Combination of fast-ion diagnostics in velocity-space tomographies

Due to the usually complicated and anisotropic nature of the fast-ion distribution function, diagnostic velocity-space weight functions, which indicate the sensitivity of a diagnostic to different fast-ion velocities, are used to facilitate the analysis of experimental data. Additionally, when velocity-space weight functions are discretized, a linear equation relating the fast-ion density and the expected diagnostic signal is formed. In a technique known as velocity-space tomography, many measurements can be combined to create an ill-conditioned system of linear equations that can be solved using various computational methods. However, when velocity-space weight functions (which by definition ignore spatial dependencies) are used, velocity-space tomography is restricted, both by the accuracy of its forward model and also by the availability of spatially overlapping diagnostic measurements. In this work, we extend velocity-space weight functions to a full 6D generalized coordinate system and then show how to reduce them to a 3D orbit-space without loss of generality using an action-angle formulation. Furthermore, we show how diagnostic orbit-weight functions can be used to infer the full fast-ion distribution function, i.e., orbit tomography. In depth derivations of orbit weight functions for the neutron, neutral particle analyzer, and fast-ion D-α diagnostics are also shown.

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“…The total beam-plasma neutrons produced, as calculated by Eq. (8), is in agreement with the predictions of TRANSP/NUBEAM and FIDASIM (Fig. 6).…”

Section: B Neutron Scintillator Orbit Weight Functionsupporting

confidence: 90%

Action-angle formulation of generalized, orbit-based, fast-ion diagnostic weight functions

Stagner

Heidbrink

2017

Physics of Plasmas

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“…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.…”

Section: Sawtooth-induced Fast-ion Transportmentioning

confidence: 99%

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

Geiger¹,

Weiland²,

Jacobsen³

et al. 2015

Nucl. Fusion

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Abstract.The neutral beam current drive efficiency has been investigated in the ASDEX Upgrade tokamak by replacing on-axis neutral beams with tangential off-axis beams.A clear modification of the radial fast-ion profiles is observed with a fast-ion Dalpha diagnostic that measures centrally peaked profiles during on-axis injection and outwards shifted profiles during off-axis injection. Due to this change of the fast-ion population, a clear modification of the plasma current profile is predicted but not observed by a motional Stark effect diagnostic.The fast-ion transport caused by MHD activity has been studied in low collisionality discharges that exhibit strong (1,1) modes. In particular due to sawtooth crashes, significant radial redistribution of co-rotating fast-ions is observed which can very well be described by the Kadomtsev model. In addition, first tomographic reconstructions of the central 2D fast-ion velocity space in the presence of sawtooth crashes allow the investigation of the pitch dependence of the mode-imposed redistribution: A stronger redistribution of mainly co-rotating fast ions is observed than of those with smaller pitch values.

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“…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.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2016

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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 D α (FIDA) spectroscopy as well as the strive for high-resolution images. In highdefinition 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.

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Combination of fast-ion diagnostics in velocity-space tomographies

Cited by 50 publications

References 65 publications

Action-angle formulation of generalized, orbit-based, fast-ion diagnostic weight functions

Action-angle formulation of generalized, orbit-based, fast-ion diagnostic weight functions

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

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

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