Search

Conductivity and ionization mechanisms in liquid helium have been investigated by using high field conduction phenomena occurring near a sharp needle electrode. When the voltage V applied between the needle and the plane is greater than some threshold voltage [V.sub.s], a corona-like discharge regime was observed. Charge carriers are generated in the high field region close to the point where an intense light emission is detected. Above [V.sub.s] the mean current is space charge limited and the mobility of charge carriers has been deduced from analysis of current-voltage characteristics. Some parameters of the corona-like region have been deduced from a spectral analysis of the light emitted during the discharge. Index Terms--liquid helium, corona, space charge, charge carrier mobility, optical spectroscopy, spectrum analysis.

This paper deals with the breakdown voltage characteristics of saturated liquid helium in the presence of a needle-shaped or spherical metallic particle to obtain insulation design data for pool-cooled, low-temperature superconducting coils and to find the predominant factor affecting the breakdown voltage of liquid helium at a given state. The results show the following. (1) The generation of bubbles at the instant of collision of a particle with an electrode is caused mainly by the kinetic energy released from the moving particle to the liquid helium, while the electrostatic energy accompanying the microdischarge between the particle and the electrode has a lesser effect on it. (2) The lowest breakdown voltage at different particle conditions appears in the case of a free needle particle. The low breakdown voltage is caused by the bubble triggered by the particle collision, the high electric field at the tip of the needle particle, and the rich initial electrons supplied by the microdischarge. Index Terms--Breakdown, microdischarge, bubble, liquid helium, particle, electrical insulation, superconducting coil

In the meson exchange model of weak nucleon-nucleon (NN) interactions, the exchange of virtual mesons between the nucleons is parameterized by a set of weak meson exchange amplitudes. The strengths […]

The formation of a superfluid when [sup.4]He is cooled below the characteristic lambda transition temperature is accompanied by intricate quantum mechanical phenomena, including the emergence of a Bose condensate. A combination of path integral and semiclassical techniques is used to calculate the single-particle velocity autocorrelation function across the normal-to-superfluid transition. We find that the inclusion of particle exchange alters qualitatively the shape of the correlation function below the characteristic transition temperature but has no noticeable effect on the dynamics in the normal phase. The incoherent structure factor extracted from the velocity autocorrelation function is in very good agreement with neutron scattering data, reproducing the width, height, frequency shift, and asymmetry of the curves, as well as the observed increase in peak height characteristic of the superfluid phase. Our simulation demonstrates that the peak enhancement observed in the neutron scattering experiments below the transition temperature arises exclusively from particle exchange, illuminating the role of Bose-statistical effects on the dynamics of the quantum liquid. semidassical dynamics | forward-backward | Bose-Einstein condensation | time correlation function | incoherent structure factor

New data of the electronic properties of liquid helium and liquid neon are discussed with respect to their relevance for the initiation of electric breakdown. It is concluded that under ideal conditions the cathode plays a decisive role in the initiation of the breakdown process. It is speculated that a Townsend type breakdown mechanism is absent in the liquid phase of these elements. Index Terms--Liquid helium, breakdown, liquid neon, delocalized electrons, heat of vaporization, Townsend process, electron multiplication, radiation.

A small HTS coil, cooled by liquid or gas helium, is described and discussed.

A procedure is proposed for examining the effect of the convection of liquid helium on a superconducting magnet.

Design and testing of the Alpha Magnetic Spectrometer (AMS), a particle physics experiemnt for the International Space Station (ISS), is described and discussed.

The paper deals with the bubble formation process in the presence of a free conducting particle in liquid helium to understand the electrical insulation environment in pool cooled superconducting devices under particle contaminated conditions. Experiments were conducted with dc stressed parallel plane electrodes containing a free spherical metallic particle of 1mm radius in saturated normal and saturated superfluid helium. Experimental results show that a single bubble is always generated at the moment of particle collision with the electrode and the bubble behavior depends strongly on the state of the liquid helium. The bubble expanding and shrinking processes were analyzed on the basis of energy balance among released electrostatic and kinetic energies of the particle at the moment of particle collision with the electrode, kinetic energy of liquid driven by bubble growth, work done on environmental pressure and internal energy of the bubble gas. The analysis shows a fairly good agreement with experiments.

Surface spark-over voltages for various solid insulators are examined in liquid N[subscript 2] and liquid helium under ac, dc, and impulse voltages. Results are summarized and discussed.

Research has been conducted on the Pd(qol) (sub)2 that has been dissolved in an n-octane matrix. The application of several spectroscopic methods at liquid helium temperatures is described.

Research concerning the formation of bubbles in superfluid helium by the use of quantum tunneling is presented. The differences which exist between quantum and thermal cavitation are discussed.

We have been studying quench-induced 'dynamic' breakdown characteristics of [LHe.sup.*] and have already found that the electrical insulation performance of LHe was degraded drastically by the thermal bubble disturbance due to quench of superconductors. In this paper, in order to improve the insulation performance of LHe under quench conditions, we measured dynamic breakdown and prebreakdown characteristics of pressurized LHe. Experimental results revealed that breakdown voltage of LHe under quench conditions at 0.2 MPa was greatly improved, reaching 2 to 4x compared with that at atmospheric pressure under both uniform and non-uniform electric fields. Moreover, for practical and efficient insulation design of superconducting power apparatus, we investigated the dynamic breakdown voltage of LHe as functions of pressure and gap length.

Practical considerations on conductor design criteria in superfluid helium are presented, discussed and compared to design criteria in supercritical helium. Results show that the operating temperature has a critical influence on the cost of the system and can be determinant in the choice of the superconducting material. Some economical analysis are given based on a reference toroidal field magnet concept.

The development of the current bus line with conductor-holder for the melt textured YBa2Cu3O(sub x) bulk conductor is described. The objective was to moderate Joule heat generation in high current transport experiments at 4.2 K. Experimental results illustrate the feasibility of the application of the melt textured YBa2Cu3O(sub x) bulk conductor to the current feed device for the Large Helical Device.

Quench tests were conducted using the time-frequency distribution of acoustic emission signals enabling a NbTi superconducting winding cooled in liquid helium to rotate. Acoustic emission converters were used for data acquisition for voltages in the superconducting winding. A comparison of the acoustic emission signals during a quench propagation with those of a non-quench case was performed to investigate the acoustic emission components resulting from only the quench propagation.

The phenomenon of superfluidity, which so far has only been observed for [sup.4]He below the transition temperature [T.sub.[Lambda]] = 2.12 K and for [sup.3]He below [T.sub.[Lambda]] = 3 x [10.sup.-3] […]

Atoms and molecules are easily picked up by large liquid helium droplets; spectra are detected through use of either laser-induced fluorescence or beam depletion following absorption. Sensitivity is high. A variety of molecules ranging from a few atoms to large organic molecules have been embedded in He droplets and studied in the infrared region or the visible region. A new interdisciplinary branch of spectroscopic research has been born, with applications to molecular physics and low-temperature physics. Helium is the only known substance to remain liquid at normal pressures down to 0 K.

A superconducting magnetic suspension system was designed for measuring drag in rotationally symmetric bodies in liquid helium. A superconducting model with diamagnetic properties was used for maintaining stability despite disturbances. A superconducting pick-up coil and a passive approach were employed for flux shift detection and drag calculation, respectively. The pickup coil may be applied to a secondary loop or in a superconducting quantum interference device.

A review is given of recent experimental studies on the onset of dissipation in the flow of superfluid 4He through submicron-size apertures. These studies have revealed the occurrence of energy loss in discrete events. The interpretation of these events in terms of the motion individual quantum vortices across the flow is discussed.

The quenching currents of two single-strand NbTi superconducting wire and a two-layer coil were measured. The experiments were conducted to determine the impact of coil geometry as well as the impact of boiling atmospheric helium and supercritical helium cooling on quenching behavior. The results, which were obtained at 0-200 Hz, showed that the ac quench currents in the Cu30%Ni matrix wire were lower in supercritical helium.

The high dielectric dc strength of liquid helium can be explained by the effect of an energetic barrier against germinal vapor bubble elongation and runaway up to vapor discharge. The barrier normally rises in front of any cathode asperity where a germinal bubble may be produced from a collapsing vacuous cavity. After reaching a peak, the barrier changes into a decreasing tail, caused by significant vapor ionization. This so-called 'antibubble barrier' can be overcome at a sufficiently high field strength. For temperatures above the normal boiling point, the barrier peak must be lower than the threshold limit. At lower temperatures, the germinal bubbles may extend already into the falling tail, which favors runaway considerably. Nominal breakdown field strengths have been calculated assuming various surface roughness condition in the temperature range 2.5 to 4.5 K. A simple engineering formula for the roughness effect estimation is given.

Information about breakdown time lags under pulsed voltage stress is prerequisite to the insulation design of superconducting devices. Experiments have been carried out using a pulse generator which provides a step voltage with 200 ns rise time and a long decay time constant of more than 1 ms. The present experiments may bridge the information gap between time lag measurements in the ns range using extremely high field strengths, and existing data found with usual lightning impulse voltage. The test device is described in the paper, and some typical results are discussed in detail.

A test facility for high-temperature superconductivity studies exists at Argonne National Laboratory in Illinois. Currently, it is used for flow, pressure, temperature and voltage measurements during current lead-heat leakage experiments. Its equipment includes a liquid-helium dewar, a dewar insert and a computer-based data-acquisition system. It accomodates current leads rated up to 100 A.

Cooling designs of superconducting generator rotors are quite important for maintaining a stable superconducting state of field windings, and it is essential to comprehend the heat transfer characteristics of helium in rotating fields. Experiments were carried out using a large-scale rotating cryostat with a cold rotor diameter of approximately 800 mm. The heat transfer characteristics of liquid and supercritical helium under conditions of gravitational and centrifugal acceleration fields (maximum: approx. 3000 g at the refrigerant outer side in the rotor) with heat-transfer surfaces horizontal upward facing and perpendicular to such fields, radial and axial channels, and dummy winding cooling surfaces were obtained.

Photographs are presented of isolated superfluid helium-4 droplets prepared on a cesium surface, the only material known that is not wetted by superfluid helium. Although thermodynamic measurements show that the cesium surface is highly uniform, the contact angle of the droplets is extremely hysteretic and depends on whether the contact line is advancing or receding. Superfluid helium-4 droplets on an inclined surface do not flow downhill but rather are strongly pinned to the surface., Superfluid He has unusual thermal and mechanical properties (1) and is well known for its ability to spread over surfaces and to flow without dissipation through even microscopic holes. Virtually [...]

The topological defects formed in superfluid 3He are analogous to cosmic strings and support the Kibble-Zurek mechanism for the formation of defects in the early Universe. Small areas of superfluid 3He are heated by a neutron-induced nuclear reaction. The rapid cooling of the liquid bubbles produces a random network of vortices. The vortices escape when the liquid is rotated and can be studied. The results of the study show that a second-order phase transition produces the vortices.

The formation of vortices in liquid helium (He) by its transitions into a superfluid state is analogous to the formation of cosmic strings due to symmetry breaking after the Big Bang. Liquid He is heated above the superfluid transition temperature by an exothermic neutron-induced nuclear reaction. The deficit in the amount of energy released as the He cools is a measure of the density of the vortex formed. The results of the study agree with those predicted by Zurek's modification of the Kibble mechanism.

The breaking of symmetry in superfluid helium-3 (He-3) and the formation of defects is used to model the dynamics of cosmological phase transitions. Superfluid He-3 is heated and rapidly cooled to make it non-superfluid. Below the critical temperature, the symmetry of He-3 is broken. A gradient in the phase produces a flow pattern whose energy defines the superfluid analogues of cosmic strings, vortex lines. Superfluids are a good medium for studying the defects caused by symmetry breaking.

An analysis of the excitation spectrum of superfluid helium confined in porous aerogel glass explains the microscopic origin of the macroscopic fluid behavior. Neutron inelastic scattering estimations of the spectrum show that the elementary excitations, rotons, influence the system's behavior near the superfluid transition temperature. The effective mass of the rotons increases and their lifetime decreases in the system as compared to an unconfined disordered system. There are no theoretical explanations regarding the changes in the system.

Research in the superfluid phases of helium-3 verified the baked Alaska model of phase transitions in superfluids. Nucleation of the superfluid's B phase in the A phase was induced via helium-3's exposure to cosmic rays. The findings are analyzed by researchers from Stanford University.

Triplet bottleneck hole-burning experiments are conducted, along with their results presented. This experiment is carried out to understand the temperature dependence and this needs a reliable value for the rate at liquid helium temperatures.

We have demonstrated, for the first time, that high-performance liquid chromatography (HPLC) can be interfaced with fluorescence line-narrowing spectroscopy (FLNS) for on-line identification and characterization of analytes. Interfacing centered primarily on the design and construction of a novel liquid helium cryostat that accommodates variable-sized quartz tubes/capillaries suitable for HPLC as well as capillary electrophoresis/electrochromatography. In addition to the high spectral resolution afforded by FLNS, analyzing the separated components at 4.2 K minimizes photodegradation from the excitation source and provides indefinite detection times for signal averaging. The proof-of-principle for the HPLC-FLNS system is first demonstrated with a mixture of four structurally similar polycyclic aromatic hydrocarbons and then applied to the analysis of DNA adducts from mouse skin exposed to the carcinogen dibenzo[a,l]pyrene. With femtomole detection limits, HPLC-FLNS can be used for real-world analyses of complex mixtures.

A quasi-two-dimensional set of electrons (1 [is less than] N [is less than] [10.sup.9]) in vacuum, trapped in one-dimensional hydrogenic levels above a micrometer-thick film of liquid helium, is proposed as an easily manipulated strongly interacting set of quantum bits. Individual electrons are laterally confined by micrometer-sized metal pads below the helium. Information is stored in the lowest hydrogenic levels. With electric fields, at temperatures of [10.sup.-2] kelvin, changes in the wave function can be made in nanoseconds. Wave function coherence times are 0.1 millisecond. The wave function is read out with an inverted dc voltage, which releases excited electrons from the surface., There is much interest in constructing analog quantum computers (AQC). Such objects would solve problems that could never be solved by any classical digital computer, even ones that were hundreds [...]

The application of negative pressure can cause the explosion of electron bubbles that are used to study the superfluidity of liquid helium. The use of focused ultrasound cyclically increases and decreases the pressure on the electron bubble with respect to ambient pressure. The pressure minima can become negative for large amplitudes and if it goes lower than -2 atmospheres the bubble explodes. This occurs due to the absence of an equilibrium bubble radius for these pressures.

Quantized vortices in superfluid 3He surrounding a smooth wire can slowly unwind themselves, precessing as they unwind, a finding derived from an experiment measurement of the quantum value of circulation. R.E. Packard and colleagues achieved this startling result by determining the quantum of circulation kappa in superfluid 3He. The experiment used a thin wire pulled taught along the axis of a 2.96 millimeter diameter brass cylinder filled with superfluid 3He.

It is shown that in the low-temperature (0.37 kelvin) environment of superfluid helium droplets, long-range dipole-dipole forces acting between two polar molecules can result in the serf-assembly of noncovalently bonded linear chains. At this temperature the effective range of these forces is on the nanometer scale, making them important in the growth of nanoscale structures. In particular, the self-assembly of exclusively linear hydrogen cyanide chains is observed, even when the folded structures are energetically favored. This suggests a design strategy for the growth of new nanoscale oligomers composed of monomers with defined dipole (or higher order) moment directions., It is well known that as two point dipoles approach from long range they prefer to orient in a head-to-tail configuration (1). An analogous and perhaps more familiar case is [...]