Electron fishbone dynamics studies in tokamaks using the XHMGC code

The electron fishbone modes are internal kink instabilities induced by suprathermal electrons. Ion fishbones were first observed experimentally (PDX) [1], opening the path to full theoretical understanding of these phenomena [4]. Stimulated by experimental evidence of electron fishbones (DIII-D, Compass-D, FTU, ToreSupra), theoretical analysis has also been extended to the case of modes excited by fast electrons [5]. It is well known that additional heating of a plasma produces suprathermal particles which, under certain conditions, could destabilize symmetry breaking modes. Moreover, the dynamics of suprathermal electrons in present days experiments has analogies to that of alpha particles in future burning plasma devices; and resonant excitation by fast electron precession resonance may provide a good test bed for understanding the similar mechanism induced by fusion alphas. For these reasons, it is important to get insights into the underlying physics processes involved in these phenomena. In this work, numerical simulations with the HMGC code [6] are systematically carried out in tokamak equilibria. On the one side, theoretical and experimental results are confirmed, while, on the other side, numerical simulations give a deeper insight into the e-fishbones dynamics. Linear and non-linear studies of e-fishbone instability have been performed for standard (peaked on-axis) [7] and inverted (peaked off-axis) suprathermal electron density profile, with moderately hollow q-profile. It is worth noting that the two situations are significantly different in terms of the characteristic resonance frequency as well as the fraction of suprathermal particles involved in the destabilization of the mode, confirming theoretical expectations. The study of e-fishbone nonlinear saturation mechanisms uses the test particle Hamiltonian method (TPHM) package [8], illuminating the complicate and unexplored dynamics of these modes.