A theory of currents induced by radio-frequency waves in toroidal plasmas

A Fokker-Planck treatment of the plasma electron current driven by radio-frequency waves is presented for the electron cyclotron and Landau damping wave absorption mechanisms. Electron-electron collisions are taken into account using the full Fokker-Planck collision operator which leads to an integro-differential equation for the perturbation of the electron distribution function produced by the wave. This equation is solved analytically in the Lorentz gas approximation and numerically in its full form. Efficient current drive is predicted for high phase velocity waves in the electron cyclotron scheme while Landau damping is efficient at both high and low phase velocities. The reduction of the current due to trapping of electrons in a Tokamak field configuration is calculated and a dramatic suppression of the current is predicted for low phase velocities. The efficiency of driving currents with waves is found to be comparable with that obtained using fast ion beams but much less than that using steady electric fields. A comparison with an idealised wave shows that the two practical schemes considered are far from optimum.