Alfven eigenmode spectroscopy by application of external magnetic-field perturbations in the compact helical system

In many tokamaks and helical devices, Alfven eigenmodes (AEs) driven by energetic ions are intensively studied because of the importance in a fusion reactor. In particular, much attention is paid to the damping rates of AEs. In order to evaluate the damping rate of AEs, AE spectroscopy system was constructed in the CHS. In the system, alternating currents are induced along the magnetic field line using two inserted electrodes. This system was applied to low temperature plasmas produced with 2.45GHz microwaves, for a basic study of damping mechanisms of AEs. The damping rate and AE frequency were derived from an analysis of frequency dependence of a transfer function measured by this system. In low temperature plasma produced at very low toroidal field Bt<0.1T, fairly large damping rates of about 5∼20% were obtained. The AE spectroscopy experiments in low temperature plasmas were carried also for helium and neon gasses by changing electron density and Bt, to study characteristics of the damping rates in the wide range of the ratio of electron thermal velocity to the Alfven one. (author)