RF current drive in tokamak parametrically turbulent plasmas

A general theory of RF current drive in tokamak plasmas due to the parametric excitations of plasma modes is developed. Both lower-hybrid and electron-cyclotron frequency regions are considered. It is assumed that the characteristic quasilinear time tsub(QL) approx. V2sub(f)/D (Vsub(f) - phase velocity of a high-frequency parametrically excited wave and D quasilinear diffusion coefficient) is much longer than the characteristic time of nonlinear parametric saturation tsub(NL) approx. 1/νsub(a) (νsub(a) - anomalous collision frequency. Accordingly, the dominant role in saturation of the growing electric field in the resonance region is played by parametric processes. Here, cascade processes are taken as the saturation mechanism of the parametrically excited plasma. It is shown that the asymmetry of parametric turbulence levels of the excited high-frequency plasma modes coupled with the ion-sound waves results in the production of the toroidal current formed by suprathermal electrons which is directed opposite to the ohmic toroidal current. (author)