Motion of ions trapped on vortices in He II

We have measured the drift velocity of ions trapped on vortex lines at temperatures down to 0.35 K. We observe several effects which cannot easily be explained in terms of existing theories, including the broadening of an ion pulse, a limiting velocity in high electric fields, and a zero-field mobility which increases, at lower temperatures, much more rapidly than expected. An attempt is made to explain qualitatively some of these effects.