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

Abstract

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.

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

“…The kick model analysis grouped RSAEs with the same toroidal mode numbers together, and all TAEs were grouped together. The AE mode amplitudes were scaled to match the ECE data at timeslices near the RSAE cascade when The initial analysis of shot #159243 was recently reported in [26] and reveals a significant complication: the modulated beam also modulates the AE amplitude, which increases transport of the background source beam particles and strongly influences the shape of the distortion in the waveforms measured by the fast-ion diagnostics. Figure 14 compares conditionally averaged signals from t = 678-894 ms for shots #159243 and #162753.…”

“…In the tangential beam case, transport by AEs leads to hollow beam density and current profiles. In [26], the kick model fast ion distribution function for #159243 was further processed with the FIDASIM code to produce FIDA signals. Excellent agreement was found, and the hollow beam density profile was confirmed by the fast ion profiles measured by the FIDA diagnostic.…”

“…While RSAEs are destabilized near ρ qmin , the observed modes have relatively broad structure (see for example, figure 11). Since typical 75 keV fast-ion orbits are large and pass through both the magnetic axis and ρ qmin (as illustrated in figure 8 of [26]), RSAEs in particular could drive substantial outward transport at small ρ, creating hollow beam density profiles. In the perpendicular beam injection case, there are fewer RSAEs, and the kick model shows that the AE transport results in relaxed, but not hollow fast-ion density profiles (blue line in figure 17(a)).…”

“…The magnitude of critical gradient transport depends on given plasma conditions and AE activity; measurements show that the fast ion density increases in TAE-dominant cases compared to mixed AE cases, and simulations of the time-evolved transported fastion distribution function for above-threshold cases with mixed AE mode types show that the peak radial critical gradient of the beam density changes as the AEs and plasma conditions evolve in time. A companion paper [26] focuses further on fast-ion transport well above the transport threshold.…”

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

“…This discharge, which is discussed in detail in Ref. 16, is a reversed-shear plasma with a toroidal field of 2.0 T and plasma current of 0.8 MA. The fast-ion population is created by deuterium neutral beams of energy 70-81 keV that are injected in both the co-current and counter-current directions.…”