Development and installation of the KSTAR Lyman alpha diagnostic

Understanding neutral density in the plasma edge is essential for advancing particle transport theory and interpreting confinement transitions. While Balmer alpha and LIF diagnostics have been widely used, the Lyman alpha diagnostics offers higher signal-to-noise ratio and reduces molecular contributions. A new Lyman alpha diagnostic system has been developed, installed, and is planned (during 2025 KSTAR campaign) for commissioning on the KSTAR tokamak to measure neutral deuterium density near a lower divertor region including the magnetic X-point and the scrape-off layer region. The system employs a compact three-mirror pinhole camera optimized for 121.6 nm VUV emission. A 20-channel AXUV photodiode array observes a 20 cm × 8 cm region centered around the magnetic X-point and aligned along the line connecting the magnetic axis and the X-point. The optical layout is designed and optimized using the V-KSTAR platform on the KAIROS supercomputer, enabling virtual prototyping of a mirror geometry, ray paths, and a field of view. An algorithm to generate synthetic data is also developed to infer neutral density from brightness profiles using EFIT equilibria and SOLPS-ITER simulation results. Vacuum leak testing and in-vessel optical alignment have been successfully completed, and final commissioning is now in progress. We anticipate that this diagnostic system will yield new insights for neutral fueling and recycling as well as edge plasma behavior during upcoming KSTAR campaigns.