Experiments on stability of equilibria of two vortices in a cylindrical trap

We observe the (r,θ) drift motion of two nearly identical magnetized electron columns bounded by a cylindrical wall. In the 2D ExB drift approximation, these columns are vortices evolving according to the Euler equation. We observe stable and unstable equilibria in which the vortices orbit about the center of the cylinder. The equilibrium positions, oscillation frequencies, and instability rates for these spatially extended vortices agree well with the predictions of integrable point vortex theory, apparently because surface waves and shape distortions do not couple to the center-of-mass motion