Preliminary design of charged particle injection for EPOS (Electrons and Positrons in an Optimized Stellarator)

Pair plasmas, with both species of identical mass, have attracted theoretical and computational attention for years as a fundamental plasma scenario. As part of the APEX (A Positron Electron eXperiment) collaboration, the EPOS device, a quasi-symmetric tabletop-sized stellarator, is being designed to confine such a pair plasma. Numerical optimization provides a number of two-field-period quasi-axisymmetric candidate equilibria, and coil sets are evaluated for the possibility of positron injection. Experiments at APEX have previously demonstrated lossless transport of positrons across the magnetic field by ExB drift injection. For EPOS, drift injection requires stray magnetic field connecting to the positron beam line, and sufficient space for tailored electrostatic potentials. At the low energies, high fields, low rotational transform, and extremely low collisionality in EPOS, simulations of individual particle trajectories can address questions of injection and confinement. These simulations are underway, and are starting to address questions of the required rotational transform and degree of quasi-symmetry.