Quantum Turbulence In Superfluid Research Articles

Vibrating Microwire Resonators Used as Local Probes of Quantum Turbulence in Superfluid $$^{4}$$He

Vibrating Microwire Resonators Used as Local Probes of Quantum Turbulence in Superfluid $$^{4}$$He

Intra-scales energy transfer during the evolution of turbulence in a trapped Bose-Einstein condensate(a)(a)Contribution to the Focus Issue Turbulent Regimes in Bose-Einstein Condensates edited by Alessandra Lanotte, Iacopo Carusotto and Alberto Bramati.

In turbulence phenomena, including the quantum turbulence in superfluids, an energy flux flows from large to small length scales, composing a cascade of energy. A universal characteristic of turbulent flows is the existence of a range of scales where the energy flux is scale-invariant: this interval of scales is often referred to as inertial region. This property is fundamental as, for instance, in turbulence of classical fluids it characterizes the behavior of statistical features such as spectra and structure functions. Here we show that also in decaying quantum turbulence generated in trapped Bose-Einstein condensates (BECs), intervals of momentum space where the energy flux is constant can be identified. Indeed, we present a procedure to measure the energy flux using both the energy spectrum and the continuity equation. A range of scales where the flux is constant is then determined employing two distinct protocols and in the same range, the momentum distribution measured is consistent with previous work. The successful identification of a region with constant flux in turbulent BECs is a manifestation of the universal character of turbulence in these quantum systems. These measurements pave the way for studies of energy conservation and dissipation in trapped atomic superfluids, and also analogies with the related processes that take place in ordinary fluids.

Nonuniform quantum turbulence in superfluids

The problem of quantum turbulence in a channel with an inhomogeneous counterflow of superfluid turbulent helium is studied. \ The counterflow velocity $V_{ns}^{x}(y)$ along the channel is supposed to have a parabolic profile in the transverse direction $y$. Such statement corresponds to the recent numerical simulation by Khomenko et al. [Phys. Rev. B \textbf{91}, 180504 (2015)]. The authors reported about a sophisticated behavior of the vortex line density (VLD) $\mathcal{L}(\mathbf{r},t)$, different from $% \mathcal{L}\propto V_{ns}^{x}(y)^{2}$, which follows from the naive, straightforward application of the conventional Vinen theory. It is clear, that Vinen theory should be refined by taking into account transverse effects and the way it ought to be done is the subject of active discussion in the literature. In the work we discuss several possible mechanisms of the transverse flux of VLD $\mathcal{L}(\mathbf{r},t)$ which should be incorporated in the standard Vinen equation to describe adequately the inhomogeneous quantum turbulence (QT). It is shown that the most effective among these mechanisms is the one that is related to the phase slippage phenomenon. The use of this flux in the modernized Vinen equation corrects the situation with an unusual distribution of the vortex line density, and satisfactory describes the behavior $\mathcal{L}(\mathbf{r},t)$ both in stationary and nonstationary situations. The general problem of the phenomenological Vinen theory in the case of nonuniform and nonstationary quantum turbulence is thoroughly discussed.

Modeling of classical turbulence by quantized vortices

Quantum turbulence in superfluids appears as a stochastic tangle of quantized vortex lines. Interest to this system extends beyond the field of superfluid helium to include a large variety of topics both fundamental and engineering problem. In the article we present a discussion of the hot topic, which is undoubtedly mainstream in this field, and which deals with the quasi-classical properties of quantum turbulence. The idea that classical turbulence can be modeled by a set of slim vortex tubes (or vortex sheets) has been discussed for quite a long time. In classical fluids, the concept of thin vortex tubes is a rather fruitful mathematical model. Quantum fluids, where the vortex filaments are real objects, give an excellent opportunity for the study of the question, whether the dynamics of a set of vortex lines is able to reproduce (at least partially) the properties of real hydrodynamic turbulence. The main goal of this article is to discuss the current state of this activity. We cover such important topics as theoretical justification of this model, and experimental and numerical evidence for quasi-classical turbulence.

Toward the In Situ Detection of Individual He2*Excimers Using a Ti TES in Superfluid Helium

Abstract—We characterize a single titanium (Ti) transitionedge sensor (TES) designed for in-situ detection of individualHe 2 * excimers. We find a critical temperature of 420 mK, anelectrothermal time constant of ∼3 µs, and a total energy reso-lution of 1.5 eV. We observe the detector response to short laserpulses, and present a successful analysis strategy for extractingdirect-TES-hit pulse areas from a much larger substrate hitbackground. We discuss near-term plans for coupling multiplesuch TESs together with a shared aluminum (Al) absorber,increasing the He 2 * collection area to millimeter scales. Finally,we briefly discuss the technical challenges (and solutions) ofinstalling a hermetic superfluid volume in a cryogen-free dilutionrefrigerator.Index Terms—Transition Edge Sensor, Helium excimer, UVSensor I. I NTRODUCTION T HE dynamics of quantum turbulence in superfluid 4 Heis a field of active research. Quantum vortices are madeobservable by trapping particles within the vortex core. Theparticles used for tagging the vortices in this way wouldideally be easily observed, long-lived, and small in diameterso as not to perturb the vortex dynamics under study. Micron-scale hydrogen ice particles have been used [1], [2], but He

Life cycle of superfluid vortices and quantum turbulence in the unitary Fermi gas

The unitary Fermi gas (UFG) offers an unique opportunity to study quantum turbulence both experimentally and theoretically in a strongly interacting fermionic superfluid. It yields to accurate and controlled experiments, and admits the only dynamical microscopic description via time-dependent density functional theory (DFT) - apart from dilute bosonic gases - of the crossing and reconnection of superfluid vortex lines conjectured by Feynman in 1955 to be at the origin of quantum turbulence in superfluids at zero temperature. We demonstrate how various vortex configurations can be generated by using well established experimental techniques: laser stirring and phase imprinting. New imagining techniques demonstrated by the MIT group [Ku et al. arXiv:1402.7052] should be able to directly visualize these crossings and reconnections in greater detail than performed so far in liquid helium. We demonstrate the critical role played by the geometry of the trap in the formation and dynamics of a vortex in the UFG and how laser stirring and phase imprint can be used to create vortex tangles with clear signatures of the onset of quantum turbulence.

Quantum turbulence in superfluids with wall-clamped normal component

In Fermi superfluids, such as superfluid (3)He, the viscous normal component can be considered to be stationary with respect to the container. The normal component interacts with the superfluid component via mutual friction, which damps the motion of quantized vortex lines and eventually couples the superfluid component to the container. With decreasing temperature and mutual friction, the internal dynamics of the superfluid component becomes more important compared with the damping and coupling effects from the normal component. As a result profound changes in superfluid dynamics are observed: the temperature-dependent transition from laminar to turbulent vortex motion and the decoupling from the reference frame of the container at even lower temperatures.

Observations of Vortex Emissions from Superfluid 4He Turbulence at High Temperatures

An immersed object with high velocity oscillations causes quantum turbu- lence in superfluid 4 He, even at very low temperatures. The continuously generated turbulence may emit vortex rings from a turbulent region. In the present work, we report vortex emissions from quantum turbulence in superfluid 4 He at high temper- atures, by using three vibrating wires as a turbulence generator and vortex detec- tors. Two detector wires were mounted beside a generator wire: one in parallel and the other in perpendicular to the oscillation direction of the generator. The detection times of vortex rings represent an exponential distribution with a delay time t0 and a mean detection period t1. The delay time includes the generation time of a fully developed turbulence and the time-of-flight of a vortex ring. At high temperatures, vortices are dissipated by relative motion between a normal fluid component and the vortices, resulting that only large vortex rings are reachable to the detectors. Using this method, we detected vortex rings with a diameter of 100 µm, comparable to a peak-to-peak vibration amplitude of 104 µm of the generator. The large vortices ob- served here are emitted anisotropically from the generator. The emissions parallel to the vibrating direction are much less than those perpendicular to the direction.

Time-of-Flight Measurements of Vortices Emitted from Quantum Turbulence in Superfluid 4He

An oscillating obstacle generates quantum turbulence in superfluids, when vortices remained attached to obstacle surfaces or vortex rings collided with it during oscillation. Turbulence provides a source of vortices; however, the characteristics of these vortices are not clear. In the present work, we report the flight of vortices emitted from quantum turbulence in superfluid 4He at low temperatures, using vibrating wires as a generator and a detector of vortices. A vortex-free vibrating wire can detect only the first colliding vortex ring, though it will be refreshed after low vibration and be able to detect a vortex ring again. By measuring a period from the start of turbulence generation to the vortex detection repeatedly, we find an exponential distribution of time-of-flights with a non-detection period t0 and a mean detection period t1, suggesting a Poisson process. Both periods t0 and t1 increase with increasing distance between a generator and a detector. A vortex flight velocity estimated from period t0 suggests that the sizes of the emitted vortex rings distribute to a range smaller than a generator thickness or a generator vibration amplitude. Vortices are emitted radially from a turbulence region, at least in the direction of oscillator vibration.

A New Device for Studying Low or Zero Frequency Mechanical Motion at Very Low Temperatures

We have developed a new “floppy wire” device for studying the motion through quantum fluids and solids at very low temperatures. The device is particularly well suited for producing large amplitudes of motion, for measuring drag forces at low frequency, and for studying “zero” frequency dynamics by measuring transient behavior. The device is very versatile and allows motion to be studied over a broad range of velocities and amplitudes. It generates negligible heat leaks and so is ideally suited for ultra low temperature experiments. The device has many potential applications in quantum fluids and solids research, including the study of drag forces at low frequencies in both the laminar and turbulent flow regimes, and the investigation of motion in (super)solid 4He. We discuss the principles and modes of operation of the device and present some preliminary measurements in vacuum, in normal liquid 3He and in superfluid 4He. We also present measurements of a “floppy grid” device, which could be used for generating large volumes of quantum turbulence in superfluids at low temperatures.

Weak turbulence of Kelvin waves in superfluid He

The physics of small-scale quantum turbulence in superfluids is essentially based on knowledge of the energy spectrum of Kelvin waves, Ek. Here we derive a new type of kinetic equation for Kelvin waves on quantized vortex filaments with random large-scale curvature which describes a step-by-step energy cascade over scales resulting from five-wave interactions. This approach replaces the earlier six-wave theory, which has recently been shown to be inconsistent owing to nonlocalization Solving the four-wave kinetic equation, we found a new local spectrum with a universal (curvature-independent) exponent, Ek∝k−5∕3, which must replace the nonlocal spectrum of the six-wave theory, Ek∝k−7∕5 in any future theory, e.g., when determining the quantum turbulence decay rate, found by Kosik and Svistunov under an incorrect assumption of locality of energy transfer in six-wave interactions.