Non-linear magnetic perturbations during edge-localized modes in TCV dominated by lownmode components

Abstract

Abstract. Edge localized modes (ELMs) are instabilities in the edge of tokamak plasmas in the high confinement regime (H-mode). Despite beneficial aspects of ELMs, in a future device the size of the energy loss per ELM must be controlled, in order to avoid intolerable divertor power flux densities. To proceed in understanding how the ELM size is determined and how ELM mitigation methods work it is necessary to characterize the non-linear evolution of ELMs.This publication presents a detailed analysis of the toroidal structure of dominant magnetic perturbations during type I ELMs in TCV. These signatures of the instability can be observed most intensely in close temporal vicinity to the onset of enhanced D α -radiation. In particular it is shown that dominant magnetic perturbations already have a rigid toroidal mode structure when they are detected with magnetic probes. This indicates that perturbations associated with this type of ELMs at TCV can not be observed in their linear phase. Furthermore it is demonstrated that the toroidal structure of dominant magnetic perturbations is most often dominated by the n = 1 component. This is in clear contrast to typical results of linear stability calculations, leading to the hypothesis that the dominant toroidal mode number from the linear to the non-linear phase has a transition from intermediate to low values. In general, the reported results show that non-linear coupling leads to a significant modification of the mode structure.

A 3-D MHD equilibrium description of nonlinearly saturated ideal external kink/peeling structures in tokamaks

“…Although the linear phase indicates that large n toroidal mode numbers are important, the nonlinear phase exhibits a dominance of n = 1 corrugations particularly when the edge J BS becomes large. This is consistent with observations of ELM behaviour in the TCV tokamak, which reports the dominance of n = 1 toroidal structures (Wenninger et al 2013), as well as numerical simulations with the JOREK code that shows n = 1 modes becoming increasingly important from the linear to the early nonlinear phases (Krebs et al 2013) and then completely dominant in the saturated nonlinear phase (Liu et al 2015). For small edge J BS and large edge p , larger n peeling/ballooning modes become relevant, but we are cannot fully resolve these with the spectrum of modes applied to converge 3-D equilibrium states with the VMEC code.…”

“…phase so that n 1 structures become dominant (Wenninger et al 2013) that are further buttressed with linear to nonlinear simulations undertaken with the JOREK code (Krebs et al 2013;Liu et al 2015). On the right-hand side of figure 6, we see that for the smaller J BS /I t = 0.307 edge bootstrap current case, the m/n = 6/4 Fourier component of B is dominant at the plasma boundary, with subdominant m/n = 7/4, 5/4 and 9/4 structures also appearing.…”

Nonlinear excitation of low-n harmonics in reduced magnetohydrodynamic simulations of edge-localized modes

“…During type-I ELMy discharges in TCV (Tokamak a configuration variable), the toroidal mode structure of the magnetic perturbations has been found to be often dominated by low mode numbers, in particular by the n ¼ 1 component. 11 The magnetic diagnostics in ASDEX Upgrade are not suitable for the detection of low-n harmonics 12 such that it is unclear at present if this phenomenon is also found here.…”

Recent progress in the quantitative validation of JOREK simulations of ELMs in JET

“…Note: the discontinuities observed at 0. 40 peeling-ballooning modes in time. This second MHD event is clearly closer to a type-I ELM than the quasilinear events used in section 3.2.…”