Density threshold for parametric instability of lower-hybrid waves in tokamaks

Lower-hybrid waves of finite wavenumber in a tokamak are susceptible to parametric instabilities when a certain threshold in density and power is exceeded. For typical tokamak temperatures, the channel of decay with a large growth rate is found to be due to the nonlinear ion-cyclotron damping of the beat wave generated by the pump and the lower-hybrid sideband. We furthermore compute the spectrum of the sideband at ω0−nωci through the nonlinear cascade of parametric interaction. The resonant decay into ion-cyclotron waves is also considered and shown to have a lower density threshold close to the critical density above which there is no current drive and occurs preferentially in a narrow region near the edge.