  A new mechanism of thermalization involving a direct energy cascade is
obtained in the truncated Gross-Pitaevskii dynamics. A long transient with
partial thermalization at small-scales is observed before the system reaches
equilibrium. Vortices are found to disappear as a prelude to final
thermalization. A bottleneck that produces spontaneous effective
self-truncation and delays thermalization is characterized when large
dispersive effects are present at the truncation wavenumber. Order of magnitude
estimates indicate that self-truncation takes place in turbulent Bose-Einstein
condensates. This effect should also be present in classical hydrodynamics and
models of turbulence.
