Numerical calculation of 3D free-boundary MHD equilibria*

An important aspect of the study of toroidal magnetic confinement devices is the numerical calculation of 3D MHD equilibria. This is done using codes that each make their own assumptions to arrive at their calculated equilibrium. This study aims to compare the same stellarator system using four different codes, VMEC, SPEC, SIESTA, and DESC, and investigate their similarities and differences. This will be done with both a theoretical configuration and an experimental reconstruction from the Compact Toroidal Hybrid (CTH). VMEC is a tokamak and stellarator Ideal MHD code which assumes nested toroidal flux surfaces (NTFS). SPEC is also used for both configurations, but it does not assume NTFS's, rather it assumes a stepped pressure profile to avoid singular currents. SIESTA similarly does not assume NTFS's, but it can switch between Ideal and Resistive MHD in its calculations. DESC is a python code that, like VMEC, assumes NTFS's, but rather than representing inverse coordinates R and Z on a grid in the radial (flux-surface label) coordinates, it uses a Fourier-Zernike basis set. Also a recent addition to SIESTA, where non-resonant magnetic islands can be observed without significant increase in computation time, will be shown. Quantitative and qualitative comparisons of the different theoretical and CTH equilibria will be presented.