Neoclassical effects on RF current drive in tokamaks

Abstract

Neoclassical effects on RF current drive which arise because of the inhomogeneity of the magnetic field in tokamak devices are analysed. A bounce averaged 2-D Fokker-Planck equation is derived from the drift kinetic equation and is solved numerically. The model features current drive due to a strong RF wave field. The efficiency of current drive by electron cyclotron waves is significantly reduced when the waves are absorbed at the low magnetic field side of a given flux surface, whereas the efficiency remains at the same level as in the homogeneous ideal plasma when the waves are absorbed at the high field side. The efficiency of current drive by fast waves (compressional Alfvén waves) with low phase velocity (v‖/vth < 1) is significantly degraded by neoclassical effects, no matter where the wave is absorbed, and the applicability of this wave seems, therefore, to be doubtful.