Observations of effects of banana trapping on neutral-beam-injected ions in plt

The horizontally scanning charge-exchange analyser on the Princeton Large Torus (PLT) was used to measure fast co-moving ions during counter-injection of neutral beams. The large flux of charge- exchange neutrals observed in the near-parallel direction under these circumstances exhibited a sharp cut-off as the analyser was scanned to a fully parallel aiming angle. At a fixed particle energy, the cut-off angle showed a strong dependence on the plasma current, which can be predicted by using finite-banana-width tokamak orbit theory. Monte-Carlo calculations using a Fokker-Planck formalism and full toroidal geometry give substantive agreement with the experimental data, indicating that banana-trapping effects on neutral-beam- injected ions were directly observed.