Alfvén eigenmodes excitation by external loop antennas in the Compact Helical System heliotron/torsatron

Abstract

Four loop antennas are installed in the Compact Helical System (CHS) heliotron/torsatron in order to externally excite Alfvén eigenmodes (AEs). These antennas are used in two different operation methods: one is as a set of four external loop antennas, and the other two pairs of electrodes inserted near the plasma boundary. For these two operations, magnetic perturbations in the frequency range of 10–250 kHz are generated in the plasma. The plasma response for the applied magnetic perturbations is measured by magnetic probe arrays. Information of AEs can be derived from the analysis of the response. In the excitation experiment of AEs using the “electrode method,” magnetic fluctuations are resonantly enhanced in a certain electron density during the afterglow of a neutral beam heated plasma, where no energetic ions exist. The fluctuations have the toroidal mode number n=1 and the frequency lies just above the lower bound of the toroidicity-induced Alfvén eigenmode (TAE) gap in the plasma core region. The excited fluctuations are thought to be related to TAEs.