Enhancing stellarator accessibility through port size optimization

Magnetic fusion devices must be designed to enable fast and reliable maintenance. Intense neutron fluxes generated by the fusion reactions damage the plasma-facing components, necessitating their periodic replacement. The flexibility in stellarator coil geometry can be utilized to find configurations that allow for large radial or vertical access ports between the coils. In this study, we propose representing access ports as closed curves on the plasma boundary and deriving a set of objective functions and constraints to optimize stellarator coils for large ports. This novel approach to designing stellarators with enhanced accessibility is demonstrated by presenting a set of coil optimization results. We show that configurations exhibiting regions with large curvature, commonly found in quasi-helically symmetric and quasi-isodynamic configurations, often allow large radial access ports. Additionally, we show that adding a few carefully optimized windowpane coils to the usual modular coils of some configurations can generally lead to solutions with significantly larger access ports.