Thermal self-interaction of electromagnetic radiation in a medium with an overheating instability

A theoretical investigation is made of the stability of a plane electromagnetic wave traversing a plasma of a non-self-sustained discharge in a relaxing gas. It is shown that an overheating instability of the plasma leads to an absolute instability of an electromagnetic wave in the presence of small-scale perturbations. The angular distribution of the instability increment is obtained. Its anisotropy is attributed to a transverse (relative to an electric field) nature of the overheating instability.