Observation of a beam-driven low-frequency mode in Heliotron J

A low-frequency electromagnetic mode (LF-mode) has been observed in a stellarator/ heliotron device, Heliotron J. The LF-mode is observed only in neutral beam heated plasmas with a co-current, which increases the rotational transform. The observed LF-mode peaks at rho = 0.86-0.91, where rho = r/a is the normalized minor radius. The LF-mode propagates in the ion diamagnetic drift direction, and the poloidal mode number is m similar to 6-8, determined with a combination of Mirnov coil arrays and 2D fast camera measurement. A high m number induces significant Doppler shift from the background ion flow. The frequency is corrected with both E x B rotation and the ion diamagnetic flow. The corrected frequency is almost consistent with the beta-induced Alfven eigenmode gap evaluated with the STELLGAP code, although very close to the accumulation point. The mode could also be a global Alfven eigenmode.