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Title: Evolution of Filament Structures during Edge-Localized Modes in the MAST Tokamak

Abstract

Edge-localized modes (ELMs) are repetitive instabilities that occur in the outer region of tokamak plasmas. This Letter provides new information on and the implications of the evolution of the filament structures observed during ELMs in the MAST tokamak. The filaments exist for the time over which particles are being released into the scrape off layer. They start off at the plasma edge rotating at the velocity of the pedestal, and then decelerate toroidally and accelerate radially outwards. As the filaments propagate radially they remain aligned with the local magnetic field line.

Authors:
; ; ; ;  [1];  [2];  [3];  [1];  [4];  [5];  [6]
  1. EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxon OX14 3DB (United Kingdom)
  2. Institut fuer Physik der Humboldt-Universitaet zu Berlin (Germany)
  3. (Germany)
  4. (Ireland)
  5. University of York, Heslington, York YO10 5DD (United Kingdom)
  6. Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, Garching (Germany)
Publication Date:
OSTI Identifier:
20775238
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Letters; Journal Volume: 96; Journal Issue: 18; Other Information: DOI: 10.1103/PhysRevLett.96.185001; (c) 2006 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; EDGE LOCALIZED MODES; EVOLUTION; FILAMENTS; MAGNETIC FIELDS; MAST TOKAMAK; PLASMA; PLASMA CONFINEMENT; PLASMA SCRAPE-OFF LAYER

Citation Formats

Kirk, A., Counsell, G., Dowling, J., Martin, R., Walsh, M., Koch, B., Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, Garching, Scannell, R., Department of Electrical and Electronic Engineering, University College Cork, Association EURATOM-DCU, Wilson, H. R., and Herrmann, A.. Evolution of Filament Structures during Edge-Localized Modes in the MAST Tokamak. United States: N. p., 2006. Web. doi:10.1103/PhysRevLett.96.185001.
Kirk, A., Counsell, G., Dowling, J., Martin, R., Walsh, M., Koch, B., Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, Garching, Scannell, R., Department of Electrical and Electronic Engineering, University College Cork, Association EURATOM-DCU, Wilson, H. R., & Herrmann, A.. Evolution of Filament Structures during Edge-Localized Modes in the MAST Tokamak. United States. doi:10.1103/PhysRevLett.96.185001.
Kirk, A., Counsell, G., Dowling, J., Martin, R., Walsh, M., Koch, B., Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, Garching, Scannell, R., Department of Electrical and Electronic Engineering, University College Cork, Association EURATOM-DCU, Wilson, H. R., and Herrmann, A.. Fri . "Evolution of Filament Structures during Edge-Localized Modes in the MAST Tokamak". United States. doi:10.1103/PhysRevLett.96.185001.
@article{osti_20775238,
title = {Evolution of Filament Structures during Edge-Localized Modes in the MAST Tokamak},
author = {Kirk, A. and Counsell, G. and Dowling, J. and Martin, R. and Walsh, M. and Koch, B. and Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, Garching and Scannell, R. and Department of Electrical and Electronic Engineering, University College Cork, Association EURATOM-DCU and Wilson, H. R. and Herrmann, A.},
abstractNote = {Edge-localized modes (ELMs) are repetitive instabilities that occur in the outer region of tokamak plasmas. This Letter provides new information on and the implications of the evolution of the filament structures observed during ELMs in the MAST tokamak. The filaments exist for the time over which particles are being released into the scrape off layer. They start off at the plasma edge rotating at the velocity of the pedestal, and then decelerate toroidally and accelerate radially outwards. As the filaments propagate radially they remain aligned with the local magnetic field line.},
doi = {10.1103/PhysRevLett.96.185001},
journal = {Physical Review Letters},
number = 18,
volume = 96,
place = {United States},
year = {Fri May 12 00:00:00 EDT 2006},
month = {Fri May 12 00:00:00 EDT 2006}
}
  • The plasma in tokamaks often exhibits a relaxation oscillation called the edge-localized mode (ELM), which is generally attributed to MHD instability driven by strong gradients at the plasma boundary. It is shown here that field-aligned currents flowing just outside the boundary may also play a role in the ELM process. The poloidal perturbation magnetic field during ELMs in the DIII-D tokamak calculated from measured currents can reproduce prominent observed features, including a narrow magnetic structure at the outboard midplane similar to filaments observed earlier in DIII-D and NSTX.
  • This article describes the poloidal plasma particle distribution of type-III edge localized modes (ELMs) in the Mega-Ampere spherical tokamak [R.-J. Akers et al., Phys. Plasmas 9, 3919 (2002)]. A fast imaging camera with 10 {mu}s exposure time is used to record the D{sub {alpha}} light coming from the entire poloidal cross section. Furthermore, three sets of probes, triggered at the same time, acquired at 1 MHz, and located at different poloidal, radial, and toroidal locations in the tokamak are used. ELMs are observed to affect the D{sub {alpha}} emission throughout the low-field scrape-off layer; on the high-field side, however, thismore » effect is found to be small. The results obtained by imaging agree with the pointwise measurements using Langmuir probes. The radial propagation is shown to occur at a speed of 250 m/s, whereas the toroidal convection from the top to the bottom of the plasma is shown to be consistent with a transport at the local sound speed. Strong correlation amplitudes are reported among the probes that are poloidally and toroidally separated by several meters. The study of the cross-correlation coefficients as a function of the frequency indicates that this correlation is caused by the low-frequency component of the signal and that the high-frequency part is not correlated. Consequently, the filamentary structures are interpreted as caused by the onset of turbulence during an ELM and do not constitute the ELM itself.« less
  • Direct measurements of the pedestal recovery during an edge-localized mode cycle provide evidence that quasi-coherent fluctuations (QCFs) play a role in the inter-ELM pedestal dynamics. Using fast Thomson scattering measurements, the pedestal density and temperature evolutions are probed on sub-millisecond time scales to show a fast recovery of the density gradient compared to the temperature gradient. The temperature gradient appears to provide a drive for the onset of quasi-coherent fluctuations (as measured with the magnetic probe and the density diagnostics) localized in the pedestal. The amplitude evolution of these QCFs tracks the temperature gradient evolution including its saturation. Such correlationmore » suggests that these QCFs play a key role in limiting the pedestal temperature gradient. The saturation of the QCFs coincides with the pressure gradient reaching the kinetic-ballooning mode (KBM) critical gradient as predicted by EPED1. Furthermore, linear microinstability analysis using GS2 indicates that the steep gradient is near the KBM threshold. Furthermore, the modeling and the observations together suggest that QCFs are consistent with dominant KBMs (although microtearing cannot be excluded as subdominant).« less
  • The plasma in tokamaks often exhibits a relaxation oscillation called the edge localized mode (ELM), which is generally attributed to MHD instability driven by strong gradients at the plasma boundary. It is shown here that field-aligned currents flowing just outside the boundary may also play a role in the ELM process. The poloidal perturbation magnetic field during ELMs in the DIII–D tokamak calculated from measured currents can reproduce prominent observed features, including a narrow magnetic structure at the outboard midplane similar to filaments observed earlier in DIII–D and NSTX.
  • The effect of gas desorption from the tokamak first wall on the pedestal recovery in the H-mode after an edge-localized-mode burst is considered. Results of FACE code simulations of hydrogen desorption from a beryllium wall are presented. It is found that the wall has a significant effect on plasma processes only at sufficiently low temperatures (of about 400 K), which agrees with qualitative estimates obtained earlier in the zero-dimensional approximation.