Superflow and Dissipation in Superfluid Helium Three.

In the past five years understanding of the flow properties of superfluid helium three has been revolutionized by experiments performed with rotating cryostats, both at Cornell University and at the Helsinki University of Technology. The work at Helsinki has concentrated on understanding the vortex structures which appear in the rotating fluid. They have concentrated on the change in the NMR spectrum during rotation, but have also worked with ion mobilities and persistent flow in a packed powder torus. At Cornell we have concentrated on the dissipation mechanisms and critical velocities of superflow. The work presented here comprises three of these experiments. The anomalously large damping of U-tube oscillations at zero Bar had cast some doubt on whether the low temperature phases of ^3 He were true superfluids, capable of sustaining persistent currents. The U-tube results were explained in terms of second viscosity. At Cornell we developed an ac gyroscope technique to measure directly the angular momentum associated with such currents. Using a 100mum foam filled torus, the first successful measurements showed a 1mm/sec critical velocity in the B-phase. A second experiment exploiting the damping, due to textural motion, of torsional oscillations of a torus about its axis of symmetry provided the most sensitive measure of persistent currents in the A and B phases. The anisotropic nature of the B-phase within five coherence lengths of the walls allows textural motion to be exploited in this phase. An unexpected decrease in the critical velocity with decreasing temperature in the B-phase at high pressure is not yet fully understood, but may be related to a transition in the nature of the vortex structures seen in several experiments in Helsinki. A third experiment measured the damping of superfluid oscillations due to the mutual friction force in ^3 He - A. A cell, essentially a fourth sound resonator, constructed by H. E. Hall and previously used to measure mutual friction in the B-phase was employed. The divergence of the coefficient of mutual friction, B rho_{n}/rho near T _{c} had been predicted by Hall to reflect the value of the intrinsic angular momentum. Our measurements showed only a weak divergence, of order (1 - T/T_{c})^{0.2}. (Abstract shortened with permission of author.).