Confinement of helically trapped fast-ion orbits in a neutral-beam-sustained heliotron E Plasma

Abstract

Perpendicular (0°) and off-perpendicular (28°) neutral-beam injectors (Einj = 26 keV) and a perpendicular neutral-particle energy analyser are used to investigate the confinement of helically trapped (‘localized’) suprathermal ions in a currentless Heliotron E plasma. High neutral-particle flux, synchronized with the injection of the 0° beam, has been observed, indicating (as was predicted by an orbit-following Monte Carlo code) that deeply trapped localized fast ions can circulate toroidally around the Heliotron E device without entering the loss cone in velocity space. Up-down asymmetries in the neutral outflux, usually related to the uncompensated toroidal drift of localized ions in non-axisymmetric systems, were examined. The up-down symmetry of the observed neutral flux was found to depend on energy. In the high-energy regime (3–4 keV < E <Einj) the profile is nearly up-down symmetric, suggesting a good confinement of energetic localized ions. In the low-energy regime (E < 3–4 keV) the profile is distorted (by a factor of three) in the ion drift direction. The lack of distortion in the ion drift direction in the high-energy regime is explained in terms of a compensation of the toroidal drift by the poloidal precessional drift due to the deep helical field ripple. Profiles of neutral outflux in the high-energy regime also showed a significant difference between 0° and the 28° injection; the 28° beam produced a hollow profile, while the 0° beam produced a peaked profile. It was found that the Monte Carlo code could reproduce this difference in the neutral-flux profiles. Both the time evolution and the up-down profiles of neutral outflux can be consistently interpreted in terms of the drift orbit theory and the classical slowing-down process of fast ions if one assumes that the observed perpendicular neutrals are emitted by fast ions which are on transit banana orbits.