Measurements of a nonlinear diocotron mode in pure electron plasmas

Measurements of frequencies and density perturbations of the large amplitude l=1, ${\mathrm{k}}_{\mathrm{z}}$=0 diocotron mode on pure electron plasma columns are presented. This mode is a nonlinear dynamical state of an off-axis electron column which is E×B drifting around the containment axis. At large amplitudes, the plasma column distorts into an oval cross section and the mode frequency shifts by an amount proportional to mode amplitude squared. The frequency shift arises because (1) the plasma is closer to its image than a linear model assumes, and (2) the shape distortion modifies the image charge distribution.