Design and operation of APEX-LD: a compact levitated dipole for the confinement of electron-positron pair plasmas

Abstract: The objective of the APEX (A Positron-Electron eXperiment) project is to magnetically confine and study electron--positron pair plasmas. For this purpose, a levitated dipole trap (APEX-LD) has been constructed. The magnetically levitated, compact (7.5-cm radius), closed-loop, high-temperature superconducting (HTS) floating (F-)coil consists exclusively of a No-Insulation (NI) Rare-earth Barium Copper Oxide (ReBCO) winding pack, solder-potted in a gold-plated-copper case. A resealable in-vacuum cryostat facilitates cooling (via helium gas) and inductive charging of the F-coil. The 70-minute preparation cycle reliably generates persistent currents of ~60 kA-turns and an axial magnetic flux density of B_0 ~ 0.5 T. We demonstrate levitation times in excess of three hours with a vertical stability of sigma_z < 20 um. Despite being subjected to routine quenches (and occasional mechanical shocks), the F-coil has proven remarkably robust. We present the results of preliminary experiments with electrons, and outline the next steps for injecting positron bunches into the device.