Pure Helium Research Articles

Investigations of Droplet‐Plasma Interaction using Multi‐Dimensional Coupled Model

AbstractRecently developed multi‐dimensional coupled fluid‐droplet model is used to investigate the behavior of complex interaction between the liquid precursor droplets and atmospheric pressure plasma (APP). The significance of this droplet‐plasma interaction is not well understood under diverse realm of working conditions in two‐phase flow. In this study, we explain the implication of vaporization of liquid droplets in APP which are subsequently responsible to control major characteristics of surface coating depositions. Coalescence of water droplets is more dominant than Hexamethyldisiloxane (HMDSO) droplets because of its sluggish rate of evaporation. A disparity in the performance of evaporation is identified in two independent mediums, such as gas mixture and discharge plasma using HMDSO precursor. The length of evaporation of droplets is amplified by an increment of gas flow rate indicating with a reduction in the gas temperature and electron mean energy. In particular, the spatio‐temporal density distributions of charged particles show a clear pattern in which the typical nitrogen impurity ions are primarily effective as compared to other helium ionic species along the pulse of droplets in APP. Finally, we contrast the behavior of discharge species in the pure helium and He‐N2 gas mixtures revealing the importance of stepwise and Penning ionization processes. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Helium gas permeability measurements of polymeric membranes using the difference method

This paper presents a new permeation measurement facility based on the difference method, which is shown to be capable of measuring gas fluxes through different polymeric membranes. In order to correct for the background pressure increase in the measurement chamber, a reference chamber has been adapted and connected to it. The permeation flux was examined by measuring the pressure difference between the reference and measurement chambers. Pure helium permeation was performed through polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET) membranes using this method. The uncertainty of the permeability was evaluated. The results show that the introduction of reference chamber can mitigate the influence of the residual pressure in the measurement chamber. This new method allows simplified and visualized measurement of the permeation fluxes.

Reduction of circulation power for helium-cooled fusion reactor blanket using additive CO2 gas

Helium (He) cooling requires large circulation power to remove high heat from plasma side and nuclear heating by high energy neutron in fusion reactors due to its low density. Based on the recent findings that the heat transfer capability of the light gas can be enhanced by mixing another heavier gas, this study adds CO2 to a reference helium coolant and evaluates the cooling performance of the binary mixture for various compositions. To assess the cooling performance, computational fluid dynamic (CFD) analyses on the KO HCML (Korea Helium Cooled Molten Lithium) TBM are conducted. As a result, it is revealed that the binary mixing of helium, which has favorable thermophysical properties but the density, with a heavier noble gas or an unreactive gas significantly reduces the required circulation power by an order of magnitude with meeting the thermal design requirements. This is attributed to the fact that the density can be highly increased with small amount of a heavier gas while other gas properties are kept relatively comparable. The optimal CO2 mole fraction is estimated to be 0.4 and the circulation power, in this case, can be reduced to 13% of that of pure helium. This implies that the thermal efficiency of a He-cooled blanket system can be fairly enhanced by means of the proposed binary mixing.

Investigation of helium barrier discharges with small admixtures of oxygen

Barrier discharges in helium and in helium with small admixtures of oxygen were investigated by electrical measurements, the spatiotemporally resolved optical emission spectroscopy and surface charge diagnostics via the electro-optic Pockels effect. As already known, in pure helium a diffuse discharge is typically formed because of the significant role of the metastable species. However, even a very small oxygen admixture (0.025 vol.%) causes the transition to a filamentary mode as a result of the effective quenching of helium metastables by oxygen molecules. This effect was indicated by a significant decrease of N the first negative system emission. The transition region was characterized by several Townsend-like discharge breakdowns becoming more and more unstable with an increasing O2 admixture. The formation of the atmospheric pressure Townsend-like discharge was confirmed by the spatiotemporally resolved emission. The development of the surface charges agrees qualitatively and quantitatively well with the transported charge during the discharge breakdown calculated from the discharge current.

Time-resolved and XUV spectroscopy of helium nanodroplets

SynopsisThe ultrafast dynamics of pure and doped helium nanodroplets is studied using VIS and XUV femto-second pump-probe spectroscopy in combination with ion and electron imaging detection.

Gaseous Cascade Amplification in He-H2O Gas Mixture in an Environmental Scanning Electron Microscope

This paper deals with the gaseous cascade amplification in He-H2O mixture in an ESEM. It is shown that such a mixture has the same behavior and gas amplification gain slightly modified with the increase of the (pd) parameter as compared to pure Helium. The use of the mixture gives rise to higher gain with respect to water vapor and more efficiency in the electrical neutralization of the specimen surface compared to He. All in all, the mixture exhibits high Townsend ionization coefficient, high gain and low operating voltage, which allows an expected increase of the signal to noise ratio.

Asteroseismology of GD358 with complex core carbon and oxygen profiles

AbstractGD 358 is the brightest (mv=13.7) and best studied helium atmosphere white dwarf pulsator. We present an analysis based on over 1000 hours of observations spanning 2007-2014 as well as archival data going back to 1982. From the complete data set, we identify a total of 27 independent frequencies and fit 14 of them as m=0 modes in our asteroseismic analysis. We add GD358 to a set of helium atmosphere white dwarfs fitted with similar models. With this consistent set, we can see a trend in the thickness of the pure helium layer that are quantitatively consistent with time-dependent diffusion calculations.

Formation of Positively Charged Liquid Helium Clusters in Supercritical Helium and their Solidification upon Compression.

Positively charged ions were produced in supercritical helium at temperatures from 6 to 10 K and up to 2 MPa using a corona discharge. Their mobility was measured via current-voltage curves, and the hydrodynamic radius was derived using Stokes law. An initial increase and subsequent decrease of hydrodynamic radius was observed and interpreted in terms of growth, compression and solidification of ion clusters. The mobility was modeled using a van der Waals-type thermodynamic state equation for the ion-in-helium mixed system and a temperature-dependent Millikan-Cunningham factor, describing experimental data both in the Knudsen and the Stokes flow region. Regions of maximum hydrodynamic radius and large compressibility were interpreted as boiling points. These points were modeled over a large range of pressures and found to match the Frenkel line of pure helium up to 0.7 MPa, reflecting similarity of density fluctuations in pure supercritical helium and gas-liquid phase transitions of ionic helium clusters.

Experimental investigation on tritium release from lithium titanate pebble under high temperature of 1073 K

The temperature of Li2TiO3 pebble breeder in a fusion DEMO blanket is assumed to be more than 1000K. For the investigation of tritium release from a Li2TiO3 pebble breeder blanket at such a high temperature, we have carried out a tritium release experiment with the DT neutron source at the JAEA-FNS. The Li2TiO3 pebble (1.0–1.2mm in diameter) of 70g was put into a stainless steel container and installed into an assembly stratified with beryllium and Li2TiO3 layers. During the DT neutron irradiation, the temperature was kept at 1073K with wire heaters in the blanket container. Helium gas including 1% hydrogen gas (H2/He) mainly flowed inside the container as the purge gas. Two chemical forms, HT and HTO, of extracted tritium were separately collected during the DT neutron irradiation by using water bubblers and CuO bed. The tritium activity in the water bubbler was measured by a liquid scintillation counter. To investigate the effect of moisture in the purge gas, we also performed the same experiments with H2O/He gas (H2O content: 1%) or pure helium gas. From our experiment at 1073K, in the case of the purge gas includes H2, it is indicated that the increasing tendency of HT release is similar to that of the dry H2/He.

Study of Earth and Jupiter-like plasmas for atmospheric entries using a non-transferred arc torch

This paper presents the results obtained by a 100 kW non-transferred arc plasma torch dedicated to the studies of plasmas characteristics of atmospheric entries of spatial probes, especially Earth and Jupiter entries. Spectra acquisition of the produced plasmas is achieved using optical emission spectroscopy. For Earth entry conditions, air plasma was obtained with a maximal temperature around 6800 K with a good agreement using atomic lines of oxygen and nitrogen (and also copper coming from electrode’s ablation) and molecular bands of N2, CN and , testifying to a good thermal equilibrium. As the first step in the study of Jupiter atmospheric entry, pure helium plasma was produced with the same maximal temperature of about 7500 K. Helium plasma was achieved for the first time using the plasma torch. Recorded spectra show a continuum, He I lines as well as copper. He II lines are not detected.

Saturated phase densities of (CO2 + H2O) at temperatures from (293 to 450) K and pressures up to 64 MPa

An apparatus consisting of an equilibrium cell connected to two vibrating tube densimeters and two syringe pumps was used to measure the saturated phase densities of (CO2+H2O) at temperatures from (293 to 450)K and pressures up to 64MPa, with estimated average standard uncertainties of 1.5kg·m−3 for the CO2-rich phase and 1.0kg·m−3 for the aqueous phase. The densimeters were housed in the same thermostat as the equilibrium cell and were calibrated in situ using pure water, CO2 and helium. Following mixing, samples of each saturated phase were displaced sequentially at constant pressure from the equilibrium cell into the vibrating tube densimeters connected to the top (CO2-rich phase) and bottom (aqueous phase) of the cell. The aqueous phase densities are predicted to within 3kg·m−3 using empirical models for the phase compositions and partial molar volumes of each component. However, a recently developed multi-parameter equation of state (EOS) for this binary mixture, Gernert and Span [32], was found to under predict the measured aqueous phase density by up to 13kg·m−3. The density of the CO2-rich phase was always within about 8kg·m−3 of the density for pure CO2 at the same pressure and temperature; the differences were most positive near the critical density, and became negative at temperatures above about 373K and pressures below about 10MPa. For this phase, the multi-parameter EOS of Gernert and Span describes the measured densities to within 5kg·m−3, whereas the computationally-efficient cubic EOS model of Spycher and Pruess (2010), commonly used in simulations of subsurface CO2 sequestration, deviates from the experimental data by a maximum of about 8kg·m−3.

Measurement of Negatively Charged Dust by Using an Electric Probe in Large RF Helium Plasmas

We investigated charged dust and its effect on RF plasma by using a planar electric probe in a large-scale device. In background plasmas, the particle density is 108 to 109 cm−3 and the electron temperature is 2 to 4 eV. When dust is contained in plasma, it is negatively charged by electrons attached to the dust. The charged dust density and the charge were calculated by comparing dusty helium plasma to pure helium plasma. Depending on the increase in the amount of dust, the charged dust density increases with the decrease in the charge due to depletion of the electrons in the background plasma. The results show that the charge changes the interactions between the dust and particles in the background plasma.

AM CANUM VENATICORUM PROGENITORS WITH HELIUM STAR DONORS AND THE RESULTANT EXPLOSIONS

We explore the outcome of mass transfer via Roche lobe overflow of pure helium burning stars in close binaries with WDs. The evolution is driven by the loss of angular momentum through gravitational wave radiation (GWR), and both stars are modeled using Modules for Experiments in Stellar Astrophysics (MESA). The donors have masses of and accrete onto WDs of mass MWD from to . The initial orbital periods (Porb) span 20–80 minutes. For all cases, the accretion rate onto the WD is below the stable helium burning range, leading to accumulation of helium followed by unstable ignition. The mass of the convective core in the donors is small enough so that the WD accretes enough helium-rich matter to undergo a thermonuclear runaway in the helium shell before any carbon–oxygen enriched matter is transferred. The mass of the accumulated helium shell depends on MWD and the accretion rate. We show that for and , the first flash is likely vigorous enough to trigger a detonation in the helium layer. These thermonuclear runaways may be observed as either faint and fast Ia SNe or, if the carbon in the core is also detonated, Type Ia SNe. Those that survive the first flash and eject mass will have a temporary increase in orbital separation, but GWR drives the donor back into contact, resuming mass transfer and triggering several subsequent weaker flashes.

Direct Monte Carlo simulation of nanoscale mixed gas bearings

The conception of sealed hard drives with helium gas mixture has been recently suggested over the current hard drives for achieving higher reliability and less position error. Therefore, it is important to understand the effects of different helium gas mixtures on the slider bearing characteristics in the head–disk interface. In this article, the helium/air and helium/argon gas mixtures are applied as the working fluids and their effects on the bearing characteristics are studied using the direct simulation Monte Carlo method. Based on direct simulation Monte Carlo simulations, the physical properties of these gas mixtures such as mean free path and dynamic viscosity are achieved and compared with those obtained from theoretical models. It is observed that both results are comparable. Using these gas mixture properties, the bearing pressure distributions are calculated under different fractions of helium with conventional molecular gas lubrication models. The outcomes reveal that the molecular gas lubrication results could have relatively good agreement with those of direct simulation Monte Carlo simulations, especially for pure air, helium, or argon gas cases. For gas mixtures, the bearing pressures predicted by molecular gas lubrication model are slightly larger than those from direct simulation Monte Carlo simulation.

Aspects of surface scanning by direct analysis in real time mass spectrometry employing plasma glow visualization.

Visual monitoring of the Direct Analysis in Real Time (DART) gas impact region during sampling was demonstrated via its metastable plasma glow. It is known that adding neon into helium for DART leads to plasma glow, but this effect has not been used in practice and discussed in the literature so far. A single quadrupole mass spectrometer with a DART SVPA ion source was used for recording of DART mass spectra from different surfaces, using galangin and p-coumaric acid as model analytes. In specific cases, the composition of the mass spectra was clarified using an Orbitrap mass spectrometer. Plasma glow visualization made it possible to track the metastable gas distributions during surface scanning. The influence on the composition of the mass spectra was studied for different carrier gases, i.e. pure helium versus a helium-neon mixture, and for the vacuum pumping rate. The spatial resolution was substantially improved via a DART cap with a narrowed internal diameter, but impaired by a decreased sensitivity. Comparably low signal intensities were obtained for analytes on porous layers due to analyte penetration and metastable gas scattering. Visualization through the plasma glow enables the optimal selection of the coordinates for DART-MS analysis and thus it will support scanning and imaging MS on surfaces, including porous planar chromatographic separation materials.

Gas Composition Monitoring in Cryogenic Fluid Transfer Lines

A range of aerospace applications require chemical sensing technology [1]. One application area is the monitoring of fuel leaks in launch vehicles. Detection of low concentrations of hydrogen and hydrocarbon fuels is critical in avoiding explosive conditions that could harm personnel and damage the vehicle. Reliable vehicle operation also depends on the timely and accurate measurement of these leaks. Further, the simultaneous measurement of oxygen as well is an important component determining if a hazardous condition exists. Detection of low concentrations of fuel, such as hydrogen or methane, is generally necessary from room temperature to cryogenic ambient, and in air or inert gas atmospheres. Thus, the development of multiple sensors (a sensor array) to determine the concentration of fuels, such as hydrogen, hydrocarbons, oxygen, and other parameters is necessary for aerospace applications. Development of Smart Sensor Systems for the detection of leaks has been on-going [1-2]. This work is based on the ability to produce microsensor platforms that can be tailored to measure a range of species. Combined with that effort has been the development of miniaturized hardware and software systems (“Lick and Stick” technology) that can be implemented in aerospace applications. These “Lick and Stick” systems include signal conditioning, data processing, power, and communication in a compact structure, for placement in multiple locations to improve the awareness of an environment. Figure 1a) shows an example this technology: a “Lick and Stick” system composed of a hydrogen, hydrocarbon, and oxygen sensor systems with a microprocessor and communication capabilities. Other sensors such as pressure and humidity can be included. Each sensor is integrated with its own temperature detector and heater for temperature control facilitated by the Smart Sensor electronics. This paper describes the extension of this Smart System leak sensor technology to the monitoring of the gas concentrations in cryogenic fluid transfer lines. In particular, cryogenic fluid transfer lines that carry liquid hydrogen to a launch vehicle are often purged with helium before the lines are exposed to the ambient. This is done in order to insure that residual hydrogen concentrations within the transfer lines are below hazardous levels before the introduction of oxygen. Pure helium is used, in contrast to, for example, nitrogen, to ensure the purge gas itself does not liquefy. Given rising costs of helium, there is a desire to minimize its use to that least amount needed in order to establish a nonhazardous level of hydrogen within the cryogenic fluid transfer line. One method to reduce helium usage is to monitor the hydrogen concentration within the cryogenic fluid transfer line and measure when nonhazdarous concentrations are achieved. This implies that a gas monitoring be mounted in contact with the interior of the cryogenic fluid transfer line and be able to withstand cryogenic temperatures. Measurements of interest include hydrogen and oxygen, but also pressure and humidity. A cryogenic fluid transfer line gas monitoring system based on chemical microsensors is under development to address this application. The core technology is based on the “Lick and Stick” sensor system of Figure 1a), but major modifications are necessary for this application. A prototype systems is shown in Figure 1b). The sensor elements, such as hydrogen, oxygen, and humidity sensors, are packaged for insertion directly into the cryogenic flow line using the fittings shown. These sensor elements do not need to operate at cryogenic temperatures, but must at least survive exposure to these conditions; sensor operation occurs later during the purge of the fluid transfer line at low temperatures closer to ambient after the cryogen flow. The sensor elements are remotely connected to the Smart Sensor electronics, which operate at ambient conditions. This paper describes the development of this prototype cryogenic fluid transfer line gas monitoring system, its initial testing, and comparison of results to reference instruments. It is concluded that the use of microsensors for this application have the potential to provide real-time monitoring capabilities in applications such as cryogenic fluid transfer lines, and thus provide a method to monitor and minimize the use of helium in such applications.

Gas Permeation Properties of Soluble Aromatic Polyimides Based on 4-Fluoro-4,4'-Diaminotriphenylmethane.

A series of new organic polyimides were synthesized from 4-fluoro-4'4"-diaminotriphenylmethane and four different aromatic dianhydrides through a one-step, high-temperature, direct polycondensation in m-cresol at 180–200 °C, resulting in the formation of high-molecular-weight polyimides (inherent viscosities ~ 1.0–1.3 dL/g). All the resulting polyimides exhibited good thermal stability with initial decomposition temperatures above 434 °C, glass-transition temperatures between 285 and 316 °C, and good solubility in polar aprotic solvents. Wide-angle X-ray scattering data indicated that the polyimides were amorphous. Dense membranes were prepared by solution casting and solvent evaporation to evaluate their gas transport properties (permeability, diffusivity, and solubility coefficients) toward pure hydrogen, helium, oxygen, nitrogen, methane, and carbon dioxide gases. In general, the gas permeability was increased as both the fractional free volume and d-spacing were also increased. A good combination of permeability and selectivity was promoted efficiently by the bulky hexafluoroisopropylidene and 4-fluoro-phenyl groups introduced into the polyimides. The results indicate that the gas transport properties of these films depend on both the structure of the anhydride moiety, which controls the intrinsic intramolecular rigidity, and the 4-fluoro-phenyl pendant group, which disrupts the intermolecular packing.

In situ high temperature oxidation analysis of Zircaloy-4 using acoustic emission coupled with thermogravimetry

Zircaloy-4 oxidation behavior at high temperature (900°C), which can be reached in case of severe accidental situations in nuclear pressurised water reactor, was studied using acoustic emission analysis coupled with thermogravimetry. Two different atmospheres were used to study the oxidation of Zircaloy-4: (a) helium and pure oxygen, (b) helium and oxygen combined with slight addition of air. The experiments with 20% of oxygen confirm the dependence on oxygen anions diffusion in the oxide scale. Under a mixture of oxygen and air in helium, an acceleration of the corrosion was observed due to the detrimental effect of nitrogen. The kinetic rate increased significantly after a kinetic transition (breakaway). This acceleration was accompanied by an acoustic emission activity. Most of the acoustic emission bursts were recorded after the kinetic transition (post-transition) or during the cooling of the sample. The characteristic features of the acoustic emission signals appear to be correlated with the different populations of cracks and their occurrence in the ZrO2 layer or in the α-Zr(O) layer. Acoustic events were recorded during the isothermal dwell time at high temperature under air. They were associated with large cracks in the zirconia porous layer. Acoustic events were also recorded during cooling after oxidation tests both under air or oxygen. For the latter, cracks were observed in the oxygen enriched zirconium metal phase and not in the dense zirconia layer after 5h of oxidation.

Anatomy of a rapid pressure swing adsorption process performance

A detailed numerical study of the individual and cumulative effects of various mass, heat, and momentum transfer resistances, which are generally present inside a practical adiabatic adsorber, on the overall separation performance of a rapid pressure swing adsorption (RPSA) process is performed for production of nearly pure helium gas from an equimolar binary (N2 +He) gas mixture using 5 A zeolite. Column bed size factor (BSF) and helium recovery (R) from the feed gas are used to characterize the separation performances. All practical impediments like column pressure drop, finite gas‐solid mass and heat transfer resistances, mass and heat axial dispersions in the gas phase, and heats of ad(de)sorption causing nonisothermal operation have detrimental impacts on the overall process performance, which are significantly accentuated when the total cycle time of a RPSA process is small and the product gas helium purity is high. These impediments also prohibit indefinite lowering of BSF (desired performance) by decreasing process cycle time alone. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2008–2015, 2015

On the Ring-Shaped Structure of Helium Plasma Jets

The results of simulation of positive streamers propagating along plasma jets formed in pure helium and in helium with 3% of air and emerged into ambient air are presented. It is shown, in agreement with the experimental data, that the addition of air leads to disappearance of ring-shaped structure of plasma jet radiation typical to jets of pure helium. The calculated radial profiles of the electron density in channels of streamers propagating in air at nonuniform radial distributions of the gas density are also given. Like in plasma jets, these profiles can have a ring-shaped structure. In both cases, the reason for the formation of ring-shaped structure is radial nonuniformity of the rate of ionization of gas species by electron impact.