Pure electron plasmas in a compact levitated dipole trap

APEX-LD (A Positron Electron eXperiment - Levitated Dipole) was recently constructed to confine electron-positron pair plasmas. The device employs a ReBCO high-temperature superconducting coil that is energized with a persistent current and magnetically levitated for up to 3 hours using active feedback stabilization. The floating coil is relatively compact (R = 7.5 cm) to minimize the number of positrons needed to achieve a pair plasma within the uninterrupted toroidal volume that surrounds it. The magnetic field of APEX-LD is similar to the dipole field that gives rise to the magnetosphere. Both exhibit large variations in magnetic field strength along the converging poloidal field lines. As in the magnetosphere, charged particles can be mirror-trapped between the poles; however, the levitated current ring also permits particles to pass through its interior and there is no loss cone. In principle, the device can magnetically confine non-neutral electron plasmas, neutral pair plasmas, or anything in between.



Preliminary confinement experiments have been performed in APEX-LD using pure electron non-neutral plasmas. A lanthanum hexaboride thermionic emitter on the edge of the confinement volume floods the trap with electrons. Large amplitude, narrow-band oscillations in the region of 100 kHz are consistently observed in wall probe measurements after the pulsed fill. The n=1 toroidal mode is typically long-lived (> 1s) and its frequency correlates with the ExB drift velocity. We attribute this feature to the dipole-equivalent of the lowest-order diocotron mode that is commonly observed in non-neutral plasmas in linear traps. Similar modes have been reported to occur in electron plasmas in the RT-1 levitated dipole trap. In addition, we have observed upward chirps and spontaneous excitation of n=2 toroidal modes. An axial electron beam probe provides time-resolved measurements of the space-charge potential of the trapped pure electron plasmas and is employed to investigate inward transport.