Helium Distributions Research Articles

Demonstration of full polarization control of soft X-ray pulses with Apple X undulators at SwissFEL using recoil ion momentum spectroscopy.

The ability to freely control the polarization of X-rays enables measurement techniques relying on circular or linear dichroism, which have become indispensable tools for characterizing the properties of chiral molecules or magnetic structures. Therefore, the demand for polarization control in X-ray free-electron lasers is increasing to enable polarization-sensitive dynamical studies on ultrafast time scales. The soft X-ray branch Athos of SwissFEL was designed with the aim of providing freely adjustable and arbitrary polarization by building its undulator solely from modules of the novel Apple X type. In this paper, the magnetic model of the linear inclined and circular Apple X polarization schemes are studied. The polarization is characterized by measuring the angular electron emission distributions of helium for various polarizations using cold target recoil ion momentum spectroscopy. The generation of fully linear polarized light of arbitrary angle, as well as elliptical polarizations of varying degree, are demonstrated.

Effect of free surfaces on localized plastic deformation in single-crystal nickel containing helium bubbles and radiation-induced self-interstitial atom clusters

This paper presents molecular dynamics (MD) simulations of plastic deformation in single-crystal Ni models containing internal distributions of helium (He) bubbles and self-interstitial atom (SIA) clusters corresponding to experimental characterization of ex-service Inconel X-750 components. We simulate single crystals with and without free surfaces at room temperature and 588K, the latter temperature corresponding to the Inconel X-750 service conditions. Both models are deformed under multiple slip to a maximum deviatoric strain of ∼10 %. A dislocation slip band forms in the model without free surfaces, but not in the model with free surfaces. However, the latter exhibits nano-twin nucleation from surfaces. Helium (He) bubbles that overlap with a slip band or a nano-twinned region deform more severely than other bubbles in the model. Nevertheless, the bubbles do not link up and do not form configurations that resemble incipient cracks.

Multiple Stellar Populations at Less Evolved Stages. III. A Possible Helium Spread in NGC 2210

Helium variations are common features of globular clusters (GCs) with multiple stellar populations. All the formation scenarios predict that secondary population stars are enhanced in helium, but the exact helium content depends on the polluters. Therefore, searching for helium variations in a star cluster is a straightforward method to understand whether it hosts multiple populations or not and to constrain the formation scenario. Although this topic has been well explored for Galactic GCs, GCs beyond the Milky Way are challenging to study because of their large distances. This work studies the helium distribution of GK-type main-sequence (MS) dwarfs in an old (∼12.5 Gyr) GC in the Large Magellanic Cloud, NGC 2210, using the deep photometry observed by the Hubble Space Telescope. We compare the observed morphology of the MS with that of synthetic populations with different helium distributions. We confirm that NGC 2210 dwarfs have a helium spread, with an internal dispersion of δ Y ∼ 0.06–0.07. The fraction of helium-enriched stars depends on the δ Y distribution. A continuous δ Y distribution would indicate that more than half of MS stars are helium enriched (∼55%). If the δ Y distribution is discrete (bimodal), a fraction of ∼30% enriched stars is able to explain the observed morphology of the MS. We also find that the He-enriched population stars are more centrally concentrated than He-normal stars.

Thermal expansion of plasma-exposed tungsten

We report results from a systematic analysis of thermal expansion of plasma-exposed tungsten based on molecular-dynamics simulations using models of tungsten with distributions of helium (He) bubbles in the tungsten matrix. We distinguish between two approaches of filling the bubbles with He, where the amount of He in the bubble can or cannot vary with temperature. In the former case, the thermal expansion coefficient decreases monotonically with the porosity and He content of the tungsten matrix, while in the latter case, the thermal expansivity increases monotonically with increasing porosity and He content. The latter condition, where the He content in the bubble is determined at the implantation temperature and remains constant with varying temperature in the tungsten matrix, is consistent with He species transport in tungsten used as a plasma-facing component (PFC) in nuclear fusion reactors and implies the development of biaxial compressive thermal strains in the PFC material that contribute to accelerating the growth of a nanostructure on PFC tungsten surfaces. Our analysis advances the fundamental understanding of thermal expansion in PFC tungsten and contributes to the development of a thermophysical property database for properly incorporating effects of realistic heat loads into modeling the dynamical response of PFC tungsten under fusion reactor operating conditions.

Dopant binding with vacancies and helium in metal hydrides

Metal hydrides are crucial for the long-term storage of tritium but suffer degradation due to the buildup and release of helium decay products. Therefore, it is of interest to explore how dopants in these metal hydrides may impact helium bubble nucleation and distributions, as well as associated fracture and helium retention. Prior studies have focused on helium behavior in pure metal hydrides or with one or two types of impurities. Analytical models have also shown that the concentration of nucleated bubbles can impact the time to fracture of materials. This study utilizes high-throughput density functional theory calculations to identify the impact of transition metal substitutional dopants on helium binding energies in face-centered cubic metal hydrides, such as erbium hydride, holmium hydride, scandium hydride, titanium hydride, yttrium hydride, and zirconium hydride. This study also explores the impact of hydrogen vacancies on the binding energy of helium near the substitutional defects. Finally, this study presents an initial assessment of dopant stability and solubility limits at many temperatures and pressures. Several metals strongly bind to helium in these metal hydrides, making them promising for influencing helium nucleation and subsequent bubble growth. However, many of these potentially beneficial substitutional defects have low solubility limits. The calculations show that some strong binding dopants may be soluble in quantities that affect the bubble concentration and impact material performance.

Modelling and measurements of gas tungsten arc welding in argon–helium mixtures with metal vapour

Argon–helium mixtures in gas tungsten arc welding of an iron workpiece are investigated using an axisymmetric computational model that includes the cathode, workpiece, and arc plasma in the computational domain. The three-gas combined diffusion coefficient method is used to treat diffusion of helium, argon, and iron vapour. Calculations for argon–helium mixtures without metal vapour are performed; good agreement with previous numerical results is found. A transition from a helium-like to an argon-like arc occurs when the argon mole fraction increases above about 0.3. Calculations for a wide range of argon–helium mixtures including iron vapour are then performed. Adding helium to argon alters the arc properties and affects the weld geometry. Iron vapour cools the arc for all argon–helium mixtures. Iron vapour is present above the workpiece, near the cathode and in the arc fringes for very low argon mole fractions. As the argon mole fraction increases, the iron vapour becomes increasingly confined to the region above the workpiece, with small amounts near the cathode tip. Emission spectroscopy measurements of arcs in argon–helium mixtures with water-cooled copper and uncooled iron workpieces were performed. The measured distributions of atomic helium and iron emission show good agreement with the predictions of the model.

Measurements of helium distributions in a scaled-down parking garage model for unintended releases from a fuel cell vehicle

Most previous studies of hydrogen fuel cell vehicle (HFCV) releases in garages have focused on residential garages. However, hydrogen releases in large underground parking garages can be completely different with more concerns about public safety. In the present work, unintended hydrogen releases were studied in a scaled-down underground parking garage model to fill the knowledge gap concerning hydrogen dispersion in a large confined space. Helium was used in place of hydrogen for safety reasons. Gas sensors were used to measure the helium concentrations along the ground and the ceiling. The nearfield concentration distributions were studied to show that the leak jet was strongly affected by the buoyancy, so the gas concentration was very low along the floor with a flammable gas mixture only appearing right under the leaking car. The gas plume driven by the buoyancy reached the ceiling and spread in all directions resulting in a flammable layer along the ceiling. The effects of neighboring cars and walls were also investigated to show that the obstacles affected the nearfield gas dispersion but did not significantly affect the farfield concentration distributions. The concentration increased linearly with the release flow rate for the flow rates in this study. The gas dispersion during the release showed self-similar characteristics, which could be used for leak detection. This database will be useful for model validation for modeling HFCV releases in large underground parking garages and also provides guidelines for gas sensor placement in large confined spaces.

Distorted surfaces of magnetic helium-peculiar stars: an application to a Cen

ABSTRACT Helium-peculiar magnetic chemically peculiar stars show variations of helium abundance across their surfaces. As a result of associated atmospheric scale height variations, the stellar surface becomes distorted, with helium-rich regions dented inwards. Effectively, on top of flux variations due to opacity effects, the depressed helium-rich surface regions become less bright in the optical regions and brighter in the ultraviolet. We study the observational effects of the aspherical surface on the light curves of a Cen. We simulate the light curves of this star adopting surface distributions of He, N, O, Si, and Fe derived from Doppler mapping and introducing the effect of distortion proportional to helium abundance. We show that while most of the optical and UV variations of this star result from flux redistribution due to the non-uniform surface distributions of helium and iron, the reduction of light variations due to the helium-related surface distortion leads to a better agreement between simulated optical light curves and the light curves observed with the BRITE satellites.

Evidence for a Dichotomy in the Interior Structures of Jupiter and Saturn from Helium Phase Separation

Abstract We examine the comparative thermal evolution of Jupiter and Saturn, applying recent theoretical results for helium’s immiscibility in fluid metallic hydrogen. The redistribution of helium in their interiors proceeds very differently for the two planets. We confirm that, based on Jupiter’s atmospheric helium depletion as observed in situ by the Galileo entry probe, Jupiter’s interior helium has differentiated modestly, and we present models reconciling Jupiter’s helium depletion, radius, and heat flow at the solar age. Jupiter’s recently revised Bond albedo implies a higher intrinsic flux for the planet, accommodating more luminosity from helium differentiation, such that mildly superadiabatic interiors can satisfy all constraints. The same phase diagram applied to the less massive Saturn predicts dramatic helium differentiation, to the degree that Saturn inevitably forms a helium-rich shell or core, an outcome previously proposed by Stevenson & Salpeter and others. The luminosity from Saturn’s helium differentiation is sufficient to extend its cooling time to the solar age, even for adiabatic interiors. This model predicts Saturn’s atmospheric helium to be depleted to Y = 0.07 ± 0.01, corresponding to a He/H2 mixing ratio 0.036 ± 0.006. We also show that neon differentiation may have contributed to both planets’ luminosity in the past. These results demonstrate that Jupiter and Saturn’s thermal evolution can be explained self-consistently with a single physical model, and emphasize that nontrivial helium distributions should be considered in future models for Saturn’s internal structure and dynamo.

Helium distributions in ocean island basalt olivines revealed by X-ray computed tomography and single-grain crushing experiments

X-ray computed tomography of individual olivine crystals in basalts from Ofu and Olosega islands, American Samoa, reveals that a small fraction of the olivines contain the vast majority of the fluid inclusions. Single-grain crushing experiments demonstrate that He and CO2 reside primarily in these inclusions. Low CO2 pressures in most grains, corresponding to depths of less than 1 km, provide evidence of ubiquitous decrepitation and associated pressure reduction in the fluid inclusions. Even so, the olivines with the highest inclusion volumes yielded sufficient He to obtain precise He concentrations and isotopic compositions. Within analytical uncertainty, 3He/4He ratios are homogeneous among the olivines from each basalt, but among basalts, the ratios range from 21 to 35 Ra. The total range in C/3He ratio within the analyzed olivines is from 3.6 × 107 to 1.5 × 1010, and varies by nearly an order of magnitude within the olivines from each basalt. We postulate that this wide range of C/3He ratios is caused by grain-scale decoupling of C and 3He due to extensive He diffusion out of fluid inclusions through the olivine lattice during magma ascent and cooling. If so, primary Ofu-Olosega magmas probably had C/3He ratios less than 4 × 108, which is lower than previous estimates for hotspot magmas.

A coupled rate theory-Monte Carlo model of helium bubble evolution in plasma-facing micro-engineered tungsten

A multiscale model of helium bubble evolution in plasma-facing materials is developed. The model links different stages of helium bubble evolution: deposition, nucleation, growth, motion, and coalescence. Helium deposition is simulated with the SRIM Monte Carlo program to give spatial information on helium and displacement damage distributions near the surface. This deposition profile is then introduced into a space-dependent rate theory of bubble nucleation and growth to describe the early stages of the distribution and size of helium bubbles. The coarsening stage of bubble evolution as a result of whole bubble motion, interaction, and coalescence is modeled by a new Object Kinetic Monte Carlo (OKMC) model, for which initial conditions are taken from the mean-field rate theory calculations. The model is compared to experimental data on low-energy helium plasma interaction with micro-engineered tungsten (W), and on high-energy helium ion deposition in flat W samples. Novel features of the multiscale model include: (1) space-dependent rate theory; (2) OKMC model of bubble motion in stress and temperature fields; and (3) application of the model to micro-engineered materials, and comparison with experiments on the same time-scale. At low helium ion energy, it is found that the mechanism of trap mutation is essential in achieving good agreement with experimental measurements. On the other hand, good agreement with experiments at high incident ion energy and temperature showed the importance of bubble coalescence and coarsening as main mechanisms. The results of the model are compared with experiments on flat W surfaces irradiated at high ion energy (30 keV), and with micro-engineered W, where the surface is coated with high-density micro-pillars at low ion energy (around 100 eV). The predicted average bubble radius and density are in qualitative agreement with experimental results.

Thermal performance analysis of a high-altitude solar-powered hybrid airship

Abstract The increasing application of hybrid airships which have been recently proposed as high altitude platforms, makes it necessary for research into the thermal performance of such airships that possess a photovoltaic module array(PVMA). In this study, a simplified thermal model of a high-altitude hybrid airship with a PVMA, was proposed that included direct solar, infrared, reflected, and scattered radiation and convective heat transfer. Based on computational fluid dynamics(CFD), a simulation methodology, using a user defined function(UDF) program, was introduced to investigate the PVMA's thermal effects on the hybrid airship. A ground experiment was also performed to validate this numerical method's effectiveness. Further simulations and discussion of temperature and velocity distributions of the hybrid airship's internal helium were conducted. The results showed that the PVMA atop the hull had large effects on the hybrid airship's thermal performance. The temperature distribution of the envelope varied greatly during the day and night because of the PVMA's influence as well as the internal helium flow. In addition, forced convection had little influence on the PVMA's output performance, which was analyzed in detail.

Gas flow rate dependence of the discharge characteristics of a helium atmospheric pressure plasma jet interacting with a substrate

A 2D (axisymmetric) computational study of the discharge characteristics of an atmospheric pressure plasma jet as a function of gas flow rate was performed. The helium jet emerged from a dielectric tube, with an average gas flow velocity in the range 2.5–20 m s−1 (1 atm, 300 K) in a nitrogen ambient, and impinged on a substrate a short distance dowstream. The effect of the substrate conductivity (conductror versus insulator) was also studied. Whenever possible, simulation predictions were compared with published experimental observations. Discharge ignition and propagation in the dielectric tube were hardly affected by the He gas flow velocity. Most properties of the plasma jet, however, depended sensitively on the He gas flow velocity, which determined the concentration distributions of helium and nitrogen in the mixing layer forming in the gap between the tube exit and the substrate. At low gas flow velocity, the plasma jet evolved from a hollow (donut-shaped) feature to one where the maximum of electron density was on axis. When the gas flow velocity was high, the plasma jet maintained its hollow structure until it struck the substrate. For a conductive substrate, the radial ion fluxes to the surface were relatively uniform over a radius of ~0.4–0.8 mm, and the dominant ion flux was that of He+. For a dielectric substrate, the radial ion fluxes to the surface peaked on the symmetry axis at low He gas flow velocity, but a hollow ion flux distribution was observed at high gas flow velocity. At the same time, the main ion flux switched from to as the He gas flow velocity increased from a low to a high value. The diameter of the plasma ‘footprint’ on the substrate first increased with increasing He gas flow velocity, and eventually saturated with further increases in velocity.

Multielectron effects in the photoelectron momentum distribution of noble-gas atoms driven by visible-to-infrared-frequency laser pulses: A time-dependent density-functional-theory approach

We present the photoelectron momentum distributions (PMDs) of helium, neon, and argon atoms driven by a linearly polarized, visible (527-nm) or near-infrared (800-nm) laser pulse (20 optical cycles in duration) based on the time-dependent density-functional theory (TDDFT) under the local-density approximation with a self-interaction correction. A set of time-dependent Kohn-Sham equations for all electrons in an atom is numerically solved using the generalized pseudospectral method. An effect of the electron-electron interaction driven by a visible laser field is not recognizable in the helium and neon PMDs except for a reduction of the overall photoelectron yield, but there is a clear difference between the PMDs of an argon atom calculated with the frozen-core approximation and TDDFT, indicating an interference of its $M$-shell wave functions during the ionization. Furthermore, we find that the PMDs of degenerate $p$ states are well separated in intensity when driven by a near-infrared laser field, so that the single-active-electron approximation can be adopted safely.

A theoretical study on collision mechanisms for low energy electron impact ionization of helium in the perpendicular geometry

Under the condition of ten different incident energies ranging from 3 eV to 80 eV above the ionization potential of helium and the outgoing electrons having equal energies, by making use of 3C model and modified 3C model, the triple differential cross sections of electron-impact single ionization of the ground state of helium in the perpendicular geometry are calculated. The result is compared with corresponding experimental result to systematically investigate the influences of various screening effects on the triple differential cross sections for helium. The collision mechanisms of the triple differential cross sections are explored. The result shows that the effects of dynamic screening in the final state can directly affect the structures of the triple differential cross sections at lower incident energy, which will unavoidably affect the angular distribution and relative amplitude of side peaks at angles =90 and =270. The screening effects of residual electron in the final state of He+ have a similar significant effect on the amplitude of triple differential cross section of helium and angular distributions and relative amplitudes of side peaks at angles =90 and =270. When the incident energy is over 84.6 eV, the screening effect of residual electron in the final state of He+ has a slight effect on the amplitude of triple differential cross section, which can be overlooked. But the effects of dynamic screening in the final state on side peaks at angles =90 and =270 need considering. In addition, by taking advantage of DS3C-Z model, the results of collision mechanism of various peaks at angles =180, =90 and =270 show that the middle peak at angle =180 is produced by a process called triple scattering mechanism and then the side peaks at angles =90 and =270 are produced by a process called double scattering mechanism. Such a collision mechanism has a direct influence on the generation and variation law of triple differential cross sections.

Simulation of a non-equilibrium helium plasma bullet emerging into oxygen at high pressure (250–760 Torr) and interacting with a substrate

A two-dimensional computational study of a plasma bullet emanating from a helium gas jet in oxygen ambient at high pressure (250–760 Torr) was performed, with emphasis on the bullet interaction with a substrate. Power was applied in the form of a trapezoidal +5 kV pulse lasting 150 ns. A neutral gas transport model was employed to predict the concentration distributions of helium and oxygen in the system. These were then used in a plasma dynamics model to investigate the characteristics of the plasma bullet during its propagation and interaction with a substrate. Upon ignition, the discharge first propagated as a surface wave along the inner wall of the containing tube, and then exited the tube with a well-defined ionization front (streamer or plasma bullet). The plasma bullet evolved from a hollow (donut-shaped) feature to one where the maximum of ionization was on axis. The bullet propagated in the gap between the tube exit and the substrate with an average speed of ∼2 × 105 m/s. Upon encountering a metal substrate, the bullet formed a conductive channel to the substrate. Upon encountering a dielectric substrate, the bullet turned into an ionization wave propagating radially along the substrate surface. For a conductive substrate, the radial species fluxes to the surface peaked on the symmetry axis. For a dielectric substrate, a ring-shaped flux distribution was observed. The “footprint” of plasma-surface interaction increased either by decreasing the gap between tube exit and substrate, decreasing the relative permittivity of an insulating substrate, or decreasing pressure. As the system pressure was lowered from 760 to 250 Torr, the discharge was initiated earlier, and the plasma bullet propagation speed increased. A reverse electric field developed during the late stages of the ramp-down of the pulse, which accelerated electrons forming a brief backward discharge.

Monte Carlo simulations of ³He ion physical characteristics in a water phantom and evaluation of radiobiological effectiveness.

(3)He ions may hold great potential for clinical therapy because of both their physical and biological properties. In this study, the authors investigated the physical properties, i.e., the depth-dose curves from primary and secondary particles, and the energy distributions of helium ((3)He) ions. A relative biological effectiveness (RBE) model was applied to assess the biological effectiveness on survival of multiple cell lines. In light of the lack of experimental measurements and cross sections, the authors used Monte Carlo methods to study the energy deposition of (3)He ions. The transport of (3)He ions in water was simulated by using three Monte Carlo codes-FLUKA, GEANT4, and MCNPX-for incident beams with Gaussian energy distributions with average energies of 527 and 699 MeV and a full width at half maximum of 3.3 MeV in both cases. The RBE of each was evaluated by using the repair-misrepair-fixation model. In all of the simulations with each of the three Monte Carlo codes, the same geometry and primary beam parameters were used. Energy deposition as a function of depth and energy spectra with high resolution was calculated on the central axis of the beam. Secondary proton dose from the primary (3)He beams was predicted quite differently by the three Monte Carlo systems. The predictions differed by as much as a factor of 2. Microdosimetric parameters such as dose mean lineal energy (y(D)), frequency mean lineal energy (y(F)), and frequency mean specific energy (z(F)) were used to characterize the radiation beam quality at four depths of the Bragg curve. Calculated RBE values were close to 1 at the entrance, reached on average 1.8 and 1.6 for prostate and head and neck cancer cell lines at the Bragg peak for both energies, but showed some variations between the different Monte Carlo codes. Although the Monte Carlo codes provided different results in energy deposition and especially in secondary particle production (most of the differences between the three codes were observed close to the Bragg peak, where the energy spectrum broadens), the results in terms of RBE were generally similar.

Development of a compact ECR ion source for various ion production.

There is a desire that a carbon-ion radiotherapy facility will produce various ion species for fundamental research. Although the present Kei2-type ion sources are dedicated for the carbon-ion production, a future ion source is expected that could provide: (1) carbon-ion production for medical use, (2) various ions with a charge-to-mass ratio of 1/3 for the existing Linac injector, and (3) low cost for modification. A prototype compact electron cyclotron resonance (ECR) ion source, named Kei3, based on the Kei series has been developed to correspond to the Kei2 type and to produce these various ions at the National Institute of Radiological Sciences (NIRS). The Kei3 has an outer diameter of 280 mm and a length of 1120 mm. The magnetic field is formed by the same permanent magnet as Kei2. The movable extraction electrode has been installed in order to optimize the beam extraction with various current densities. The gas-injection side of the vacuum chamber has enough space for an oven system. We measured dependence of microwave frequency, extraction voltage, and puller position. Charge state distributions of helium, carbon, nitrogen, oxygen, and neon were also measured.

A self‐consistent model of helium in the thermosphere

AbstractWe have found that consideration of neutral helium as a major species leads to a more complete physics‐based modeling description of the Earth's upper thermosphere. An augmented version of the composition equation employed by the Thermosphere‐Ionosphere‐Electrodynamic General Circulation Model (TIE‐GCM) is presented, enabling the inclusion of helium as the fourth major neutral constituent. Exospheric transport acting above the upper boundary of the model is considered, further improving the local time and latitudinal distributions of helium. The new model successfully simulates a previously observed phenomenon known as the “winter helium bulge,” yielding behavior very similar to that of an empirical model based on mass spectrometer observations. This inclusion has direct consequence on the study of atmospheric drag for low‐Earth‐orbiting satellites, as well as potential implications on exospheric and topside ionospheric research.

Helium ion distributions in a 4 kJ plasma focus device by 1 mm-thick large-size polycarbonate detectors

Abstract Helium ion beam profile, angular and iso-ion beam distributions in 4 kJ Amirkabir plasma focus (APF) device were effectively observed by the unaided eyes and studied in single 1 mm-thick large-diameter (20 cm) polycarbonate track detectors (PCTD). The PCTDs were processed by 50 Hz–HV electrochemical etching using a large-size ECE chamber. The results show that helium ions produced in the APF device have a ring-shaped angular distribution peaked at an angle of ∼ ± 60 ° with respect to the top of the anode. Some information on the helium ion energy and distributions is also provided. The method is highly effective for ion beam studies.