Measurements of Non-neutral Plasma Confinement and Mode Behavior in a Dipole Magnetic Field

A magnetic dipole has the necessary symmetry to take advantage of canonical angular momentum conservation for confining non-neutral plasma. Previous theory and computation identified stable global thermal equilibrium states that require tailored electrostatic boundary conditions (Steinbrunner, 2023). The Lawrence Non-neutral Dipole (LND), a trap with a supported 3” diameter disk with a surface magnetic field of 2.3 kG on axis, aims to determine whether these states can be experimentally accessed. The magnet is biased negatively relative to the vacuum chamber to confine electrons that are sourced from a LaB6 emitter. Four copper wall probes located on the equatorial plane of the magnet are used to monitor image charge signals created by electron plasma propagation. Mode frequencies of 200 ~ 250 kHz are dominant. Characteristics of this mode resemble those of diocotron modes that have been experimentally observed in cylindrical geometry. Such measurements support the development of a theory for dipole geometry diocotron modes. Destructive electron population measurements are made by switching the magnet bias to a positive value and observing the current pulse created by collecting electrons. The effects of varying the aspect ratio of the chamber are explored by installing movable electrodes.



This work is based upon work supported by the National Science Foundation under Grant No. 2206620.

P. Steinbrunner, T. M. O’Neil, M. R. Stoneking, and D. H. E Dubin, J. Plasma Phys. 89, 935890401 (2023).