Lower hybrid parametric instabilities: saturation by cascading and pump depletion

Abstract

Parametric instabilities near the lower hybrid frequency and associated electron and ion heating have been observed experimentally for some years, most recently in radio-frequency heating experiments on the Adiabatic Toroidal Compressor Tokamak. Here, the nonlinear evolution and saturation by cascading of the parametric decay instability near the lower hybrid frequency is considered by solving a weak-turbulence wave kinetic equation. The principal results are that (1) a steady state solution exists only for pumps near threshold when the pump frequency is several times the lower hybrid frequency; (2) the decay instability saturates when the energy in decay waves is roughly equal to the pump energy; (3) most of the pump energy is deposited in super thermal electrons when ..omega../sub 0/ much greater than ..omega../sub lh/ and (4) pump depletion in the low-density surface plasma will be a serious problem unless the pump power is kept near threshold for large tokamaks.