EFFECT OF HE-3 IMPURITIES ON THE NUCLEATION OF PHASE SLIPS IN SUPERFLUID HE-4

The critical value of the dc current of superfluid He-4 through a micr o-aperture is found to decrease at low temperatures in the presence of He-3 impurities. We measure the dependence of the critical current on the pressure difference across the micro-aperture. In the temperature region where the He-3 affected decrease in the critical current is de tected, we measure a large increase in critical current with pressure difference. The effect can be explained by the finite rate of arrival of ballistic He-3 atoms to a nucleation site. The critical velocity in creases at high pressure difference when the rate of phase slippage be comes comparable to the flux of He-3 atoms.