During neutral beam injection coherent magnetohydrodynamic activity occurs at low and medium P, which is driven by the energetic beam particles. An interpretation is given in terms of marginally stable global Alfven eigenmodes (GAE), which are destabilized by Landau resonances of the fast circulating particles. Toroidicity induced gaps for toroidal Alfven eigenmodes do not occur in W7-AS, but GAE modes in gaps below the shear Alfven continua can be excited. This is investigated within a gyrofluid model, which has been used successfully for tokamak cases previously.

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Core, edge and scrape-off-layer plasma behaviour is studied principally under conditions of an ι a = 5/9 boundary island configuration-which is relevant for the upcoming W7-AS divertor campaign-but for now with ten inboard sector limiters. The major focus is on compatibility between good core confinement and attainment of high recycling at the limiter. At low input power P in 0.4 MW, operation at densities necessary to attain effective divertor action in the future invariably leads to a transition to the ELM-free H-mode accompanied by lower edge densities and increased core radiation until radiation collapse ensues. Thereby, enhancement factors in τ E of nearly two above the international stellarator confinement scaling are transiently achieved. The threshold density nthr e , necessary to attain the H-mode increases with heating power, such that at 2 MW NBI heating power the H-mode is completely suppressed and peak densities at the limiter exceeding 1.5 × 10 20 m −3 are realized. The efficacy of newly-installed control coils designed to manipulate the island geometry is tested. Their influence on the core plasma is verified. Due to geometrical effects associated with the mutual shadowing of the inboard limiters, statements regarding the influence on island physics must await the divertor configuration.

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