Modeling symmetry breaking islands in the Compact Toroidal Hybrid (CTH) using SIESTA

The Compact Toroidal Hybrid (CTH) is a five field-period stellarator tokamak hybrid device. The addition of plasma current enables the device to access regimes with higher rotational transform than a pure stellarator device can achieve. Consequently, several low-order resonances can be introduced which break stellarator symmetry. Experimental measurements indicate the presence of rotating magnetic islands. Since the presence of plasma current significantly alters the shape of magnetic surfaces, these islands cannot be effectively modeled using vacuum fields. The Scalable Iterative Equilibrium Solver for Toroidal Applications (SIESTA), a tool in the Stellarator-Tools code suite, is a fast equilibrium solver that allows for non-nested magnetic topologies. SIESTA starts from a VMEC equilibrium to provide an initial guess for the fields and to define a background computational grid. A helical perturbation is applied at a resonance to break the nested VMEC topology and open islands. Recent work has added sparse Fourier mode spectra capability which enables rapid and reliable solutions for low order islands which break field periodicity. In this work, we present a new enhancement to control the island phase necessary to enable island equilibrium reconstruction in V3FIT. Adding a phase term to the Fourier modes breaks symmetric equilibrium into symmetric and asymmetric contributions. We will show symmetry breaking equilibria in the nominally stellarator symmetric CTH device.