Experimental dynamics of 2-D vorticity rings

A direct analogy exists between the dynamics of a 2-D inviscid, incompressible fluid, and a magnetized pure electron plasma, with the fluid vorticity corresponding to the plasma density. Thus the U.C. Berkeley electron trap is a powerful tool for controlling and measuring the ''vorticity'' of an evolving fluid system. Here we study the evolution of a hollow ring of vorticity. Through the Kelvin-Helmholtz instability of fluid mechanics, and the equivalent diocotron instability of electron beam theory, a symmetric ring develops into individual vortices. A sufficiently asymmetric ring, however, undergoes a ''winding'' instability, resulting in a spiral shape. We show various images detailing the stages of the dynamics for various initial ring parameters. (author) 6 refs., 6 figs