Characterization of fast ion behaviour during tangential neutral beam injection in the Advanced Toroidal Facility

The confinement characteristics and behaviour of energetic ions injected during tangential neutral beam injection (NBI) experiments on the Advanced Toroidal Facility (ATF) are examined using several experimental and computational methods. Measurements by a two dimensional scanning neutral particle analyser of the fast ion, slowing down spectra of the injected ions have been used to examine the characteristics of the fast ion distribution in low density plasmas (n_e ≤ 1.0 × 10¹⁹ m ^-3). The energetic ions are found to behave in a manner consistent with the classical slowing down process embodied in the fast ion Fokker-Planck equation. Neutron measurements have been used to extend these results to higher density plasmas (n_e ≤ 7.5 × 10¹⁹ m^-3). Because of experimental uncertainties, it is difficult to determine whether the measured neutron rates are consistent with classical predictions; however, there are indications in some ATF magnetic configurations that the measured neutron source rates are much lower than expected, indicating an enhanced orbit loss of fast ions in these configurations. In addition, computational studies suggest that the effectiveness of tangential NBI in certain operating regimes in ATF may be limited owing to the presence of loss regions at the trapping boundary.