Two-Dimensional Stability of an Electromagnetic Wave Obliquely Incident on a Nonuniform Plasma

For an electromagnetic wave obliquely incident on an inhomogeneous plasma, the dominant instability at critical density is a radiating decay instability whose associated ion-wave frequency, even in regimes where the growth rate greatly exceeds the real frequency, is approximately ${\left({{\lambda }_{\mathrm{D}}}^{2}L\right)}^{-\frac{1}{3}}{c}_s\equiv K_{min}{c}_s$. The stimulated radiation peaks between the normal and backscatter angles and may be an important factor in explaining present experiments. A nonlinear calculation at modest power shows that the nonlinear state involves a large-amplitude ion wave through which the excited waves can compress the pump wave.