Electron Plasma in the Lawrence Non-neutral Dipole

The Lawrence Non-neutral Dipole (LND) device uses a supported neodymium magnet (3’’ diameter disk with a surface magnetic field of 2.3 kG on axis) to confine pure electron plasma in a magnetic dipole field. Two primary diagnostics in LND include 1) positively biasing the magnet to collect electrons onto it, known as the “magnet dump” method, and 2) studying the image charge signal from the copper wall probes. We measure the number of electrons that are collected inside the chamber by changing the magnet bias to “dump” the electrons. As the magnet bias changes from negative to positive, the electrons that are confined are collected as they fall on the magnet. Using the wall probes, we monitor the voltage signals generated across a 10 kΩ resistor by the flow of plasma image charge. We apply Fast Fourier Transform (FFT) as well as spectrogram analysis to study the plasma mode frequency. Observations are consistent with Deller (2025) regarding the toroidal mode propagation in the APEX (A Positron- Electron eXperiment) levitated dipole trap and suggest that this is dipole version of the m=1 diocotron mode.

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

A. Deller, et al Plasma Phys. Control. Fusion, 67, 015030 (2025).