EPOS: Stellarator design for pair plasmas*

The EPOS device (Electrons and Positrons in an Optimized Stellarator) is being designed to confine a low-density, low-temperature pair plasma. This exotic target requires different considerations compared to the typical fusion goal. Due to the low availability of antimatter, the plasma density is limited by the total number of positrons. In order to achieve plasma densities such that a Debye length is 1/10th of the minor radius, EPOS must be as small as reasonably possible. Typical stellarator coil tolerances become unmanageable at small size but can be improved with stochastic optimization. To avoid positron loss, plasma temperatures must be kept below 1-5 eV, motivating a 2T magnetic field for effective radiative cooling. This large, steady-state field requires superconducting magnets. Coil design for EPOS includes strain optimization for high-temperature superconducting (HTS) tape, validated with a series of test coils. Finally, positron fueling requires charged particle injection across the magnetic field. The positron beamline will connect to EPOS's stray magnetic field, and electrodes will generate ExB drifts into the confinement region. EPOS requires expanded coils to produce this "weave lane" and allow space for the electrode structures. This contribution discusses the integration and progress on the above tasks towards a HTS tabletop-sized quasi-axisymmetric stellarator.