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

Abstract

To perform realistic simulations of radio-frequency current drivein tokamaks in the ion cyclotron range of frequencies,the wave fields evaluated by the toroidal full-wave code TORIChave been used to build the quasilinear electron diffusion coefficient,which, in turn, is used by a recently developed solver of thebounce-averaged quasilinear Fokker-Planck kinetic equation, QLEFOPS.Applications to possible fast wave current drive experimentsin ASDEX Upgrade are presented.The effects of varying the target electron temperature,the background Ohmic current, and, within the relatively limitedpossibilities, the directivity of the antenna have beeninvestigated. The results are in reasonable agreement with thepredictions of the parametrization of the current drive (CD)efficiency proposed by Ehst and Karney based on the adjoint methodfor the solution of the kinetic equation. The detailed analysis which was necessary to guarantee consistencybetween the two codes, on the other hand, has put into evidencecharacteristics of this CD scheme, which,to some extent, are likely to limit the efficiencyachievable in practice. In particular, the normalized quasilineardiffusion coefficient for realistic power densities turns out to befar too small for the development of a quasilinear plateau. In theabsence of suprathermal electrons, sufficient absorption can beachieved only by positioning the spectrum of parallel phase velocitiesexcited by the antenna close to the thermal region, where the theoreticalefficiency is poor.