Imaging key aspects of fast ion physics in the DIII-D tokamak

Abstract

Visible imaging has been used to provide the 2D spatial structure and temporal evolution of the profile of highenergy neutrals introduced by neutral beam injection, the fast ion profile and a variety of plasma instabilities in DIII-D plasmas; the combination of these techniques form a comprehensive fast ion physics diagnostic suite. The injected neutral profile is imaged in Doppler shifted D α light induced by collisional excitation. Fast ion profile information was obtained through imaging of Doppler shifted fast ion D α light (FIDA) emitted by re-neutralized energetic ions. Imaging of FIDA emission during sawtooth events shows a large central depletion following sawtooth crashes-indicative of a broad redistribution of fast ions. Two examples of instability structure measurements are given. Measurements of the detailed 2D poloidal structure of rotating tearing modes were obtained using spectrally filtered fast imaging of broadband visible bremsstrahlung emission, a method which is capable of imaging with high resolution the structure of coherent oscillations in the core of current and next-step fusion plasma experiments and can be applied to virtually any mode with a finite perturbed bremsstrahlung emissivity and frequency in the laboratory frame. Measurements are also presented of the n = 0 energetic particle geodesic acoustic mode which were made by observing fluctuations in active emission.

Tomography of fast-ion velocity-space distributions from synthetic CTS and FIDA measurements

“…Our tomographic and theoretical results contradict the conventional wisdom that at least two CTS or FIDA views would necessarily be required for tomography of fast-ion velocity distribution functions [12,[22][23][24][25][26][27][28][29][30][31][32]. In an idealized situation in fact just one single CTS or FIDA view suffices to compute an accurate tomography.…”

“…In [24] reconstructions from two and three synthetic CTS views have been shown to contain salient features of the underlying 2D fast-ion velocity distribution functions in idealized situations. It has since become conventional wisdom that a 2D velocity distribution function could not be found from one single 1D CTS or FIDA view and that at least two CTS or FIDA views with different projection directions would be necessary for that [12,[22][23][24][25][26][27][28][29][30][31][32]. We demonstrate that in fact just one single 1D CTS or FIDA view theoretically suffices to compute tomographies of almost the entire discrete 2D velocity distribution function under idealized conditions.…”

Progress in modelling fast-ion D-alpha spectra and neutral particle analyzer fluxes using FIDASIM

“…The latter is an unavoidable background signal in FIDA spectra and is, for instance, helpful to check the absolute intensity calibration of a FIDA system. The local bremsstrahlung per unit wavelength is calculated using [19] dN B dλ = 7.57 × 10 −9 g n 2 e Z ef f λT 1/2 e e −hc/λTe ,…”

Recent and future upgrades to the DIII-D tokamak

“…Advances in fast-framing camera technology have allowed the development of high-resolution visible imaging for studying phenomena at the core of plasmas in DIII-D. A variety of highenergy phenomena are studied by imaging emissions from various contributors that include bremsstrahlung, injected neutrals, and Doppler-shifted D˛light from fast ions [5,6]. For example, the fast ion D˛diagnostic (FIDA) [7] uses charge-exchange recombination spectroscopy on fast ions interacting with injected neutral beams.…”