Speaker
Description
Recurrent bursting Alfvénic instabilities can severely degrade energetic-particle (EP) confinement and impose transient loads on plasma-facing components. We recently reported the first comprehensive simulations of ICRF-heated tokamak plasmas on the slowing-down timescale with the kinetic-MHD hybrid code MEGA, where shear Alfvén wave induced minority ion transport is self-consistently retained during high-energy-tail formation [1]. Both bursting and non-bursting toroidal Alfvén eigenmodes (TAEs) were obtained: on-axis or inboard ICRF resonance yields bursting TAEs, which appear as a set of radially separated discrete modes at distinct, nearly fixed frequencies without chirping, whereas outboard heating always produces a single broad structure whose harmonics share nearly the same frequency, independent of RF power.
This work clarifies the mechanism through EP phase-space analysis. The deeply trapped ICRF-accelerated minority ions resonant with a fixed-frequency TAE occupy a highly localized region of ($E$, $\mu$) phase space. For on-axis/inboard heating, TAE transport carries these ions away from the RF layer, so replenishment is weak. The resulting discrete TAEs keep the resonances isolated, free energy accumulates, and once the growing amplitudes force neighboring resonance islands to overlap, a burst is triggered, consistent with the resonance-overlap (Chirikov) avalanche picture [2]. For outboard heating, the resonant ions stay close to the RF layer, where the strong velocity-space diffusion sharply raises the replenishment rate νeff and restores them faster than the wave flattens the local gradient, suppressing isolated phase-space structures. This high-νeff picture also explains why the non-bursting state sustains a higher EP beta than the bursting one, which a critical-gradient argument alone cannot.
Building on this mechanism, we report for the first time simulations in which an on-axis-driven bursting state is suppressed by an additional low-field-side ICRF wave. The auxiliary wave introduces velocity-space diffusion where the burst-transported ions accumulate, driving the resonant population into the high-νeff regime and mitigating the bursts. To our knowledge this is the first comprehensive simulation to demonstrate phase-space engineering of energetic ions [3] for the control of bursting Alfvénic activity in magnetic confinement fusion.
Acknowledgments: This work was supported by JSPS KAKENHI with Grant Number JP21H04973, JP24K17032 and JP24H00207, the NINS program of Promoting Research by Networking among Institutions (Grant Number 01422301).
References
[1] J. Wang et al., Nucl. Fusion (2026), accepted (doi:10.1088/1741-4326/ae72a4).
[2] H.L. Berk et al., Nucl. Fusion 35, 1661(1995).
[3] J.P. Graves et al., Nat. Commun. 3, 624 (2012).