We show that vortex nucleation in superfluid 3He by rapid thermal quench
in the presence of superflow is dominated by a transverse instability of the
moving normal-superfluid interface. Exact expressions for the instability
threshold as a function of supercurrent density and the front velocity are
found. The results are verified by numerical solution of the Ginzburg-Landau
equation.Comment: 4 Pages, 4 Figure, submitted to Phys. Rev. Let