Optical design of the Lyman alpha based beam emission spectroscopy (LAB) diagnostic on the HL-2A tokamak

In this article, we present the optical design of a novel diagnostic on the HL-2A tokamak, i.e. the 20-channel edge Lyman-alpha beam emission spectroscopy, which is a promising solution for edge density turbulence research on tokamaks, as it offers the possibility of density fluctuation measurement with a 3.3 mm spatial resolution while maintains a high temporal resolution of 1 μs. The optical path, including the reflective collection optics, the high-dispersion spectrometer, and the linear detector array, is carefully optimized to obtain a good image quality and a high throughput. The maximum root mean square radius of the collection optics is 64 μm. The detected photon flux is estimated to be about 10¹¹ photons/s/channel.