CHARACTERIZATION OF FAST-ION BEHAVIOR DURING TANGENTIAL NEUTRAL BEAM INJECTION IN THE ADVANCED TOROIDAL FACILITY

The confinement characteristics and behaviour of energetic ions inject ed during tangential neutral beam injection (NBI) experiments on the A dvanced Toroidal Facility (ATF) are examined using several experimenta l and computational methods. Measurements by a two dimensional scannin g neutral particle analyser of the fast ion, slowing down spectra of t he injected ions have been used to examine the characteristics of the fast ion distribution in low density plasmas (n(e) less than or equal to 1.0 x 10(19) m(-3)). The energetic ions are found to behave in a ma nner consistent with the classical slowing down process embodied in th e fast ion Fokker-Planck equation. Neutron measurements have been used to extend these results to higher density plasmas (n(e) similar or eq ual to 7.5 x 10(19) m(-3)). Because of experimental uncertainties, it is difficult to determine whether the measured neutron rates are consi stent with classical predictions; however, there are indications in so me ATF magnetic configurations that the measured neutron source rates are much lower than expected, indicating an enhanced orbit loss of fas t ions in these configurations. In addition, computational studies sug gest that the effectiveness of tangential NBI in certain operating reg imes in ATF may be limited owing td the presence of loss regions at th e trapping boundary.