Modeling Magnetic Islands on the Compact Toroidal Hybrid with SIESTA

Nested magnetic flux surfaces typically result in improved performance in toroidal plasmas. Due to 3D magnetic fields in stellarators they are not guaranteed, and one must account for magnetic islands in the equilibrium. Several codes have been developed to study magnetic islands in plasma equilibria, such as SPEC, HINT and SIESTA. In this study we have used the SIESTA code to model magnetic island behavior. For topological changes to occur in SIESTA an initial, user defined, 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 as opposed to other codes where magnetic islands occur with less direct user involvement. To quantify the effect the perturbation has on the islands we have located and O- and X-points of magnetic islands and calculated their size through a proxy known as the Greene residue for different stellarator configurations with varying perturbation magnitudes. The perturbation size is observed to alter the radial location and size of the islands. These tools in conjunction with equilibrium reconstructions and other diagnostics will be used to study stellarator symmetry-breaking islands observed on CTH.