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Four loop antennas are installed in the Compact Helical System (CHS) heliotron/torsatron in order to externally excite Alfve´n 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 Alfve´n eigenmode (TAE) gap in the plasma core region. The excited fluctuations are thought to be related to TAEs. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A system for Alfve´n-eigenmode spectroscopy in the compact helical system heliotron/torsatron is presented. In this system, four electrodes inserted near the last closed flux surface are employed as the exciter of Alfve´n eigenmodes (AEs). A set of magnetic probes is employed as the detectors of excited magnetic fluctuations. An alternating current path along the confinement magnetic field line is generated in the plasma edge between two electrodes which intersect with a specified magnetic field line. Another two electrodes also produce another current path. The polarity between these two current paths is adjusted to specify the toroidal mode number of the excited field perturbations. The frequency of the alternating current is in the range of 10–250 kHz. The possibility of the excitation of AEs by this system is discussed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]