Nonlinear dynamics of shear Alfvén fluctuations in divertor tokamak test facility plasmas

Abstract

Following the analysis on linear spectra of shear Alfvén 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 Alfvén 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, passingEP transport is analyzed in detail for toroidal Alfvén 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.

Nonlinear reversed shear Alfvén eigenmode saturation due to spontaneous zonal current generation

“…the ITER; Tomabechi et al 1991) are expected to operate at steady state with a substantial non-inductive current fraction (Gormezano et al 2007), which generally renders a reversed shear scenario (Huang et al 2020). In this circumstance, the reversed shear Alfvén eigenmode (RSAE; also known as Alfvén cascade) is frequently observed (Sharapov et al 2001) as driven unstable by energetic particles (EPs) (Zonca et al 2002;Fasoli et al 2007;Chen et al 2014;Chen & Zonca 2016) in present-day tokamaks, and is expected to play significant roles in future reactors (Wang et al 2018), in transporting fusion alpha particles to the tokamak edge (Wang et al 2019), which has deleterious effects on plasma self-heating, and may damage the plasma-facing components (Ding et al 2015). The RSAE is a branch of the shear Alfvén wave (SAW) eigenmode localized radially near the minimum of the safety factor q-profile (Sharapov et al 2002), which is denoted as q min .…”

“…Typically, RSAEs are dominated by one or two poloidal harmonics, with the radial width ∝ q/(r 2 0 q ) (see section III of Zonca et al (2002)) with r 0 being the radial location of q min and q ≡ ∂ 2 r q. Despite the fairly good understanding of the linear physics, the nonlinear dynamics of RSAE is still interesting and valuable to research, especially in view of the EP as well as thermal particle transport induced by the associated electromagnetic field perturbations (Wang et al 2019(Wang et al , 2020Shi et al 2020;Wei et al 2021). The transport rate is closely related to the perturbation amplitude (Chen 1999;Zonca et al 2021); and in reactor-relevant cases with many SAWs simultaneously driven unstable by EPs, the EP orbit could become chaotic and eventually lost in the presence of many low-amplitude SAWs (threshold value δB/B ∼ O(10 −4 )) (White et al 2010a,b).…”

Nonlinear excitation of a geodesic acoustic mode by toroidal Alfvén eigenmodes and the impact on plasma performance

“…Thus, understanding the nonlinear dynamics of TAE, including saturation, is crucial for understanding the properties of burning plasmas in future reactors, and was under extensive investigation in the past decades [6,[17][18][19][20][21][22]. Nonlinear evolution of Alfvénic fluctuations, including TAE, can occur along two "routes" [23]; i.e., they can saturate through either nonlinear wave-particle phase space dynamics [17][18][19][20]24] and/or nonlinear mode-mode coupling processes [21,22,[25][26][27][28], as reviewed in Ref. 6.…”

Dynamics of reversed shear Alfvén eigenmode and energetic particles during current ramp-up

“…Accordingly, on one hand, in order to fully capture the frequency sweeping feature of RSAE, it is necessary to take into account the finite equilibrium evolution in simulation setup. On the other hand, a self-consistent, nonperturbative description of SAW-EP interactions is also crucially needed, since the non-perturbative effect of EPs plays a significant role in RSAE dynamics when the driven fluctuation deviates from instability threshold [21,43,44,45]. Here, the nonperturbative effect corresponds to the important contribution of EPs in determining mode structure and real frequency on both linear and nonlinear dynamics, such that SAW-EP resonance condition is best maintained and corresponding power transfer is maximized [46,47,3,48,49,50,51,52,53,1,44,45].…”

“…On the other hand, a self-consistent, nonperturbative description of SAW-EP interactions is also crucially needed, since the non-perturbative effect of EPs plays a significant role in RSAE dynamics when the driven fluctuation deviates from instability threshold [21,43,44,45]. Here, the nonperturbative effect corresponds to the important contribution of EPs in determining mode structure and real frequency on both linear and nonlinear dynamics, such that SAW-EP resonance condition is best maintained and corresponding power transfer is maximized [46,47,3,48,49,50,51,52,53,1,44,45]. Indeed, the deviation of RSAE spectra from MHD limit has been identified in experiments with high power input [54,55,56,9,57,32,37], suggesting the important role of non-perturbative EPs.…”