The impact of fishbones on neutron rate in beam-heated plasmas in MAST

Abstract

A plasma instability has been associated with loss of fusion rate in tokamaks since the introduction of beam-heating in the 1980’s: the Fishbone. Losses of fusion rate on tokamaks during the heating phase present a significant obstacle in reaching the ignition condition required to harness nuclear fusion as a power source. If the conditions exacerbating fusion rate losses during these events can be understood, they can be mitigated. We examine a wide selection of fishbone events from MAST to determine the characteristics associated with greater losses in fusion rate. Quantities for expressing the behaviour of fusion rate are discussed, and statistically significant changes in fusion rate as a function of fishbone phase are examined. Using correlation analysis and sub-classifying the dataset based on observed loss characteristics indicates that fishbone behaviour is significantly more complex than previously believed. Correlation analysis using the full dataset found no reliable quantity that relates plasma conditions during the fishbone to changes in the fusion rate. To study a fishbone undergoing loss, a reduced model of wave-particle resonant interaction between fishbone harmonics and fast ions was built. This models the physical process of fast ions losing energy to the plasma by redistributing the fast ion distribution function in velocity-space. When resonant interactions with higher fishbone harmonics are included, the reduced-model accesses regions of the fast ion distribution function at higher velocities. This results in a larger effect on the modelled fusion rate. Small changes in the fast ion distribution in these regions produces changes in fusion rate that match experimentally observed losses. These results suggest the possibility of developing inference diagnostics for the fast ion distribution function using MHD spectroscopy.