Abstract Details
Abstracts
Author: Jacob M Halpern
Requested Type: Poster
Submitted: 2025-03-12 07:11:34
Co-authors: N. C. Logan, E. J. Paul, C. Paz-Soldan
Contact Info:
Columbia University
100 Morningside Dr, Apt 2L
New York, New York 10027
United States
Abstract Text:
We use self-consistent 3D equilibria generated by VMEC to determine the correct reference frame when calculating error fields in perturbed equilibrium codes subject to n=1 asymmetries. Error fields ten thousand times weaker than the axisymmetric field in a tokamak reduce plasma rotation through neoclassical torque and create magnetic islands which can lock to the plasma, ultimately causing a disruption. Perturbed equilibrium codes are typically used to model the plasma response to error fields and set tolerances on coil installation. These perturbative codes must assume a reference axis for axisymmetry, which can have significant impacts on the predicted plasma response. In this study, we analyze cases of radial/vertical shifts of the SPARC tokamak coil set, as well as the case of a tilted centerpost in the NSTX-U spherical tokamak. First, we use the resonant response in VMEC to reiterate the importance of the relative shift of the coils, which is independent of the choice of reference frame, and demonstrate that this response scales linearly with applied normal fields. We then use a combination of the magnetic axis position and normal displacement of magnetic field lines to determine the correct reference frame. We show that this correct frame is dominantly set by radial toroidal field coil displacements. Furthermore, we discuss a method which can be used to identify the limits of the linear theory of perturbed equilibria in terms of the magnitude of coil asymmetry when in the correct reference frame. This study enables engineers to confidently use 3D perturbative models for determining assembly tolerances and provides insight into the correct application of the theory.
Characterization: 1.0
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