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

Journal Article · Wed May 05 00:00:00 EDT 2021 · Review of Scientific Instruments

DOI:https://doi.org/10.1063/5.0043095· OSTI ID:1781877

Truong, D. D. ; McKee, G. R. ; Yan, Z.; Jaehnig, K.; Winz, G. R. ; Fonck, R. J. ; Geiger, B.

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.

• Sponsoring Organization: USDOE
• Grant/Contract Number: FC02-04ER54698; FG02-08ER54999; FG02-89ER53296
• OSTI ID: 1781877
• Journal Information: Review of Scientific Instruments, Journal Name: Review of Scientific Instruments Vol. 92 Journal Issue: 5; ISSN 0034-6748
• Publisher: American Institute of Physics
• Country of Publication: United States
• Language: English