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

The mission of the APEX (A Positron-Electron eXperiment) collaboration is to magnetically confine and study electron-positron pair plasmas in either a compact levitated dipole (APEX-LD), or an optimized stellarator (EPOS, under construction). Implementing APEX-LD posed several engineering challenges, including, (1) the need for the compact (R=7.5 cm) superconducting "floating coil" (F-coil) to repeatedly make and break thermal contact with cryogenically cooled components in a vacuum environment, (2) excitation of the persistent current in the F-coil, (3) long-duration, feedback-stabilized levitation of the F-coil, and (4) a demand for robustness to repeated quenches and possible mechanical shocks. This talk presents solutions to these challenges and the operational highlights, including, efficient persistent current induction to ~0.5 T on-axis, levitation times in excess of three hours with a stability of σ_z=18 µm, and "gentle" quenching of the fully-soldered, no-insulation (NI) rare-earth barium copper oxide (ReBCO) high-temperature superconducting (HTS) coil. Finally, we describe results from first experiments (i.e., visualization of electron injection) and next steps for future injection of cold, dense pulses of positrons.