Integrated 2D beam emission spectroscopy diagnostic at the Huan-Liuqi-2A (HL-2A) tokamak

Abstract

Two newly developed, eight-channel, integrated Beam Emission Spectroscopy (BES) detectors have been installed at Huan-Liuqi-2A tokamak, which extends the existing 16 single-channel modular BES system with additional 16 spatial channels. The BES collects the Doppler-shifted Balmer D α emission with a spatial resolution of 1 cm (radial) × 1.5 cm (poloidal) and a temporal resolution of 0.5 µs to measure long-wavelength ( k⊥ ρ i < 1) density fluctuations. Compared to the modular BES, the dark noise of the integrated BES is reduced by 50%–60% on average. The signal-to-noise ratio of the integrated BES system is optimized by the high light throughput front-end optics, high quantum efficiency photodiodes, high-gain, low-noise preamplifiers, and sufficient cooling capacity provided by the thermoelectric cooling (TEC) units that maintain the detectors at −20 °C. Crosstalk between channels that share the same optical system is found to be negligible. High-quality density fluctuation data enables 2D (radial–poloidal) imaging of turbulence, which allows for multi-channel spectral analysis, multi-channel cross-correlation analysis and velocimetry analysis. Preliminary results show that BES successfully captures the spatiotemporal features of the local turbulence and obtains statistically consistent turbulence characterization results.