Design and operation of APEX-LD: A compact levitated dipole for a positron-electron experiment

The objective of the APEX (A Positron-Electron eXperiment) project is to magnetically confine and study positron-electron 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 floating (F-)coil consists exclusively of a no-insulation rare-earth barium copper oxide 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-min preparation cycle reliably generates persistent currents of ∼60 kA-turns and an axial magnetic flux density of B0 ≈ 0.5 T. We demonstrate levitation times in excess of 3 h with a vertical stability of σz < 20 μm. 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.