Measurement of Spatiotemporal Structures of Density Fluctuations Using Two-Directional Beam Emission Spectroscopy in LHD

A beam emission spectroscopy (BES) system for density fluctuation measurements having the sight lines passing through the plasma in the toroidal direction was developed in the Large Helical Device (LHD). The coverage of the area sampled by 15 × 100 optical fibers is nearly from the core to the edge on the horizontally elongated poloidal cross section with the spatial pitch of around 1.0 cm, and 32 channels is detected simultaneously with the 4 × 8 pixel Avalanche Photodiode Detector camera. In order to improve the directional sensitivity for the propagation of the density fluctuation in the radial direction or the poloidal direction, we propose the use of slit-shaped sight lines. An initial result of the density fluctuation measurements for MHD activity in the edge region and its cross-correlation analysis is presented.