Study of the vortex-ring transition in superfluidHe4

Extensive experimental results on the vortex-ring transition in liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ between 0.3 and 0.7°K are presented. These include high-field mobilities and vortex-ring creation rates for positive and negative ions at pressures up to the melting pressure and at various ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ impurity concentrations. The significance of our observations is discussed in terms of possible models of the transition mechanism.