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

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 x 10(-10) Torr and purified helium is injected to pressures greater than or equal to 5 x 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/B-3/2 instead of the expected 1/B-2 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, (C) 1999 American Vacuum Society, [S0734-2101(99)16604-1].