We study the relaxation dynamics of an isolated zero temperature
quasi-two-dimensional superfluid Bose-Einstein condensate (BEC) that is
imprinted with a spatially random distribution of quantum vortices. Following a
period of vortex annihilation, we find that the remaining vortices
self-organise into two macroscopic coherent `Onsager vortex' clusters that are
stable indefinitely. We demonstrate that this occurs due to a novel physical
mechanism --- the evaporative heating of the vortices --- that results in a
negative temperature phase transition in the vortex degrees of freedom. At the
end of our simulations the system is trapped in a non-thermal state. Our
computational results provide a pathway to observing Onsager vortex states in a
superfluid Bose gas.Comment: 10 pages, 7 figure
