Non-neutral Plasma in Dipole Fields

Non-neutral plasmas have historically been studied in uniform magnetic field (Penning-Malmberg) traps. Excellent confinement is assured in such devices due to cylindrical symmetry and the conservation of canonical angular momentum. Purely poloidal field traps such as the dipole also have the necessary symmetry to take advantage of angular momentum conservation. Recent theory and computation have identified promising states that satisfy the requirements for global thermal equilibrium and therefore ought to be absolutely stable (Steinbrunner 2023). Two experiments are under construction to test the experimental accessibility of these states. A supported dipole trap is under construction at Lawrence University. In this device, a permanent magnet (3” diameter disk with magnetic moment of 68 Am²) is mounted on a central rod. Some of the equilibrium states predicted by theory reside exclusively on the outboard midplane and are therefore likely to be accessible in a supported dipole trap with appropriately tailored boundary conditions. Second, the construction of the APEX (A Positron-Electron eXperiment) – levitated dipole is nearing completion. It makes use of a 15 cm diameter high-temperature superconducting (HTS) coil (I < 60 kAt) that is inductively charged using a second HTS coil (I < 164 kAt). Stable levitation is achieved with active feedback on the lifting coil current for times that exceed one hour. Electrons are injected at the edge to create a suitable target electron plasma into which positrons will be introduced to create an electron-positron plasma.