R. Matsutani

Magnetic islands are formed at the plasma edge in Heliotron J, a stellarator/heliotron device, by controlling the rotational transform, and the effect of the magnetic islands on the poloidal flow profile was investigated using a Q-band Doppler reflectometer. Comparative measurements were performed for the magnetic configurations with magnetic islands at the plasma edge, referred to as ‘island configuration,’ and those without the magnetic islands, referred to as ‘non-island configuration.’ In all configurations, poloidal flow in the electron diamagnetic direction was observed at the edge in electron cyclotron heated plasmas. In the non-island configuration, the sheared flow maximum was inside the last closed flux surface (LCFS), whereas in the island configuration, the sheared flow was formed around the magnetic island independent of the LCFS position, and increases in electron temperature, T e , and density, n e , were observed at the edge. The fine control of the magnetic island position around the LCFS shows that the poloidal flow structure, particularly the flow shear position, correlates with the position of magnetic islands. An increase in T e and n e is observed at the edge only when the magnetic islands are located outside the LCFS. These results indicate that the edge magnetic islands are a possible means to control the poloidal flow shear position and the edge plasma parameters.

In this study, we observed a distinct low-frequency (LF) fluctuation in the n/m = 4/9 configuration under electron cyclotron heating (ECH) and neutral beam injection heating—conditions favorable for the L–H transition in Heliotron J [Sano et al., Nucl. Fusion 45, 1557 (2005)]. This LF mode, with a frequency of approximately 14 kHz, was detected during the L–H transition using Doppler reflectometry (DR), electron cyclotron emission (ECE), magnetic probes (MP), Dα emission, and edge divertor probes (DP). It modulates edge turbulence amplitude and extends radially, with toroidal and poloidal mode numbers of n/m = 1/3. It is found that the LF mode detected by ECE is correlated with the perturbation of the Dα emission, suggesting the mode contributes to the cross-field edge transport. At the L–H transition, while the LF mode persists, modulation of edge turbulence is suppressed, as evidenced by the reduced amplitudes in Dα emission and DP signals. These findings reinforce the importance of turbulence in the edge transport modulation induced by the LF mode.