Assessing confinement limitations and scaling for a toroidal electron plasma using the m=1 diocotron frequency

Confinement issues for toroidal electron plasmas are experimentally addressed using the Lawrence Nonneutral Torus. Electron plasmas confined in a partially toroidal trap ($B=200\mbox{G},$ $R_0=43\mbox{cm}$ $a=5\mbox{cm}$) exhibit an unstable $m=1$ diocotron mode. Suppression of this mode is necessary to prevent plasma loss via scrape off on the wall. By detecting the flow of image charge to and from a wall probe, an active feedback circuit suppresses the growth of the mode, extending the confinement time to the order of 40ms. By modulating the feedback and allowing small amplitude diocotron oscillations to grow, measurement of the evolution of the total charge is made. This technique is non-destructive and implies an initial charge of $12\mbox{nC}$ or a mean density of $5\times10^{6}\mbox{cm}^{-3}$. The effects of the magnetic field on the confinement time and charge evolution were measured by scanning the field strength between 100G and 200G. Stronger magnetic fields yielded longer confinement times. Data was also taken for pressures between $2.0\times10^{-7}\mbox{ Torr}$ and $1.2\times10^{-5}\mbox{ Torr}$. Higher pressures resulted in less trapped charge. This work is supported by NSF and USDOE.