Nested magnetic flux surfaces typically result in improved performance in toroidal plasmas. Due to the inherent 3D magnetic fields in stellarators, nested flux surfaces are not guaranteed in an equilibrium and this allows for magnetic islands and field-line chaos to be present. In this study, the Scalable Iterative Equilibrium Solver for Toroidal Applications (SIESTA) code is used to model magnetic island behavior in a magnetically confined plasma equilibrium. For topological changes to occur in SIESTA, a user-inputted magnetic field perturbation is applied to the system in tandem with an artificial resistivity. These arbitrarily chosen values lead to uncertainty in the accuracy of the islands present in the calculated equilibrium. To quantify the perturbation's effect on the islands, we have implemented an algorithm to locate O- and X-points of magnetic islands and used the Cary–Hanson method to calculate their size for two different shot reconstructions of the compact toroidal hybrid with varying perturbation magnitudes. Increasing the perturbation magnitude is observed to enlarge the magnetic islands, as well as shift their radial location in the plasma, with the island center moving inwards. A relationship between the resonance location relative to the plasma boundary and the island size was also observed and further investigated. Furthermore, we describe a convenient method for interpolating the magnetic field while keeping its divergence-free nature.
Saudeau et al. (2026) studied this question.