Abstract At higher electric fields, the drifting ions are not in thermal equilibrium with the gas of the elementary excitations of the superfluid, and the mobility depends on the field. This chapter presents the experimental results of the ion mobility in the superfluid at higher fields and larger temperatures. The field dependence of the mobility is explained in terms of roton scattering in presence of an enhanced roton density around the charge. Emphasis is given on the discovery of the giant mobility discontinuity at which the excess energy is dissipated by ions creating quantized vortex rings. Ions are captured by them and drift together as a new, single unit called a charged vortex ring. The puzzling issue of the drift velocity discontinuities is also addressed.
Measurements of the positive-ion velocity as a function of electric field in dilute solutions of
3He in
4He at 18 mK show a discontinuity which suggests the existence of two kinds of vortex rings in addition to the bare ion. The assignment of circulation quantum numbers is discussed.
Measurements of the velocity of charge carriers in several dilute solutions of
3He in
4He in the temperature range 0.017-1.0 K are reported. The low-field negative-ion mobility shows a ${T}^{\ensuremath{-}\frac{1}{2}}$ dependence below 0.08 K characteristic of hard-sphere scattering. A maximum is observed in the mobility in the vicinity of 0.4 K. This can be interpreted in terms of a
3He condensation on the bubble surface. The positive-ion mobility is nearly temperature independent below 0.7 K. This has recently been interpreted in terms of a momentum-dependent polarization scattering. The field dependence of the positive-carrier velocity shows a discontinuity in the vortex-ring regime. The discontinuity can be shown to result from a size-dependent drag coefficient and a maximum is observed for very small rings (\ensuremath{\sim}16 \AA{}).