Lower hybrid wave produced supra-thermal electrons and fishbone-like instability in FTU

Abstract

The fishbone-like internal kink instability driven by supra-thermal electrons generated by lower hybrid current drive is an important issue of burning plasma research. Indeed, the trapped particle averaged bounce characterizing the interaction of trapped alpha particles with low-frequency MHD modes in burning plasmas depends on energy, not on mass, hence the charged fusion product effects can be usefully modelled by the analogous effect induced by the fast electrons on the low-frequency MHD modes. Fishbone-like internal kink instabilities driven by electrons were observed during experiments on Frascati Tokamak Upgrade (FTU) and interpreted in terms of an oscillating ‘fixed point’ activity followed by one of ‘limit cycle’, produced by suprathermal electrons in the presence of a q-profile with q _min ≈ 1. As an interesting behaviour of the fast electron population produced by the LH power when the q-profile meets the condition q _min ≈ 1, the fast electron bremsstrahlung data of FTU show a marked redistribution in space across layers centred at r/a ≈ 0.20–0.33. This redistribution occurs during and in phase with the fishbone oscillation observed by the MHD diagnostic, with the same time scale and at the same radial position of the peak emission seen by x-ray tomography.