Nonlinear dynamics of Shear Alfv\'en fluctuations in Divertor Tokamak Test facility plasmas

TL;DR

This paper investigates the nonlinear behavior and saturation mechanisms of shear Alfvén fluctuations in DTT plasmas using hybrid simulations, revealing how energetic particle transport is affected by different mode types and stability regimes.

Contribution

It provides new insights into the nonlinear dynamics and mode saturation of shear Alfvén fluctuations, highlighting the role of resonance structures and stochasticity in EP transport.

Findings

Saturation of reversed shear Alfvén eigenmode is mainly due to radial decoupling of trapped EPs.

EP transport occurs on a scale similar to the mode width for trapped EPs.

Passing EPs exhibit weak redistribution in weakly unstable regimes and meso-scale diffusion in strongly unstable regimes.

Abstract

Following the analysis on linear spectra of shear Alfv\'en fluctuations excited by energetic particles (EPs) in the Divertor Tokamak Test (DTT) facility plasmas [T. Wang et al., Phys. Plasmas 25, 062509 (2018)], in this work, nonlinear dynamics of the corresponding mode saturation and the fluctuation induced EP transport is studied by hybrid magnetohydrodynamic-gyrokinetic simulations. For the reversed shear Alfv\'en eigenmode driven by magnetically trapped EP precession resonance in the central core region of DTT plasmas, the saturation is mainly due to radial decoupling of resonant trapped EPs. Consistent with the wave-EP resonance structure, EP transport occurs in a similar scale to the mode width. On the other hand, passing EP transport is analyzed in detail for toroidal Alfv\'en eigenmode in the outer core region, with mode drive from both passing and trapped EPs. It is shown that passing EPs experience only weak redistributions in the weakly unstable case; and the transport extends to meso-scale diffusion in the strongly unstable case, due to orbit stochasticity induced by resonance overlap. Here, weakly/strongly unstable regime is determined by Chirikov condition for resonance overlap. This work then further illuminates rich and diverse nonlinear EP dynamics related to burning plasma studies, and the capability of DTT to address these key physics.