Relaxation of 2D turbulence to vortex crystals

A magnetically confined electron column evolves in (r, θ) as an essentially inviscid, incompressible 2D fluid with a single sign of vorticity. Turbulent initial states with 50-100 vortices relax due to vortex merger and filamentation, in general agreement with recent scaling theories. However, this 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-scale viscosity, but strongly dependent on the strength of the weak background vorticity. A new "restricted maximum fluid entropy" theory predicts the crystal patterns and background vorticity distribution, by assuming conservation of the robust flow invariants and preservation of the intense vortices.