Observation of Critical-Gradient Behavior in Alfvén-Eigenmode-Induced Fast-Ion Transport

Abstract

Experiments in the DIII-D tokamak show that fast-ion transport suddenly becomes stiff above a critical threshold in the presence of many overlapping small-amplitude Alfvén eigenmodes (AEs). The threshold is phase-space dependent and occurs when particle orbits become stochastic due to resonances with AEs. Above threshold, equilibrium fast-ion density profiles are unchanged despite increased drive, and intermittent fast-ion losses are observed. Fast-ion Dα spectroscopy indicates radially localized transport of the copassing population at radii that correspond to the location of midcore AEs. The observation of stiff fast-ion transport suggests that reduced models can be used to effectively predict alpha profiles, beam ion profiles, and losses to aid in the design of optimized scenarios for future burning plasma devices.

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

“…Figure 6 reproduces a figure that was published in Ref. 6. The modulated flux measured by an NPA diagnostic jumps suddenly above a threshold of about 4 MW.…”

“…Bursts of this sort are detected by a fast-ion loss detector. 6 Detailed discussions of these data will appear in a future publication.…”

“…Unstable toroidal Alfven eigenmodes (TAE) and reversed-shear Alfven eigenmodes (RSAE) are common. 14 Conditions with multiple unstable AEs occur both in L-mode plasmas during the current ramp 6,14,25 and in H-mode steady-state scenario plasmas with elevated q profiles. 26,27 Fast-ion diagnostics include the volume-averaged neutron rate, 28 which is dominated by beam-plasma reactions in the L-mode plasmas, FIDA diagnostics with oblique sightlines, 29 and solid-state neutral-particle analyzers (NPA) operated in current mode 30 (Fig.…”

“…3(a)). Most recently, 6 the FIDA profiles were measured during a systematic power scan. Initially, as the power increased, the fast-ion profile grew larger but, for higher power levels where the AE activity was strong, the FIDA profile was "clamped" (Fig.…”

“…5). For the fast ions, recent work 6 shows that the threshold for appreciable transport occurs at a threshold that is well beyond the linear stability threshold. The threshold occurs when orbits become stochastic in the portion of phase space interrogated by the fast-ion diagnostic.…”