Optimization and coil development for a tabletop HTS pair plasma stellarator*

The EPOS (Electrons and Positrons in an Optimized Stellarator) device will be a high-temperature superconducting (HTS), tabletop-sized, quasi-axisymmetric stellarator designed to confine a low-density, low-temperature, 𝑒⁺𝑒^- pair plasma. Despite this exotic target, the magnetic configuration optimization is similar to that for fusion plasmas in many ways.

Due to the low availability of antimatter, EPOS will be small (~10-liter plasma volume) and will take advantage of precise quasisymmetry to ensure good particle confinement and reach plasma densities (𝑎/𝜆_𝐷 > 10). Typical stellarator coil tolerances become even more difficult at small size, but this can be improved with a 3D-printed multi-coil support shell and stochastic optimization. The ~2-T, steady-state magnetic field will be generated by non-insulated rare-earth barium copper oxide (ReBCO) coils. To enable this, we are designing, manufacturing, and testing a series of coils; these range from planar manufacturing demos to a full-size, full-current, non-planar coil cooled to 20K. Including these requirements has motivated the use of single-stage optimization with stochastic, finite-build coils to balance the small device size, construction tolerances, and HTS strain limits.