Fast ion confinement during high power tangential neutral beam injection into low plasma current discharges on the ISX-B tokamak

The beam ion thermalization process during tangential neutral beam injection in the ISX-B tokamak is investigated. The classical model is tested in co- and counter-injected discharges at low plasma current, a regime where large orbit width excursions enhance the importance of the loss regions. To test the model, experimental charge exchange spectra are compared with the predictions of an orbit following Monte Carlo code. Measurements of beam-plasma neutron emission and measured decay rates of the emission following beam turnoff provide additional information. Good agreement is found between theory and experiment. Furthermore, beam additivity experiments show that, globally, the confinement of beam ions remains classical, independently of the injected beam power. However, some experimental evidence suggests that the fast ion density in the plasma core did not increase with beam power in a way consistent with classical processes.