Vortex-Ring Creation by Negative Ions in Dilute Mixtures ofHe3-He4

We describe an experiment which indicates that the addition of ${\mathrm{He}}^3$ impurities to superfluid helium has a pronounced effect on the velocity at which negative ions produce vorticity (vortex rings). The ${\mathrm{He}}^3$ impurities have no effect, however, on the creation of vorticity by positive ions.