Speaker
Description
The APEX (A Positron Electron eXperiment) collaboration is developing a “tabletopsized”
stellarator, EPOS (Electrons and Positrons in an Optimized Stellarator). In this
work, we present ReBCO non-planar, non-insulated (NI) coils designed to confine pairplasmas
within a quasi-axisymmetric magnetic field of 2.0 T on axis. The stellarator’s
small size enhances its utility by reaching higher plasma densities with relatively low
amounts of available positrons. Both aspects enable long-lived matter-antimatter plasmas.
HTS tape was selected for its ability to achieve high field strengths with efficient cryogenic
operation. The design minimizes binormal, torsional strains on the HTS tapes while
simultaneously ensuring their structural integrity, and critical current retention under
operational loads. Additionally, trade-offs between field accuracy and coil shapes —
e.g. exclusion of concave sections, ensuring enough coil-to-coil spacing, and robustness
to manufacturing errors — were carefully managed during the conception process. This
balance was achieved through iterative stochastic single stage optimization to ensure that
the plasma shape/topology is compatible with buildable coils. The final configuration
was chosen after an initial selection process, result of a reduced parameter scan in the coil
currents and major radius. This method allowed to push further than initially expected
the accuracy of the magnetic field while maintaining manufacturability and mechanical
resilience.