A transition phenomenon characterized by rapid decrease of H_α intensity signal and the increase of line-averaged density was observed in the neutral beam heated plasma in the Compact Helical System (CHS) when the heating power exceeds a certain threshold [1]. We have applied a Beam Emission Spectroscopy (BES) [2] for the measurement of the local density variations or perturbations with high time resolution. Spatial channels of BES consist of 8 optical fibers with object lenses viewing over the plasma minor radius in CHS.

The intensity of the emission from the collisionally excited neutral beam atoms (denoted as "beam emission") for the discharge obtained from the channel observing near the plasma edge increases rapidly at the transition and decreases at the back transition [3]. The signal in the edge region can be considered to correspond to the temporal evolution of the local plasma density because the beam attenuation is negligible near the plasma edge. On the other hand, the signal decreases by approximately 35 % at the transition near the plasma center. Since beam attenuation is enhanced by 10-20 % , the decrease of the core density is about 10-20 %. These results indicate the formation of the particle transport barrier in the edge region.

The power spectra of the fluctuation in the beam emission signals shows a coherent-like fluctuation around 5 kHz only during the barrier-formation phase. The phase of this perturbation is opposite between in the edge and core region. These results suggest that the perturbation is originated in the edge region and has an effect on the beam density in the downstream of the beam line. We suspected that this fluctuation is enhanced by the edge pressure or density gradient increased as the transport barrier is formed.

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