Direct Observation of Nonlinear Coupling between Pedestal Modes Leading to the Onset of Edge Localized Modes

Abstract

Prior to eruptive events such as edge localized modes (ELMs), quasi-coherent fluctuations, referred to as pedestal modes, are observed in the edge of fusion devices. We report on the investigations of nonlinear coupling between these modes during quasi-stationary inter-ELM phases leading to the ELM onset. Three dominant modes, with density and magnetic signatures, are identified as the key players in the triggering mechanism of certain class of ELMs. We demonstrate that one of these mode is generated by the two others through three waves interactions. The generated mode is radially shifted relative to the other two modes towards the last-closed flux surface as the ELM event approaches. Our results suggest that nonlinear coupling of pedestal modes, associated with radial distortions pushing out of the pedestal, is a possible mechanism for the triggering low frequency ELMs relevant for future fusion devices..

Time-dependent experimental identification of inter-ELM microtearing modes in the tokamak edge on DIII-D

“…The later-time evolution of the MTM amplitude is observable in this case due to the motion of the q = 16/13 rational surface across ρ max(ω e * ) during the pedestal recovery. Additional variation in the mode amplitude as a function of time is likely the result of nonlinear coupling between various pedestal modes [65] and will be the subject of future work.…”

Phase-mixing v.s. phase synchronization in the dynamics of flow-shear induced edge transport barrier

“…Such edge transport barriers are not stationary, but relax quasi-periodically. Several models have been proposed to account for such relaxations [3,4,5,6,7,8]. In addition, nonlinear resistive-MHD simulations observed similar nonlinear oscillations [9].…”

Perturbative Determination of Plasma Microinstabilities in Tokamaks

“…However, as the q surface moves inwards, the drive, amplitude and frequency drop until the n = 3 mode dominates. Power is transferred to more unstable rational surfaces during this transition through non-linear coupling between various pedestal modes 7 . This manifests in the magnetics measurements as a disappearance of the n = 4 signature coincident with a peak in the n = 3 amplitude around ∼ 60 ms after the jogging event (see figure 4(a)).…”

“…Second, various metastable microinstabilities induce transport across the pedestal despite the high levels of turbulent shear, controlling the evolution of the pedestal structure 6 . If left unmitigated, non-linear interactions between these microinstabilities periodically spark global explosive events 7,8 called edgelocalized modes (ELMs) 9 which can melt and erode the machine wall 10 . As such, understanding the details of these microinstabilities is not only for crucial for the optimization of fusion parameters but also for successful plasma control 11,12 .…”