Thresholds of parametric instabilities near the lower-hybrid frequency

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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.