Preparation of a beam emission spectroscopy diagnostic based on a Lyman-alpha line to diagnose core and edge-plasma density fluctuation on the HL-2A tokamak

In this article, the design of a Lyman-alpha-based beam emission spectroscopy (LAB) diagnostic on the HL-2A tokamak has been proposed for the first time. The purpose of this novel diagnostic is to measure density fluctuations of tokamak plasma. The light-collection system of LAB, which consists of the first mirror and two groups of coaxial double-mirror telescopes, can realize a two-segmented viewing field of ρ = 0‒0.2 and ρ = 0.75‒1, which is optimized to measure plasma density fluctuation, not only in the edge transport barrier region but also in the internal transport barrier region, to investigate the underlying physics of turbulence in tokamaks. Spectrometers are developed to separate out the Doppler-shifted target line (122.03 and 122.17 nm) from the background Lyman-alpha line (121.53 nm). Here, 30 Core-LAB channels and 30 Edge-LAB channels are under development on the HL-2A tokamak. It has high radial spatial resolutions of about 2.7 mm and 3.3 mm for the core and edge channels, respectively. Taking the high light intensity of this Lyman-alpha line into account, temporal resolution of 200 kHz can be ensured by broad bandwidth amplifiers. This high spatio-temporal resolution makes LAB a potential keen tool to experimentally investigate tokamak plasma physics.