Two-dimensional beam emission spectroscopy for hydrogen isotope negative neutral beam in Large Helical Device

A new beam emission spectroscopy system that has improved lines of sight is installed in the Large Helical Device (LHD), and routine measurement has been started in the 21st LHD experiment campaign in 2019-2020. The new system is optimized for hydrogen isotope experiments by equipping a rotatable large-diameter interference filter to be compatible with either the hydrogen or the deuterium beam emission component. An avalanche photo diode detector array having 8 × 8 pixels is used for obtaining a radial-vertical image of electron density fluctuation covering the mid-radius to the plasma periphery. Spatial resolution and wavenumber cutoff are derived from equilibrium reconstruction and plasma kinetic profiles. Obtained fluctuation data is presented for a low field high beta discharge. The spatiotemporal structure of the fluctuations is clearly shown by Fourier correlation analyses.