The initial transient response of a collisionless plasma to a high positive voltage step is investigated. Four different pulses are observed. An electron plasma wave pulse is followed by an ion burst. The latter is overtaken and absorbed by a highly supersonic ion pulse. Thereafter, an ion rarefaction pulse with roughly the ion acoustic velocity propagates into the other direction. The supersonic pulse is tentatively explained as the result of a transient Buneman instability. Together with the rarefaction pulse, it forms the first cycle of an instability which is customarily considered a current‐ driven ion acoustic one, but is, in fact, a potential relaxation instability.