Experimental Realization of Nearly Steady-State Toroidal Electron Plasmas

Non-neutral plasmas are routinely confined in the uniform magnetic field of a Penning-Malmberg trap for arbitrarily long times and approach thermal equilibrium. Theory predicts that dynamically stable and therefore long-lived equilibria exist for non-neutral plasmas confined in the curved, non-uniform field of a \emph{toroidal} trap, but that ultimately thermal equilibrium states do not exist. On long timescales, the poloidal $\mathbf{E}\times \mathbf{B}$ rotation through the non-uniform toroidal magnetic field leads to magnetic pumping transport. A new experiment has, for the first time, demonstrated the existence of a stable, long-lived (\emph{i.e.} nearly steady-state) toroidal equilibrium for pure electron plasmas and is poised to observe the magnetic pumping transport mechanism.\footnote{J.P. Marler and M.R. Stoneking, Phys. Rev. Lett. \textbf{100}, 155001 (2008).} Electron plasmas with densities of order $10^6$ cm$^{-3}$ are trapped in the Lawrence Non-neutral Torus II for several seconds. LNT II is a high aspect ratio ($R_o/a \approx 10$), partially toroidal trap (a 270$^\circ$ arc with $B_o=670$ G). The $m=1$ diocotron mode is launched and detected using isolated segments of a fully-sectored conducting boundary and its frequency is used to determine the total trapped charge as a function of time. The observed confinement time ($\approx 3$ s) approaches the theoretical limit ($\approx 6$ s) set by the magnetic pumping transport mechanism of Crooks and O'Neil.\footnote{S.M. Crooks and T.M. O'Neil, Phys Plamas \textbf{3}, 2533 (1996).} We also present equilibrium modeling and numerical simulation of the toroidal $m=1$ mode constrained by experimental data. Future work includes the identification of the dominant transport mechanisms via confinement scaling experiments and measurement of the $m=2$ mode frequency, and development of a strategy for making a transition to fully toroidal confinement.