Suprathermal electron driven fishbone instability in the TCV tokamak

Observations of fishbone oscillations driven by energetic electrons have been reported in several tokamaks, but experimental evidence of the interaction between electrons and the mode has remained limited. In the TCV tokamak, a state-of-the-art multi-channel hard x-ray spectroscopy system equipped with digital pulse detection allows the study of the redistribution of electron cyclotron current drive (ECCD)-created suprathermal electrons at high temporal resolution. For the first time, the response of hard x-ray profiles to the internal kink mode is observed at the frequency of the mode. This work demonstrates the role of suprathermal electrons in destabilizing the fishbone mode and in particular the interaction of trapped electrons with the mode. The mode is destabilized after the ramp-up of suprathermal electrons and then trapped suprathermal electrons preferentially interact with the mode and are expelled during the mode oscillation phase. The experimentally observed mode frequency and rotation direction are in good agreement with the solution of the linear fishbone dispersion relation, where the electron distribution function is modelled using a Fokker-Planck code coupled with a hard x-ray synthetic diagnostic. The frequency and stability predicted by the modeling also agree well with data from an ECCD power scan, as the varying suprathermal electron distribution affects the mode behavior.