Development of a vacuum ultraviolet diagnostic system with millimeter resolution for density fluctuation measurements in tokamaks

In this work, we present the first Lyman-Alpha-Based Beam Emission Spectroscopy (LAB) diagnostic system. This system achieves a significantly enhanced radial spatial resolution of ∼3.3 mm compared to conventional BES. The LAB diagnostic monitors the vacuum ultraviolet Lyman-alpha line (n = 2 → 1 transition, 121.53 nm) and has been implemented on the HL-2A tokamak with twenty spatial channels covering the edge plasma region (ρ = 0.73–1.01) for standard HL-2A discharges. Beyond spatial resolution improvements, the LAB system shows over tenfold signal amplification relative to traditional BES. The entire optical assembly has been securely assembled and underwent rigorous testing within a dedicated vacuum chamber interfaced with HL-2A’s main vacuum system. The monochromator provides linear dispersion of ∼0.09nm/mm and successfully resolves adjacent hydrogen isotope emission lines (HI: 121.567 nm and DI: 121.534 nm) using a 100 μm slit. With a temporal resolution of 1 μs, the LAB diagnostic is a powerful tool for investigating localized, small-scale, high-frequency plasma phenomena in tokamaks.