Effects of plasma indentation and neutral beam injection orientation on MHD instabilities in PBX

In investigating the role of neutral beam orientation and plasma geometry in stabilizing fishbone activity in PBX, comparisons were made between near-perpendicular and tangential injection, and discharges with and without significant (>10%) indentation. Periodic spikes in the fast neutral flux and drops in the neutron production, reminiscent of sawtooth oscillations observed during tangential injection in PLT, were observed during tangential injection in PBX. Changes in the fast neutral signal and neutral emissivity were also measured in association with high frequency (120-210 kHz) instabilities in these discharges. Although neither phenomenon has been correlated yet with a mechanism like fishbones for expelling beam ions from the plasma, their depletion from the center of the discharge suggests the need for controlling them to ensure efficient heating of the plasma center with tangential beams. (author)