Title: Thresholds of parametric instabilities near the lower hybrid frequency

Parametric instabilities excited in non-uniform plasmas by spatially localized pump fields oscillating near the local lower-hybrid frequency are analytically investigated. Corresponding threshold conditions, temporal growth rates, and spatial amplification factors are obtained for the oscillating-two-stream instability and the decay instabilities due to nonlinear electron and ion Landau dampings.

Resonant decay instabilities of a pump wave with frequencymore » $omega$$sub 0$ near the lower-hybrid frequency theta/subl//subh/ are analyzed with respect to the wavenumber k of the decay waves and the ratio $omega$$sub 0$/$omega$/subl// subh/ 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 $omega$$sub 0$< (2$omega$/subl//subh/)$sup 1$/$sup 2$, it was found that these decay processes can occur and have faster growth than ion quasi-modes provided the drift velocity (cE$sub 0$/B$sub 0$) 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 (krho/subi/ >1) waves. The experimental results are consistent with the linear theory of parametric instabilities in a homogeneous plasma. (AIP)« less

Experimental data are presented which shows that during lower hybrid radio frequency heating of the Princeton ATC Tokamak, parametric instabilities are excited, and the ion heating correlates with the presence of the parametric spectra. A theoretical interpretation of the parametric instabilities is presented.