Development of a new beam emission spectroscopy with interchangeable optical filters in LHD

MHD fluctuations in H-mode plasma are crucial for performance. Our research focuses on the correlation between particle transport and MHD fluctuation through coherence analyses of density fluctuation, measured by Beam Emission Spectroscopy (BES), and magnetic fluctuation, captured by magnetic probes. Previously, we discovered some noteworthy discharges where the higher harmonics of a low frequency (~2kHz) density fluctuation exhibit low cross-coherence with their corresponding magnetic fluctuation harmonics, despite the high coherence at the fundamental mode. Our current BES system uses a 2D optic array to cover the radial and poloidal distribution. Recognizing the fact that the mode structure in the magnetic fluctuations does not necessarily match the density fluctuation, we considered more measurements from a different toroidal position to be useful. In the last Large Helical Device (LHD) experimental campaign, we installed a new BES system alongside the current system at a different toroidal angle, with optical channels arranged to optimize radial profile resolution. The ability to switch the probing neutral beam by substituting the optical filter is unique to the LHD due to its distinctive neutral beam configuration. Specifically, neutral beams #1, #2, and #5 intersect at approximately the same area, which is visible through the 9-O diagnostic port. In this presentation, we will introduce the components and capabilities of the new BES system, as well as some preliminary measurement results.