Fluctuative Mechanism of Vortex Nucleation in the Flow of ⁴He

We propose a mechanism of a vortex nucleation in a flow of a superfluid $^4He$. The mechanism is related to the creation by critical fluctuations of a "plasma" of half-vortex rings located near the wall. The "plasma" screens the attraction of the vortex to the wall and permits vortex nucleation. In the spirit of Williams-Shenoy theory we derive the scaling laws in the critical region and estimate the scaling relation and the critical exponent $p$ for critical velocity; we find $V_c\sim V_{0}(1-T/T_c)$, so that $p=1$. Various applications of the obtained results are discussed.