Fishbone activity in JET

Abstract

During auxiliary heating experiments in JET, periodic bursts of MHD oscillations resembling 'fishbones' have been observed in the signals of several diagnostics. The bursts have repetition times of 10–40 ms and oscillation frequencies ranging from 1 kHz to more than 20 kHz. While the amplitude of these oscillations increases at high poloidal and toroidal beta, they are also observed in plasmas with modest values of beta. Moreover, they occur in discharges with either neutral beam injection or ion cyclotron resonance heating, which is consistent with the idea that they are due to an instability driven by energetic ions. However, there is little evidence in JET that the bursts have a significant impact on fast ion containment or energy confinement. The paper describes the characteristics of the bursts and gives an analysis of the regimes over which they occur. In addition, the evidence for the interaction of the oscillations with high energy particles is considered and discussed in the light of theoretical models of the instability which is believed to be responsible for the bursts.