Non-standard high energy ion orbits

During intense ICRF-heating in JET, the resonating minority ions are often accelerated up to energies in the Mev range. For such ions, the standard ''banana'' orbits are distorted near the magnetic axis into ''potato shaped'' orbits, with a characteristic width δp=(2qv/Ω R)2/3R. For a 1 MeV hydrogen ion in a JET-like environment, δp ∼30 cm. Since, in JET, the width of the ICRF absorption region is typically of the order of or less than 30 cm, most minority ions acquire non-standard potato orbits during on-axis high power ICRF-heating. We present an analytic solution of the Fokker-Planck equation for perpendicular neutral injection of very energetic ions. Ions accelerated by ICRF can be expected to behave in a similar manner. The fast ions pressure profiles and a non standard fast ion current are discussed here. Furthermore, the stabilising influence of fast ions with large orbit widths on internal kink modes is also investigated. It is found that the maximum stabilising effect can be obtained by focusing the ICRF field as much as possible on the magnetic axis. (author) 3 refs., 6 figs