Observation of an unstablel=1diocotron mode on a hollow electron column

Stable and unstable diocotron modes varying as $e^{\mathrm{il}\theta }$, with l=1, are measured on a magnetized, partially hollow electron column. The unstable mode is observed to grow exponentially over several decades, in contradiction to present 2 D fluid theory. This l=1 instability can dominate the evolution well into the nonlinear regime, resulting in cross-field transport to a stable, monotonically decreasing density profile. To the extent this process is described by 2D E×B drifts, it is isomorphic to the Kelvin-Helmholtz shear-flow instability of inviscid fluids.