Characterization of off-axis fishbones

Abstract

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 amplitude.

Energetic ion loss detector on the Alcator C-Mod tokamak

“…This represents an advancement in design since future wallintegrated detectors have the potential to allow the creation of FILD arrays that provide increased phase space coverage by collecting lost ions across a greater number of toroidal and poloidal positions. Energetic ion losses are known to occur with toroidal and poloidal wall-strike dependencies due to propagating modes, 57 and simulations predict similar effects in ITER resulting from assorted magnetic symmetrybreakings. 8 While present FILD designs do not scale well, in terms of cost and port space, to arrays consisting of more than a few probes, a wall-integrated solution could alleviate those issues.…”

Compact solid-state neutral particle analyzer in current mode

“…During so-called off-axis fishbones, where modes are driven by energetic trapped particles at the fastion precession frequency, significant burst signals are readily detected on ssNPA as well as other energetic fast ion measurements. 20 After the successful DIII-D ssNPA tests, the four-channel array existing on NSTX (Refs. 9 and 10) was adapted to current mode operation, using 25 mm 2 AXUV sensors and extended apertures.…”

Kinetic effects of trapped energetic particles on stability of external kink modes with a resistive wall

“…This coupling of FLMs with different mode numbers may provide a partial interpretation of the bursting mode distortion, observed in the recent DIII-D experiments. 26,37 V. SUMMARY AND DISCUSSIONS Kinetic effects of trapped EPs on the external kink modes, in the presence of a resistive wall, are investigated, using the kinetic RWM dispersion relation. Our results show that, at a 0 B 0 ¼ 0:99 in our study, when the perpendicular beta b à of trapped EPs exceeds a critical value b à c , the FLM instability with an external kink eigenstructure is triggered, which rapidly grows with increasing b à (> b à c ) and eventually becomes a dominant instability.…”