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

Abstract

We compute tomographies of 2D fast-ion velocity distribution functions from synthetic collective Thomson scattering (CTS) and fast-ion D α (FIDA) 1D measurements using a new reconstruction prescription. Contradicting conventional wisdom we demonstrate that one single 1D CTS or FIDA view suffices to compute accurate tomographies of arbitrary 2D functions under idealized conditions. Under simulated experimental conditions, single-view tomographies do not resemble the original fast-ion velocity distribution functions but nevertheless show their coarsest features. For CTS or FIDA systems with many simultaneous views on the same measurement volume, the resemblance improves with the number of available views, even if the resolution in each view is varied inversely proportional to the number of views, so that the total number of measurements in all views is the same. With a realistic four-view system, tomographies of a beam ion velocity distribution function at ASDEX Upgrade reproduce the general shape of the function and the location of the maxima at full and half injection energy of the beam ions. By applying our method to real many-view CTS or FIDA measurements, one could determine tomographies of 2D fast-ion velocity distribution functions experimentally.

Fast-ion transport and neutral beam current drive in ASDEX upgrade

“…For a more detailed analysis that includes all accessible wavelength ranges and viewing geometries, a tomographic inversion has been applied that is based on a truncated singular value decomposition as in previous work [34,35,36]. In this method the forward model to calculate FIDA spectra is formulated in the form of a matrix larger values for positive pitches than for negative pitches, explained by the co-current NBI injection geometry.…”

High-definition velocity-space tomography of fast-ion dynamics

“…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]. The 2D velocity distribution function is spatially localized in a small measurement volume which we take to be a single point in position space.…”

Multi-view fast-ion D-alpha spectroscopy diagnostic at ASDEX Upgrade

“…In the future, installing additional viewing geometries with angles to the magnetic field lines close to 40 • and 80 • will permit the direct measurement of the fast-ion velocity space distribution using tomographic inversion procedures as proposed by 31,32,34 . Moreover, the FIDA spectrometer could be equipped with a narrow blocking wire close to the CCD chip.…”

“…This allows, for example, the effect of sawtooth crashes 30 on the different parts of the velocity space, to be investigated. In addition, the acquisition of FIDA spectra with multiple viewing geometries, as presented here, will open up the possibility to infer 2D velocity-space distribution functions from the spectra by tomographic inversion in velocity space as presented in 31,32 . The FIDA diagnostic's exposure time of 2 ms permits the analysis of the evolution of measured FIDA intensities with good temporal resolution.…”