Upstream ion wave excitation in an ion-beam–plasma system

Plasma normal modes in ion-beam–plasma systems were experimentally investigated previously only for the waves propagating in the downstream(along the beam)direction.In this paper,the ion wave excitation and propagation in the upstream(against the beam)direction in an ion-beam–plasma system were experimentally studied in a double plasma device.The waves were launched by applying a ramp voltage to a negatively biased excitation grid.Two kinds of wave signals were detected,one is a particle signal composed of burst ions and the other is an ion-acoustic signal arising from the background plasma.These signals were identified by the dependence of the signal velocities on the characteristics of the ramp voltage.The velocity of the burst ion signal increases with the decrease of the rise time and the increase of the peak-to-peak amplitude of the applied ramp voltage while that of the ion-acoustic signal is independent of these parameters.By adjusting these parameters such that the burst ion velocity approaches to the ion-acoustic velocity,the wave–particle interaction can be observed.