Alfven Eigenmodes and Geodesic Acoustic Modes Driven by Energetic Ions in an LHD Plasma with Non-monotonic Rotational Transform Profile

摘要

In the Large Helical Device (LHD), a reversed magnetic shear (RS) configuration having non-monotonic rotational transform profile was formed by intense counter neutral beam (NB) current drive. In the RS configuration helical plasma, the reversed shear Alfven eigenmode (RSAE) with n=1 toroidal mode number was identified together with energetic-ion-driven geodesic acoustic mode (GAM) with n=0. Temporal sweeping of the RSAE frequency was well-explained by ideal MHD theory without introducing non-perturbative energetic ion effects. The minimum value of RSAE frequency in the sweeping phase agrees well with GAM frequency. Nonlinear interaction between the RSAE and GAM generates a lot of driven modes. In thus produced low density RS configuration, bulk ion temperature in the plasma center T_(io) starts to increase linearly in time for more than 10 times of global energy confinement time just after the local minimum of the rotational transform has passed through the particular rational value τ/2π=1/3. When energetic ion driven AEs and GAM are reexcited appreciably, T_(io) decreases with slower time scale than that of the rise. During the rising phase of T_(io) , plasma potential measured by heavy ion beam probe becomes deeper in the plasma core region. Then, it becomes shallow in the decay phase of T_(io).