Anomalous Diffusion of Fast Ions in the Presence of Fishbone Instabilities in Tokamaks
Fast ions, whether produced by fusion reactions, or by ionisation of neutral beam, are expected to play a major role in the heating of burning plasmas. Therefore, the study of the fast ion’s behaviour in tokamaks is important for the future burning plasma experiments such as ITER. Understanding the effect of neutral beam injection on the current profile is also important for current profile control and for achieving steady state scenarios where non-inductive current drive is necessary. Several plasma discharges have been carried out in the MAST tokamak to investigate to which extent the q-profile may be modified by neutral beam current drive (NBCD). Transp simulations of the beam deposition during steady state experiments, with off-axis NBCD, have been carried out. It has been found that an anomalous diffusion (with a diffusion coef- ficient of roughly Db ∼ 0.5 m2 /s) of the fast ion is needed to explain the significantly lower neutron rate measured than predicted by the Transp code using an assumption of classical beam deposition and collisional ther- malisation. Transp simulations show that this diffusion broadens the fast ion deposition profile and may help to avoid harmful instabilities . This anomalous diffusion is suspected to be caused by fishbone instabilities, as the time of the largest discrepancy between simulated and measured neutron rates correlates well with the highest magnitude of fishbone activity. The aim of this work is to investigate, with simulations of the Hagis code, if the interaction between fast ion resulting from off-axis NBCD and fishbone instabilities may be responsible for the fast ion anomalous diffusion needed to explain the observed neutron rate.