Development and validation of a critical gradient energetic particle driven Alfven eigenmode transport model for DIII-D tilted neutral beam experiments

Waltz, R. E.; Bass, E.M.; Heidbrink, W. W.; Vanzeeland, M.

doi:10.1088/0029-5515/55/12/123012

Cited by 27 publications

(49 citation statements)

References 22 publications

(59 reference statements)

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“…21 In this paper, the use of a modulated source provides new information about the fast-ion transport by AEs above the stochastic threshold. Section II summarizes the experimental conditions from the perspective of the critical-gradient paradigm.…”

Section: Introductionmentioning

confidence: 99%

Fast-ion transport by Alfvén eigenmodes above a critical gradient threshold

et al. 2017

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Experiments on the DIII-D tokamak have identified how multiple simultaneous Alfvén eigenmodes (AEs) lead to overlapping wave-particle resonances and stochastic fast-ion transport in fusion grade plasmas [C. S. Collins et al., Phys. Rev. Lett. 116, 095001 (2016)]. The behavior results in a sudden increase in fast-ion transport at a threshold that is well above the linear stability threshold for Alfvén instability. A novel beam modulation technique [W. W. Heidbrink et al., Nucl. Fusion 56, 112011 (2016)], in conjunction with an array of fast-ion diagnostics, probes the transport by measuring the fast-ion flux in different phase-space volumes. Well above the threshold, simulations that utilize the measured mode amplitudes and structures predict a hollow fast-ion profile that resembles the profile measured by fast-ion Dα spectroscopy; the modelling also successfully reproduces the temporal response of neutral-particle signals to beam modulation. The use of different modulated sources probes the details of phase-space transport by populating different regions in phase space and by altering the amplitude of the AEs. Both effects modulate the phase-space flows.

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Section: Introductionmentioning

confidence: 99%

Fast-ion transport by Alfvén eigenmodes above a critical gradient threshold

et al. 2017

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“…An early implementation of a critical gradient model [27] assumed that if the fast-ion gradient β ∂ ∂r EP / exceeds the stability threshold of the fastest growing mode, then fast-ion transport becomes large, and particles are transported to flatten the profile and maintain marginal stability. Another early model [28,29] uses a threshold associated with microturbulence that exceeds the linear stability threshold. The measurements reported here and in [25] show that the onset of significant fast-ion transport exceeds the linear AE stability threshold and is associated with the presence of multiple, overlapping AEs that cause particle orbits to become stochastic.…”

Section: Introductionmentioning

confidence: 99%

Phase-space dependent critical gradient behavior of fast-ion transport due to Alfvén eigenmodes

et al. 2017

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Experiments in the DIII-D tokamak show that many overlapping small-amplitude Alfvén eigenmodes (AEs) cause fast-ion transport to sharply increase above a critical threshold in beam power, leading to fast-ion density profile resilience and reduced fusion performance. The threshold is above the AE linear stability limit and varies between diagnostics that are sensitive to different parts of fast-ion phase-space. Comparison with theoretical analysis using the nova and orbit codes shows that, for the neutral particle diagnostic, the threshold corresponds to the onset of stochastic particle orbits due to wave-particle resonances with AEs in the measured region of phase space. The bulk fast-ion distribution and instability behavior was manipulated through variations in beam deposition geometry, and no significant differences in the onset threshold outside of measurement uncertainties were found, in agreement with the theoretical stochastic threshold analysis. Simulations using the 'kick model' produce beam ion density gradients consistent with the empirically measured radial critical gradient and highlight the importance of including the energy and pitch dependence of the fast-ion distribution function in critical gradient models. The addition of electron cyclotron heating changes the types of AEs present in the experiment, comparatively increasing the measured fast-ion density and radial gradient. These studies provide the basis for understanding how to avoid AE transport that can undesirably redistribute current and cause fast-ion losses, and the measurements are being used to validate AE-induced transport models that use the critical gradient paradigm, giving greater confidence when applied to ITER.

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“…AEs are driven by the EP pressure gradient, which has a threshold for marginally stable AE. The existence of the threshold was confirmed by theory 23,24 and experiment. 25 Above the threshold, AE amplitudes increase rapidly, which induces EP stiff transport, 6 until the local EP gradient declines to the threshold.…”

Section: Introductionmentioning

confidence: 60%

Prediction of the energetic particle redistribution by an improved critical gradient model and analysis of the transport threshold

et al. 2022

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Based on the theory of critical gradient model (CGM) and following the simulation method proposed by Waltz et al. [Nucl. Fusion 55, 123012 (2015)], a combination of TGLFEP and EPtran code is employed to predict the energetic particle (EP) transport induced by Alfvén eigenmodes (AEs). To be consistent with the experiment, recent improvements to the simulation method include consideration of threshold evolution and orbit loss due to finite orbit width. The revised CGM is applied to simulate two DIII-D experimental discharges (#142111 and #153071). It well reproduces the experimental profiles with multiple unstable AEs and large-scale EP transport. Discharge #142111 had previously been simulated using a nonlinear MHD-kinetic code MEGA [Todo et al., Nucl. Fusion 55, 073020 (2015)] with a transport mechanism based on stochasticity induced by overlapping AE. By comparing the simulated EP profiles, we find that the AE transport threshold is approximated by both the MEGA nonlinear stability threshold and the proposed CGM threshold (error <5% for single n and <17% for multiple n simulation). Both of them are larger than the linear stability threshold of the most unstable AE mode by a quantity of the order of the flux needed to sustain EP transport by the background turbulence. We have also applied the improved CGM to simulate the α particle redistribution for a China Fusion Engineering Test Reactor steady state scenario. Because of the clear separation between the AE unstable region and the loss cone, only a moderate α particle loss of ∼9.6% is predicted.

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Development and validation of a critical gradient energetic particle driven Alfven eigenmode transport model for DIII-D tilted neutral beam experiments

Cited by 27 publications

References 22 publications

Fast-ion transport by Alfvén eigenmodes above a critical gradient threshold

Fast-ion transport by Alfvén eigenmodes above a critical gradient threshold

Phase-space dependent critical gradient behavior of fast-ion transport due to Alfvén eigenmodes

Prediction of the energetic particle redistribution by an improved critical gradient model and analysis of the transport threshold

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