Omnigenous umbilic stellarators

Abstract: To better understand the dependence of the magnetic field structure in the plasma edge due to plasma boundary shape, we define and develop umbilic stellarators. These equilibria are characterized by a single continuous high-curvature edge on the plasma boundary that goes around multiple times toroidally before meeting itself. We develop a technique that allows us to simultaneously optimize the plasma boundary along with a curve lying on the boundary on which we impose a high curvature while imposing omnigenity -- a property of the magnetic field that ensures trapped particle confinement throughout the plasma volume. After generating omnigenous umbilic stellarators, we design coil sets for some of these equilibria and calculate and understand the field line structure in the edge. Finally, we propose an experiment to modify an existing tokamak to a stellarator using this technique and explore a potential way to convert a limited tokamak into a diverted stellarator.