Resonant superbanana and resonant banana losses of injected fast ions in Heliotron E and Wendelstein VII-A: effects of the radial electric field

Abstract

The role of the radial electric field Er in energetic trapped particle confinement was investigated in nonaxisymmetric toroidal devices with orbit following Monte Carlo beam ion thermalization codes. A comparative study was performed in Wendelstein VII-A (Max-Planck-Institut fur Plasmaphysik, Garching) and in Heliotron E (Kyoto University) to explain the efficient heating achieved with perpendicular neutral beam injection. Re-entering of fast ions was examined in the presence of the Er field with multiple assumptions on the loss boundary of particles. Monte Carlo simulations showed that an inward Er field improves the heating efficiency in W VII-A but tends to deteriorate it in Heliotron E. This difference was interpreted in terms of fast ion loss regions induced by two different branches of the E × B drift resonance. The efficient heating in W VII-A was explained by a counter-side shift of the trapping boundary of resonant banana orbits due to the E × B resonance. The increased fast ion loss in Heliotron E was explained by the resonance occurring between the E × B drift and the poloidal drift of helically trapped ions. However, the resulting resonant superbanana loss was found to be small when the vacuum vessel wall was used as the boundary. The effects of re-entering of ions on the ion heating rate and the charge exchange loss were also clarified. It is concluded that the efficient heating observed in Heliotron E experiments is compatible only with Monte Carlo simulations that take re-entering of fast ions into account