Design of an arrangement of cubic magnets with discrete polarizations for a quasi-axisymmetric stellarator experiment*

The usage of permanent magnet arrays to shape the confining magnetic fields of stellarators has been a topic of recent interest due to the potential of permanent magnets to reduce or eliminate the need for non-planar coils. As a proof-of-concept for this idea, we have developed a design for an arrangement of cubic permanent magnets that works in tandem with a set of planar toroidal-field coils to confine a quasi-axisymmetric plasma with a field on axis of 0.5 T. All of the magnets in the design are constrained to have identical geometry and one of three polarization types in order to reduce fabrication costs while still producing sufficient field accuracy. Their spatial layout was developed in tandem with a support structure and assembly scheme. We present some of the key considerations leading to the design, including the procedure for choosing the optimal polarization of each magnet from a discrete set, the choice of polarization types to be used, and the partitioning of the magnets into wedge-like modules that enable a radial insertion concept.