Ion temperature and rotation fluctuation measurements with ultra-fast charge exchange recombination spectroscopy (UF-CHERS) in the DIII-D tokamak

• Published in: Review of scientific instruments 2021-05, Vol.92 (5), p.053513-053513
• Main Authors: Truong, D. D., McKee, G. R., Yan, Z., Jaehnig, K., Winz, G. R., Fonck, R. J., Geiger, B.
• Format: Article
• Language: English
• Subjects: Amplification; Avalanche diodes; Bandwidths; Broadband; Charge exchange; High gain; Ion recombination; Ion temperature; Ion transport; Light diffraction; Neutral beams; Noise; Optical components; Photodiodes; Photons; Plasmas (physics); Preamplifiers; Scientific apparatus & instruments; Signal to noise ratio; Spectrum analysis; Turbulence
• ISSN: 0034-6748; 1089-7623
• DOI: 10.1063/5.0043095

An upgraded detector and several optimizations have significantly improved the Ultra-Fast Charge Exchange Recombination Spectroscopy (UF-CHERS) diagnostic sensitivity to ion temperature and parallel velocity fluctuations at turbulence relevant spatio-temporal scales. Normalized broadband ion temperature and parallel velocity fluctuations down to x̃x∼1% (x = Ti, v∥) and up to ∼450 kHz have been measured in a variety of plasmas. The multi-field nature of the CHERS technique also allows measurements of the cross-phase angles of the fluctuating fields. UF-CHERS is optimized to observe emissions from the electron exchange reaction between intrinsic C6+ and hydrogenic neutral beam injected particles near 529 nm. UF-CHERS consists of two chords separated by ∼1 cm radially, less than the turbulence correlation length in DIII-D plasmas, which enables correlated measurements to suppress incoherent electronic and photon noise. The optical components of the spectrometer include a volume-phase-holographic grating with >90% transmission between 528 and 530 nm and f/2 200-mm lenses, selected to maximize the optical efficiency and photon flux. Diffracted light from each chord is collected in eight spectral bins, each with a bandwidth of ∼0.25 nm, and detected and amplified by chilled avalanche photodiodes and custom high-gain, wide bandwidth low-noise preamplifiers to achieve the optimal signal-to-noise ratio. The resulting signals are digitized at 1 MHz, 103–104× faster than the conventional CHERS diagnostics. Spatial coverage is achieved by repositioning a motorized fiber tray between plasmas. UF-CHERS measurements will advance the understanding of turbulent ion transport and contribute to the validation of transport models and simulations.