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

Four loop antennas are installed in the Compact Helical System (CHS) heliot ron/torsatron in order to externally excite Alfven eigenmodes (AEs). These antennas are used in two different operation methods: one is as a set of fo ur external loop antennas, and the other two pairs of electrodes inserted n ear the plasma boundary. For these two operations, magnetic perturbations i n the frequency range of 10-250 kHz are generated in the plasma. The plasma response for the applied magnetic perturbations is measured by magnetic pr obe arrays. Information of AEs can be derived from the analysis of the resp onse. In the excitation experiment of AEs using the "electrode method," mag netic fluctuations are resonantly enhanced in a certain electron density du ring the afterglow of a neutral beam heated plasma, where no energetic ions exist. The fluctuations have the toroidal mode number n=1 and the frequenc y lies just above the lower bound of the toroidicity-induced Alfven eigenmo de (TAE) gap in the plasma core region. The excited fluctuations are though t to be related to TAEs. (C) 2001 American Institute of Physics.