Magnetic Diagnostics for Numerical MHD Equilibria

The development of three-dimensional MHD equilibria is aided by the development of numerical diagnostics that analyze magnetic quantities given an arbitrary representation of the magnetic field. The VMEC code [S. P. Hirshman and H. K. Meier, Phys. Fluids 28, 1387 (1985)] and the DESC code [D. W. Dudt and E. Kolemen, Phys. Plasmas 27, 102513 (2020)] both assume nested toroidal flux surfaces and directly calculate the Fourier harmonics of the geometry of the flux surfaces. The SIESTA code [S. P. Hirshman, et al., Phys. Plasmas 18, 062504 (2011)] and the SPEC code [S. R. Hudson, et al., Phys. Plasmas 19, 112502 (2012)] both allow for magnetic islands to develop in the equilibrium and calculate the vector potential. In this work, a set of magnetic diagnostics is developed and utilized that calculates the location of the magnetic axis, flux surfaces, rotational transform, magnetic island width (if any are present) from the Fourier components of either the magnetic field or the vector potential. These tools will be applied to a test case for comparison. Comparison of the diagnostic results are presented for both vacuum and finite-pressure equilibria.