Normal Helium Research Articles

The Nature of the Chemical Anomalies of the Sirius A

On the basis of the analysis of the observed equivalent line widths of FeI, FeII, and TiII in the spectra of Sirius A and Omicron Peg and calculation of abundances of these elements, the oscillator strengths of the lines used are refined. With the improved oscillator strengths, the iron and titanium abundances in the atmosphere of Sirius A are obtained with a higher accuracy than previously:lgN(FeI) = 7.899 ±0.011,lgN(FeII) = 7.908±0.010,lgN(TiII) =5.30±0.02.The improved accuracy allowed one to conclude that the surface magnetic field is absent in the atmosphere of Sirius A: H =3D 0±100 Gs. The equivalent widths of 7 helium lines of the red region of the spectrum are measured. The calculations of these lines (with allowance for their blending with lines of other elements) show normal helium abundance. Following a comparison of features of Sirius A and Omicron Peg, a suggestion is advanced on possible reasons for the existence of Am stars.

Optimized growth conditions for solid Ar and Kr moderators in slow positron beam

Solid rare-gas moderators can be used for producing slow positrons. The most natural choices for this kind of moderators are Ar and Kr because they are easily available, and their solidification temperature can be achieved by a normal two-stage helium cryopump. In this work an encapsulated 22Na source and a cone-shaped moderator were installed in a helium cryopump for testing the solid Ar and Kr moderators. As a result we found a moderation efficiency of (9.5 ± 1.0) × 10 −4 for Ar and (1.2 ± 0.1) × 10 −3 for Kr. the yield of these moderators decreased 12% and 7%, respectively, in 10 h.

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.

Electron transport behavior in normal and abnormal direct current discharge cathode sheath

This paper studies the electron transport behavior in normal and abnormal helium direct current (dc) glow discharges using a Monte Carlo simulation, in which the influences of the electron initial energy and cathode surface reflection on the electron swarm behavior are considered. The electron-neutral atom collision processes and the electron swarm parameters (such as the number of collisions, collision rates, ionization coefficient, mean drift velocity, electron density, etc.) are analyzed. The results show that the variations of the initial energy of the electron emitted from the cathode and the cathode reflection cause a weak effect on the electron transport parameters in the two discharges. The results also indicate that the electron transport parameters are quite different in normal and abnormal discharges.

A search for stable strange quark matter nuggets in helium

A search for stable strange quark nuggets has been conducted in helium and argon using a high sensitivity mass spectrometer. The search was guided by a mass formula for strange quark nuggets which suggested that stable strange helium might exist at a mass around 65 u. The chemical similarity of such “strangelets” to noble gas atoms and the gravitational unboundedness of normal helium result in a large enhancement in the sensitivity of such a search. An abundance limit of no more than 2 · 10 −11 strangelets per normal nucleus is imposed by our search over a mass region from 42 to 82 u, with much more stringent limits at most (non-integer) masses.

‘Underflow’ introduction system for solid and liquid samples in bench‐top gas chromatographic/mass spectrometric systems

AbstractA simple, inexpensive inlet system was developed for the introduction of liquid and solid samples, to be employed in commercial gas chromatographic/mass spectrometric instruments, and not requiring sample injection through the chromatographic column. The system does not need any modifications of the ion source housing and pumping system and is based on admitting the sample vapour as an underflow to the normal helium flow through the chromatographic column at a location near the transfer line.

Electric-field dependence of muonium formation in liquid helium

The influence of electric fields on the formation of muonium in liquid helium (4He,3He, and mixture of4He + 0.2%3He) has been studied. It was found that the relative distribution of muon-electron pairs is anisotropic. The maximum muon density is shifted with respect to the electrons in the direction of the initial muon momentum. Due to the anisotropy the muonium asymmetry in normal liquid helium is enhanced by a factor of 3 in an electric fieldE=1 kV/cm.

Development of the 19 T high field magnet system

A high field magnet system, up to 19 tesla at 1.8K, with magnet bore of 75mm was developed. The magnet consists of Nb/sub 3/Sn solenoids (3 sections) and NbTi solenoids (2 sections). A liquid helium vessel is divided into two parts (4.2K upper part and 1.8K lower part) by a fiberglass reinforced plastics separator. The central field is 17 tesla at 4.2K with normal liquid helium and 19 tesla at 1.8K with pressurized superfluid helium. This magnet can be operated in persistent mode and the field stability at that time is less than 10ppm/hour. The field homogeneity is better than 0.1% at 50 mm sphere volume. The stored energy is about 3MJ. A small refrigerator was installed on the top flange of the cryostat to reduce the evaporation rate of liquid helium and to maintain the superconducting magnet at low temperature while pausing. This system is used for the measurement of critical current and other physical properties of superconducting wire and other materials such as high T/sub c/ superconductors. >

Thermal stratification of liquid helium in the SHOOT dewars

The Superfluid Helium On-Orbit Transfer (SHOOT) project was a shuttle payload designed to demonstrate the technology needed to transfer superfluid helium between two Dewars in space. It was launched with normal liquid helium which was pumped to superfluid temperatures on-orbit. During the 93h prelaunch ground hold, the liquid was thermally stratified, with temperatures of ≈4.3 K at the top and rising slowly from 2.2K at the bottom of each Dewar over time. This stratification was helpful, as it allowed for more efficient removal of the parasitic heat inputs. There were also temperature gradients in the Dewars during the on-orbit pumpdown, with a much higher degree of beneficial stratification in the port Dewar than in the starboard. We describe the liquid temperature profiles during ground hold and on-orbit pumpdown, discussing the effects of this stratification and comparing the performances of the two Dewars. We also discuss the results of a computer model of the isothermal pumpdown of a SHOOT Dewar on-orbit.

Thermal response of normal and superfluid helium to an AC heat flux

Measurements of the thermal response, ΔT(ω), across a layer of helium in its normal and superfluid phases have been made for an AC heat source, Q(t)=Q oexp(iωt) . Previous DC measurements below T λ have indicated a dependence on the layer thickness, d, for κ eff at low concentrations, X, suggesting an intrinsic length scale within the fluid. A ΔT(ω) profile is ideally suited to reveal any internal length scales of the fluid. The apparatus also permits d to be varied continuously and in situ from zero to 3mm.

The transfer of superfluid helium in space

The Superfluid Helium On-Orbit Transfer (SHOOT) Flight Demonstration waspart of the space shuttle STS-57 mission in June, 1993. The experiment objectives included: transfer of superfluid between two dewars m a low gravity environment at different flow rates; operation of two different liquid acquisition devices within the dewars; liquid/vapor phase separation for normal liquid helium as well as superfluid at varying venting rates; accurate mass gauging and flow metering; and autonomous control of the transfer process by an expert system aboard a computer on the shuttle. This paper summarizes the findings of the SHOOT experiment and the performance of individual components, with particular emphasis on the anomalous behavior of one of the phase separators.

Low gravity thermal stratification of liquid helium on SHOOT

The Superfluid Helium On-Orbit Transfer (SHOOT) Flight Demonstration is a Shuttle attached payload designed to demonstrate the technology needed to transfer superfluid helium in space. It is a unique liquid helium payload in that it will launch with normal helium which will be pumped to superfluid temperatures after orbit insertion by means of a liquid/vapour phase separator capable of operating with both He-I and He-II at its inlet. Tests of this phase separator in one gravity have shown that when the phase separator provides cooling to the bottom of the Dewar (minimizing convection) the bulk liquid thermally stratifies. The stratification can be so pronounced that a distinct phase boundary appears between He-I at 4.2 K above He-II just a T λ which exhibits behaviour characteristic of a liquid/vapour boundary or that between two liquids. In the very low gravity environment of space it is expected that such an interface will occur and, once established, will persist due to the difference in surface tension between He-I and He-II and the poor thermal conductivity of He-I. For SHOOT, thermal stratification is a benefit both during ground hold on the launch pad and after launch. When stratification occurs during ground hold parasitic heat inputs are more efficiently removed. After launch, the pressure inside the Dewars will be enhanced if the liquid stratifies, resulting in higher mass flow rates through the phase separator and consequently shorter pumpdown times. Estimates of the extent and impact of thermal stratification will be presented as well as predictions of the bheaviour of the He-I/He-II boundary.

Unified open tubular column chromatography: Sequential gas chromatography, at normal pressures, and supercritical fluid chromatography on the same column

AbstractA unified chromatograph was constructed in which open tubular column gas chromatography, at normal helium carrier gas pressures, followed by supercritical fluid chromatography with carbon dioxide mobile phase was carried out in a single run using a 100‐μm i. d. column. Sample introduction was accomplished by means of a pneumatically‐actuated rotary injection valve. A second rotary valve was used post‐column to direct the flow of column effluent to the flame ionization detector through an uncoated fused silica restrictor during the GC separation mode or to the FID through a frit restrictor during the SFC separation mode. This novel valve switching arrangement allowed system operating parameters of temperature and pressure programs and linear velocities to be optimized independently of each separation mode. The unified GC/SFC system was demonstrated for practical applications in various fields of process and product quality control.

Sealing performance of gaskets and flanges against superfluid helium

A series of tests has been carried out on the sealing characteristics of different combinations of commercially available gaskets and flanges against superfluid helium. The aluminium Helicoflex gasket with a stainless steel or aluminium alloy flange was effective against leakage of normal liquid helium but exhibited leakage with superfluid helium. However, copper-, silver-, nickel- and indium-coated aluminium Helicoflex gaskets with a stainless steel or aluminium alloy flange were found to prevent leakage of superfluid helium.

Quench propagation in a cable-in-conduit force-cooled superconductor-preliminary results

A preliminary test was performed to measure quench propagation in a cable-in-conduit superconductor. Although the data are not extensive, the behavior of the sample was similar to that reported by T. Ando et al. (1989) for tests performed at the same current densities (though at 7-T field). The propagation increased with time, a phenomenon that can only be explained by thermal hydraulics of the coolant. The maximum propagation velocity was about 5 m/s at a current density of 100 A/mm/sup 2/. The propagation velocity (tens of meters per second) predicted by others was not observed. Based on the measured initial normal zone hot helium expansion velocity, the condition for use of the finish time formula of L. Dresner was not met in either the present experiment or in Ando's experiment. It is not clear whether the observed slightly higher power dependence of normal zone velocity on elapsed time is due to changes in helium expansion velocity or is a result of THQ (thermal hydraulic quenchback). Further studies, both analytical and experimental, are needed before the existence of THQ can be verified.

Longitudinal propagation of a normal phase along a superconducting wire and its transient heat evacuation to a normal helium bath

The longitudinal destruction of the superconducting state along a wire by an electrical subcritical current can occur if a strong enough thermal disturbance appears somewhere along the wire. The longitudinal expansion velocity of the normal phase along the wire is then strongly dependent on the different heat production and evacuation processes. As for a wire in a superfluid helium bath, a steady boundary thermal conductance across the wire wall and the normal helium bath can be deduced by adjusting the theoretical data with the experimental data. For the temperature range considered in this paper, the steady boundary thermal conductance seems to be independent of the helium bath temperature but not of the wire surface morphology. It is also shown that the experimental and theoretical data are in strong disagreement unless a transient term in the heat transferred across the wire wall and the liquid He I separating interface is introduced in the theoretical model. A transient heat transfer coefficient is defined, and its dependence on the different wire characteristics is investigated. An empirical relationship between the transient heat transfer coefficient and the wire characteristics is deduced for a given bath temperature. The transient heat transfer coefficient is shown to depend on the maximum thermal flux released into the bath across the wire wall and not on the wire surface morphology.

Pressure drop and He II flow through fine mesh screens

Fluid acquisition systems for He II transfer devices will utilise gallery arms to ensure that the fluid encounters the pump inlet. In near term experiments such as Superfluid Helium on Orbit Transfer (SHOOT), the preferred configuration consists of several rectangular channels which have one side made from a Dutch weave stainless steel screen having 325 × 2300 wires per inch. The effective pore diameter for this screen is about 5 μm. The present paper reports on measurements of pressure drop across a screen when it is subjected to a flow of liquid helium. The experiment measures the time rate of change of the level in two different helium reservoirs connected by a screen-blocked channel. Results with normal helium are compared with predictions based on the Armour-Cannon equations. The He II data show considerable deviation from the classical result. A discussion of the He II pressure drop results in terms of two fluid hydrodynamics is included.

Calculation of normal zone propagating velocities and transient thermal effects dependence on the superconducting wire parameters

Normal phase propagation velocities along superconducting wires immersed in a cooling normal helium bath were investigated experimentally and theoretically. The measurements have been done for bare tin wires with different radii and resistivities. The theoretical calculations of the normal zone propagation velocity versus the circulating current assume a transient mechanism for heat transfer between the wire wall and the normal helium bath. The transient-heat-transfer term is of the form alpha theta /sup 2/ delta theta / delta t, where theta is the temperature relative to the normal helium bath temperature. The transient-heat-transfer coefficient, alpha , is then defined, and good agreement between the theoretical and experimental data is obtained. The coefficient depends on the bath temperature and, nonrandomly, on the maximum power per unit surface released to the normal helium bath, which in turn depends on the wire radius, critical current, and resistivity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Cavitation in flowing superfluid helium

Flowing superfluid helium cavitates much more readily than normal liquid helium, and there is a marked difference in the cavitation behaviour of the two fluids as the lambda point is traversed. Examples of cavitation in a turbine meter and centrifugal pump are given together with measurements of the cavitation strength of flowing superfluid helium. We attribute the unusual cavitation behaviour of superfluid helium to its immense thermal conductivity.

Mechanical pumps for superfluid helium transfer in space

Two alternate mechanical pump concepts have been identified for the transfer of superfluid helium in space. Both pumps provide flow at sufficient head and have operating characteristics suitable for the Space Infrared Telescope Facility (SIRTF) refill mission. One pump operates at a relatively low speed and utilizes mechanical roller bearings, while the other operates at a higher rotational speed using either electromagnetic or tilting pad gas-dynamic bearings. The use of gas bearings requires transfer of normal helium so that the gas pressure within the pump casing is high enough to operate the bearings. The operating characteristics of both pumps are predicted, the dimensions are estimated and major technology issues are identified. The major issues for each pump design are cavitation performance and bearing development. Roller bearings require quantified reliability for operation in space while electromagnetic bearings require basic development as well as a complex control system. The low speed pump has significantly poorer hydraulic efficiency than the high speed pump.