Parametric excitation of ion-sound quasimodes during lower-hybrid heating experiments in tokamaks

Abstract

Parametric decay of a lower-hybrid pump wave into another lower-hybrid wave and a low-frequency ion-sound quasimode (ωR≊k∥vti) is studied. Such an instability may be excited during high-power lower-hybrid heating experiments in tokamak plasmas and may lead to strong modification of the incident n∥ spectrum near the plasma periphery. Such an instability could explain the broadened and downshifted frequency spectrum and phase-independent heating observed in the Alcator A tokamak experiments. Although the growth rate for this decay increases like γ+Γ2∝E20 for powers slightly above threshold (Γ2 is the linear damping rate at the lower sideband and E0 is the pump electric field), for powers well above threshold (such that γ≳ωR)γ∝E2/30, and the convective thresholds are rather high. However, for inaccessible n0∥, the pump wave power is expected to accumulate on the outer surface of the plasma column where the growth rate is large and the convective thresholds are significantly reduced. In such a case the threshold pump power can become quite low and may be exceeded in experiments such as Alcator A.