Relaxation of 2D turbulence to vortex crystals

A magnetically confined electron column evolves in (r, theta) as an essenti ally inviscid, incompressible 2D fluid with a single sign of vorticity. Tur bulent initial states with 50-100 vortices relax due to vortex merger and f ilamentation, in general agreement with recent scaling theories. However, t his relaxation is sometimes halted when 3-20 vortices "anneal" into a fixed pattern, or "vortex crystal." 2D vortex-in-cell simulations reproduce this effect, demonstrating that the vortex "cooling" is independent of fine-sca le viscosity, but strongly dependent on the strength of the weak background vorticity. A new "restricted maximum fluid entropy" theory predicts the cr ystal patterns and background vorticity distribution, by assuming conservat ion of the robust how invariants and preservation of the intense vortices.