G. R. Pickett

The mechanical behavior of a vibrating wire in superfluid3He-B in the ballistic limit

An experimental procedure is described for the characterization of the response of very small vibrating wires to liquid3He at temperatures down to 120 µK. The relative scales of the mean free path in the liquid and the radius of the wire play a significant role in the interpretation of the results. It is shown that the same ideas concerning the transition from hydrodynamic to ballistic behavior as the temperature is reduced can be applied both to saturated3He-4He solutions and to superfluid3He-B.

New methods for nuclear cooling into the microkelvin regime

We describe the philosophy and practice of a new method of nuclear cooling in which the copper refrigerant is immersed directly in the3He sample to be cooled using a guard cell configuration. The method has been used to cool liquid3He to ∼120 µK. We also describe a variant of the method intended for cooling metallic samples, by which a platinum NMR thermometer has been cooled to ∼13 µK. Finally, in an appendix we suggest a very simple nuclear cooling method utilizing the copper flakes used in the manufacture of paint, which will cool liquid3He to around 1 mK with a minimum of cryogenic effort.

A microscopic calculation of the force on a wire moving through superfluid3He-B in the ballistic regime

When a macroscopic object moves through superfluid3He, it experiences a force arising from the effect of quasiparticle scattering. We develop a three-dimensional microscopic model to calculate the force on a smooth cylinder moving at constant velocityv as a model of a vibrating wire. At large (subcritical) wire velocity, the force tends to an asymptotic value as 1/v2, rather than exponentially as in a one-dimensional calculation. At lowv the force is linear inv. We briefly discuss the agreement of our calculations with experimental measurements on a vibrating wire below 0.2Tc, where the quasiparticle trajectories are ballistic.

Application of a SQUID magnetometer to NMR at low temperatures

Using a SQUID magnetometer, changes in the nuclear magnetization of copper are observed when the nuclear spins absorb rf power at resonance. Since the detected flux change is directly proportional to the change in static magnetizationΔM, this method offers higher sensitivity at low frequencies than conventional NMR which responds todM/dt. The experimental arrangement for observingΔM at temperatures below1 K is described. The variation of signal amplitude with temperature and rf power is found to agree well with predictions from the Bloch equations. A comparison of the sensitivity of the SQUID magnetometer and conventional NMR methods shows the advantage of the new technique at low frequencies and for nuclear systems with broad lines and long spin-lattice relaxation times.

Turbulence: Suddenly it's chaos

Injecting vortices into a rotating sample of superfluid helium-3 shows a sudden switch from smooth to chaotic behaviour, and throws light on turbulence — one of the last unsolved problems of classical physics.

Generation, evolution, and decay of pure quantum turbulence : a full Biot-Savart simulation

A zero-temperature superfluid is arguably the simplest system in which to study complex fluid dynamics, such as turbulence. We describe computer simulations of such turbulence and compare the results directly with recent experiments in superfluid He-3-B. We are able to follow the entire process of the production, evolution, and decay of quantum turbulence. We find striking agreement between simulation and experiment and gain insights into the mechanisms involved.

An Advanced Dilution Refrigerator Designed for the New Lancaster Microkelvin Facility

We have constructed a large new dilution refrigerator for use with the new Lancaster nuclear cooling facility. The machine is housed in a purpose-rebuilt suite of rooms and has been designed to have a very low base temperature, a very low heat leak environment and to run for long periods between refrigerant refills. The machine has been operated in continuous mode down to ∼1.75 mK and can run for 10 days between refills. Preliminary nuclear cooling experiments suggest that even with an experiment attached the mixing chamber can still run below 2 mK.

A dilution refrigerator combining low base temperature, high cooling power and low heat leak for use with nuclear cooling

Abstract We describe the design philosophy, design, construction and performance of a dilution refrigerator specifically intended for nuclear cooling experiments in the submillikelvin regime. Attention has been paid from the outset to minimizing sources of heat leaks, and to achieving a low base temperature and relatively high cooling power below 10 mK. The refrigerator uses sintered silver heat exchangers similar to those developed at Grenoble. The machine has a base temperature of 3 mK or lower and can precool a copper nuclear specimen in 6.8 T to 8 mK in 70 h. The heat leak to the innermost nuclear stage is

Vortex generation in superfluid 3He by a vibrating grid

Recently we have found that a vibrating wire resonator produces turbulence in superfluid 3He-B at low temperatures when driven above its pair-breaking critical velocity. The vorticity is produced along with a beam of excitations from pair breaking. Here, we discuss preliminary measurements of turbulence generated from an oscillating grid at low temperatures. The grid oscillator is made from a goal-post shaped vibrating wire resonator supporting a fine copper mesh. While the dissipation by a conventional wire resonator is dominated by pair-breaking at the velocities required for turbulence generation, the dissipation of the grid oscillator appears to be dominated by turbulence. This allows us to generate turbulence without the unwanted effects of a quasiparticle beam. Preliminary measurements suggest that the grid turbulence has a rather different behaviour from that generated by conventional wire resonators.

Towards Superfluidity of 3He Diluted by 4He

Search for the superfluid state of dilute 3He dissolved to 4He is one of the major remaining problems of low temperature physics. We describe our two experiments designed to pursue the lowest achieved temperature in such mixtures essentially below the values reported before.