Measurements and modelling of fast-ion redistribution due to resonant MHD instabilities in MAST

Jones, O.; Cecconello, M.; McClements, K. G.; Klimek, Iwona; Akers, R.; Boeglin, W.; Keeling, D.; Meakins, A.; Perez, R. V.; Sharapov, S. E.; Turnyanskiy, M.

doi:10.1088/0741-3335/57/12/125009

Cited by 16 publications

(16 citation statements)

References 38 publications

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“…Fast NBI ion redistribution with a concomitant drop in neutron rates has been observed in DIII-D discharges in the presence of a large magnetic island [24]. In MAST bursts of n = 1, m = 1 fishbone modes and TAEs have been reported to cause drops in the neutron rate [25]. TAEs are not observed in JET discharges heated only by NBI, as investigated here.…”

Section: Sawteeth Elms and Ntmssupporting

confidence: 48%

The ‘neutron deficit’ in the JET tokamak

et al. 2017

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The measured D-D neutron rate of neutral beam heated JET baseline and hybrid H-modes in deuterium is found to be between approximately 50% and 100% of the neutron rate expected from the TRANSP code, depending on the plasma parameters. A number of candidate explanations for the shortfall, such as fuel dilution, errors in beam penetration and effectively available beam power have been excluded. As the neutron rate in JET is dominated by beamplasma interactions, the 'neutron deficit' may be caused by a yet unidentified form of fast particle redistribution. Modelling, which assumes fast particle transport to be responsible for the deficit, indicates that such redistribution would have to happen at time scales faster than both the slowing down time and the energy confinement time. Sawteeth and edge localised modes are found to make no significant contribution to the deficit. There is also no obvious correlation with magnetohydrodynamic activity measured using magnetic probes at the tokamak vessel walls. Modelling of fast particle orbits in the 3D fields of neoclassical tearing modes shows that realistically sized islands can only contribute a few percent to the deficit. In view of these results it appears unlikely that the neutron deficit results from a single physical process in the plasma.

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Section: Sawteeth Elms and Ntmssupporting

confidence: 48%

The ‘neutron deficit’ in the JET tokamak

et al. 2017

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“…A similar example of this kind of model validation has recently been carried out at MAST. In this case, a set of complementary fast ion diagnostics, including the MAST neutron camera, were used in conjunction with modeling to study fast ion behavior during TAEs and fishbone instabilities [34,35]. The MAST neutron camera has also been used in a similar way to study sawtooth redistribution [36].…”

Section: Model Validation and Parameter Fittingmentioning

confidence: 99%

Measuring fast ions in fusion plasmas with neutron diagnostics at JET

Eriksson

Hellesen

Binda

et al. 2018

Plasma Phys. Control. Fusion

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Fast ions in fusion plasmas often leave characteristic signatures in the neutron emission from the plasma. In this paper, we show how neutron measurements can be used to study fast ions and give examples of physics results obtained on present day tokamaks. The focus is on measurements with dedicated neutron spectrometers and with compact neutron detectors used in each channel of neutron profile monitors. A measured neutron spectrum can be analyzed in several different ways, depending on the physics scenario under consideration. Gross features of a fast ion energy distribution can be studied by applying suitably chosen thresholds to the measured spectrum, thus probing ions with different energies. With this technique it is possible to study the interaction between fast ions and MHD activity, such as toroidal Alfvén eigenmodes and sawtooth instabilities. Quantitative comparisons with modeling can be performed by a direct computation of the neutron emission expected from a given fast ion distribution. Within this framework it is also possible to determine physics parameters, such as the supra-thermal fraction of the neutron emission, by fitting model parameters to the data. A detailed, model-independent estimate of the fast ion distribution can be obtained by analyzing the data in terms of velocity space weight functions. Using this method, fast ion distributions can be resolved in both energy and pitch by combining neutron and gamma-ray measurements obtained along several different sightlines. Fast ion measurements of the type described in this paper will also be possible at ITER, provided that the spectrometers have the dynamic range required to resolve the fast ion spectral features in the presence of the dominating thermonuclear neutron emission. A dedicated high-resolution neutron spectrometer has been designed for this purpose.

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“…Many FIDA systems have two or three viewing directions rather than the five available at ASDEX Upgrade. DIII-D has three FIDA views [46,47] and MAST [48,49], NSTX [50], EAST [51], and LHD have two views [52,53]. In figure 8 we reconstruct the six cases from figure 7 using just two FIDA views to show that this approach could be useful for other machines.…”

Section: Inversions Using Numerical Simulations As Prior Informationmentioning

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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Measurements and modelling of fast-ion redistribution due to resonant MHD instabilities in MAST

Cited by 16 publications

References 38 publications

The ‘neutron deficit’ in the JET tokamak

The ‘neutron deficit’ in the JET tokamak

Measuring fast ions in fusion plasmas with neutron diagnostics at JET

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

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