Progress of Lyman-alpha-based beam emission spectroscopy (LyBES) diagnostic on the HL-2A tokamak

An edge Lyman-alpha-based beam emission spectroscopy (LyBES) diagnostic, using a heating NBI (neutral beam injection) system, is currently under development on the HL-2A tokamak. The 20-channel edge LyBES, which is intended to measure the density fluctuation in plasma edge (from R = 1960 mm to R = 2026 mm) with an improved spatial resolution of 3.3 mm, is a complement to the existing conventional beam emission spectroscopy (BES) diagnostic. In this article, we introduce the progress of LyBES diagnostic, including the collection optics, the monochromator, and the detector system. The reflectance of the collection mirrors is measured to be ~82% at 122 nm, and the aberration geometrical radius of the collection optics is tested to be ~150 μm in the aimed area. The linear dispersion of the LyBES monochromator is designed to be ~0.09 nm mm⁻¹. The bandwidth of the detector system with the 5×10⁷ V A⁻¹ preamplifier gain is measured to be ~280 kHz, and the peak-to-peak noise of the detector system is tested to be ~16 mV. The finalized design, components development and testing of the LyBES diagnostic have been completed at present, and an overall performance of the LyBES diagnostic is to be confirmed in the next HL-2A campaign.