PROPAGATION OF A LARGE-AMPLITUDE RAREFACTIVE ION-ACOUSTIC PULSE

In an experiment carried out in a linear, unmagnetized plasma, evoluti on and propagation of nonlinear, rarefactive ion-acoustic pulses have been investigated. The nonlinear rarefactive pulse is observed to unde rgo fissioning as it propagates. Measurements of the ion velocity dist ribution function within the pulse indicate strong modifications of th e distribution functions at velocities in the neighbourhood of the ion acoustic speed. Wave-particle interaction is thus shown to play an im portant role in the observed behaviour. Large-amplitude rarefactive pu lses, while showing multiple fissioning, also exhibit a large-amplitud e, low-frequency (approximate to 30 kHz) oscillatory tail, downstream of the main pulse. The oscillatory tail is unlike the expected Airy fu nction that normally accompanies a weakly nonlinear rarefactive pulse. The observations suggest the existence of ion-hole-like structures on the downstream side. The mechanism of the excitation seems to be thro ugh modification of the ion phase space due to trapping of ions in the negative potential well of the rarefactive pulse.