Investigation of low-frequency fluctuation modulating turbulence during L–H transition in Heliotron J

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.