Inviscid damping of an elliptical vortex in an external strain flow*

A 2D elliptical vortex undergoes rotation or nutation around its equilibrium when subjected to an applied external strain flow. When a small non-uniform peripheral vorticity is present, the amplitude of vortex oscillatory motion is observed to damp toward the non-axisymmetric elliptical equilibrium. This damping mechanism is studied experimentally by imposing ExB drift motion of an electron plasma in a Penning-Malmberg trap, which is analogous to the dynamics of a rotating vortex in a 2D inviscid, incompressible fluid. The external strain flow is generated by applying voltages to the sectors of the cylindrical boundary of the trap. The amount of peripheral vorticity is controlled by varying the plasma fill time. Without strain, a perturbed vortex with a smoothly decreasing profile exterior to the core experiences critical-layer damping. While external strain does not significantly change the measured decay rate, it reduces the rotation frequency of the mode about equilibrium. At large strain, the trapping oscillations are reduced by particles near the cat's eye going over the separatrix. These results are compared to particle-in-cell simulations and available theoretical results.