shock are found. A simple model is in a good agreement with simulation data.
Yu. V. Medvedev (Fri,) studied this question.