  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.
