Mechanism of color center formation during hydrogen photoinjection into nanodimensional MoO<Subscript>3</Subscript> films

The formation of paramagnetic Mo⁵⁺ centers in strongly disordered nanodimensional MoO₃ films as a result of the photoinjection of hydrogen at room and helium temperatures has been studied. The room-temperature concentration of paramagnetic Mo⁵⁺ centers quite rapidly reaches saturation, whereas at helium temperatures, it exhibits continuous growth during the entire hydrogen photoinjection process. Free radicals have been found at helium temperatures, which appear as a result of hydrogen atom detachment from adsorbed molecules (acting as hydrogen donors). The rate of radical formation decreases approximately by half during irradiation, while the rate of formation of the Mo⁵⁺ centers in the irradiated films exhibits a more than tenfold drop. This difference is related to the formation of [Mo⁵⁺ Mo⁵⁺] pair centers.     

Luminescence quenching in erbium-doped phosphate glass films irradiated with hydrogen ions

We have studied the effect of irradiation with hydrogen and helium ions on the photoluminescence (PL) of phosphate films doped with ytterbium and erbium. The irradiation with hydrogen ions leads to more effective quenching of the PL from erbium ions and, for the ion doses Φ ≥5×10¹⁶cm⁻², to a more significant decrease in the PL lifetime as compared to that in the films irradiated with helium ions. The results are interpreted within the framework of a model assuming the formation of OH groups in ion-irradiated glasses, which offer an effective channel of nonradiative recombination of the electron excitations in the material studied.     

Surfactants for Exploring the Liquid–Solid Surface Free Energy

Helium-3 and helium-4 can be used as surfactants to modify the observed wetting transitions of hydrogen and deuterium on rubidium. The effect of these surfactants on the wetting temperature is calculated. Prewetting induced by ⁴ He in H ₂ on Rb is studied, and found to be re-entrant. Experiments can use the effect to measure the difference in temperature and concentration dependence of a free solid surface and a liquid/solid interface. This may help to resolve some unresolved questions pertaining to the wetting behavior of helium isotopic mixtures on cesium.     

Reduction of hydrogen and helium retention in stainless steel by argon glow discharge

Although wall conditioning using Ar glow discharges is occasionally employed to reduce helium and hydrogen retention in the inner walls of the vacuum chamber of nuclear fusion devices, the effect of an Ar glow discharge on the reduction of helium and hydrogen retention has never been systematically investigated. In the present experiment, Ar glow discharges were applied to the stainless steel walls of a glow discharge device. Residual gas analysis (RGA) clearly showed that the Ar glow discharge significantly reduced the amount of helium and hydrogen retained. It was found that the fraction of the amount of desorbed helium to the initial retained amount of helium was as high as 13% and the fraction of the amount of desorbed hydrogen to the initial amount of hydrogen was as high as 36%.     

Cryogenically pumped laboratory metallurgical installation

The paper is concerned with the laboratory metallurgical installation designed for heat treatment and zone melting of refractory metals without a crucible. A 10kW electron-beam gun serves as a heater. The installation is evacuated from the atmospheric pressure to the ultimate vacuum using cryopumps only. Particular attention is given to hydrogen pumping. Performances of helium adsorption and condensation cryopumps with an efficiency up to 10⁴/s^?1 for hydrogen have been studied experimentally. The consumption of liquid helium was found to be 40–50cm³ h^?1 for adsorption pumps and 7cm³h^?1 for condensation pumps. It is shown that the application of the condensation pump with a surface cooled down to 2.3 K seems to be most efficient.     

Active roles of helium in the growth of hydrogenated microcrystalline silicon germanium thin films

A combination of hydrogen and helium dilutions was introduced when the microcrystalline silicon germanium (μc-SiGe:H) thin films were prepared by very high frequency plasma enhanced chemical vapor deposition on a low-temperature substrate. An optimum helium flow rate was found to achieve the structural uniformity in the growth direction, while Ge content was found to nearly keep constant with varying flow rates of helium. An abundance of atomic H was detected in plasma due to the attendance of helium and no obvious photosensitivity deterioration was observed on the thin film with a high crystalline volume fraction. The active roles of helium were identified by analyzing the mechanism in the plasma, where both metastable He_m^? and He⁺ can accelerate the diffusion of Ge related radicals and passivation of the dangling bonds on the growth surface, respectively. These phenomena have been revealed by experimental results. Therefore, a combination of hydrogen and helium dilutions can improve the structure of the μc-SiGe:H thin films with little degradation of photo-electronic properties.     

氢氦混合工质脉管制冷性能研究

由于氢气具有优越的传热和流动特性,有望减少脉管制冷机回热器的流动和传热损失,从而可获得高于纯氦工质的制冷性能.从氢组分对循环热力学性能、流体流动与传热性能等因素的影响角度出发,开展了氢-氦混合工质脉管制冷性能研究,并在30 K温区进行了实验验证,获得了比纯氦更好的制冷性能.     

One-step synthesis of MoO3 and MoO3-x nanostructures by condensation in gas: effect of the carrier gas

MoO3 and MoO3-x nanostructures were grown in a simple one-step process by direct evaporation of MoO3 pellets from a tungsten resistive source in presence of helium or hydrogen at pressures from 100 to 1200 Pa. This method uses no templates, catalysts or oxidizing agents. It leads to one dimensional (1-D) crystalline nanostructures mixed with amorphous material in variable ratios. Amorphous structures grew preferentially when hydrogen was used as carrier gas while crystalline material predominated when helium was used. In fact, only crystalline structures were found when the evaporation was carried out under a helium pressure of 600 Pa with source temperatures between 763 and 910 degrees C. Hydrated MoO3 phases with different water concentrations were preferentially formed using hydrogen. X-ray photoelectron spectroscopy detected only molybdenum in its +6 oxidation state in the samples grown under helium, exhibiting the same chemical composition of the source material. Molybdenum in its +6 as well as its +5 oxidation states was detected in the samples obtained under hydrogen at 600 Pa. Hydroxyl groups were identified in samples grown using both gases. The effect of the helium pressure on the growth kinetics and crystallinity of the samples is discussed according to the kinetics conditions (supersaturation, evaporation, cooling and convection rates) driving to the formation of nanostructures in the inert-gas condensation. Finally, the effect of hydrogen on the growth of MoO3 is discussed.     

The generation of particles to observe quantized vortex dynamics in superfluid helium

We describe a method to prepare a sample of superfluid helium-4 with hydrogen particles suspended within it. The method is to dilute hydrogen gas with helium at room temperature, and bubble the mixture through liquid helium at a temperature above the superfluid phase transition temperature, T_λ ≈ 2.17 K. The procedure yields a suspension of micron-sized particles whose total volume is about 10⁵ times smaller than the fluid volume. The fluid and suspension are then cooled to a temperature below T_λ. We show that the particles, so prepared in superfluid helium, are useful for studying superfluid flows and, in particular, the dynamics of quantized vortices. In addition, the particle-superfluid helium system is rich in not yet fully explained interactions. We review preliminary investigations that include observing the vortex lattice in rotating helium, vortex reconnection in quantized vortex turbulence, and vortex ring decay. These data illustrate the basic mechanisms of dissipation in superfluid turbulence.     

Helium detection in gas mixtures by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been evaluated as a tool for monitoring trace levels of helium in gas mixtures consisting mostly of hydrogen. Calibration data for helium in hydrogen was investigated at different helium concentration levels. At high concentrations of helium (>7.25%), the LIBS signal is quenched due to Penning ionization. The hydrogen alpha line (656.28 nm) was observed to broaden as the concentration of helium impurities in the hydrogen gas mixture increased. The helium line at 587.56 nm was selected as the analyte line for helium impurity detection. The effects of laser energy, the delay time between the laser pulse and data acquisition, and the gas pressure on the LIBS signal of helium were investigated to determine the optimum conditions for helium detection. The LIBS signal from the helium line at 587.56 nm shows good linear correlation with helium concentration for He concentrations below 1%. Thus, LIBS can be reliably used to detect the low levels of helium. The limit of detection for helium was found to be 78 ppm.     

Thermal Compression of Atomic Hydrogen on Helium Surface

We describe experiments with spin-polarized atomic hydrogen gas adsorbed on liquid ⁴He surface. The surface gas density is increased locally by thermal compression up to 5.5 × 10¹² cm⁻² at 110 mK. This corresponds to the onset of quantum degeneracy with the thermal de-Broglie wavelength being 1.5 times larger than the mean interatomic spacing. The atoms were detected directly with a 129 GHz electron-spin resonance spectrometer probing both the surface and the bulk gas. This and the simultaneous measurement of the recombination power, allowed us to make accurate studies of the adsorption isotherm and the heat removal from the adsorbed hydrogen gas. From the data, we estimate the thermal contact between 2D hydrogen gas and phonons of the helium film. We analyze the limitations of the thermal compression method and the possibility to reach the superfluid transition in 2D hydrogen gas.     

The energetics of helium and hydrogen atoms in β-SiC: an ab initio approach

Silicon carbide is a prime candidate for plasma-facing materials in future fusion reactors. The formation energies of various interstitial configurations of helium and hydrogen atoms in β-SiC were estimated based on density functional theory. Special consideration was given to the helium and hydrogen interstitials as the bubble seeds in β-SiC. From an energetic point of view, only one helium atom could position itself into the tetrahedral sites. However, up to three hydrogen atoms could easily position themselves into the tetrahedral sites by forming a stable H₂ molecule or a 3H-trimer that was newly identified in this study. Based on the different behaviors of helium and hydrogen, an explanation is proposed for the experimental observations of bubble formation in irradiated β-SiC.     

Hydrogen trapping in helium damaged metals: a theoretical approach

A model which explains the trapping of hydrogen around or near helium bubbles is presented. According to this model, hydrogen atoms are attracted toward the bubbles due to positive stresses created by the very high pressure (350 kbar) existing inside the bubbles. The extreme trapping energy of hydrogen atoms around helium bubbles has been theoretically calculated and found to be 0.71 eV atom^–1. It is shown that most of the hydrogen atoms are trapped in a very small volume located very close to the bubble surface. The total hydrogen quantity was found to be in the range of 45–76 atoms per bubble for a wide range of hydrogen atom concentration, C. The good agreement between the theoretical results and data based on many experimental measurements reinforces the assumptions underlying the very basis of the suggested mechanism. The model proposed in this study can lead to better understanding of failure mechanisms.     

Wetting in Binary Fluid Mixtures: Recent Resugts in H2/He on Cesium

We present the resugts of an experiment on the wetting of liquid hydrogen on a cesium substrate using helium as a surfactant. In contrast to the previously studied case of ³He-⁴He mixtures on cesium, there is less ambiguity about the concentration dependence of the liquid-solid surface tension, suggesting that the resugts may be more easily interpreted; however, the relatively high temperatures and pressures complicate both experiment and calculation. The thermal expansion of the liquid turns out to be a major contribution to the temperature variation of the liquid-solid surface tension. When this is taken into account, there is excellent agreement between the observed variation of the wetting temperature with helium concentration (?.31±0.01 K/bar) and the calculated value (?.30 K/bar).     

Thermophysicochemical Reaction of ZrCo–Hydrogen–Helium System

Nuclear fusion energy, which is clean and infinite, has been studied for more than half a century. Efforts are in progress worldwide for the demonstration and validation of nuclear fusion energy. Korea has been developing hydrogen isotope storage and delivery system (SDS) technologies including a basic scientific study on a hydrogen storage medium. An SDS bed, which is a key component of the SDS, is used for storing hydrogen isotopes in a metal hydride form and supplying them to a tokamak. Thermophysicochemical properties of the ZrCo–H\(_{2}\)–He system are investigated for the practical utilization of a hydriding alloy system. The hydriding reaction, in which \(\hbox {ZrCoH}_{\mathrm{x}}\) is composed as ZrCo absorbing hydrogen, is exothermic. The dehydriding reaction, in which \(\hbox {ZrCoH}_{\mathrm{x}}\) decomposes into ZrCo and hydrogen, is endothermic. The heat generated through the hydriding reaction interrupts the hydriding progress. The heat loss by a dehydriding reaction impedes the dehydriding progress. The tritium decay product, helium-3, covers the ZrCo and keeps the hydrogen from contact with ZrCo in the SDS bed. In this study, we designed and fabricated a ZrCo bed and its performance test rig. The helium blanketing effect on a ZrCo hydrogen reaction with 0 % to 20 % helium content in a gaseous phase and a helium blanket removal method were studied experimentally. In addition, the volumetric flow rates and temperature at the beginning of a ZrCo hydrogen reaction in a hydrogen or helium atmosphere, and the cooling of the SDS bed by radiation only and by both radiation and natural convection related to the reuse cycle, were obtained.     

Surface tension of liquid inert gases

The surface tension of liquid inert gases, including ³He and ⁴He helium isotopes, has been calculated using a method based on determining the work that must be done to remove an atom from the liquid surface. This approach provides quite satisfactory agreement with the available experimental data. Calculations of the surface tension in liquid helium also take into account the quantum zero-point oscillations of atoms. The role of the dimensional effect in the surface tension for nanodroplets is discussed.     

Nanoscale Quantum Solvation of para-H2 Around the Linear OCS Molecule Inside 4He Droplets

We present a microscopic analysis of the quantum solvation structures of para-H₂ around the OCS molecule when embedded in low temperature ⁴He droplets. The structures of clusters containing M=5 and 6 para-H₂ molecules are compared with corresponding structures for M=1 (OCS-H₂ complex) and M=17 (a full solvation shell), as well as with the clusters in the absence of helium. We find that the helium has negligible effect on the structures for the small and large OCS(H₂) _M clusters, but that it modifies the cluster structure for M=6. We discuss implications of these resugts for the onset of superfluidity in the solvating hydrogen shell and for spectroscopic measurements.     

Measurement of helium metastable atom densities in a plasma-based ambient ionization source

We describe direct imaging of the densities of helium metastable atoms in the afterglow of a helium dielectric-barrier discharge (He-DBD) using collisionally assisted laser-induced fluorescence (LIF). For the conditions tested, comparison of fluorescence images of a He-DBD with analogous maps of emission from highly excited helium atoms revealed that helium metastable atom densities did not correlate well with emission from the plasma. Fluorescence images also showed that helium metastable atom densities increased substantially when a glass slide was placed 10.0 mm from the discharge capillary in a geometry typical for desorption-ionization experiments. We also studied the effect hydrogen has on the helium metastable atom densities. The hydrogen severely quenched the metastable state leaving it virtually undetectable. Emission was quenched as well, but to a lesser extent. The addition of 1% H(2) to the helium in the source provided nearly a factor of 2 improvement in the sensitivity of the signal for coumarin 47 when the plasma was used to ionize the dye under ambient conditions, despite the quenching of the helium metastable atom population.