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Tokamak Plasma High Field Side Response to an n = 3 Magnetic Perturbation: A Comparison of 3D Equilibrium Solutions from Seven Different Codes



Allan Reiman, Antoine Cerfon, Geoffrey B. McFadden


In comparing equilibrium solutions for a DIII-D shot that is amenable to analysis by both stellarator and tokamak 3D equilibrium codes, a significant disagreement has been seen between solutions of the VMEC stellarator equilibrium code and solutions of tokamak perturbative 3D equilibrium codes. The source of that disagreement has been investigated, and that investigation has led to new insights into the domain of validity of the different equilibrium calculations, and to a finding that the manner in which localized screening currents at low order rational surfaces are handled can affect global properties of the equilibrium solution. The perturbative treatment has been found to break down at surprisingly small perturbation amplitudes due to overlap of the calculated perturbed flux surfaces, and that treatment is not valid in the pedestal region of the DIII-D shot studied. The perturbative treatment is valid, however, further into the interior of the plasma, and flux surface overlap does not account for the disagreement investigated here. Calculated equilibrium solutions for simple model cases and comparison of the 3D equilibrium solutions with those of other codes indicate that the disagreement arises from a difference in handling of localized currents at low order rational surfaces, with such currents being absent in VMEC and present in the perturbative codes. The significant differences seen in the global equilibrium solutions associated with the presence or absence of delta function screening currents at rational surfaces suggests that it may be possible to extract information about localized currents from appropriate measurements of global equilibrium plasma properties. That would require improved diagnostic capability on the high field side of the tokamak plasma, a region difficult to access with diagnostics.
Nuclear Fusion


tokomak, magnetohydrodynamics, plasma physics, DIII-D shot, equilibrium calculation, magnetic surfaces


Reiman, A. , Cerfon, A. and McFadden, G. (2015), Tokamak Plasma High Field Side Response to an n = 3 Magnetic Perturbation: A Comparison of 3D Equilibrium Solutions from Seven Different Codes, Nuclear Fusion, [online],, (Accessed July 13, 2024)


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Created May 14, 2015, Updated October 12, 2021