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

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.

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

“…Other examples of a transport threshold will appear in a forthcoming publication. 7 Evidence that the transport threshold exceeds the linear stability threshold is also found in steady-state scenario plasmas. Figure 4 of Ref.…”

“…Details of the comparison between the measured threshold and stochasticity theory are the focus of a forthcoming paper. 7 Because different modes interact with fast ions in different parts of phase space, the measured threshold depends sensitively on both the diagnostic and on the fast-ion population that drives the modes. 7 Detailed probing of transport in phase space utilizes the combination of a modulated source and an array of fast-ion diagnostics.…”

“…7 Because different modes interact with fast ions in different parts of phase space, the measured threshold depends sensitively on both the diagnostic and on the fast-ion population that drives the modes. 7 Detailed probing of transport in phase space utilizes the combination of a modulated source and an array of fast-ion diagnostics. Similarities and differences between thermalplasma and fast-ion modulation experiments are discussed in a recent paper.…”

Verification and validation of integrated simulation of energetic particles in fusion plasmas

“…Multiple unstable RSAEs and TAEs are excited using early deuterium beam power injected at 70-81 keV, with 4.0 MW of co-current, on-axis NBI, 0.7 MW of cocurrent, off-axis NBI, and 1.7 MW of counter-current, on-axis NBI. This discharge has excellent diagnostic coverage and was examined extensively in studies [3,4,15] of AE-induced fast-ion transport in critical gradient experiments. Figure 1(a) shows the spectrogram of electron cyclotron emission (ECE) data during the current ramp for shot #159243, along with calculated RSAE frequency evolution from an ad hoc model [16].…”

Machine Learning Characterization of Alfvénic and Sub-Alfvénic Chirping and Correlation With Fast-Ion Loss at NSTX

“…Furthermore, experiments and simulations in the literature 2,15,22,23 have shown that rapid long range frequency chirping across a wide range of frequency chirps (mode 'avalanching') is correlated with high amounts of fast ion loss. Other work has shown that mode-mode destabilisation may play a role in Alfvénic frequency chirping -activity in the KTF frequency range (∼ 1 kHz to 30 kHz) may instigate Alfvénic activity, and vice-versa.…”