Normal Helium Research Articles

The momentum distribution of normal and superfluid liquid 4He

Path-integral methods can be employed to calculate properties of boson systems at any temperature. We have performed a number of simulations of liquid helium and have calculated the momentum distribution as a function of temperature. We find that liquid helium has a condensate below 2 K but not above. The momentum distribution is found to be non-Gaussian even in normal liquid helium. We have also calculated the difference in the momentum distribution between a boson system and a distinguishable particle system.

Propagation of a normal zone along superconducting wires suspended in a liquid helium bath

Experimental and theoretical normal zone velocities have been obtained for superconducting tin wires in a normal and superfluid helium bath. Published theoretical calculations assume a flat normal-superconductor (N-S) boundary. A shape of the N-S boundary that results from a radial destruction of superconductivity in the moving front is calculated. For the sample in superfluid helium, the calculations are in good agreement with the experimental data. In normal helium no agreement between experimental and theoretical data can be obtained without introducing a transient heat transfer coefficient between the wire and the bath.

Transient heat transport in superfluid helium in cylindrical geometry

Heat transport in cylindrical space from a round superconductor immersed in a finite bath of superfluid has bean analyzed both numerically and analytically. The computer and closed analytical results are essentially the same. Analytical equations are given for the temperature of the helium bath as a function of radius, time, and heat flux from the conductor and for the time to reach the superfluid to normal helium transition temperature at the conductor as a function of heat flux.

Propagating Normal Zones and Transient Heat Transfer Effects in Superconducting Wires

Theoretical and experimental data of normal zone propagating velocities along superconducting wires immersed in a normal helium bath are presented. Longitudinal destruction of the superconductivity arises from thermal instabilities along the superconducting wire. Propagating normal zone velocities are thus strongly dependent on the thermal transfer mechanism between the helium bath and the superconducting wire. Transient heat transfer effects are necessary to obtain good agreement between experiment and theory. The used transient heat transfer coefficient depends on the bath temperature and on the sample resistivity as it can be seen from our results.

Horizontal-branch stars in NGC 6752 with strong helium lines

view Abstract Citations (7) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Horizontal-Branch Stars in NGC 6752 with Strong Helium Lines Crocker, Deborah A. ; Rood, Robert T. ; Oconnell, Robert W. Abstract Two hot horizontal-branch (HB) stars have been observed in the globular cluster NGC 6752. Both stars have strong He I lines at 4026A which indicate an abundance which is similar to that of Population I stars. These are the only observations of globular cluster HB stars showing apparently normal helium abundance. How these observations may be related to some unsolved problems of horizontal-branch morphology is considered. Publication: The Astrophysical Journal Pub Date: October 1986 DOI: 10.1086/184753 Bibcode: 1986ApJ...309L..23C Keywords: Abundance; Globular Clusters; Helium; Horizontal Branch Stars; Line Spectra; Stellar Spectrophotometry; Stellar Rotation; Astrophysics; CLUSTERS: GLOBULAR; STARS: ABUNDANCES; STARS: HORIZONTAL-BRANCH; STARS: ROTATION full text sources ADS | data products SIMBAD (5)

Centrifugal pump for superfluid helium

This Paper summarizes and correlates the liquid helium pump data obtained previously in two separate test programs. In normal helium the second set of data shows a large performance improvement over the first set as a result of changes in measurement methods and in the pump itself. Peak pump efficiencies of 46% were measured. The pump appeared to perform approximately the same in He II as in He I; however, the He II data are not adequate for system design or analyses. Therefore, a new pump test program is planned to test the improved version of the pump in an apparatus designed specifically for He II.

極低温におけるオーステナイト系ステンレス鋼の変形挙動と試料温度の変化

Measurements were made on the temperature rise of tensile test specimens of austenitic stainless steels undergoing plastic deformation and discontinuous flow in normal liquid helium and superfluid helium. As the strain rate increased, the ultimate tensile strength and elongation decreased and the temperature rise increased. Compared with the results in normal liquid helium, the frequency of stress drop increased and the temperature rise was suppressed in superfluid helium.

The effect of He3 on CO2 laser power

Substitution of He3 for normal helium in a sealed-off conventional CO2 laser resulted in power output increases as high as 14%. No such effect was observed in a narrow-bore waveguide laser, indicating that the observed improvements were due to more efficient heat conduction to the tube wall.

Temperature rise during the tensile test in superfluid helium

Measurements were made on the temperature rise of tensile test specimens undergoing plastic deformation and discontinuous flow in superfluid helium. Compared with the results in normal liquid helium, the frequency of load drop increased, and the temperature rise was suppressed.

Simple thermo-level meter for HeI and HeII by a dynamic method

A simple liquid helium level monitoring device has been developed using pairs of carbon resistors mounted on a stainless steel ‘dipstick’. The device described has a resolution of 50 mm and negligible power dissipation. The level of both superfluid and normal helium, and the liquid temperatures, can be determined. The device has no moving parts and after some years of use has proved to be sturdy and durable. A similar device is proposed with a resolution of 12 mm and a microcomputer data recording system.

A survey of spectral morphology and rotational velocities among the helium-rich stars

A spectroscopically homogeneous listing of all known members of the helium-rich B star category is presented together with uniform estimates of their rotational velocities. Equivalent widths of hydrogen, helium, and metal lines in the spectra of the helium-rich and comparison stars are given. The distribution of rotational velocities is found to be indistinguishable from that of normal early B dwarfs, except for a possible excess of rapid rotators. The existence of metal abundance anomalies in the helium-rich spectra is critically discussed, as is the identification of helium-rich stars as massive Population I objects with normal main sequence cores and surface helium enhancements, or as low mass highly evolved objects with processed cores.

Subharmonic phenomena in normal and superfluid helium

We have previously reported on observations of subharmonics occurring in parametrically driven shallow water gravity waves [R. Keolian and I. Rudnick, J. Acoust. Soc. Am. Suppl. 1 72, S13 (1982)]. The method depends on oscillating an annular resonator vertically with high enough amplitude so it exceeds the threshold for subharmonic generation. The apparatus has been adapted for low temperature work and liquid helium is now used instead of water. Our preliminary results are similar to those obtained in water including the occurrence of deep and ultradeep subharmonics and the appearance of chaos. Temperatures ranged above and below the lambda point. [Work supported by ONR.]

Studies of differential cross sections for electron impact ionisation of helium

Triple differential cross sections are obtained using different wavefunctions of the normal helium atom and with different expressions for the effective charges of the outgoing electrons. The results obtained are compared with experimental findings at intermediate and high energies. The exchange effect is considered at low energy. Double differential cross sections are calculated with a hydrogen-like wavefunction for helium and results are compared with experimental observations.

超流動ヘリウムＩＩでの気密シール

A series of of tests on the sealing characteristics of a number of gaskets and flanges against the superfluid helium below 2.17°K has been made. Commercially available aluminum Helicoflex with stainless steel and aluminum alloy flanges is tight to the normal liquid helium but leaks at the superfluid temperatures. However copper, silver and indium coated aluminum Helicoflex and Sureket (a gasket made of stainless steel tubing with silver coating) with stainless steel and aluminum alloy flanges are found to be tight to the superfluid helium.

Resistance creep of Allen-Bradley resistors at low temperatures

A simple liquid helium level monitoring device has been developed using pairs of carbon resistors mounted on a stainless steel ‘dipstick’. The device described has a resolution of 50 mm and negligible power dissipation. The level of both superfluid and normal helium, and the liquid temperatures, can be determined. The device has no moving parts and after some years of use has proved to be sturdy and durable. A similar device is proposed with a resolution of 12 mm and a microcomputer data recording system.

The stability of Nb 3Sn-composite conductors cooled by normal and superfluid helium

The stability of Nb 3Sn superconductors, which were additionally stabilized by soldering copper to the prereacted cable, was investigated. Cooling channels in a sample coil provided access to LHe in the 4.2 K and the 1.85 K temperature range. The measurements show that a heat flow rate of 0.6 W cm −2 at 4.2 K in the Nb 3Sn composite conductor guarantees a stable operation even at local heat inputs of about 0.5 J onto the surface of the conductor. In superfluid helium, recovery up to the take-off values of current in magnetic fields between 8 T and 13 T was found. A theoretical analysis was performed to explain the experimental results with respect to the cooling channel geometry, magnetic fields, and the local energy inputs to initiate a normal conducting region.

Critical current behavior in bending and tensile stressed Nb<inf>3</inf>Sn composites at temperatures below 4.2 K

Critical current measurements were obtained on bronze-diffused reacted Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn multifilamentary conductors as a function of mechanical stress loads and applied transverse magnet fields up to 13 T for temperatures in the range between 4.2 K and 1.9 K. To simulate the winding handling of reacted Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn wires for superconducting magnets, operated in normal or superfluid helium the effects of room temperature bend strain on current carrying capacities of monolithic and flat cable conductors were investigated by controlled winding processes using mandrels with various diameters. The results show a field-dependent degradation in critical current with increasing bend strain for both conductor types. Only a very small precompression effect (< 0.5%) on critical current is observed. At higher strain, beginning at 0.4%, the critical current decreased rapidly, reaching at 13 T by more than 50% at a bending strain of 0.9% calculated on overall conductor cross section. Both optical and scanning electron microscopy were used to examine the samples before and after testing. Additional data on the mechanical properties (tensile stress-strain diagrams) of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn samples are presented, along with some preliminary critical current versus uniaxial tensile stress measurements at 2.2 K and 4.2 K, respectively. First results indicate that the increase in critical current at 2.2 K is relatively larger (about 25%) than that at 4.2 K. This fact is interesting for Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn magnets cooled with superfluid helium to obtain a broad strain tolerance.

Radiative collisional quenching of metastable muonic hydrogenpμ−(2s)and the metastable muonic helium ionαμ−(2s)

Fully quantum-mechanical calculations have been performed to determine the radiative collisional cross sections for quenching of metastable muonic hydrogen $p{\ensuremath{\mu}}^{\ensuremath{-}}(2s)$ by normal hydrogen atoms and for quenching of the metastable muonic helium ion $\ensuremath{\alpha}{\ensuremath{\mu}}^{\ensuremath{-}}(2s)$ by normal helium atoms. The interatomic potential curves and the radiative transition rates for the mixed electronic-muonic systems are calculated using molecular-structure methods. The nuclear motion is treated as adiabatic and the interaction is written in terms of a complex potential. The cross sections are evaluated by direct numerical integration of the Schr\"odinger equation. The two-body thermal (300 K) quenching rate constants obtained are 3.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ ${\mathrm{cm}}^{3}$/sec for $p{\ensuremath{\mu}}^{\ensuremath{-}}$ and 6.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{3}$/sec for $\ensuremath{\alpha}{\ensuremath{\mu}}^{\ensuremath{-}}$.

Theoretical pulsation of metallic-line stars

The linear-theory radial-pulsation stability of low-helium delta Scuti variable models (1.0--2.5 Msun) has been investigated to see if metallicism and pulsation can occur simultaneously. Metallicism, which occurs in slowly rotating stars after the gravitational settling of He and the loss of the He II convection zone and its deep mixing for Y< or approx. =0.1, can then be established rapidly compared with the evolution time scale. Pulsation can still occur with driving due to the residual helium and the enhanced hydrogen. With the reduced helium giving no connection zone, the pulsation instability strip, whose blue and edges are estimated in this paoer, is about half as wide as with a normal helium abundance. Zero helium in the surface driving regions, however, produces blue edges so red that probably no instability strip exists at all. The red edge, predicted theoretically on the basis of the importance of convection in the outer zone, agrees well with the observational one. Cool, low-helium and metallic-line stars are then predicted to pulsate in a 200--500 K wide strip that is widest between the main-sequence luminosity of 5 Lsun and 15 Lsun. This strip reasonably includes the observed pulsating delta Del and mild Am stars, butmore » there may be conflicts. Since blue edges for varying ionization-zone helium content occur across the entire instability strip, bluer first and higher overtone pulsations are also predicted everywhere from less than 7000 K to over 8000 K, the redder ones probably showing metallicism.« less

Formation of excited helium atoms from helium molecular ions

It is proposed that the helium atoms in the second quantum levels which are produced by the recombination of helium molecular ions arise from collisions between normal helium atoms and excited helium molecules during their passage downward from the high Rydberg levels first entered to the metastable level.