A compact high-T_c superconducting levitated dipole experiment for creation and study of positron-electron plasma

We present design and construction progress for a compact high-T_c superconducting (HTS) levitated dipole experiment to confine and study a magnetized positron-electron plasma. The confinement volume (0.01 m³) will be loaded with >10¹⁰ positrons (and electrons) to produce a low-temperature (~ 1 eV) pair plasma with a Debye length ~1 cm.  The closed floating coil (15 cm diam.) has 150 turns of HTS tape (GdBaCuO, T_c = 92 K) each with >400 A at the design magnetic field (< 2 T at the coil).  The coil is inductively charged using a (164 kA-t) HTS open-lead coil that is fixed to a coldhead and incorporates a sealable/openable chamber into which He gas (~ 1 Torr) can be introduced to control the cooling of the floating coil.  Tests of the cooling and inductive charging strategy verified the feasibility of the technique.  The feedback levitation system required to stably levitate the floating coil has been tested.  Levitation times of order an hour are anticipated with a cooled (80 K) copper radiation shield. Construction of all major components of the final system is underway; cooling, charging, and levitation tests are anticipated in 2021 with electron plasma and positron injection experiments starting in 2022.  Experiments performed in a prototype dipole, as well as simulations of the final setup, proved the feasibility of using an ExB drift technique to inject positrons.  Efficient positron injection into an electron plasma (n ~ 4x10¹² m^-3) was demonstrated.