Thresholds of parametric instabilities near the lower-hybrid frequency

Abstract

Resonant decay instabilities of a pump wave with frequency ω0 near the lower-hybrid frequency ϑlh are analyzed with respect to the wavenumber k of the decay waves and the ratio ω0/ωlh to determine the decay process with the minimum threshold. It was found that the lowest thresholds are for decay into an electron plasma (lower-hybrid) wave plus either a backward ion-cyclotron wave, an ion Bernstein wave, or a low-frequency sound wave. For ω0< (2ωlh)1/2, it was found that these decay processes can occur and have faster growth than ion quasi-modes provided the drift velocity (cE0/B0) is much less than the sound speed. In many cases of interest, electromagnetic corrections to the lower-hybrid wave rule out decay into all but short wavelength (kρi ≳1) waves. The experimental results are consistent with the linear theory of parametric instabilities in a homogeneous plasma.