Fast‐Ion Response to Energetic‐Particle‐Driven MHD Activity in Heliotron J

Abstract

AbstractIn Heliotron J, low magnetic shear configuration, instabilities with frequency chirping in the frequency range of Alfvén eigenmodes have been observed in tangentially injected neutral beam plasmas. These modes are induced by energetic‐particle driven magnetohydrodynamic (MHD) instabilities such as global Alfvén eigenmode or energetic particle mode. A hybrid directional Langmuir probe system has been installed into Heliotron J to investigate the response of fast‐ion fluxes to the MHD modes. A high coherent response of the ion flux to the bursting modes has been observed not only by the co‐directed probe but also by the counter‐directed one. A linear correlation between the response of the co‐directed ion flux and the mode amplitude has been found. The radial profile of the response of the co‐directed ions has decreased with the minor radius and has not been obtained significantly outside last closed flux surface. These results indicate that the fast‐ion response is due to a resonant convective oscillation. The ion flux response of the counter‐directed probe has appeared in the growth phase of the mode burst. Its phase relation is different from that of co‐directed one and magnetic probe located at the Heliotron J vacuum vessel. Two candidates of the detected ion flux of the counter‐directed probe have been discussed (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)