Three dimensional boundary displacement due to stable ideal kink modes excited by external n = 2 magnetic perturbations

Abstract

Abstract.In low-collisionality (ν ) scenarios exhibiting mitigation of edge localized modes (ELMs), stable ideal kink modes at the edge are excited by externally applied magnetic perturbation (MP)-fields. At ASDEX Upgrade these modes can cause three-dimensional (3D) boundary displacements up to the centimeter range. These displacements have been measured using toroidally localized high resolution diagnostics and rigidly rotating n = 2 MP-fields with various applied poloidal mode spectra. These measurements are compared to non-linear 3D ideal magnetohydrodynamics (MHD) equilibria calculated by VMEC. Comprehensive comparisons have been conducted, which consider for instance plasma movements due to the position control system, attenuation due to internal conductors and changes in the edge pressure profiles.VMEC accurately reproduces the amplitude of the displacement and its dependencies on the applied poloidal mode spectra. Quantitative agreement is found around the low field side (LFS) midplane. The response at the plasma top is qualitatively compared. The measured and predicted displacements at the plasma top maximize when the applied spectra is optimized for ELM-mitigation. The predictions from the vacuum modeling generally fails to describe the displacement at the LFS midplane as well as at the plasma top. When the applied mode spectra is set to maximize the displacement, VMEC and the measurements clearly surpass the ‡ matthias.willensdorfer@ipp.mpg.de arXiv:1702.03177v2 [physics.plasm-ph] 24 Jul 2017 3D boundary displacement due to stable ideal kink modes excited by external n=2 MPs 2 predictions from the vacuum modeling by a factor of four. Minor disagreements between VMEC and the measurements are discussed. This study underlines the importance of the stable ideal kink modes at the edge for the 3D boundary displacement in scenarios relevant for ELM-mitigation.

Characterisation of the fast-ion edge resonant transport layer induced by 3D perturbative fields in the ASDEX Upgrade tokamak through full orbit simulations

“…Several experiments have been carried out in AUG to show the impact of MPs poloidal mode spectra on ELM mitigation, in particular, shot #33143 with high normalised β N = 2.4, low collisionality ν * e = 0.2 and q 95 = 3.8. A magnetic perturbation with a toroidal mode number n = 2 was generated by the ELM control coils installed in AUG, where currents flowing through the lower set of coils were kept constant in time while the upper currents were slowly rotating at 2 Hz, introducing a phase shift (∆ϕ U L = φ U −φ L ) that resulted in a continuous scan in the poloidal mode spectra [18]. The source of energetic particles was introduced by means of 5 MW neutral beam injection (NBI) distributed on 2 sources with 2.5 MW from 2 NBI boxes, one at 60 kV (NBI#3) the other at 93 kV (NBI#8).…”

Field-Line Localized Destabilization of Ballooning Modes in Three-Dimensional Tokamaks

“…Experimental setup -The presented example is from a series of ASDEX Upgrade experiments to measure the displacement by combining rigidly rotating MP-fields with the toroidal mode number n = 2 and toroidally localized diagnostics [17]. In this work, we primarily use electron cyclotron emission (ECE) diagnostics, which deliver the electron temperature (T e ) around the low field side (LFS) midplane from the measured radiation temperature (T rad ) [18].…”

“…To model ξ r , we employ the 3D ideal MHD equilibrium code VMEC [21]. If no strong resistive MHD modes are active, VMEC is able to predict ξ r at the edge [10,17]. For details about its setup for ASDEX Upgrade discharges and the accuracy of VMEC in the presence of rational surfaces, we refer to Ref.…”

“…For details about its setup for ASDEX Upgrade discharges and the accuracy of VMEC in the presence of rational surfaces, we refer to Ref. [17,18,22] and Ref. [23,24], respectively.…”

“…Their appearance seems to be correlated with a sufficiently large ξ r around LFS midplane and normalized beta (β N ). Although these experiments have different applied poloidal mode spectra set by ∆ϕ UL [17], the modes always occur around the same field-lines in the 3D geometry, which experience the zero-crossing ξ r from positive to negative values when the MP-field is rotated in positive toroidal direction. Moreover, the 2.0 T experiments in which the MP-field amplitude, normalized to the background magnetic field, is larger, show coupling of the modes to the external MP-field resulting in a vanishing phase velocity relative to the n = 2 motion.…”