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 ${\ensuremath{\beta}}_{T}q>~0.045$ 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 \ensuremath{\le} 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.