The fishbone instability in the DIII-D tokamak

Abstract

ABSTRACT. Although most DIII-D plasmas are stable to the fishbone instability, fishbones are sometimes observed when /3 p £ 1.5 and ^ 5 5 x 10 13 cm" 3 . These bursts are usually of minor significance operationally; however, under one condition, over 50% of the beam power was lost. The angle of beam injection has little effect on the virulence of the instability, suggesting that the fishbone instability in DIII-D is the ion diamagnetic branch of the internal kink.

The tokamak Monte Carlo fast ion module NUBEAM in the National Transport Code Collaboration library

“…Sawtooth oscillations [26], fishbone instabilities [27][28][29], and magnetic ripple [30] cause perturbations of the magnetic field and distortions of the fast ions paths, which result in either rearrangement of fast ions within the plasma or ejection of the ions from the plasma region [30]. Magnetic ripple is the non-axisymmetric variation in the strength of magnetic field and the non-axisymmetric displacement of magnetic field lines around the toroidal circumference of tokamak as a result of the discrete toroidal field coils.…”

Energetic Ion Experiments in DIII-D

“…27 Ordinarily, the fishbone instability barely perturbs the D-D fusion reaction rate, indicating that the instability has little effect on beam-ion confinement. The observations 28 suggest that the DIII-D mode is a fluid instability 29 rather than an energetic particle mode. 30 In Ref.…”

“…30 In Ref. 28 it was erroneously reported that fishbones occasionally cause large beam-ion losses, but subsequent measurements 31 showed that TAE instabilities were primarily responsible.…”

“…The fishbone instability is sometimes seen, but beam-ion effects seem unimportant. 28 Several beamdriven instabilities are observed but have not yet been studied thoroughly, including Alfvén cascades, 37 ellipticity-induced Alfvén eigenmodes, 15,38 compressional Alfvén eigenmodes or global Alfvén eigenmodes with frequencies just below the cyclotron frequency, 39 and ion cyclotron emission. 31 …”

Infernal Fishbone Mode

“…[1][2][3][4] It can be very dangerous, leading to expulsion of a considerable fraction (up to 50%) of energetic ions. 5 But, on the other hand, sometimes it is useful, being a trigger of formation of the internal transport barrier. 6 Typically, the fishbone instability is associated with an m = n = 1 perturbation (m and n are the poloidal and toroidal mode numbers, respectively) in the plasma core.…”