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

Ion cyclotron resonance heating in a toroidal plasma not only increases the perpendicular energy of the resonating ions but also results in their spatial transport. Depending on the direction of propagation of the waves, the ions will either drift inwards or outwards giving rise to an RF induced rotation with the toroidal torque component in the co-current or counter-current directions, respectively. It is found that the spatial transport induced by the RF field, the topology of the ion drift orbits and a wave field consistent with ion absorption are important for determining the distribution function of the heated species. Studies of ICRF heating with the self-consistent code SELFO reveal new features such as the formation of non-standard passing orbits residing on the low field 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 and others only to an outward drift. The lack of cancellation of the drift terms is further enhanced by the self-consistent coupling, increasing the absorption for waves propagating parallel to the plasma current, but not for waves propagating in the antiparallel direction. This results in a strong inward pinch also for symmetric wave spectra as well as for typical experimental spectra, with the dominant peak in the counter-plasma-direction.