A. M. Guénault

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.

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.

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.

A compact dilution refrigerator with vertical heat exchangers for operation to 2 mK

We describe a compactly designed dilution refrigerator with closely packed, vertical heat exchangers. The refrigerator reaches a temperature of 2 mK and is easily constructed, since the sintered heat exchangers are straight units. Vibrating wire resonators are employed in the mixing chamber as diagnostic tools, which may act as both thermometers and phase-boundary level indicators. There is a design problem in the vertical arrangement, namely, the sumps on the concentrated phase side that can slowly fill with dilute phase and degrade the performance. The problem is solved by draining the superfluid4He component in any collected dilute phase through superleaks into the mixing chamber.

Spatial extent of quantum turbulence in non-rotating superfluid 3He-B

Abstract Quantum turbulence has been shown to reflect a beam of quasiparticles in the B-phase of superfluid 3 He by Andreev processes. We have investigated the evolution of the turbulence generated by a vibrating wire resonator driven at high velocities and temperatures down to ∼0.1Tc. The vibrating wire produces vorticity together with the expected quasiparticle beam whenever the wire velocity exceeds the critical pair breaking velocity. By using an array of detector wires we are able to investigate the development of the turbulence both in space and time. We observe that the turbulence propagates preferentially along the direction of the quasiparticle beam and drops off in a roughly exponential manner with a decay length of the order of 2 mm .

Thirty-Minute Coherence in Free Induction Decay Signals in Superfluid 3He-B

The persistent induction signal (PIS) is an extremely long lived NMR mode in the B-phase of superfluid 3He. The first signals observed lived for up to 25s following a moderate NMR tipping pulse, but were found to be highly irreproducible. Here we present new measurements obtained in a smooth sapphire cell designed to achieve the lowest possible temperatures. The signals observed in this cell are quite reproducible but are only excited over a restricted range of magnetic field gradients. The free induction signals exceed 30 minutes at the lowest temperatures.

Breaking the superfluid speed limit in a fermionic condensate

An experiment reports the unexpected behaviour of an object in uniform motion in superfluid helium-3 above the Landau critical velocity — the limit above which it can generate excitations at no energy cost.

The Thermal Conductivity of Superfluid 3He in Aerogel: A Measurement of the Energy Gap

Transport properties of superfluid 3He in aerogel are governed by a fixed mean free path set by the typical dimensions of the strand separation. We describe preliminary measurements of the thermal conductivity of superfluid 3He confined in 98% silica aerogel. The majority of the measurements are made in relatively high magnetic fields where the superfluid within the aerogel is in the A-phase like state. Since the quasiparticle mean free path is fixed, the temperature dependence of the thermal conductivity depends only on the superfluid energy gap and on the texture in the case of the A-phase. The results so far obtained are broadly consistent with the aerogel-confined superfluid having a temperature dependent energy gap close to the BCS prediction although there are significant deviations at the lowest temperatures.