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

Abstract

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 ~ 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 × B rotation and the ion diamagnetic flow. The corrected frequency is almost consistent with the beta-induced Alfvén eigenmode gap evaluated with the STELLGAP code, although very close to the accumulation point. The mode could also be a global Alfvén eigenmode.

Magnetohydrodynamic hybrid simulation of Alfvén eigenmodes in Heliotron J, a low shear helical axis stellarator/heliotron

“…Due to the low magnetic shear, n/m = 2/4 global Alfvén eigenmode (GAE) is normally observed along with the n/m = 1/2 energetic particle mode (EPM) [10,11]. In a recent experiment, a low frequency Alfvénic mode is observed in the medium bumpiness configuration, if the coplasma current reaches certain threshold [12]. The candidate of this observed mode is either GAE or beta-induced Alfvén eigenmode (BAE).…”