Vortex nucleation in superfluid He-4

W F Vinen suggested in 1961 that the creation of quantized vortex line in s uperfluid He-4 is impeded by an energy barrier Measurements of the rate nu at which negative ions nucleate vortices in isotopically pure 4He have vind icated this idea. Barrier heights derived from the measured temperature dep endence of nu are in good agreement with values calculated by Muirhead, Vin en and Donnelly (MVD) on the assumption that the nascent vortex first appea rs as a small loop at the equator of the moving ion. The MVD model can in a ddition provide a satisfactory explanation for the extraordinary sensitivit y of nu to tiny traces of He-3 observed in experiments. A prediction that v ortices can also be created by a fast adiabatic expansion of liquid 4He thr ough the lambda (superfluid) transition-perhaps modelling the creation of c osmic strings at a cosmological phase transition in the early universe-has been tested. The results imply line densities that are smaller by a factor of at least 100 than those predicted. It is pointed out that Vinen's contri butions to the understanding of superfluidity have been substantial and tha t their influence is likely to be felt far into the future.