Steady-State Confinement of Non-neutral Plasmas by Rotating Electric Fields

Abstract

We apply ``rotating wall'' electric fields to spin up a non-neutral plasma in a Penning-Malmberg trap, resulting in steady-state confinement (weeks) of up to ${10}^{9}{\mathrm{Mg}}^{+}$ ions. The resulting ion columns can be near global thermal equilibrium, with near-uniform temperature and rotation frequency. The equilibrated plasma $\mathbf{E}\ifmmode\times\else\texttimes\fi{}\mathbf{B}$ rotation rate ${f}_{E}$ is observed to be somewhat less than the drive frequency ${f}_{w}$, with slip $\ensuremath{\Delta}f\ensuremath{\equiv}{f}_{w}{\ensuremath{-}f}_{E}$ depending on temperature as $\ensuremath{\Delta}f\ensuremath{\propto}{T}^{1/2}$ for $0.05\ensuremath{\lesssim}T\ensuremath{\lesssim}5\mathrm{eV}$. Dynamic measurements of applied torque versus slip frequency show plasma spin up and compression for $\ensuremath{\Delta}f>0$ and plasma slowing and expansion for $\ensuremath{\Delta}f<0$. By gradually increasing ${f}_{w}$, density compression up to 20% of the Brillouin density limit has been achieved. Heating resonances and hysteresis in plasma parameters are also observed.