The effect of the toroidal wavenumber spectrum on the high energy tail formation during ICRH

The influence of finite orbit width effects and wave induced spatial diffusion on the resonant ion distribution function in the presence of ion cyclotron resonance heating (ICRH) is studied. In particular, it is found that the toroidal wavenumber spectrum affects the fast ion energy density and the local formation of the high energy tail. For high energy ions interacting with waves having a large toroidal wavenumber, the wave induced spatial diffusion plays an equal or even more important role than the finite orbit width. Only in the limit of artificially high plasma current and low toroidal wavenumber are results from two dimensional (2-D) orbit averaged Fokker-Planck models recovered. The finite orbit width and the wave induced spatial diffusion also produce ions that follow loss orbits, resulting in localized high energy ion losses to the wall. (author). 20 refs, 6 figs. Letter-to-the-editor