Pure Helium Research Articles

Residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion as a function of process atmosphere and component design

The influence of the process gas, laser scan speed, and sample thickness on the build-up of residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion was studied. Pure argon and helium, as well as a mixture of those (30% helium), were employed to establish process atmospheres with a low residual oxygen content of 100 ppm O2. The results highlight that the subsurface residual stresses measured by X-ray diffraction were significantly lower in the thin samples (220 MPa) than in the cuboid samples (645 MPa). This difference was attributed to the shorter laser vector length, resulting in heat accumulation and thus in-situ stress relief. The addition of helium to the process gas did not introduce additional subsurface residual stresses in the simple geometries, even for the increased scanning speed. Finally, larger deflection was found in the cantilever built under helium (after removal from the baseplate), than in those produced under argon and an argon-helium mixture. This result demonstrates that complex designs involving large scanned areas could be subjected to higher residual stress when manufactured under helium due to the gas’s high thermal conductivity, heat capacity, and thermal diffusivity.

L–H transition threshold studies in helium plasmas at JET

We present a study of the power threshold for L–H transitions (P LH) in almost pure helium plasmas, obtained in recent experiments at JET with an ITER-like wall (Be wall and W divertor). The most notable new result is that the density at which P LH is minimum, , is considerably higher for helium than for deuterium and hydrogen plasmas. We discuss the possible implications for ITER in its pre-fusion operating power phase.

Reduction of incandescent spatter with helium addition to the process gas during laser powder bed fusion of Ti-6Al-4V

The effect of the process gas during laser powder bed fusion (L-PBF) was investigated using high-speed shadowgraphy while melting Ti-6Al-4V powder under high purity argon, helium, and a mixture of both, on a laboratory-scale machine. These recordings reveal that the generation of incandescent spatters can be reduced by at least 60% using pure helium and by ∼30% using addition of helium to argon in comparison to the use of traditional argon. The quantity of colder spatters appeared unaffected by the change of process gas. Different configurations of gas flow versus laser scanning direction were investigated and revealed that fumes and spatters are less accumulated at the laser spot with helium addition. Furthermore, the use of the argon–helium mixture proved to be as efficient as pure argon in the dragging and extraction of the fumes. Shadowgraphs revealed the more rapid expansion of fumes in helium-rich atmospheres, limiting the accumulation of scattering objects close to the laser spot and thus melt pool instability. These results were correlated to process snapshots on an industrial-scale system, confirming the reduction of hot spatter generation. Finally, the findings put in evidence the more rapid cooling of spatters with helium addition to the process gas – a promising aspect to limit powder bed degradation during L-PBF. In addition, the use of mixtures of helium and argon would be economically interesting compared to pure helium, typically more expensive than the traditionally used argon.

A novel cobalt-free and CO2-stable LDC-LSCF membrane with high oxygen permeability

Cobalt (Co)-free and carbon dioxide (CO2)-stable Ce0.8La0.2O2−δ –La0.1Sr0.9Cu x Fe1−x O3−δ (LDC-LSCF) dual-phase oxygen-permeable membranes have been successfully developed through the one-pot ethylenediaminetetraacetic acid–citric acid method. The appropriate amount of copper (Cu) doping can increase the content of oxygen (O) vacancies and improve the oxygen (O2) permeability of the materials. At 900°C, the oxygen permeation flux of 60 wt.% Ce0.8La0.2O2−δ –La0.1Sr0.9Cu0.2Fe0.8O3−δ (60LDC-LSCF-0.2) membrane reaches 0.58 (ml/min)/cm2 with pure helium (He) as the sweep gas, which is the highest oxygen permeation flux among dual-phase membranes with different copper-doping contents. The effects of different LDC contents on the static stability, conductivity and oxygen permeability of the dual-phase membranes are studied. The oxygen permeation flux of the 60LDC-LSCF-0.2 membrane remains 0.37 (ml/min)/cm2 with pure carbon dioxide as the sweep gas during the long-term oxygen permeation test. The present results indicate that LDC-LSCF oxygen-permeable membranes are promising membranes for oxygen production and carbon dioxide capture.

Study on the Effect of Helium on the Dielectric Strength of Medium-Voltage Vacuum Interrupters

This paper presents the results of a comparative analysis of the dielectric strength of disconnecting vacuum interrupters operating on air and helium. The breakdown voltage Ud was measured in the pressure range from 8.0 × 10−4 Pa to 3.0 × 101 Pa for air and from 8.0 × 10−4 Pa to 7.0 × 102 Pa for helium, while varying the interelectrode distance from 1.0 to 5.0 mm. Dedicated laboratory workstations were used to determine the actual pressure values in the vacuum interrupters tested and to precisely measure and record the dielectric strength results of the test object. It was found that the helium-filled vacuum interrupter maintains its full dielectric strength in significantly larger pressure ranges, while the air-filled vacuum interrupter loses its insulating properties. Thus, it is possible to make vacuum interrupters based on the working medium associated with pure helium, with larger working pressure ratings. Under such conditions, it is easier to maintain the tightness of the device and to limit cut-off currents and overvoltages associated with vacuum switchgear.

Electron Swarm Parameters and Dielectric Properties of the Superconducting Binary Mixtures of He-H2

In this study, the electron energy distribution function EEDF, the electron swarm parameters, the effective ionization coefficients, and the critical field strength (dielectric strength) in binary He-H2 gas mixture which used as cryogenic for high-temperature superconducting power application, are evaluated by using two-term approximation of the Boltzmann equation over the range of E/N ( the electric field to gas density) from 1 to 100 Td ( 1 Td=10-17 Vcm2) at temperature 77 K and pressure 2MPa, taking into account elastic and inelastic cross-section. Using the calculated EEDF, the electron swarm parameters (electron drift velocity, mean electron energy, diffusion coefficient, electron mobility, ionization and attachment coefficient) are calculated. At low reduced electric field E/N, the EEDF close Maxwellian distribution, at high E/N, due to vibrational excitation of H2 the calculated distribution function is non-Maxwellian. Besides, in the He-H2 mixture, it is found that increasing small amount of H2 enhances to shift the tail of EEDF to the lower energy region, the reduced ionization coefficient α/N. reduced effective ionization coefficient (α-η)/N) decreases, while, reduced attachment coefficient η/N, reduced critical electric field strength (E/N)crt. and critical electric field Ecrt. Increases, because of hydrogen’s large ionization cross-sections. The dielectric strength of 5% H2 in mixture is in good agreement with experimental values, it is found that dielectric strength depend on pressure and temperature. The electron swarm parameters in pure gaseous helium (He) and hydrogen (H2), in satisfying agreement with previous available theoretical and experimental values. The validity of the calculated values has been confirmed by two-term approximation of the Boltzmann equation analysis.

High-fidelity modeling of breakdown in helium: initiation processes and secondary electron emission

Understanding the role of physical processes contributing to breakdown is critical for many applications in which breakdown is undesirable, such as capacitors, and applications in which controlled breakdown is intended, such as plasma medicine, lightning protection, and materials processing. The electron emission from the cathode is a critical source of electrons which then undergo impact ionization to produce electrical breakdown. In this study, the role of secondary electron yields due to photons (γ ph) and ions (γ i) in direct current breakdown is investigated using a particle-in-cell direct simulation Monte Carlo model. The plasma studied is a one-dimensional discharge in 50 Torr of pure helium with a platinum cathode, gap size of 1.15 cm, and voltages of 1.2–1.8 kV. The current traces are compared with experimental measurements. Larger values of γ ph generally result in a faster breakdown, while larger values of γ i result in a larger maximum current. The 58.4 nm photons emitted from He(21P) are the primary source of electrons at the cathode before the cathode fall is developed. Of the values of γ ph and γ i investigated, those which provide the best agreement with the experimental current measurements are γ ph = 0.005 and γ i = 0.01. These values are significantly lower than those in the literature for pristine platinum or for a graphitic carbon film which we speculate may cover the platinum. This difference is in part due to the limitations of a one-dimensional model but may also indicate surface conditions and exposure to a plasma can have a significant effect on the secondary electron yields. The effects of applied voltage and the current produced by a UV diode which was used to initiate the discharge, are also discussed.

Non-linear effects and electron heating dynamics in radio-frequency capacitively coupled plasmas with a non-uniform transverse magnetic field

A non-uniform transverse magnetic field is used to increase the plasma density and create an asymmetry in radio frequency capacitively coupled plasmas for plasma sputtering and plasma vapor deposition. Based on one-dimensional particle-in-cell/Monte Carlo collision simulations, the effect of the magnetic field magnitude on the non-linear behavior and the electron heating dynamics is studied for a pure helium plasma at a pressure of 30 mTorr. The results show that increasing the magnetic field magnitude can generate a more positive DC self-bias. As a result, non-linear oscillations of the electron current density and the electric field close to the grounded electrode are enhanced. An electric field reversal is induced when the powered electrode sheath collapses to balance electron and ion fluxes toward this boundary due to the strong confinement of electrons. Anomalous energetic electron beams are observed propagating from the collapsed sheath toward the plasma bulk. It is shown that such beams are reflections of the beams originating from the opposite expanding sheath based on the analysis of single particle motions. We show that energetic electron beams can be reflected by the transverse magnetic field.

Key chemical reaction pathways in a helium-nitrogen atmospheric glow discharge plasma based on a global model coupled with the genetic algorithm and dynamic programming

Determination of the key chemical reaction pathways in cold atmospheric plasmas (CAPs) is of great importance not only for understanding the spatiotemporal evolutions of the key plasma parameters during discharges but also for improving the plasma materials processing qualities. In this paper, a novel chemical reaction reduction method (CRRM) is proposed by using the global fluid model coupled with the genetic algorithm and the dynamic programming technique. With the aid of this newly developed CRRM, the key chemical reaction pathways can be automatically screened with a high computational efficiency under a pre-set critical calculation accuracy for the atmospheric pure helium and helium–nitrogen glow discharge plasmas. By comparing the calculated key plasma parameters, e.g., the species number densities, electron temperatures, voltage–current characteristics, based on the simplified models and their corresponding full models with those of the experimentally measured data, the reliability of the CRRM itself and the established key chemical reaction database for the atmospheric pure helium and helium–nitrogen CAPs are validated. This research also provides a general method for screening the key chemical reaction pathways for various low-temperature plasma sources.

Comparison of atmospheric pressure argon producing O(1S) and helium plasma jet on methylene blue degradation

A solution of methylene blue dye was degraded under an atmospheric pressure plasma jet operating in a linear field configuration with pure argon or pure helium as working gases. Optical emission spectroscopy was carried out to understand the reactive species present with and without dye treatment. Both plasma jets contain reactive species such as OH, N2, and atomic oxygen (O). However, atomic oxygen takes a greatly different form depending on the working gas. In the argon plasma jet, we observe that most of the atomic oxygen produced is the O(1S)–O(1D) transition that also leads to the green colored plasma plume. On the other hand, the helium plasma jet produces the well known triplet states of oxygen at 777 and 844 nm. The absorption spectra confirmed the faster and more energy efficient degradation of the methylene blue dye when treated by the argon plasma jet. Argon plasma with enhanced atomic oxygen content can be utilized as a cheaper and efficient method for waste water treatment.

Two competing mechanisms of plasma action on a jet flow

The influence of the plasma generation and propagation in a pulsed atmospheric pressure plasma jet on the gas flow is experimentally studied. The discharge was powered with pulsed voltage signals of both polarities with amplitudes that ranged from 2.8 to 10.8 kV and with the frequencies of 1–5 kHz. It operated in a pure helium and helium with a small admixture of oxygen and nitrogen. The measurements of the flow changes are performed by means of a laboratory constructed manometer with a free membrane. Two plasma action mechanisms have been identified and described: the electrohydrodynamic force mechanism and thermal mechanisms due to the fast localized heating. These two mechanisms appear to be counterbalancing or supporting depending on the experimental conditions. Besides, it was shown that plasma can cause a decrease in the gas velocity, probably depending on the dominant presence of the negative or positive ions in the jet.

Influence of nitrogen and oxygen admixture on the development of helium atmospheric-pressure plasma jet

This paper presents the experimental and numerical results about the influence of nitrogen (N2) and oxygen (O2) admixture on the development of a helium (He) atmospheric-pressure plasma jet (APPJ) in a long dielectric tube. The results revealed that the jet length and the propagation velocity are strongly affected by introducing N2 or O2 into the He flow. Specifically, it was observed that a higher N2/O2 admixture led to the decrease in the density of both energetic and relative low-energy electrons outside the grounded electrode, which corresponds to the shortening of the jet length. In the He/O2 mixture, the electrons are easily captured by O2/O in the region of the plasma bulk. In the He/N2 mixture, the jet propagation characteristics will change since N2 has many low-level excitation states that consumed a large number of energetic electrons. The simulation shows that the magnitude of the axial electric field in the jet head depends strongly on the amount of N2 and/or O2 in the gas flow. In both cases, the peak electric field is on the order of 5 kV/cm, which is significantly higher than that in pure helium of 3 kV/cm even if the admixture is low (less than 4% N2 or 2% O2 in this work). Positive charges of higher density in the jet head are needed to induce a stronger electric field for the jet propagating in N2(x%)/He and O2(x%)/He mixtures compared with that in pure He.

Investigation of a new energy‐efficient cryogenic process configuration for helium extraction and liquefaction

This study's main focus is to simulate and design a novel cryogenic system that produces liquid helium. The novelty of this research has two main parts: first, this system is able to produce large amounts of liquid helium at atmospheric pressure, and second, from an energy consumption point of view, it performs better than other processes of liquid helium production. The first and second laws of thermodynamic are used to investigate this process. In this novel cryogenic process, liquid helium is produced at three stages; first, the mainstream contains some helium enter to helium upgrader to increase helium concentration. After that, the stream with high helium concentration enters to pressure swing adsorption process, and pure helium is obtained. Finally, pure helium enters to cryogenic process and is liquified in an expander. The network consumption in this system is equal to 18.66 MW, and the proposed system's exergy efficiency is obtained at 23.4%. Also, the results show that this system can produce 196.5 kg/h liquid helium. In this research, Aspen HYSYS V10 and MATLAB software are used to simulate this cryogenic process. Finally, numerical assessment is done on the key parameters that influence the performance of the process.

Effect of helium xenon as working fluid on centrifugal compressor of power conversion unit of closed Brayton cycle power plant

Closed Brayton cycle (CBC) having single-shaft, centrifugal type compressor is considered as an efficient energy-conversion option associated for gas-cooled reactor (GCR) heat source. In terrestrial power plants and space power systems noble gases are considered as an efficient working fluid for most of the GCR's and CBC engines. The effectiveness of various noble gases as working fluid in closed cycle power plants for the power conversion units is of imperative concern. Although pure helium is relatively difficult to compress nonetheless it is measured as the best coolants for closed Brayton cycle power plants due to its better transport properties. Due to compression properties, its use resulted in the requirement of more mass, bigger size, higher cost and relatively more dynamic problems of rotatory machines in energy conversion system. The mixture of xenon with helium up to a molecular weight of <40 g/mol resulted in an increase of the coefficient of heat transfer as well as the significant increase in the loading of the compressor. Therefore, performance analysis is conducted for a novel design of helium xenon centrifugal compressor. The performance analysis is conducted with different molecular weight mixtures of helium xenon using similarity criteria. It is concluded that the use of helium xenon 40 g/mol is the optimum choice for space applications and 15 g/mol for closed Brayton cycle terrestrial power plants.

Simulation of cold spray coating for powder pre-heat and impact velocity

In the present work, Computational Fluid Dynamics (CFD) simulation was carried out to comprehend the role of varying pressure and gas mixture. The simulation results gave the pre-heat temperature and impact velocity of feedstock powder particles in cold spray (CS) coating process technology. The simulation was carried out using Fluent and the geometry has been drawn using SolidWorks. The pressure-based, standard k-ɛ two-equation model was used in the present simulation work. The results come out with the best-suited pressure of 7 MPa to attain high impact velocity and the pre-heat temperature. The best possible gas or gas mixture that comes out for attaining the maximum impact velocity was pure helium. The gas mixture with 80% N2 & 20% He gives the maximum pre-heat temperature. The high impact velocity improves the coating properties and pre-heating of the particles improves the bonding strength and first layer deposition in the cold spray coating process. The presence of helium gas in low concentration improves the pre-heat temperature, with high concentration tremendously improves the impact velocity of feedstock powder particles.

Electron parameters in a hollow cathode discharge plasma in He and He:H2O mixture

The electron energy distribution functions (EEDFs) were measured in the discharge plasma in pure helium and in helium with the addition of water vapor He:H2O (2:1). The studies were carried out in the discharge gap between the rectangular hollow cathode and the mesh anode using the original probe measurement system. It was established, in particular, that the EEDF in He had pronounced maxima in the regions of ∼4 eV and ∼15 eV, while in the He:H2O mixture there was no local maximum in the high-energy region. The EEDF calculations in the local approximation for experimental conditions were performed. It turned out that the calculated distribution functions differ noticeably from the measured ones. The possible physical causes leading to the discrepancy between the measured and calculated EEDFs were discussed.

Nucleosynthesis imprints from different Type Ia supernova explosion scenarios and implications for galactic chemical evolution

We analyze the nucleosynthesis yields of various Type Ia supernova explosion simulations including pure detonations in sub-Chandrasekhar mass white dwarfs; double detonations and pure helium detonations of sub-Chandrasekhar mass white dwarfs with an accreted helium envelope; a violent merger model of two white dwarfs; and deflagrations and delayed detonations in Chandrasekhar mass white dwarfs. We focus on the iron peak elements Mn, Zn, and Cu. To this end, we also briefly review the different burning regimes and production sites of these elements, as well as the results of abundance measurements and several galactic chemical evolution studies. We find that super-solar values of [Mn/Fe] are not restricted to Chandrasekhar mass explosion models. Scenarios including a helium detonation can significantly contribute to the production of Mn, in particular the models proposed for calcium-rich transients. Although Type Ia supernovae are often not accounted for as production sites of Zn and Cu, our models involving helium shell detonations can produce these elements in super-solar ratios relative to Fe. Our results suggest a re-consideration of Type Ia supernova yields in galactic chemical evolution models. A detailed comparison with observations can provide new insight into the progenitor and explosion channels of these events.

Positron scattering and transport in liquid helium

In previous papers we have proposed a method for the ab initio calculation of fully differential cross-sections for electron scattering in liquids and applied it to liquid argon, xenon and krypton. In this paper, we extend the procedure to the consideration of positron scattering in liquid helium, which is complicated by the annihilation process as well as the fact that the electron definition for the region ‘owned’ by a target atom used previously does not have a positron analogue. We explore several physically motivated definitions to obtain effective positron scattering in the dense fluid. We find that our calculations of a pure helium system cannot precisely match experimental measurements, however by including a small admixture (<0.1%) of an impurity, we can obtain reasonable agreement in the dense gas phase. In contrast, the comparison between our calculations and liquid phase measurements is less satisfactory. This provides motivation to explore further multiple scattering effects in the theory.

Radiation damage of tungsten surface irradiated with high-energy hydrogen and helium beams of plasma focus device

Tungsten (W) surface damages due to high-energy irradiations of a plasma focus device, including pure Hydrogen (H) ions, pure Helium (He) ions, and 50−50 H +He mixture was assessed. W is one of the candidate materials for use in the first-wall of nuclear fusion reactors such as tokamaks. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) were utilized to analyze these damages. SEM images, in a good agreement with blister formation mechanisms proposed by large-scale atomic/molecular massively parallel simulator (LAMMPS) simulations, illustrated three different states. Due to the ability of He ions to increase the join probability of tiny blisters, the intermediate state of H+He mixture irradiation, including large individual blisters perches between two limit states of pure H and pure He irradiations, including tiny individual blisters and large clusters of blisters, respectively. AFM analyses confirm the role of He to provide the intermediate state based on values of roughness in these three states. XRD analyses show the considerable changes in the position and intensity of peaks. The shifted peak positions toward the higher angles present a mean value in the intermediate state of H+He mixture irradiation. Also, the medium peak intensity of the intermediate state clarifies the role of He in modulating the sharp behavior of H in the irradiation of W sample. The calculated compressive stresses show a moderate value for the irradiation with the H+He mixture.

Design Optimization and Analysis of Thermoacoustic Refrigerators

Thermoacoustic refrigeration, a novel technology, uses eco-friendly gases like helium, air or the mixture of noble gases as working substances in the absence of moving parts. The design, optimization and analysis of thermoacoustic refrigerators using helium and air as oscillating gases are discussed. Pure helium is chosen since it is proven as the best and economical working gas compared to the alternate pure or the mixture of noble gases. Air is chosen since it is abundant in nature and the least cost of the pressurized dry air cylinders. The design optimization strategies discussed in this paper serve as a guide for aspiring researchers in the design and development of thermoacoustic coolers. Cooling power as a function of stack diameter is discussed. Theoretical results of the optimized coolers are compared with DeltaEC simulation results for validation and are in agreement with each other.