Design, construction, and commissioning of a levitated dipole trap for electron-positron pair plasma studies*

Magnetic dipole traps have demonstrated good confinement properties for both non-neutral and quasi-neutral plasmas, making this a highly suitable type of trap for the creation and study of low-temperature, long-lived electron-positron pair plasmas.  To generate such a plasma, the APEX (A Positron-Electron eXperiment) Collaboration is planning to inject positrons (supplied by the reactor-based beam NEPOMUC, then collected into pulses in a buffer-gas trap) into a dipole magnetic field, which is previously loaded with a comparable population of electrons.

 

The "floating coil" (F coil) is a 15-cm-diameter high-temperature superconducting (HTS) closed coil, which when charged and levitated forms our dipole trap. The F coil is cooled in a small sub-chamber within the vacuum vessel which is transiently pressurized with helium, providing thermal contact with cryogenic surfaces (~20 K).  A second HTS "charging coil" (C coil, I = 151 kAt), which is integrated into the walls of this sub-chamber, inductively charges the F coil. The geometry of the sub-chamber aligns the F & C coils, enabling efficient current induction of I = 56 kAt in the F coil, producing a magnetic field strength of B_axis= 0.5 T.

 

By levitating the charged F coil, we prevent magnetic field lines from intersecting material surfaces (e.g. mechanical supports). A FPGA-based PID-feedback levitation controller stabilises the vertical displacement by varying the current in a secondary “lifting coil” (L coil) based off the signal from three laser rangefinders. The current levitation record is 3.5 hrs, limited by the maximum L coil current. The future addition of an actively cooled thermal radiation shield surrounding the trapping region will slow the resistive decay due to thermal warming, therefore increasing levitation time.

 

Simulations in the single-particle regime have shown that a technique for injecting positrons across magnetic flux surfaces (by means of strategically applied ExB drifts) that was previously demonstrated in a prototype trap based on a supported permanent magnet is expected to be transferrable to the higher fields and symmetric geometry of the levitated dipole trap.  The commissioning of the levitated dipole trap is underway, with initial electron injection experiments anticipated to take place this summer (2023).