Abstract
Velocity-space tomography of the fast-ion distribution function in a fusion plasma is usually a photon-starved tomography method due to limited optical access and signal-to-noise ratio of fast-ion Dα (FIDA) spectroscopy as well as the strive for high-resolution images. In high-definition tomography, prior information makes up for this lack of data. We restrict the target velocity space through the measured absence of FIDA light, impose phase-space densities to be non-negative, and encode the known geometry of neutral beam injection (NBI) sources. We further use a numerical simulation as prior information to reconstruct where in velocity space the measurements and the simulation disagree. This alternative approach is demonstrated for four-view as well as for two-view FIDA measurements. The high-definition tomography tools allow us to study fast ions in sawtoothing plasmas and the formation of NBI peaks at full, half and one-third energy by time-resolved tomographic movies.
Highlights
It is often convenient to split ion distribution functions in magnetized fusion plasmas into two parts
As Ti and ni are nearly constant on a flux surface, bulk-ion measurements often refer to flux surface measurements of these fundamental parameters [1,2,3,4]
We have previously shown that it is possible to correctly reconstruct the injection energy in plasma heated by neutral beam injection (NBI), and good agreement with TRANSP predictions was found in the absence of strong magnetohydrodynamic (MHD) activity [24, 27]
Summary
It is often convenient to split ion distribution functions in magnetized fusion plasmas into two parts. Velocity-space tomography allows us to infer the fundamental 2D fast-ion velocity distribution functions from measured spectra in analogy to bulk-ion measurements [20,21,22,23,24,25,26,27,28,29,30, 31]. In highdefinition velocity-space tomography we make up for the lack of data through various types of prior information This improves results for the five-view FIDA diagnostic at the tokamak ASDEX Upgrade [38], but it allows the use of inversion techniques for more common FIDA systems with two or three views.
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