Dynamics of ideal modes and subsequent ELM crashes in 3D tokamak geometry from external magnetic perturbations

Abstract

The impact of three-dimensional (3D) tokamak geometry from external magnetic perturbations on edge instabilities has been examined in high confinement mode plasmas with edge localised modes (ELMs) in ASDEX Upgrade. The 3D geometry has been probed using rigidly rotating MP fields. The measured distortions of the plasma boundary are compared to single-fluid ideal magnetohydrodynamic (MHD) equilibria using VMEC and MARS-F applying ideal and resistive MHD, whereas VMEC uses only ideal MHD. Both codes accurately reproduce the measured radial displacements of the edge density and temperature profiles in amplitude, toroidal phase and their dependence on the applied poloidal mode spectrum.The induced 3D geometry distorts the local magnetic shear, which locally reduces the stabilising effect from field-line bending at certain most unstable field lines. Around these field lines, we observe additional stable ideal MHD modes with clear ballooning structure in-between ELMs. After their immediate appearance, they saturate and then grow on timescales of the pedestal pressure recovery. The subsequent ELMs show strongly localised magnetic Plasma Physics and Controlled Fusion

Observation of accelerated beam ion population during edge localized modes in the ASDEX Upgrade tokamak

“…This observation is interpreted as the acceleration of beam ions in the presence of parallel electric fields arising during ELMs. Bursts in the electron cyclotron emission and soft x-ray diagnostics are also observed at the onset of these ELMs [7], similar to previous observations in MAST, which are indicative of electron acceleration [8], thus supporting the hypothesis of the presence of electric fields. Although previous investigations have reported ion acceleration in magnetically confined plasmas during internal reconnection events [9,10] and merging-compression during plasma start-up [11], this is the first experimental indication of ion acceleration during ELMs in a tokamak.…”

“…In addition to the measurement of fast-ion losses, spikes in soft X-ray emission [24] and bursts in electron cyclotron emission (ECE) [25] signals have also been observed at the onset of some of these ELMs [7]. This is shown in Fig.7, where (a) shows the timetrace of the divertor current and the FILD signal during an ELM.…”

Pedestal electron collisionality and toroidal rotation during ELM-crash suppression phase under n = 1 RMP in KSTAR

“…Such a contradiction may be resolved by a recent RMP study considering three-dimensional geometry, where a local distortion of RMP-induced magnetic shear can drive highly localized ballooning modes. 35,36 However, the localized destabilization is not sufficient to explain the bifurcated onset of the ELM-crash suppression phase entirely. In the KSTAR database, the I RMP conditions and the equilibrium profiles in the early phase of ELM-crash suppression are not significantly different from those of the mitigation phase right before transition.…”

Parametric decay instabilities near the second-harmonic upper hybrid resonance in fusion plasmas

“…As microwave bursts in connection with ELMs have been observed under a number of different experimental conditions in the past [6,20,21,[25][26][27][28][29][30][31][32][33], we provide a brief overview of these bursts and their connection, or lack thereof, to PDIs. Many of the earlier observations of microwave bursts during ELMs occurred without ECRH [33] and additionally had frequencies proportional to the magnetic field strength [29,33].…”

“…As microwave bursts in connection with ELMs have been observed under a number of different experimental conditions in the past [6,20,21,[25][26][27][28][29][30][31][32][33], we provide a brief overview of these bursts and their connection, or lack thereof, to PDIs. Many of the earlier observations of microwave bursts during ELMs occurred without ECRH [33] and additionally had frequencies proportional to the magnetic field strength [29,33]. This is a strong indication that they do not originate from PDIs, which require an ECRH beam to drive the instabilities and lead to emissions with frequencies depending on the ECRH frequency, but rather from ECE associated with fast electrons.…”