Study of high-beta magnetohydrodynamic modes and fast-ion losses in PDX

Strong magnetohydrodynamic activity has been observed in PDX neutral-beam--heated discharges. It occurs for ..beta../sub T/q> or =0.045 and is associated with a significant loss of fast ions and a drop in neutron emission. As much as 20%--40% of the beam heating power may be lost. The instability occurs in repetitive bursts of oscillations of < or =1 msec duration at 1--6 msec intervals. The magnetohydrodynamic activity has been dubbed the ''fishbone instability'' from its characteristic signature on the Mirnov coils.