Simulation of fast wave current drive in tokamaks in the ion cyclotron frequency range

To perform realistic simulations of radio-frequency current drive in tokamaks in the ion cyclotron range of frequencies, the wave fields evaluated by the toroidal full-wave code TORIC have been used to build the quasilinear electron diffusion coefficient, which, in turn, is used by a recently developed solver of the bounce-averaged quasilinear Fokker-Planck kinetic equation, QLEFOPS. Applications to possible fast wave current drive experiments in ASDEX Upgrade are presented. The effects of varying the target electron temperature, the background Ohmic current, and, within the relatively limited possibilities, the directivity of the antenna have been investigated. The results are in reasonable agreement with the predictions of the parametrization of the current drive (CD) efficiency proposed by Ehst and Karney based on the adjoint method for the solution of the kinetic equation. The detailed analysis which was necessary to guarantee consistency between the two codes, on the other hand, has put into evidence characteristics of this CD scheme, which, to some extent, are likely to limit the efficiency achievable in practice. In particular, the normalized quasilinear diffusion coefficient for realistic power densities turns out to be far too small for the development of a quasilinear plateau. In the absence of suprathermal electrons, sufficient absorption can be achieved only by positioning the spectrum of parallel phase velocities excited by the antenna close to the thermal region, where the theoretical efficiency is poor. (author)