Anisotropic fast neutral particle spectra in the MAST spherical tokamak

The behaviour of fast ions, generated by high energy neutral beams, is investigated in the MAST spherical tokamak using a spatially scanning neutral particle analyser (NPA), which has views that detect charge exchange neutrals from both passing and trapped ions. Almost featureless neutral particle spectra are produced, characteristic of thermalization of the neutral beam energy components, when viewing the charge exchange neutrals from passing fast ions in both L- and H-modes. Using NPA views with low tangency radius, R-NPA, both passing and trapped fast ions can be monitored. In L-mode, similar featureless spectra are observed in contrast to H-mode, where they are highly structured. All these aspects have been faithfully reproduced using particle transport codes showing, in particular, that the H-mode spectra found at low R-NPA arise from a small fraction of poorly confined trapped fast ions at the edge, an effect enhanced by the relatively high edge ion and neutral densities.