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Title: Validation of conducting wall models using magnetic measurements

Abstract

The impact of conducting wall eddy currents on perturbed magnetic field measurements is a key issue for understanding the measurement and control of long-wavelength MHD stability in tokamak devices. As plasma response models have growth in sophistication, the need to understand and resolve small changes in these measurements has become more important, motivating increased fidelity in simulations of externally applied fields and the wall eddy current response. In this manuscript, we describe thorough validation studies of the wall models in the MARS-F and VALEN stability codes, using coil–sensor vacuum coupling measurements from the DIII-D tokamak. The valen formulation treats conducting structures with arbitrary threedimensional geometries, while mars-f uses an axisymmetric wall model and a spectral decomposition of the problem geometry with a fixed toroidal harmonic n. The vacuum coupling measurements have a strong sensitivity to wall eddy currents induced by timechanging coil currents, owing to the close proximities of both the sensors and coils to the wall. Measurements from individual coil and sensor channels are directly compared with valen predictions. It is found that straightforward improvements to the valen model, such as refining the wall mesh and simulating the vertical extent of the DIII-D poloidal field sensors, lead to goodmore » agreement with the experimental measurements. In addition, couplings to multi-coil, n = 1 toroidal mode perturbations are calculated from the measurements and compared with predictions from both codes. Lastly, the toroidal mode comparisons favor the fully three-dimensional simulation approach, likely because this approach naturally treats n > 1 sidebands generated by the coils and wall eddy currents, as well as the n = 1 fundamental.« less

Authors:
 [1];  [1];  [1];  [2];  [1];  [2];  [2]
  1. Columbia Univ., New York, NY (United States)
  2. General Atomics, San Diego, CA (United States)
Publication Date:
Research Org.:
Columbia Univ., New York, NY (United States); General Atomics, San Diego, CA (United States)
Sponsoring Org.:
USDOE Office of Nuclear Energy (NE)
OSTI Identifier:
1354790
Grant/Contract Number:
FC02-04ER54698; FG02-95ER54309; FG02-04ER54761
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 56; Journal Issue: 10; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; applied classical electromagnetism; electrostatics; magnetohydrodynamics; tokamaks; electric and magnetic measurements

Citation Formats

Hanson, Jeremy M., Bialek, James M., Turco, Francesca, King, J., Navratil, Gerald A., Strait, Edward J., and Turnbull, Alan. Validation of conducting wall models using magnetic measurements. United States: N. p., 2016. Web. doi:10.1088/0029-5515/56/10/106022.
Hanson, Jeremy M., Bialek, James M., Turco, Francesca, King, J., Navratil, Gerald A., Strait, Edward J., & Turnbull, Alan. Validation of conducting wall models using magnetic measurements. United States. doi:10.1088/0029-5515/56/10/106022.
Hanson, Jeremy M., Bialek, James M., Turco, Francesca, King, J., Navratil, Gerald A., Strait, Edward J., and Turnbull, Alan. 2016. "Validation of conducting wall models using magnetic measurements". United States. doi:10.1088/0029-5515/56/10/106022. https://www.osti.gov/servlets/purl/1354790.
@article{osti_1354790,
title = {Validation of conducting wall models using magnetic measurements},
author = {Hanson, Jeremy M. and Bialek, James M. and Turco, Francesca and King, J. and Navratil, Gerald A. and Strait, Edward J. and Turnbull, Alan},
abstractNote = {The impact of conducting wall eddy currents on perturbed magnetic field measurements is a key issue for understanding the measurement and control of long-wavelength MHD stability in tokamak devices. As plasma response models have growth in sophistication, the need to understand and resolve small changes in these measurements has become more important, motivating increased fidelity in simulations of externally applied fields and the wall eddy current response. In this manuscript, we describe thorough validation studies of the wall models in the MARS-F and VALEN stability codes, using coil–sensor vacuum coupling measurements from the DIII-D tokamak. The valen formulation treats conducting structures with arbitrary threedimensional geometries, while mars-f uses an axisymmetric wall model and a spectral decomposition of the problem geometry with a fixed toroidal harmonic n. The vacuum coupling measurements have a strong sensitivity to wall eddy currents induced by timechanging coil currents, owing to the close proximities of both the sensors and coils to the wall. Measurements from individual coil and sensor channels are directly compared with valen predictions. It is found that straightforward improvements to the valen model, such as refining the wall mesh and simulating the vertical extent of the DIII-D poloidal field sensors, lead to good agreement with the experimental measurements. In addition, couplings to multi-coil, n = 1 toroidal mode perturbations are calculated from the measurements and compared with predictions from both codes. Lastly, the toroidal mode comparisons favor the fully three-dimensional simulation approach, likely because this approach naturally treats n > 1 sidebands generated by the coils and wall eddy currents, as well as the n = 1 fundamental.},
doi = {10.1088/0029-5515/56/10/106022},
journal = {Nuclear Fusion},
number = 10,
volume = 56,
place = {United States},
year = 2016,
month = 8
}

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  • A method for validating conducting wall models used in magnetohydrodynamic (MHD) stability and control simulations is described. Measurements of frequency-dependent vacuum couplings between magnetic sensors and nonaxisymmetric coils in the DIII-D tokamak [J. L. Luxon, et al., Fusion Science and Technology 48 (2005) 807] are compared with predictions from the valen and marsf codes. The valen formulation treats conducting structures with arbitrary threedimensional geometries, while mars-f uses an axisymmetric wall model and a spectral decomposition of the problem geometry with a fixed toroidal harmonic n. The vacuum coupling measurements have a strong sensitivity to wall eddy currents induced by time-changingmore » coil currents, owing to the close proximities of both the sensors and coils to the wall. Measurements from individual coil and sensor channels are directly compared with valen predictions. It is found that straightforward improvements to the valen model, such as refining the wall mesh and simulating the vertical extent of the DIII-D poloidal field sensors, lead to good agreement with the experimental measurements. In addition, couplings to multi-coil, n = 1 toroidal mode perturbations are calculated from the measurements and compared with predictions from both codes. The toroidal mode comparisons favor the fully three-dimensional simulation approach, likely because this approach naturally treats n > 1 sidebands generated by the coils and wall eddy currents, as well as the n = 1 fundamental.« less
  • Plasma confinement devices frequently employ a conducting wall to confine a pulsed magnetic field. Holes in the wall produce perturbations of the uniform field which can affect the plasma equilibrium and field topology. The magnetic field perturbation around such a hole has been calculated using the quasistatic approximation for various cases by analytic and numerical methods, and has been measured for one case experimentally. Simple analytic approximations are presented. The perturbations near a conducting interface are found to be of a much longer range than would be true for fields of the same symmetry in free space.