Second-sound Shock Waves Research Articles

Multichannel system for fast measurement of temperatures at 1-4.2K

The report presents a set of electronics designed for investigation of fast thermal processes taking place in He I and He II. Its main features are as follows. First, temperature field is measured at several distanced points in short time. This is useful, for example, while dealing with second-sound shock waves, superfluid turbulence dynamics, etc. Second, minimal wiring is sufficient for multipoint temperature measurement what is principal, especially, in the case of forced flow cryostats. Third, the measurement procedure allows to achieve rather high precision with the use of rather low-sensitive Ge thermometers. Warm and cold tests have shown the system efficiency and reliability.

Producing and recording converging second-sound shock waves

Two different designs of convergent counterflow channels are presented. The imploding second-sound wave was produced by a vacuum-deposited chromium film on the inner curved surface of these channels, and the temperature was measured by a bolometer whose distance from the heated surface could be externally adjusted. If the average heat input is small enough to avoid the production of superfluid vorticity, which is the main dissipative process, acoustic cylindrical waves can be observed. The temporal evolution of the perturbation temperature follows the geometrical predictions.

The evolution of axisymmetric rectangular second-sound (heat) pulses in superfluid helium

Very little attention has been given, so far, to the problem of the propagation of axisymmetric second-sound shock waves, although they may occur in practical applications. In this paper, research on the propagation of moderate cylindrical heat pulses in superfluid helium is reported. A theoretical model is built based on the experimental observation of the evolution of the structure of rectangular heat pulses. The agreement between the experimental measurements and the theoretical calculations of the evolution of the main parameters is very satisfactory.

Schlieren photography of second-sound shock waves in superfluid helium

A conventional schlieren system has been used to study second-sound shock waves produced in a shock tube with optical windows. Initially planar shocks are observed to remain so, even after several reflections from the end walls. No interaction of the shocks with the side wall boundary layers is seen, attesting to the thinness of these layers. Second-sound shocks are reflected from the liquid–vapor interface, producing transmitted gasdynamic shocks and reflected second-sound shocks. Several strong shocks are fired with short separation times, generating observable fluctuations in the fluid.

A HeI-HeII Interface and Second Sound Shock Waves near the Superfluid Transition

Etude basee sur le modele phenomenologique de Guinzburg-Landau dependant du temps. L'interface He I-He II et les ondes de choc du second son pres de la transition superfluide sont des solutions kinks des equations

Using second-sound shock waves to probe the intrinsic critical velocity of liquid helium II

A critical velocity truly intrinsic to liquid helium II is experimentally sought in the bulk fluid far from the apparatus walls. Termed the ‘‘fundamental critical velocity,’’ it necessarily is caused by mutual interactions which operate between the two fluid components and which are activated at large relative velocities. It is argued that flow induced by second-sound shock waves provides the ideal means by which to activate and isolate the fundamental critical velocity from other extraneous fluid–wall interactions. Experimentally it is found that large-amplitude second-sound shock waves initiate a breakdown in the superfluidity of helium II, which is dramatically manifested as a limit to the maximum attainable shock strength. This breakdown is shown to be caused by a fundamental critical velocity. Secondary effects include boiling for ambient pressures near the saturated vapor pressure or the formation of helium I boundary layers at higher ambient pressures. When compared to the intrinsic critical velocity discovered in highly restricted geometries, the shock-induced critical velocity displays a similar temperature dependence and is the same order of magnitude.

Experiments on the circulation and propagation of large-scale vortex rings in He II

The normal-fluid and superfluid vorticity, produced by forcing He II out of a tube 0.8 cm in inner diameter at velocities of about 10 cm/sec, is investigated in the region in front of the tube orifice. The method of circulation measurement uses the flow-induced variations of the traveling times of second-sound shock waves. The evolution of large-scale vortex rings with a nearly homogeneous vorticity distribution is observed and measured. Their translational velocity and circulation are measured at temperatures 1.28 K≤T≤2.02 K in the region 1–7 orifice diameters downstream from the orifice. Strong evidence for a marked dependence of the normal-fluid circulation in the vortex ring on temperature and time is found. It points out the importance of the mutual friction force for both macroscopic velocity fields.

Experimental investigations on fast gold-tin metal film second-sound detectors and their application

In order to optimize gold-tin metal film probes as bolometers for measurement of fast second-sound signals in He II, the influence of their composition and of annealing on their electrical properties at low temperatures was investigated systematically. Measurements of the dependence of the voltage-current characteristics on temperature were supplemented by electron microscopy of the film surfaces and by measurement of the resistivity of the composed films at room temperature as a function of the atomic percentage of tin. Fast interdiffusion, which took place after evaporating typically 1000 A of tin onto 200 A of gold, resulted in the development of different phases of the Au-Sn system in the layer. Due to proximity effects between grains of these phases the transition temperature of such a film is reduced to the temperature range of He II. Different types of weak-link behavior which may govern the current-induced breakdown of superconductivity are discussed. As an example of the application of the metal film probes, the circulation of a macroscopic vortex ring was measured using the flow-induced variation of the traveling times of second-sound shock waves. Values of up to several cm2/sec were thus obtained to an accuracy of some 10−2 cm2/sec.

New experimental results obtained with second-sound shock waves

Second-sound shock waves are used to study the nonlinear dynamics of superfluid helium II and, especially, intrinsic interactions arising between the two-fluid components. Three new results are summarized: 1) formation of double-shock pulses; 2) formation of helium I boundary layers; and 3) identification of a fundamental critical velocity.

Experimental investigation of the circulation of large scale vortex rings in HE II

The vorticity produced by forcing He II out of a tube 8 mm in diameter was investigated in the region in front of the tube orifice. By a non-disturbing method of measurement based on the flow-induced differences in running times of second-sound shock waves, the formation of large scale vortex rings was observed and measured. The circulation of their normalfluid velocity field was found to be not conserved.

A fast response superconducting thin-film probe for detection of second-sound shock waves in superfluid helium

Fast response probes are needed for studying the formation and propagation of second-sound shock waves (and for applying such waves to special measuring purposes) in superfluid helium. Newly developed superconducting thin-film probes enable shock-front rise times of down to 0.3 μs to be detected at signal-to-noise ratios higher than about 100. Using high vacuum evaporation techniques, such probes are relatively easy to produce. Their main body consists of a cylindrical quartz glass rod 1.5 mm in diameter with one end face polished to a plane of optical quality. The sensor strip is deposited onto this plane face as a two-component film of 0.02 mm width and 1 mm length. The temperature variations due to second sound cause changes in the resistance of the film and thus, at constant bias current, variations of the voltage drop across it. The temperature where the film undergoes its steep transition to superconductance and where, therefore, the probe works at its greatest sensitivity, is primarily fixed by the ratio of the two components (tin and gold) of the film, but can be adjusted to special values via the magnetic field produced by the adjustable bias current. The high resolution in time which is achievable by this probe makes it useful for accurate measurement of even small variations of the running time of second-sound shock waves. Such variations may be caused by flows, as is shown in the case of a flow produced by a rotating vane; their measurement may, therefore, serve as a tool for flow investigation.

Experiments on second-sound shock waves in superfluid helium

The waveform and velocity of second-sound waves in superfluid helium have been studied experimentally using superconducting, thin-film probes. The second-sound waves were generated with electrical pulses through a resistive film. Variations in pulse power, pulse duration, and bath temperature were examined. As predicted theoretically, the formation of a shock was observed at the leading or trailing edge of the waves depending on bath temperature. Breakdown of the theoretical model was observed for large pulse powers. Accurate data for the acoustic second-sound speed were derived from the measurements of shock-wave velocities and are compared with previous results.