Non-neutral plasma expansion induced by electron-neutral collisions in a Malmberg–Penning trap

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Single species non-neutral plasmas have very robust confinement properties because the conservation of canonical angular momentum in a system with azimuthal symmetry provides a powerful constraint on the allowed radial positions of the particles. If no external torques act on the plasma, the plasma cannot expand radially to the wall. However, collisions with a background neutral gas will exert a torque on the rotating plasma thus allowing the mean-square radius to increase. In the electron diffusion gauge experiment, a pure electron plasma is confined in a Malmberg–Penning trap and the radial density profile is measured as a function of time. The base pressure is 5×10−10 Torr and purified helium is injected to pressures ⩾5×10−9 Torr. The magnetic field is varied between 100 and 600 G. The experimentally measured radial density profile shape is found to match closely the theoretically predicted (expanding) equilibrium profile, where a single free parameter proportional to the electron temperature T is varied to best fit the experimental data. The best-fit value of the temperature T is found to stay approximately constant even as the plasma expands and the electrostatic energy decreases. The measured plasma expansion rate is found to scale with magnetic field strength as 1/B3/2 instead of the expected 1/B2 scaling. This modification in scaling may be caused by field asymmetries, which are believed to be an important factor in plasma expansion for the pressure ranges examined here. Nevertheless, the expansion rates are observed to increase with increasing background pressure, and the absolute scaling with pressure is consistent with theoretical predictions.