Pedestal fluctuation measurements with charge exchange imaging at the DIII-D tokamak.

A new high radial resolution 2D multichannel Charge eXchange Imaging (CXI) diagnostic is under development for deployment at DIII-D. The diagnostic system will measure low-to-intermediate radial wavenumber carbon density fluctuations by observing the n = 8 - 7 (λ = 529.06 nm) C-VI emission line, resulting from charge exchange collisions between heating neutral beam atoms and the intrinsic carbon ion density. The new CXI diagnostic will provide measurements with ΔR ∼ 0.4 cm to access higher kr instabilities (kr < 8 cm-1) predicted to arise in the steep-gradient region of the H-mode pedestal. The CXI system will feature 60 fiber bundles in a 12 × 5 arrangement, with each bundle consisting of four 1 mm fibers. A custom optical system has been designed to filter and image incoming signals onto an 8 × 8 avalanche photodiode array. Additionally, a novel electronics suite has been designed and commissioned to amplify and digitize the relatively low-intensity carbon signal at a 2 MHz bandwidth. Forward modeling results of the active C-VI emission suggest sufficient signal to noise ratios to resolve turbulent fluctuations. Prototype measurements demonstrate the ability to perform high frequency pedestal measurements.

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

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 Dα 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⊥ρi < 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.

Conceptual design and performance predictions for 2D beam emission spectroscopy turbulence measurements at Wendelstein 7-X.

A conceptual design for a 2D beam emission spectroscopy diagnostic system to measure ion gyro-scale plasma turbulence at Wendeslstein 7-X is described. The conceptual design identifies field-aligned viewing geometries and ports for cross-field turbulence measurements in the neutral beam volume. A 2D sightline grid covers the outer plasma region, and the grid configuration provides sufficient k-space coverage in radial and poloidal directions for ion temperature gradient and trapped-electron mode turbulence measurements. Emission intensity estimates, optical transmission losses, and detector noise levels indicate that the measurements will be sensitive to plasma density fluctuations as small as δn/n ≈ 0.5% with a bandwidth of 1 MHz. Implementation challenges include a small beam emission Doppler shift due to nearly radial heating beams and reduced optical throughput due to collection aperture limitations.

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

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.

Initial beam emission spectroscopy diagnostic system on HL-2A tokamak.

A beam emission spectroscopy system is being developed and deployed on the HL-2A tokamak to measure local low wavenumber (k ⊥ ρ i < 1) density fluctuations by measuring the Doppler-shifted emission from a 50 kV deuterium heating neutral beam. High spatial resolution (Δr ≤ 1 cm, Δz ≤ 1.5 cm) measurements are achieved with customized in-vacuum optics. High frequency, high-gain preamplifiers sample the light intensity at a Nyquist frequency of 1 MHz and achieve a high S/N ratio via high optical throughput, low-noise preamplifiers, and high quantum efficiency photodiodes. A first set of 16 detector channels [configured in an 8 (radial) × 2 (poloidal) array] has been installed and tested at HL-2A, covering the radial range r/a = 0.8-1.1. The frequency and wavenumber spectra have been measured under different plasma conditions. Initial measurements have demonstrated the capability of measuring edge plasma density fluctuation spectra and the poloidal flow velocity fields with a high S/N ratio.

Low-noise, high-speed detector development for optical turbulence fluctuation measurements for NSTX.

A new beam emission spectroscopy (BES) diagnostic is under development. Photon-noise limited measurements of neutral beam emissions are achieved using photoconductive photodiodes with a novel frequency-compensated broadband preamplifier. The new BES system includes a next-generation preamplifier and upgraded optical coupling system. Notable features of the design are surface-mount components, minimized stray capacitance, a wide angular acceptance photodiode, a differential output line driver, reduced input capacitance, doubling of the frequency range, net reduced electronic noise, and elimination of the need for a cryogenic cooling system. The irreducible photon noise dominates the noise up to 800 kHz for a typical input power of 60 nW. This new assembly is being integrated into an upgraded multichannel optical detector assembly for a new BES system on the NSTX experiment.