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Ultra-fast charge exchange spectroscopy for turbulent ion temperature fluctuation measurements on the DIII-D tokamak (invited)

Uzun Kaymak, İlker Ümit
Fonck, R. J.
McKee, G. R.
A novel two-channel, high throughput, high efficiency spectrometer system has been developed to measure impurity ion temperature and toroidal velocity fluctuations associated with long-wavelength turbulence and other plasma instabilities. The spectrometer observes the emission of the n = 8-7 hydrogenic transition of C+5 ions (lambda(air) = 529.06 nm) resulting from charge exchange reactions between deuterium heating beams and intrinsic carbon. Novel features include a large, prism-coupled high-dispersion, volume-phase-holographic transmission grating and high-quantum efficiency, high-gain, low-noise avalanche photodiode detectors that sample emission at 1 MHz. This new diagnostic offers an order-of-magnitude increase in sensitivity compared to earlier ion thermal turbulence measurements. Increased sensitivity is crucial for obtaining enough photon statistics from plasmas with much less impurity content. The irreducible noise floor set by photon statistics sets the ultimate sensitivity to plasma fluctuations. Based on themeasured photon flux levels for the entire spectral line, photon noise levels for (T) over tilde (i)/T-i and (V) over tilde (i)/V-i of similar to 1% are expected, while statistical averaging over long data records enables reduction in the detectable plasma fluctuation levels to values less than that. Broadband ion temperature fluctuations are observed to near 200 kHz in an L-mode discharge. Cross-correlation with the local beam emission spectroscopy measurements demonstrates a strong coupling of the density and temperature fields, and enables the cross-phase measurements between density and ion temperature fluctuations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4733548]