Dynamics of the m=1 diocotron mode in the electron diffusion gauge experiment

The evolution of the m=1 diocotron mode is studied experimentally over a wide range of system parameters for a pure electron plasma confined in a Malmberg - Penning trap. The frequency of the m=1 diocotron mode is monitored and found to be higher than that predicted by Levy [Phys. Fluids 11, 920 (1968)] for an infinite-length plasma column. However, the experimental measurements of the m=1 mode frequency reported here are found to be in excellent agreement with the theoretical predictions for a finite-length plasma column. The growth and damping rates of the m=1 diocotron mode are also carefully measured and compared with theoretical predictions. Probably the strongest factor affecting the stability of the m=1 diocotron mode in a nonneutral plasma with a monotonically decreasing radial density profile is resistive-wall destabilization. The measured resistive growth rates are found to agree with theory, at least for moderate values of resistance. Even after minimizing the resistive-wall instability, and maintaining the background gas pressure sufficiently low that pressure effects on the m=1 diocotron mode are negligible, it is observed experimentally that there is an open-quotes anomalousclose quotes damping of the m=1 diocotron mode that depends on the magnetic field, B, and the line density, NL, through the ratio NL/B, which is proportional to the oscillation frequency of the m=1 diocotron mode. Although the cause of this damping is not understood from first principles, its effects can be minimized by tuning the frequency of the diocotron mode. Finally, although electron collisions with background neutral atoms had been expected to result in an unstable m=1 diocotron mode, it was observed that increasing the background gas pressure causes a damping of the m=1 mode. copyright 1999 American Vacuum Society