The influence of finite drift orbit width on ICRF heating in toroidal plasmas

Abstract

Ion cyclotron resonance heating in a toroidal plasma not only increases theperpendicular energy of the resonating ions but also results in their spatial transport.Depending on the direction of propagation of the waves, the ions will eitherdrift inwards or outwards giving rise to an RF induced rotation with thetoroidal torque component in the co-current or counter-current directions,respectively. It is found that the spatial transport induced by theRF field, the topology of the ion drift orbits and a wave fieldconsistent with ion absorption are important for determining the distributionfunction of the heated species.Studies of ICRF heating with the self-consistent code SELFOreveal new featuressuch as the formation of non-standard passing orbits residing on the lowfield side of the magnetic axis.For a symmetric spectrum the drift terms will in general not cancel.Some classes of orbit will be subjected only to an inward drift andothers only to an outward drift.The lack of cancellation of the drift termsis further enhanced by the self-consistent coupling, increasing theabsorption for wavespropagating parallel to the plasma current,but not for waves propagating in the antiparallel direction.This results in a strong inward pinchalso for symmetric wave spectra as well as for typical experimentalspectra, with the dominant peak in the counter-plasma-direction.