Effect of toroidal Alfvén Eigenmode nonlinearity on edge localized mode in HL-2A NBI heated H-mode plasmas

Abstract The present paper reports the observation of shear Alfvénic wave fluctuations prior to the edge localized mode (ELM) crash in HL-2A NBI H-mode plasmas. These Alfvénic fluctuations are principally attributed to the contributions of unstable toroidal Alfvén eigenmodes (TAEs). Strong nonlinear mode couplings (NMCs) are identified between the dominant TAEs with n = 3 − 4 and low frequency magnetohydrodynamic (MHD) mode (kink or fishbone) with n = 1 before the ELM crash. TAEs exhibit pitch-fork phenomena, growing explosively into a bursty instability. These explosive events have two distinct fine structures, i.e. multi-mode and pitch-fork. The two kinds of structures can coexist, but the intense NMC leads to weakening or disappearance of the pitch-fork structure. The modes then blow-up instantaneously. Such nonlinear events indicate that the NMC may redistribute energetic particles (EPs), destroy hole-clump pairs in phase-space and potentially induce nonlinear three-wave mixing. Consequently, we find and identify the first evidence that the TAE nonlinear dynamics triggers the onset of ELMs and pedestal collapse within several hundred Alfvén time and that this process correlates with the irregular-shaped ELM featured by multiple peaks in D α signals. The experimental results manifest that the nonlinear dynamics of Alfvén fluctuations may furnish an additional perturbation with the EP pressure on the pedestal. This pressure perturbation can move closer to the MHD limit so that the ELM trigger is determined by the NMC and edge stability. These findings can advance our understanding of the triggering mechanism of an ELM event, and further highlight the significance of incorporating kinetic effects of EPs in the present models of ELM physics, such as the famous code BOUT++.