First (Non-neutral) Plasma in the APEX Levitated Dipole

A Positron Electron eXperiment Levitated Dipole (APEX-LD) is designed to study magnetically confined low energy electron-positron plasma. Its compact design (10 liter confinement volume and 7.5 cm coil radius) and use of a high-temperature superconducting coil (60 kAt) to produce a strong magnetic field (~1 T) are tailored to achieve small Debye lengths (~1 cm) with ~10¹⁰ positrons and electrons (at T~1 eV). Stable levitation of the floating coil for more than 3 hours is now reliably achieved. Since the dipole configuration is well-suited to confining plasmas at any level of non-neutrality, and as a key step toward production and study of electron-positron plasma we conducted initial non-neutral (pure electron) plasma experiments in APEX-LD. A lanthanum hexaboride (LaB₆) thermionic emitter was used to inject electrons from an equatorial position near the edge of the device. A toroidally propagating (n = 1) mode is observed using a pair of wall probes; features of this mode are consistent with a modified version of the diocotron mode, and theoretical work is developing to examine this connection. The oscillations are in the 100-500 kHz range of frequencies, depending on the fill time and contain many harmonics. Frequent, and as yet unexplained, chirping events are regularly observed. Evidence for confined plasma persists for as long as 400 seconds, indicating that confinement is very good. Ongoing work is directed at determining the total space charge and its spatial distribution in order to compare to theoretical equilibrium states for non-neutral plasma in a dipole field.



This material is based upon work supported by NSF (No. 2206620), the ERC (ERC-2016-ADG no. 741322), the DFG (HU 978/20-1, STE 2614/2-1), the U.S. DOE (DE-SC0016532), the UCSD Foundation, and the Helmholtz Association (VH-NG-1430).