S. N. Fisher

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.

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.

Excimers He2* as Tracers of Quantum Turbulence in 4He in the T=0 Limit

We have studied the interaction of metastable 4He2* excimer molecules with quantized vortices in superfluid 4He in the zero temperature limit. The vortices were generated by either rotation or ion injection. The trapping diameter of the molecules on quantized vortices was found to be 96±6  nm at a pressure of 0.1 bar and 27±5  nm at 5.0 bar. We have also demonstrated that a moving tangle of vortices can carry the molecules through the superfluid helium.

Diffuse scattering model of the thermal damping of a wire moving through superfluid3He-B at very low temperatures

We present a microscopic model of the scattering of quasiparticles in superfluid3He-B by a moving solid surface. This is used to calculate the thermal damping of a wire resonator in the low temperature regime. The calculated damping force is in good agreement with experimental results when the quasiparticles are assumed to be scattered diffusely by the wire.

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.

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.

Andreev reflection, a tool to investigate vortex dynamics and quantum turbulence in 3He-B

Andreev reflection of quasiparticle excitations provides a sensitive and passive probe of flow in superfluid 3He-B. It is particularly useful for studying complex flows generated by vortex rings and vortex tangles (quantum turbulence). We describe the reflection process and discuss the results of numerical simulations of Andreev reflection from vortex rings and from quantum turbulence. We present measurements of vortices generated by a vibrating grid resonator at very low temperatures. The Andreev reflection is measured using an array of vibrating wire sensors. At low grid velocities, ballistic vortex rings are produced. At higher grid velocities, the rings collide and reconnect to produce quantum turbulence. We discuss spatial correlations of the fluctuating vortex signals measured by the different sensor wires. These reveal detailed information about the formation of quantum turbulence and about the underlying vortex dynamics.

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.