COLLAPSE AND WINDING OF AN ASYMMETRIC ANNULUS OF VORTICITY

The dynamics of an asymmetric annulus of vorticity in an incompressibl e, inviscid two-dimensional fluid are experimentally studied using a p ure electron plasma. A strict fluid analogy requires that the plasma s ystem behave like an ideal fluid in a frictionless cylindrical contain er. For certain parameters the asymmetric annulus undergoes a complex evolution which is quite different from that of a symmetric annulus. D uring the first 'active', phase the symmetries grow until the annulus collapses, leaving a large vortex at the device centre. In the second, 'passive', phase the remainder of the annulus winds around this centr al vortex into an ever tighter spiral. Finally, slow shear instabiliti es destroy the structure of the highly evolved spiral.