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Velocity-space tomography provides a way of diagnosing fast ions in a fusion plasma by combining measurements from multiple instruments. We use a toroidally viewing and a vertically viewing fastion D-alpha diagnostic installed on the mega-amp spherical tokamak (before the upgrade) to perform velocity-space tomography of the fast-ion distribution function. To make up for the scarce amount of data, prior information is included in the inversions. We impose a non-negativity constraint, suppress the distribution in the velocity-space region associated with null-measurements, and encode the belief that the distribution function does not extend to energies significantly higher than those expected neoclassically. This allows us to study the fast-ion velocity distributions and the derived fast-ion densities before and after a sawtooth crash.
Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
The Mega Amp Spherical Tokamak (MAST) was a low aspect ratio device (R/A = 0.85/0.65 ~ 1.3) with similar poloidal cross-section to other medium-size tokamaks. The physics programme concentrates on addressing key physics ___________________________________________________________________________
Understanding the effect of Alfvén eigenmodes (AEs) and neoclassical tearing modes (NTMs) on fast ions is highly important for fusion reactors due to potentially strong resonant interactions between the fast ions and the modes. Here, we use the four-view fast-ion D-alpha (FIDA) diagnostic installed in the DIII-D tokamak to reconstruct the fast-ion velocity distribution at two radial positions during two sequential discharges with strong and weak mode activity, respectively. The velocity-space coverage of the diagnostics, however, only allows reliable reconstructions of fast ions with positive pitches. Therefore, we suggest new tomographic inversion methods relying on prior information outside the well-diagnosed region. We find that within the population of fast ions with positive pitches, ions at all energies are transported away from the measurement volumes. Comparisons between the reconstructions and kick model simulations, where the mode activity is considered, reveals that low-frequency modes such as the NTMs and low-frequency AEs contribute significantly to the positivepitch fast-ion transport in the central measurement volume, whereas TAEs and EAEs become important farther out and are responsible for decreased fast-ion confinement.
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