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

Abstract

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 10 11 photons/s/channel.

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

“…3 mm for an Edge-LAB channel ( 43.12 a » ) at R = 2.05 m. For a core channel (R = 1.73 m), where α ≈ 54.05°, LAB has a higher upper spatial resolution of R LAB | upp = υ D •τ Lym •cosα ≈ 2.7 mm. The misalignment between the sightline and the magnetic field line in the plasma edge is evaluated to be similar to that in [17] for the reference shot on HL-2A.…”

“…To meet the diagnosing requirement to investigate the underlying physics of the double transport barrier, the viewing area of LAB ranges from the tokamak edge to the core. This viewing range makes the simple preparation of the 20channel Edge-LAB optics in [17] inadequate.…”

Optical design of a novel near-infrared phase contrast imaging (NI-PCI) diagnostic on the HL-2A tokamak

“…In tokamak plasmas, experimental data show that radial transport exceeds the predictions of the neoclassical theory by more than several orders of magnitude, and this anomalous transport phenomenon is widely believed to be caused by turbulence driven by plasma density and temperature gradients. Typical turbulences due to drift wave instabilities are ion temperature gradient mode (ITG), trapped electron mode (TEM), and electron temperature gradient mode (ETG) [1][2][3][4]. A key difference among these turbulence types is that they have different wavenumber ranges.…”

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

“…Therefore, the visible light BES system does not have adequate capacity for research at the smallest spatial and temporal scales, such as the analysis of ELM and edge coherent oscillation (ECO) in the pedestal area on HL-2A [14]. For the purpose of improving the performance of beam emission spectroscopy, the idea of Lyman-alpha-based BES (LyBES) was proposed theoretically twenty years ago [16]. Compared with the conventional visible light BES diagnostic, LyBES could have a theoretically improved spatial resolution of several millimeters while maintaining the temporal resolution of 1 μs, which results mainly from the short lifetime of the Lyman-alpha line (n = 2→1, λ 0 = 121.53 nm).…”

“…Compared with the conventional visible light BES diagnostic, LyBES could have a theoretically improved spatial resolution of several millimeters while maintaining the temporal resolution of 1 μs, which results mainly from the short lifetime of the Lyman-alpha line (n = 2→1, λ 0 = 121.53 nm). Furthermore, LyBES could have greater signal intensity than that of visible light BES, benefited from a larger transition rate, state density, and fluctuation sensitivity of the Lyman-alpha line [16][17][18]. However, there is no further progress reported on LyBES, probably owing to the greater technical and engineering difficulties of vacuum ultraviolet spectroscopy.…”