Repetitive bursting instabilities with strong frequency chirping occur in highbeta, beam-heated plasmas with safety factor q > 1 in the DIII-D tokamak.
Although the mode structures differ, in many ways, the off-axis fishbones
are similar to the q = 1 fishbones first observed on the Poloidal Divertor
Experiment (PDX). The modes are driven by energetic trapped ions at the fastion precession frequency. During a burst, the frequency changes most rapidly
as the mode reaches its maximum amplitude. Larger amplitude bursts have
larger growth rates and frequency chirps. Unlike PDX fishbones, the decay
phase is highly variable and is usually shorter than the growth phase. Also,
the waveform is highly distorted by higher harmonics during the latter portion
of a burst. The radial mode structure alters its shape during the burst. Like
PDX fishbones, the modes expel trapped ions in a ‘beacon’ with a definite
phase relationship relative to the mode. Seven types of loss detectors measure
the beacon. The losses scale linearly with mode amplitude. The neutron rate
changes most rapidly at maximum mode amplitude but, depending on the loss
diagnostic, the losses often peak a few cycles later. The non-ambipolar fast-ion
losses cause a sudden change in toroidal rotation frequency across the entire
plasma. In addition to an overall drop, the neutron signal oscillates in response
to the wave. Unlike the beacon of lost particles, which maintains a fixed phase
relative to the mode, the phase of the neutron oscillations steadily increases
throughout the burst, with the greatest phase slippage occurring in the highly
nonlinear phase near maximum mode amplitudeUS Department of Energy SC-G903402, DE-FC02-04ER54698, DE-FG02-07ER5491
