Linear ray-optics theory of the radiation-driven ion-acoustic decay instability in flowing, inhomogeneous plasmas | CU Experts

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 planning and analysis of experiments. Equations are developed for the instability threshold in a homogeneous plasma and for the amplification in plasmas that may be inhomogeneous in more than one spatial direction and/or in time. These equations are formulated so that prior WKB theory of 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 finds that, for typical conditions, large amplification is possible over a wide range of angles. In addition, the flow properties of the plasma are found to be crucial in determining the angular distribution of the amplification produced by the instability. In particular, the magnitude of the plasma flow, when the flow is subsonic, directly controls the angle of maximum WKB amplification.

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