Parametric instabilities in the fast-wave heating of tokamaks

Abstract

A high-power whistler wave launched into a tokomak is susceptible to resonant decay into ion-cyclotron and lower-hybrid waves when the lower-hybrid resonance layer ωlh=ω0 exists in the outer region of the plasma x2/a2≳0.6, where ω0 is the frequency of the whistler, ωlh is the lower-hybrid frequency, x is the distance away from the center of the plasma, and a is the minor radius. For Princeton Large Torus parameters, this requires a line average density n≳6×1013 cm−3 when the whistler frequency ω0≂800 MHz and the threshold pump power for the onset of the instability is 100 kW. The threshold power is determined by the convection losses and it increases nonlinearly with the temperature and very rapidly with decreasing density. The parametric instability would tend to deposit the momentum and energy of the pump wave in the outer region of the plasma.