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.

We present results from an analysis of the ion cyclotron range of frequencies (ICRF) wave interactions with flute-interchange modes. The analysis is valid for an arbitrary wave vector of the ICRF waves, and shows that these modes can be stabilized by ICRF sideband coupling to them. The modes can be stabilized in a variety of ways, depending on details of the ICRF wave structure. We also find a new rf-driven fluid instability for a range of parallel (to the magnetic field) rf wavelengths.PACS numbers: 52.35.Py, 52.35.Mw, 52.55.Jd Recent experimental results of tandem mirror research have stimulated interest in theoretical studies of rf (radio frequency) effects in the ion cyclotron range of frequencies (ICRF) on the stability of fluteinterchange modes. These studies have been motivated by observations on Phaedrus 1 that ICRF power can be used to stabilize the central cell. Similar observations have been made on experiments in Japan. 2 Theory has predicted two distinct physical effects that can lead to ICRF stabilization of interchange modes. Ponderomotive forces produced by radial gradients in the ICRF energy is one mechanism. 1 ' 3 Stability can be achieved when the radial ponderomotive force produces azimuthal ion drifts opposite to and larger than those produced by the centrifugal force from the curvature of the magnetic field lines. This condition can be written approximately asis its "radial" gradient scale length, and o> 0 is its frequency; B 0 is the applied magnetic field strength, R c is its radius of curvature, and Vf and

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