Search results for "LIQUID HELIUM"
showing 10 items of 54 documents
Vortex density waves and high-frequency second sound in superfluid turbulence hydrodynamics
2010
In this paper we show that a recent hydrodynamical model of superfluid turbulence describes vortex density waves and their effects on the speed of high-frequency second sound. In this frequency regime, the vortex dynamics is not purely diffusive, as for low frequencies, but exhibits ondulatory features, whose influence on the second sound is here explored.
Hydrodynamic equations of anisotropic, polarized and inhomogeneous superfluid vortex tangles
2008
We include the effects of anisotropy and polarization in the hydrodynamics of inhomogeneous vortex tangles, thus generalizing the well known Hall-Vinen-Bekarevich-Khalatnikov equations, which do not take them in consideration. These effects contribute to the mutual friction force ${\bf F}_{ns}$ between normal and superfluid components and to the vortex tension force $\rho_s{\bf T}$. These equations are complemented by an evolution equation for the vortex line density $L$, which takes into account these contributions. These equations are expected to be more suitable than the usual ones for rotating counterflows, or turbulence behind a cylinder, or turbulence produced by a grid of parallel th…
On the operation of silicon photomultipliers at temperatures of 1–4 kelvin
2016
Abstract SiPM operation at cryogenic temperatures fails for many common devices. A particular type from Zecotek with deep channels in the silicon substrate instead of quenching resistors was tested at liquid helium temperature. Two similar types were thoroughly characterized from room temperature down to liquid nitrogen temperature by illuminating them with low light levels. At cryogenic temperatures the SiPMs show an unchanged rise-time and a fast recovery time, practically no after-pulses, and exhibit no increased cross-talk probability. Charge collection spectra were measured to extract the pixel gain and its variation, both comparable to room temperature at the same over-voltage. The qu…
Longitudinal counterflow in turbulent liquid helium: velocity profile of the normal component
2013
In this paper, the velocity profile of the normal component in the stationary flow of turbulent superfluid helium inside a cylindrical channel is determined, making use of a one-fluid model with internal variables derived from Extended Thermodynamics. In the hypothesis of null barycentric velocity of the fluid (the so-called counterflow situation) it is seen that, in the presence of a sufficiently high vortex length density, the velocity profile of the normal component becomes very flat in the central region of the channel. Thus, a central flat profile of the normal fluid does not necessarily imply that the flow of the normal component is turbulent.
Transition to superfluidity in liquid 4He
2012
In this work the transition from normal liquid helium I to superfluid liquid helium II, controlled by temperature and pressure, is studied in the simplified assumption of absence of viscosity. A macroscopic thermodynamical model is presented, which chooses as new independent fields the heat flux q and a phase field function f. For the heat flux a modification of Cattaneo equation is written, while for the function f a time dependent Ginzburg-Landau equation is proposed.
Thermal Excitation of Fourth Sound in Liquid Helium II
1974
In narrow channels filled with helium II two wave modes propagate, fourth sound1–3 and the fifth wave mode.4–6 According to new results of theoretical studies,7–8 it has been predicted that it should be possible to excite both wave modes mechanically by vibrating the diaphragm of a condenser microphone as well as thermally by periodically heating the surface of a solid body (e.g., a resistance layer). Shapiro and Rudnick3 have produced and detected fourth-sound signals mechanically. In the present contribution it will be experimentally verified that it is also possible to excite fourth sound thermally. Since the theory7,8 gives the result that fourth sound can be generated thermally only if…
Production of zero energy radioactive beams through extraction across superfluid helium surface
2003
A radioactive Ra-223 source was immersed in superfluid helium at 1.2-1.7 K. Electric fields transported recoiled Rn-219 ions in the form of snowballs to the surface and further extracted them across the surface. The ions were focussed onto an aluminium foil and alpha particle spectra were taken with a surface barrier spectrometer. This enabled us to determine the efficiency for each process unambiguously. The pulsed second sound wave proved effective in enhancing the extraction of positive ions from the surface. Thus we offer a novel method for study of impurities in superfluid helium and propose this method for production of zero energy nuclear beams for use at radioactive ion beam facilit…
Microscopic study of He2-SF6 trimers
2003
The He2-SF6 trimers, in their different He isotopic combinations, are studied in the framework of both the correlated Jastrow approach and the correlated hyperspherical harmonics (CHH) expansion method. The energetics and structure of the He-SF6 dimers are analyzed, and the existence of a characteristic rotational band in the excitation spectrum is discussed, as well as the isotopic differences. The binding energies and the spatial properties of the trimers, in their ground and lowest lying excited states, obtained by the Jastrow ansatz are in excellent agreement with the results of the converged CHH expansion. The introduction of the He-He correlation makes all trimers bound by largely sup…
The development of the KATRIN magnet system
2006
The Karlsruhe Tritium Neutrino Experiment KATRIN aims to measure the mass of the electron neutrino with unprecedented accuracy. For this experiment a special magnet system with about 30 LHe bath cooled superconducting magnets grouped in 10 different sections needs to be developed. The magnetic fields required for the electron transport and spectrometer resolution have a level between 3 and 6 T and must be constant in time over months. Further requirements for field homogeneity and tritium compatibility lead to a unique magnet system. A challenging task of this system is to keep the 10 m beam tube of the source magnet at a constant temperature of 30 K with extremely high temperature stabilit…
Velocity of the fourth sound in liquid helium II via extended thermodynamics
2003
This work continues a study begun in previous works, where, using Extended Thermodynamics, a monofluid model of liquid helium II is formulated. The wave propagation in bulk liquid helium II is studied in the hypothesis that the thermal dilatation is not zero. The propagation of fourth sound, studied previously neglecting both the thermal dilatation and finite volume of the powder, is studied without these simplified hypotheses: a scattering correction n is introduced to take into account the porosity. The model is more general than the standard two-fluid model because it allows that a small amount of entropy is associated with helium when it flows through a very thin capillary or a porous m…