Design of a Lyman-Alpha-Based BES for edge plasma density diagnosing on the HL-2A tokamak

•The Lyman-Alpha-Based BES (LAB), a novel diagnostic for plasma density fluctuation measurement on tokamaks.•LAB can measure density fluctuation with a high spatial resolution of 3.3 mm, which is much higher than any of the BES diagnostics worldwide (centimeters). A 20-channel edge Lyman-Alpha-Based Beam emission spectroscopy (LAB) system, which diagnoses edge density fluctuations by measuring the Doppler-shifted Lyman-alpha line (n= 2 → 1, 121.53 nm) emitting via the interaction of neutral beams and plasma particles, is under developing on the HL-2A tokamak. The radial observation range covers the main plasma from R= 1.98 m to 2.05 m with a spatial resolution of about 0.33 cm and a temporal resolution of 1 μs. The overall design for this system is completed, including the reflective collection optics, the spectrometer with a reflective toroidal grating, the detector array, and the vacuum system. Compared with the traditional beam emission spectroscopy (BES) which measures the Balmer-alpha line (n= 3 → 2, 656.10 nm), LAB promises to achieve higher signal-to-noise measurements of smaller-scale fluctuations and therefore detect previously unobserved low-amplitude pedestal turbulence.