LINEAR RAY-OPTICS THEORY OF THE RADIATION-DRIVEN ION-ACOUSTIC DECAY INSTABILITY IN FLOWING, INHOMOGENEOUS PLASMAS

The linear Wentzel-Kramers-Brillouin (WKB) or ray-optics theory of the ion-acoustic decay instability is developed for flowing inhomogeneous , fluid plasmas of arbitrary ionic charge, as is needed in the plannin g and analysis of experiments. Equations are developed for the instabi lity threshold in a homogeneous plasma and for the amplification in pl asmas that may be inhomogeneous in more than one spatial direction and /or in time. These equations are formulated so that prior WKB theory o f three-wave parametric instabilities may be directly applied to this problem. The results are compared with prior work on this instability, and are applied to several circumstances of practical interest. One f inds that, for typical conditions, large amplification is possible ove r a wide range of angles. In addition, the flow properties of the plas ma are found to be crucial in determining the angular distribution of the amplification produced by the instability. In particular, the magn itude of the plasma flow, when the flow is subsonic, directly controls the angle of maximum WKB amplification.