  We study computationally dynamics of quantised vortices in two-dimensional
superfluid Bose-Einstein condensates confined in highly oblate power-law traps.
We have found that the formation of large scale Onsager vortex clusters
prevalent in steep-walled traps is suppressed in condensates confined by
harmonic potentials. However, the shape of the trapping potential does not
appear to adversely affect the evaporative heating efficiency of the vortex
gas. Instead, the suppression of Onsager vortex formation in harmonic traps can
be understood in terms of the energy of the vortex configurations. Furthermore,
we find that the vortex-antivortex pair annihilation that underpins the vortex
evaporative heating mechanism requires the interaction of at least three
vortices. We conclude that experimental observation of Onsager vortices should
be the most apparent in flat or inverted-bottom traps.
