Description
Electron-positron “pair plasmas” – comprising two species with equal mass but opposite charge – are an experimental “grand challenge” in fundamental plasma science. Trapping these unusual matter-antimatter hybrids in the laboratory will open up opportunities to benchmark theory and simulation predictions for plasma behavior in the limit of mass symmetry. The APEX (A Positron Electron eXperiment) Collaboration aims to do this in two complementary magnetic geometries: in our levitated dipole trap (APEX-LD) and in a small, quasiaxisymmetric stellarator.
Toward this end, the EPOS (Electrons and Positrons in an Optimized Stellarator) project has adopted and adapted recent developments in stellarator optimization to design a tabletop-sized device. It was found to indeed be possible to simultaneously achieve key criteria for the coil "buildability" and magnetic field quality of the EPOS design. The leading EPOS configuration has a major radius of 19 cm, produced by 22 coils, with feasible engineering tolerances of ~1 mm. This includes the two larger coils that facilitate e+ beam guiding to the device, into which they can be injected via switched, tailored potentials that generate the necessary ExB drifts across flux surfaces. Alongside the optimization, engineering studies have demonstrated key milestones, such as experimental verification of winding angle optimization and feasibility of non-planar, coldhead-cooled, 10-cm-scale coils. Construction of the device is planned to commence after performance of a full-field “model coil” has been demonstrated.
At the same time, the project has contributed back to the stellarator optimization community – in particular, via the the open-source SIMSOPT framework – on relevant topics of common interest. Among these are metrics for material limits of the HTS tapes (which pose a challenge for non-planar coils), as well as a combined optimization using single-stage (in which equilibrium and coils are optimized together) and stochastic (in which a "cloud" of perturbed configurations is used in the optimization process) techniques. Despite being much smaller and targeting different plasmas, EPOS has found significant overlap with fusion stellarator optimization and engineering questions.