P. M. Walmsley

Quantum and quasiclassical types of superfluid turbulence

By injecting negative ions in superfluid 4He in the zero-temperature limit (T<or=0.5 K), we have generated tangles of quantized vortex line with negligible large-scale flow. For this quantum regime of superfluid turbulence, the vortex line length L was found to decay at late time t as L proportional to t{-1}, the prefactor being independent of the initial value of L. The corresponding effective kinematic viscosity is 0.1 kappa, where kappa is the circulation quantum. At T>0.7 K, a jet of ions generates quasiclassical tangles identical to those produced by mechanical means.

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.

Dynamics of quantum turbulence of different spectra

Turbulence in a superfluid in the zero-temperature limit consists of a dynamic tangle of quantized vortex filaments. Different types of turbulence are possible depending on the level of correlations in the orientation of vortex lines. We provide an overview of turbulence in superfluid 4He with a particular focus on recent experiments probing the decay of turbulence in the zero-temperature regime below 0.5 K. We describe extensive measurements of the vortex line density during the free decay of different types of turbulence: ultraquantum and quasiclassical turbulence in both stationary and rotating containers. The observed decays and the effective dissipation as a function of temperature are compared with theoretical models and numerical simulations.

Reconnections of Quantized Vortex Rings in Superfluid 4He at Very Low Temperatures

Collisions in a beam of unidirectional quantized vortex rings of nearly identical radii R in superfluid 4He in the limit of zero temperature (0.05 K) were studied using time-of-flight spectroscopy. Reconnections between two primary rings result in secondary vortex loops of both smaller and larger radii. Discrete steps in the distribution of flight times, due to the limits on the earliest possible arrival times of secondary loops created after either one or two consecutive reconnections, are observed. The density of primary rings was found to be capped at the value 500  cm-2R-1 independent of the injected density. This is due to collisions between rings causing the piling up of many other vortex rings. Both observations are in quantitative agreement with our theory.

Turbulent vortex flow responses at the A B interface in rotating superfluid 3He-B

In a rotating two-phase sample of 3He-B and magnetic-field stabilized 3He-A the large difference in mutual friction dissipation at 0.20Tc gives rise to unusual vortex flow responses. We use noninvasive NMR techniques to monitor spin down and spin up of the B-phase superfluid component to a sudden change in the rotation velocity. Compared to measurements at low field with no A phase, where these responses are laminar in cylindrically symmetric flow, spin down with vortices extending across the AB interface is found to be faster, indicating enhanced dissipation from turbulence. Spin up in turn is slower, owing to rapid annihilation of remanent vortices before the rotation increase. As confirmed by both our NMR signal analysis and vortex filament calculations, these observations are explained by the additional force acting on the B phase vortex ends at the AB interface.

A compact rotating dilution refrigerator

We describe the design and performance of a new rotating dilution refrigerator that will primarily be used for investigating the dynamics of quantized vortices in superfluid (4)He. All equipment required to operate the refrigerator and perform experimental measurements is mounted on two synchronously driven, but mechanically decoupled, rotating carousels. The design allows for relative simplicity of operation and maintenance and occupies a minimal amount of space in the laboratory. Only two connections between the laboratory and rotating frames are required for the transmission of electrical power and helium gas recovery. Measurements on the stability of rotation show that rotation is smooth to around 10(-3) rad s(-1) up to angular velocities in excess of 2.5 rad s(-1). The behavior of a high-Q mechanical resonator during rapid changes in rotation has also been investigated.

Coexistence of quantum and classical flows in quantum turbulence in the T=0 limit

Tangles of a quantized vortex line of initial density L(0)∼6×10^{3}  cm^{-2} and a variable amplitude of fluctuations of flow velocity U(0) at the largest length scale are generated in superfluid ^{4}He at T=0.17  K, and their free decay L(t) is measured. If U(0) is small, the excess random component of the vortex line length first decays as L∝t^{-1} until it becomes comparable with the structured component responsible for the classical velocity field, and the decay changes to L∝t^{-3/2}. The latter regime always ultimately prevails, provided the classical description of U holds. A quantitative model of coexisting cascades of quantum and classical energies describes all regimes of the decay.

Rotating quantum turbulence in superfluid 4He in the T=0 limit

Observations of quantum turbulence in pure superfluid 4He in a rotating container are reported. Large-scale forcing through rotational oscillations of the cubic container and detecting of turbulence via monitoring ion transport along the axis of rotation were used. Near the axial walls, with increasing forcing the vortex tangle grows without an observable threshold. This tangle gradually develops into bulk turbulence at a characteristic amplitude of forcing that depends on the forcing frequency and rotation rate. At higher amplitudes, the total vortex line length increases rapidly. Resonances of inertial waves are observed in both laminar and turbulent bulk states. On such resonances, the turbulence appears at smaller amplitudes of forcing.

A field-emission source of charges based on nanotubes for low-temperature experiments

Methods for the production of field-emission sources of charges on the basis of carbon nanotubes are described. These sources can be used to study the properties of injected charges in cryogenic liquids and crystals (the area of the source surface is about several square millimeters, the dissipated power is <10−6 W). The first and second batches of sources were produced by means of the deposition of nanotubes from an arc discharge on a flat copper substrate and via mechanical rubbing of nanotubes into a porous metal, respectively. Tests of sources from the first batch in a diode with a gap of 0.5 mm showed that in superfluid He-II, the current of negative charges at a level of 10−12 A arises when the voltage at the cathode is U = −140 V and increases to 10−9 A, when U rises to 170 V. When the voltage polarity changes, the current of positive charges arises in the diode for the voltage U ≥ 240 V. A source from the first batch was used to observe the movement of negative and positive charges in solid-helium samples at T < 75 mK. For second-batch sources, the current of negative charges at a level of 10−12 A in superfluid He-II arises at U = −260 V.

Nanotube-based source of charges for experiments with solid helium at low temperatures

Methods of preparation of the field-emission sources of charges from carbon nanotubes suitable for study of injected charges in solid helium at low temperatures T < 1 K are presented. The sources have been prepared by arc discharge deposition of nanotubes onto a flat copper substrate or by mechanical rubbing of nanotubes into porous metal surface. The test study of the voltage-current characteristics of a diode cell with the nanotube source in superfluid He II have shown that at voltages above 120 V one can observe a relatively large current I ≥ 10–13 A of negative charges in liquid helium. The field and temperature dependences of positive and negative currents in solid 4He were studied in samples grown by the blocked capillary technique. Usage of the nanotube based source of injected charges had permitted us for the first time to observe motion of the positive charges in solid helium at temperatures below 0.1 K. The current-voltage dependence could be described by a power law I∼Uα, with the value of the ...