Collapse of Vapor-Filled Bubbles in Liquid Helium

Multielectron bubbles (MEBs) are charged cavities in liquid helium which provide an interesting platform for the study of electrons on curved surfaces. Very recently, we have reported an experiment to trap these objects in a two-dimensional Paul trap, where they could be observed from ten to hundreds of milliseconds. During this time, the vapor inside the bubble condensed which resulted in a steady reduction in their size such that beyond a certain time the MEBs could no longer be detected. In this paper, we present experimental data on the lifetime of the bubbles as a function of their initial radius and compare the results with a theoretical model.     

Fission of Multielectron Bubbles in Liquid Helium Under Electric Fields

Multielectron bubbles (MEBs) are cavities in liquid helium which contain a layer of electrons trapped within few nanometres from their inner surfaces. These bubbles are promising candidates to probe a system of interacting electrons in curved geometries, but have been subjected to limited experimental investigation. Here, we report on the observation of fission of MEBs under strong electric fields, which arises due to fast rearrangement of electrons inside the bubbles, leading to their deformation and eventually instability. We measured the electrons to be distributed unequally between the daughter bubbles which could be used to control the charge density inside MEBs.     

Thermal acoustic oscillations in triple point liquid hydrogen systems

Thermal acoustic oscillations are often observed in tubes which penetrate a cryogenic system and are closed at the warm end and open at the cold end. Such tubes are genrally used for filling or vetning the tank, providing relief pressure or inserting instruments taps. Large amounts of heat (of the order of ten to a thousand times more than by normal heat conduction) can be transferred into a cryogenic system when such thermaloscillations occur. A number of studies examining thermal acoustic oscillations in liquid helium systems have been performed by Rott et al. However, only minimal consideration has been given to such oscillationsin liquid and sluch hydrogen systems. This study extends Rott's theory to the stability aspects of thermal acoustic oscillations for a straight tube closed at the warm end and inserted into a Dewar flask filled with triple point liquid hydrogen when the cold open end is located above the liquid surface. These results can also be applied to a slush hydrogen when the pressure in the Dewar flask is reduced to the triple point pressure of hydrogen. Numerical results have been obtained in this study for developing stability curves, establishing oscillation frequency characteristics and identifying critical configurations for initiating such oscillation. The mechanisms associated with the two branches of the stability curves for thermal acoustic oscillations have also been investigated.     

Thermodynamics and nucleation of bubbles in normal and superfluid liquid helium-4 at negative pressures

We report on a theoretical investigation of liquid helium-4 at negative pressures. For normal liquid helium we estimate the thermodynamic functions for negative pressure via extrapolation of measurements made for positive pressures. We determine the free energy as a function of density and temperature and find the location of the liquid-vapor spinodal. The results of these calculations are used to construct a temperature-dependent density-functional scheme to describe the inhomogeneous liquid. This density functional is then applied to calculate the rate at which bubbles nucleate in the liquid at negative pressures. We include a discussion of the properties of the superfluid phase based on the use of Landau's quasi-particle model.     

Nucleation of bubbles in liquid helium

We give a brief survey of experiments that have been performed to study the nucleation of bubbles in liquid helium at negative pressures.     

Empirical correlations for the solubility of pressurant gases in cryogenic propellants

We have analyzed data published by others reporting the solubility of helium in liquid hydrogen, oxygen, and methane, and of nitrogen in liquid oxygen, to develop empirical correlations for the mole fraction of these pressurant gases in the liquid phase as a function of temperature and pressure. The data, compiled and provided by NIST, are from a variety of sources and covers a large range of liquid temperatures and pressures. The correlations were developed to yield accurate estimates of the mole fraction of the pressurant gas in the cryogenic liquid at temperature and pressures of interest to the propulsion community, yet the correlations developed are applicable over a much wider range. The mole fraction solubility of helium in all these liquids is less than 0.3% at the temperatures and pressures used in propulsion systems. When nitrogen is used as a pressurant for liquid oxygen, substantial contamination can result, though the diffusion into the liquid is slow.     

Heterogeneous Cavitation in Liquid Helium 4 Near a Glass Plate

We have used a focused acoustic wave to study cavitation, i. e. the nucleation of bubbles, in liquid helium 4 near a clean glass plate. From the reflectance of light at the glass/helium interface, we measured the amplitude of the acoustic wave in the focal region and the nucleation pressure. From an analysis of the transmitted light we also measured the nucleation probability. We observed three different regimes with different statistics and threshold pressures in the range 0 to –3 bar, significantly less negative than for homogeneous cavitation.     

Multielectron Bubbles in Helium and Wigner Crystallization

Multielectron bubbles (MEBs) in superfluid helium support a two-dimensional electron gas (2deg) on the surface of a sphere. In principle, electron densities can range from ∼ 10¹⁰ to 10¹⁴/cm². Wigner crystallization has been observed on a flat bulk surface of helium, but the maximum density of ∼ 2 × 10⁹ is insufficient to observe quantum melting. MEBs have a number of fascinating properties. They should not only allow observation of the Wigner lattice and quantum melting, but at very high densities a new type of electron localization into a lattice called the ripplopolaron lattice is predicted. We discuss a number of phenomena and experimental techniques aimed at long-time stabilization of MEBs.     

Plasma Modes of the Three-Chain Electron System Over Liquid Helium

We simulate the surface electron system over liquid helium subjected to a confinement potential in the electron layer plane. In the solid phase, the two-dimensional system is observed to transform first into a multi-chain system and, finally, into a three chain, a zigzag structure and a single chain. Both longitudinal and transversal plasma oscillations of the three-chain electron system are determined. One of the longitudinal oscillation modes is acoustic, whereas the other two are optical. On turn, all transversal oscillations branches are optical. The theoretical results obtained can be used to identify experimentally the configurations of the low-dimensional electron system over helium under confinement potential.     

Superfluid plug as a control device for helium coolant

New results have been obtained on the characteristics of the superfluid plug as a nonmechanical control device for supplying cold helium vapour on demand from a container of superfluid helium. The superfluid plug is a device which has been proposed for space applications to serve as a phase separator for liquid helium in the superfluid state. Typical plugs are made of a porous material having pores of one to ten μm in diameter. The experimental arrangement is such that one side of the plug is in contact with the superfluid liquid helium while vapour at a low pressure (of order 1 to 10 torr) is maintained on the other side.The data reported here are for a plug with approximately 5 μm diameter pores. Temperatures, pressures, and flow rate were monitored during the experiment. A theoretical background and steady state data are presented on mass flow rates and pressures as a function of liquid temperature.The typical response of the flow rate to a change in heat input from a heater is an exponential rise or fall to an equilibrium flow rate which is proportional to the amount of heat input. The time constants of the exponential changes were measured for two heater control modes under study. The study has included an investigation of the important parameters effecting the dynamic response of the plug including the superfluid properties, plug material properties, plug pore size and plug permeability. Operating temperatures from 1.5 K to the lambda point were investigated and heating rates up to two watts were applied. These tests serve to demonstrate that the superfluid plug can be employed as a flow control device in a control system designed to provide coolant on demand.     

Ionic currents modulated by film flow in superfluid helium

The oscillations of two helium baths coupled by the saturated film have been studied with a new method employing positive ions. The sensitivity of such a method for the variation of liquid-level height is found to be of the order of 10^–6 cm. The film current was observed to be modulated with a frequency double that of the level oscillations. This interesting new feature is discussed in the framework of the present theory of liquid helium.     

Theory of thermal waves

An analogy between thermal and electromagnetic waves is established. The conditions under which thermal oscillations can give rise to acoustical oscillations are derived. The coefficients of thermal conductivity in liquid helium are calculated as a function of the temperature below the lambda point.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 43, No. 3, pp. 431–438, September, 1982.     

Possible Sound Mode Conversion in “Superfluid 4He-97% Open Aerogel” System

We have studied the acoustic properties of liquid helium filled in various aerogels. The longitudinal ultrasound velocity and attenuation were measured at the frequency of 10 MHz with aerogels that had porosity from 92 to 97%. The mode intermediate between first and fourth sound was observed. The attenuation of this mode decreased with decreasing temperature for dense aerogels. However, an attenuation maximum was observed around 1.6 K for 97% open aerogel at various liquid pressures. In the present work, we discuss the possibility of the sound modes conversion between first, second sound in superfluid and aerogel sound mode in this composite system.     

Mass Flow through Solid 4H

We continue our solid ⁴He flow experiments in which we grow solid helium samples at constant temperature in the hcp region of the phase diagram. We exploit the properties of liquid helium in a confined geometry (porous Vycor glass), and induce a mass flow through the solid at pressures higher than the bulk melting pressure. We previously observed flow, but our temperature was limited to T≥350 mK. At T≈400 mK it was found that the flow ceased at P≈27 bar, and no flow was observed for T>550 mK. We have begun to extend our measurements to lower temperatures, and our data show that at lower temperatures we observe flow at higher pressures.     

Characteristics of germanium thin film thermometers for use at low temperatures

Germanium, thin films to be used as resistance thermometers have been tested at liquid helium temperature. Germanium is deposited in vacuum on insulated substrates and then silver is deposited onto the germanium films as ohmic contacts. Thermometers with desired resistances and suitable sensitivities can be easily fabricated by choosing proper deposition conditions. These thermometers have a quick response time and can be used to measure rapidly changing surface temperatures.The film resistance can be expressed as a function of temperature by a simple correlation, log R = C₀ + C₁ log T, between 4.2 K and 20 K. The sensitivity is approximately 20 Ω/K^?1 at 4.2 K. After 50 thermal cycles, the resistance at liquid helium temperature increased slightly, but is reproducible to within 0.8%. An estimated heat capacity of the germanium film is 2 × 10^?8 J K^?1, and a thermal relaxation time is of the order of 10^?12 s at 4.2 K.A stainless steel ribbon with the thermometers deposited on its surfaces is heated by a direct current in a liquid helium pool, and the surface temperature is measured. Large temperature oscillations due to occasional liquid solid contacts are observed.     

Stability tests on the euratom LCT conductor

The stability of the conductor against local heat pulses was measured as a function of magnetic field, transport current, temperature and cooling conditions. Comparing liquid helium cooling at 1 bar and supercritical cooling at 4 bar, the stability was found to be higher in liquid helium for transport currents above ≈ 70% /_c(B). Below that current value supercritical helium cooling yielded a higher stability.     

Thermodynamics of freezing and melting of4He in Vycor

Recently it was discovered that helium in confined geometries remains a liquid at pressures considerably above bulk melting. By combining a special technique for measuring pressures in small pores with thermal measurements, the phase diagram has been obtained for freezing and melting of⁴He in Vycor glass. The measurements cover the range of temperature from 0.8 to 2.5 K and extend to pressures of 6.0 MPa. At 1 K,⁴He is found to remain a liquid up to 4.2 MPa. The change in volume and latent heat on freezing are found and a complete analysis is given of the thermodynamics of freezing of helium in the Vycor.     

Gas-liquid chromatography with a volatile "stationary" liquid phase

A unique type of gas-liquid chromatography is described in which both mobile and "stationary" phases are composed of synthetic mixtures of helium and carbon dioxide. At temperatures below the critical point of the binary mixture and pressures above the vapor pressure of pure liquid carbon dioxide, helium and carbon dioxide can form two immiscible phases over extended composition ranges. A binary vapor phase enriched in helium can act as the mobile phase for chromatographic separations, whereas a CO2-rich liquid in equilibrium with the vapor phase, but condensed on the column wall, can act as a pseudostationary phase. Several examples of chromatographic separations obtained in "empty" capillary columns with no ordinary stationary liquid phase illustrate the range of conditions that produce such separations. In addition, several experiments are reported that confirm the proposed two-phase hypothesis. The possible consequences of the observed chromatographic phenomenon in the field of supercritical fluid chromatography with helium headspace carbon dioxide are discussed.     

The effect of high magnetic fields on metal foil strain gauges at 4.2 K

The behaviour of some metal foil strain gauges in magnetic fields up to 6 T at room temperature and at the boiling points of nitrogen and helium has been investigated. At room temperature the Constantan alloy gauges showed a significant magnetically induced negative zero shift. Although at room temperature and liquid nitrogen temperature the Karma-type alloy gauges tested showed no observable magnetoresistance, they all showed a significant positive magnetoresistance at liquid helium temperature.