Experimental investigation of hemispherical ion-beam modes

Abstract

The authors have studied in-beam modes that have been produced in a double plasma device in which the driver and target plasmas are separated by a hemispherical grid biased negatively with respect to both plasmas. Both single pulses and continuous pulse trains were excited in the target plasma by application of a voltage pulse to a hemispherical grid. Detection was accomplished with a positively biased spherical probe capable of axial and rotational movement. Spatial evolution of the wave amplitude along the axis of the hemispherical grid was detected over a distance of more than two grid radii. An increase in wave amplitude and wave phase velocity as the wave approached the geometrical focal point of the grid is predicted by the equations for wave amplitude and phase velocity for spherical ion-beam modes that the authors derived using the fluid equations. The dependence of the wave phase velocity on the ion-beam speed indicates that the waves found were hemispherical ion-beam modes. >