Motion of Ions in Helium II

The general motion of ions in He II is investigated assuming that nucleation of and escape from vortex rings is a random thermally activated process. The mean drift velocity $v_d$ is calculated as a function of temperature, applied field, and ionic species. It is shown that lowfield data up to and just beyond the giant discontinuity can be explained, provided careful attention is paid to the friction forces on small rings, by assuming that $v_d$ is the equilibrium drift velocity. The transition between bare-ion and vortex-ring behavior is discussed in some detail. At higher fields one must take into account vortex-ring dynamics and the possibility of escapes. In general $v_d$ is larger than the equilibrium velocity and, for very large fields, increases with field. Predictions of the theory are compared with experimental drift-velocity data. Also considered are the characteristics of ion currents in nonuniform fields. In particular, predictions are made for the "persistence current" observed when ions propagate first through a region of constant field, then through a region of zero or retarding field.