Electron plasmas with mean densities of $5.0\times {10}^6{\mathrm{c}\mathrm{m}}^{-3}$ have been confined for as long as 18 ms in a partially toroidal trap with a purely toroidal magnetic field ($B_0=196\mathrm{G}$, $R_o=43\mathrm{c}\mathrm{m}$, $a=5\mathrm{c}\mathrm{m}$). Confinement is limited to 2.0 ms unless feedback is employed to suppress the growth of a toroidal version of the $m=1$ diocotron mode. The confinement time is much longer than all characteristic single-particle drift time scales and therefore confirms the existence of an equilibrium in which the space-charge-generated $\mathbf{E}\mathbf{\times }\mathbf{B}$ drift acts as an effective rotational transform.

• Received 10 September 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.095003