Nonresonant electron velocity diffusion by lower hybrid waves in tokamaks

On the basis of the quasilinear theory of current drive in tokamaks by lower hybrid waves, a velocity diffusion process is introduced for electrons that are not in Landau resonance with the waves. This effect is connected with the time dependence of the wave field autocorrelations which can be caused by nonlinear phenomena or by the geometric effect of a wave beam of finite toroidal width. In both cases the role of nonresonant electron velocity diffusion is important in correctly evaluating the driven current and the absorbed power. It also implies the presence of a fast RF runaway electron tail above the range in which Landau resonance occurs. While this tail requires a treatment including relativistic effects, as well as 2-D velocity space and cross field transport of fast electrons, simplified considerations indicate that it does not depend much on the wave spectrum (or on the accessibility limit). The temperature scaling of the current drive efficiency is found to be different from the result obtained from the usual quasilinear theory. (author)