Abstract
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.
Highlights
In tokamaks, Alfvén eigenmodes can exchange energy with fast-ions through resonant mode-particle interactions, causing fast-ions to move away from the core of the plasma
It is found that the Alfvén eigenmodes (AEs) transport results in mainly redistribution, not loss, of non-inductive current drive. These results show that treatment of the AEs in phase space is important in accurately predicting fast ion density and current profiles, highlighting the importance of including the velocity dependence of the fast-ion distribution function in critical gradient models which are currently commencing [43]
Experiments in the DIII-D tokamak show that many overlapping, small-amplitude Alfvén eigenmodes cause stiff fast-ion transport above a critical threshold
Summary
Alfvén eigenmodes can exchange energy with fast-ions through resonant mode-particle interactions, causing fast-ions to move away from the core of the plasma. Redistribution of fast ions can affect the plasma profiles, beam deposition, and current drive, and losses induced by AE transport can lead to localized heating that can damage the vessel walls [1]. Small-amplitude AEs are expected to be destabilized in ITER, where alpha losses must be less than O(5%) to avoid excess heat load of the first wall [2]. In DIII-D, significant fast-ion transport due to AEs has been observed in high qmin steady-state reactor scenarios where the measured neutron rate approaches 60% of the classical rate predicted by the TRANSP NUBEAM code [3] assuming no anomalous transport, limiting the achievable βN. Some scenarios with qmin closer to one still have AE activity, yet fast ions behave classically [4, 5]. Understanding the regimes where AEs lead to losses and reduced fusion performance is important for developing a practical long-pulse fusion power plant
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