Linear properties of the cross-field ion acoustic instability in a double plasma device.

Abstract

This thesis deals with the dependence of the linear spatial growth rate of the cross-field ion acoustic instability on various plasma parameters. A kinetic theory model, with elastic and inelastic ion-neutral collisions included, is presented and used to conduct a numerical survey of the instability. The growth rate is computed as a function of distance into the plasma, taking into account the attenuation of the ion beam by charge exchange collisions. Further calculations show the variation in growth rate as a function of the following quantities: electron and ion beam temperature, electron density, beam velocity, background ion temperature, magnetic field, the angle between magnetic field direction and wave vector and the finite width of the plasma. The instability was observed in a double plasma device where an ion beam was passed through a background of stationary magnetized electrons. The magnetic field was sufficiently weak to allow approximately rectilinear ion motion. The growth rate of the wave was studied using interferometer techniques. It was identified by the dispersion relation as the cross-field ion acoustic wave propagating as the slow mode of the beam. It was found that the background ions play an important role in determining the phase velocity. Experimental data of the growth rate dependence on wave number, beam velocity and magnetic field strength were found to be well described by the theoretical model. The growth rate dependence of magnetic field direction on plasma width was furthermore found to be in qualitative agreement with the model.