Hydrogen-rich Layer Research Articles

The Effects of Thomson Scattering and Chemical Mixing on Early-time Light Curves of Double-peaked Type IIb Supernovae

Previous numerical simulations of double-peaked Type IIb supernova (SN IIb) light curves have demonstrated that the radius and mass of the hydrogen-rich envelope of the progenitor star can significantly influence the brightness and timescale of the early-time light curve around the first peak. In this study, we investigate how Thomson scattering and chemical mixing in the SN ejecta affect the optical light curves during the early stages of the SNe IIb using radiation hydrodynamics simulations. By comparing the results from two different numerical codes (i.e., STELLA and SNEC), we find that the optical brightness of the first peak can be reduced by more than a factor of 3 as a result of the effect of Thomson scattering that causes the thermalization depth to be located below the Rosseland mean photosphere, compared to the corresponding case where this effect is ignored. We also observe a short-lived plateau-like feature lasting for a few days in the early-time optical light curves of our models, in contrast to typical observed SNe IIb that show a quasi-linear decrease in optical magnitudes after the first peak. A significant degree of chemical mixing between the hydrogen-rich envelope and the helium core in SN ejecta is required to reconcile this discrepancy between the model prediction and observation. Meanwhile, to properly reproduce the first peak, a significant mixing of 56Ni into the hydrogen-rich outermost layers should be restricted. Our findings indicate that inferring the SN IIb progenitor structure from a simplified approach that ignores these two factors may introduce substantial uncertainty.

An observed population of intermediate-mass helium stars that have been stripped in binaries

The hydrogen-rich outer layers of massive stars can be removed by interactions with a binary companion. Theoretical models predict that this stripping produces a population of hot helium stars of ~2 to 8 solar masses ( M ☉ ), however, only one such system has been identified thus far. We used ultraviolet photometry to identify potential stripped helium stars then investigated 25 of them using optical spectroscopy. We identified stars with high temperatures (~60,000 to 100,000 kelvin), high surface gravities, and hydrogen-depleted surfaces; 16 stars also showed binary motion. These properties match expectations for stars with initial masses of 8 to 25 M ☉ that were stripped by binary interaction. Their masses fall in the gap between subdwarf helium stars and Wolf-Rayet stars. We propose that these stars could be progenitors of stripped-envelope supernovae.

Hydrogenation of p+ poly-Si by Al2O3 nanolayers prepared by atomic layer deposition

Polysilicon (poly-Si) passivating contacts have enabled some of the highest lab-scale crystalline silicon (c-Si) solar cell conversion efficiencies, largely due to their excellent surface passivation quality, which can be aided by means of hydrogenation treatments. One frequently applied method is to use hydrogen-rich capping layers, such as Al2O3 or SiNx, which can provide hydrogen to the poly-Si/SiOx/Si interface upon annealing. In this work, the effect of the deposition conditions of the Al2O3 layers, fabricated by various types of atomic layer deposition (ALD), on the hydrogenation of p+ poly-Si is investigated, and it is compared to the direct passivation of c-Si by the same Al2O3 layers. It is found that excellent hydrogenation of the p+ poly-Si by Al2O3 could be reached by a wide range of ALD conditions, but higher annealing temperatures were required as compared to passivation of c-Si. The different ALD conditions result in Al2O3 layers with varying refractive indices, O/Al ratios, and hydrogen content, although these material properties become highly similar upon annealing. Furthermore, the p+ poly-Si layer appears to act as a reservoir for hydrogen, which alters the hydrogen effusion profiles of the Al2O3 layers. The results show that a wide range of Al2O3 layers are highly suitable for hydrogenation of p+ poly-Si and provide more insight into the hydrogenation process of poly-Si passivating contacts.

The intermediate neutron capture process

Results from observations report a growing number of metal-poor stars showing an abundance pattern midway between the s- and r-processes. These so-called r/s-stars raise the need for an intermediate neutron capture process (i-process), which is thought to result from the ingestion of protons in a convective helium-burning region, but whose astrophysical site is still largely debated. We investigate whether an i-process during the asymptotic giant branch (AGB) phase of low-metallicity low-mass stars can develop and whether it can explain the abundances of observed r/s-stars. At the beginning of the AGB phase, during the third thermal pulse, the helium driven convection zone is able to penetrate the hydrogen-rich layers. The subsequent proton ingestion leads to a strong neutron burst with neutron densities of $\approx 4.3 \times 10^{14}$ cm$^{-3}$ at the origin of the synthesis of i-process elements. The nuclear energy released by proton burning in the helium-burning convective shell strongly affects the internal structure: the thermal pulse splits and after approximately ten years the upper part of the convection zone merges with the convective envelope. The surface carbon abundance is enhanced by more than 3 dex. This leads to an increase in the opacity, which triggers a strong mass loss and prevents any further thermal pulse. We show that specific isotopic ratios of Ba, Nd, Sm, and Eu can represent good tracers of i-process nucleosynthesis. Finally, an extended comparison with 14 selected r/s-stars show that the observed composition patterns can be well reproduced by our i-process AGB model.

On the kinetics of the exchange of hydrogen between hydrogen–boron pairs and hydrogen dimers in crystalline silicon

Silicon samples after fast-firing with a hydrogen-rich silicon nitride layer on their surfaces can contain high concentrations of hydrogen (up to 6 × 1015 cm−3 in this study). Directly after fast-firing, this hydrogen is mostly present in a neutral dimeric state. Subsequent dark annealing applied in a temperature range between 140 and 175 °C leads to a conversion of dimers into HB pairs, which means that a significant number of boron acceptors are electrically inactive. The concentration of inactive boron, the hydrogen–boron (HB) pair concentration [HB], can thus be determined by measuring the change in specific resistivity before and after annealing. In our study, after the initial anneal for HB pair formation, the same samples are subsequently annealed at stepwise increasing higher temperatures, which leads to a partial backward conversion of HB pairs into neutral hydrogen dimers. This is indicated by a gradual reduction of the resistivity upon increasing the annealing temperature. By measuring the transient curves [HB](t) during each temperature step on samples with different boron content, we extract the parameters for the exchange between the HB pairs and the hydrogen dimers within the framework of our presented physical model. Apart from the backward formation of hydrogen dimers, from HB pairs upon increasing temperature, prolonged annealing at the same temperature leads to a transition of HB pairs into a more stable form of hydrogen dimers, compared to the initial form after fast-firing. This transition is also included in our proposed defect model.

Evaluation of temperature and flow area variations through the fuel degradation and relocation of the SFD Test 1–4

We used the MAAP5 code and evaluated temperature and flow area variations through fuel degradation and fuel relocation in the Severe Fuel Damage (SFD) Test 1–4. The measured temperatures in the fuel bundle were predicted by considering the steam diffusion-limited effect within the hydrogen-rich boundary layer near the fuel cladding. After confirming that the temperatures in the fuel bundle could be predicted, we clarified the effect of two different binary phase diagrams between UO2 and Zr on the computed results of relocation and flow area. One was the binary phase diagram based on MATPRO, and the other was the diagram drawn using a thermodynamic database. We found that the height of the relocated U-Zr-O mixture evaluated using the binary phase diagram drawn with the thermodynamic database was slightly lower than that evaluated using the phase diagram based on MATPRO, but the difference was small in the high temperature and diluted steam conditions of the SFD Test 1–4.

Hydrogen-induced ductilization in a novel austenitic lightweight TWIP steel

This study reported a novel hydrogen-induced ductilization in an austenitic lightweight twinning-induced plasticity (TWIP) steel (Fe-1C-25Mn-5.5Al-3Si, in wt. %). This phenomenon was irrelevant to strain rate or global work hardening rate. In fact, the stacking fault energy (SFE), 49.5 mJm−2, of the steel is an appropriate value for hydrogen-induced ductilization. This SFE was reduced by hydrogen and deformation twin was enhanced in the hydrogen-rich surface layer during deformation. Such twinning-strengthened surface limits formation of strain localization and allows extra deformation. Besides, this SFE is high enough so that martensitic transformation and hydrogen-enhanced intergranular cracking were absent, making the steel free from hydrogen embrittlement. This work provides a new concept that SFE/twinability engineering not only prevents hydrogen embrittlement but enables hydrogen-induced ductilization.

Compositional partitioning during early stages of oxidation of a uranium-molybdenum alloy

Compositional partitioning during uranium alloy oxidation was studied via complementary ex situ - in situ atom probe tomography. Nanoscopic volumes of uranium- 22 at. % molybdenum were exposed to air at room temperature/atmospheric pressure for 30–60 minutes (ex situ), and 300 ∘C - 10−5 mbar O2 gas for 2–5 minutes in a chemical reaction chamber attached to an atom probe system (in situ). For all environmental conditions, a hypostoichiometric uranium oxide is formed. Reaction fronts are observed at oxide/metal, oxide/hydride, and outer oxide/environment interfaces. Results reveal Mo redistributes across the oxide/metal interface, with a tendency for enrichment in the outer oxide. The formation of a hydrogen-rich subsurface layer between the oxide and base alloy accompanies oxidation in both air and oxygen gas environments. Carbon and silicon impurity elements also redistribute to the outer oxide, contributing to oxide film composition.

Reaction Mechanism and Selectivity Control of Si Compound ALE Based on Plasma Modification and F-Radical Exposure.

In this work, atomic layer etching (ALE) of Si compounds using H2 or N2 plasma modification followed by fluorine radical exposure is discussed. It is shown that the H2 plasma modification process promotes the selective etching of SiN, SiC, and SiCO versus SiO2. The N2 plasma modification, on the other hand, enables the selective etching of SiC and SiCO versus SiN and SiO2. The origin of the etching selectivity between different Si compounds is investigated using a combination of in situ SE and FTIR supported by several ex situ analysis techniques. It is shown that the formation of a hydrogen-rich layer after plasma modification is essential to enable the ALE process. The hydrogen-rich layer can be formed due to ion and radicals of the modification plasma (H2 plasma modification) or be a result of the reconfiguration of hydrogen that is already present in the film (N2 plasma modification). The obtained insights are expected to further enhance the etching selectivity of Si compound ALE processes. Furthermore, it is anticipated that the process can be extended to many other compound materials such as Ti and Hf, as well as enable selective etching between their oxides, carbides, and nitrides.

CFD simulations of stratified layer erosion in MiniPanda facility using the tailored CFD solver containmentFOAM

Turbulent gas mixing in a buoyant flow with density gradient is an important physical mechanism in many applications ranging from industrial gaseous emissions into the atmosphere to nuclear reactor containment flows. With relevance to nuclear reactor safety, erosion of a stratified light gas layer by a momentum-driven or buoyancy-driven jet is often studied owing to the possibility of flammable hydrogen rich layer formation and associated combustion risks in the containment as it occurred at the Fukushima Daiichi accident. In the recent developments of CFD models for containment flows, it is identified that buoyancy contribution in the turbulence production and dissipation terms irrespective of the turbulence model plays a significant role.Owing to its open-source code and rapid developments in OpenFOAM, a tailored solver containmentFOAM based on OpenFOAM libraries is currently being developed at Forschungszentrum Jülich with significant improvements and modeling choices. In the present paper, the MiniPanda experiments performed at ETH Zürich, Switzerland, are chosen for performing the URANS simulations with containmentFOAM to evaluate its capabilities while giving specific attention to the transient evolution of the flow field. In addition to the classical validation of helium concentration profiles, the temperature measurements extracted from TWMS (Temperature Wire Mesh Sensors) and the jet penetration analyses are utilized to enhance the understanding of turbulent heat and mass transfer. The impact of turbulence production and dissipation terms due to buoyancy on the turbulent gas mixing and thermal transport is demonstrated in detail. Their consideration reveals a visible impact on the mixing process, while the model formulation SGDH or GGDH has a minor impact in this flow configuration. Furthermore, the impact of the mixture model for the fluid transport properties is investigated. It is clearly demonstrated that the Wilke mixture law provides more consistent results in the case of He-air mixing.

The intermediate neutron capture process

Context. Results from observations report a growing number of metal-poor stars showing an abundance pattern midway between the s- and r-processes. These so-called r/s-stars raise the need for an intermediate neutron capture process (i-process), which is thought to result from the ingestion of protons in a convective helium-burning region, but whose astrophysical site is still largely debated. Aims. We investigate whether an i-process during the asymptotic giant branch (AGB) phase of low-metallicity low-mass stars can develop and whether it can explain the abundances of observed r/s-stars. Methods. We computed a 1 M⊙ model at [Fe/H] = −2.5 with the stellar evolution code STAREVOL, using a nuclear network of 1091 species (at maximum) coupled to the transport processes. The impact of the temporal and spatial resolutions on the resulting abundances was assessed. We also identified key elements and isotopic ratios that are specific to i-process nucleosynthesis and carried out a detailed comparison between our model and a sample of r/s-stars. Results. At the beginning of the AGB phase, during the third thermal pulse, the helium driven convection zone is able to penetrate the hydrogen-rich layers. The subsequent proton ingestion leads to a strong neutron burst with neutron densities of ∼4.3 × 1014 cm−3 at the origin of the synthesis of i-process elements. The nuclear energy released by proton burning in the helium-burning convective shell strongly affects the internal structure: the thermal pulse splits and after approximately ten years the upper part of the convection zone merges with the convective envelope. The surface carbon abundance is enhanced by more than 3 dex. This leads to an increase in the opacity, which triggers a strong mass loss and prevents any further thermal pulse. Our numerical tests indicate that the i-process elemental distribution is not strongly affected by the temporal and spatial resolution used to compute the stellar models, but typical uncertainties of ±0.3 dex on individual abundances are found. We show that specific isotopic ratios of Ba, Nd, Sm, and Eu can represent good tracers of i-process nucleosynthesis. Finally, an extended comparison with 14 selected r/s-stars show that the observed composition patterns can be well reproduced by our i-process AGB model. Conclusions. A rich i-process nucleosynthesis can take place during the early AGB phase of low-metallicity low-mass stars and explain the elemental distribution of most of the r/s-stars, but cannot account for the high level of enrichment of the giant stars in a scenario involving pollution by a former AGB companion.

Enhanced adhesion strength and solid lubricity of graphite like amorphous carbon films by hydrogen implantation

Hydrogen-free graphite-like carbon (GLC) films with columnar nanostructure were modified by hydrogen plasma immersion ion implantation (PIII) at a relatively high pulse voltage of 30 kV. The transition from a columnar nanostructure to a glassy structure in the top layer of the implanted films was firstly observed by transmission electron microscopy . Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was employed to survey the depth profile information of hydrogen (H) and carbon‑hydrogen (C H) bonds, and the results showed that the implanted hydrogen intensively distributed in the glassy layer. The results of Raman spectra showed that disorder of carbon bonds increased and a diamond like carbon structure was derived near the films surface through hydrogen ions implantation. The adhesion strength and tribological performance in nitrogen of GLC were enhanced significantly by hydrogen implantation. • Nano-columnar transferred into glassy structure in hydrogen implanted GLC films. • A hydrogen-rich layer induced by hydrogen implantation. • Adhesion strength enhanced by the hydrogen implantation. • Ultralow stable friction of GLC films in dry nitrogen realized through hydrogen implantation.

Hydrogen atom-ion synergy in surface lattice modification at sub-threshold energy

Low-energy hydrogen-isotope (HI) plasma is widely applied for industrial surface processing. Here we demonstrate that HI plasma - even with ion energies far below the threshold for stable Frenkel pair production - can strongly modify crystalline materials by forming heavily lattice-distorted surface layers with a thickness of several nanometers and a HI content of several atomic percent. We experimentally reproduce the identical lattice modification at deuterium and hydrogen plasma-irradiated tungsten (W) surfaces when the sub-threshold HI ion energies are adjusted such as to transfer equal amounts of kinetic energy in collisions with W lattice atoms. A physical model for the low-energy generation of primary defects is proposed, which involves the synergy between temporary Frenkel pair creation by sub-threshold HI ion collisions and vacancy stabilization by trapping of solute HI atoms. Such synergistic defect generation is generally expected upon injection of energetic projectiles (ions, neutrons) into HI-containing solids and likely contributes to material degradation. As a means of surface modification, the fabrication of nanometer-scale hydrogen-rich surface layers on materials (even those with negligible hydrogen solubility) via low-energy H plasma exposure promises utilization potential in catalysis and electrochemistry.

Light-induced lifetime degradation effects at elevated temperature in Czochralski-grown silicon beyond boron-oxygen-related degradation

The effect of ‘Light and elevated Temperature Induced Degradation’ (LeTID) of the carrier lifetime is well known from multicrystalline silicon (mc-Si) wafers and solar cells. In this contribution, we perform a series of carrier lifetime measurements to examine, whether the same effect may also be observable in boron-doped Czochralski-grown silicon (Cz-Si). The Cz-Si samples of our study are illuminated (i) at room temperature, (ii) under standard regeneration conditions eliminating the boron-oxygen (BO) related defect (i.e. at 185 °C) and (iii) at a temperature of 80 °C, typical for the examination of the LeTID effect in mc-Si. We observe the typical decay of the carrier lifetime due to the activation of the BO-related defect. Beyond the BO degradation, applying standard solar cell processes, there is no indication for the activation of a second defect. On samples, whose surfaces are passivated by fired hydrogen-rich silicon nitride layers, an additional bulk lifetime degradation effect on a long timescale is observed in the Cz-Si material. However, defect generation rate and injection dependence of the lifetime suggest another defect type than the mc-Si-specific LeTID defect. We conclude that by applying processing steps that trigger LeTID in mc-Si, the same defect does not occur in the Cz-Si samples examined in this study. On a long timescale, however, a hitherto unknown type of defect is activated, which is different from the mc-Si-specific LeTID defect. A careful differentiation between the various kinds of recombination centres which may form during illumination at elevated temperatures is hence of utmost importance.

R-mode oscillations in accreting white dwarfs in cataclysmic variables

Some cataclysmic variables show short-period ($\sim$200 to $\sim$2000 s) light variations, which are attributable to nonradial pulsations of the accreting primary white dwarf. We regard these periodic variations as r-mode (global Rossby-wave) oscillations which are presumably excited mechanically and confined into hydrogen-rich layers in the white dwarf. Often observed modulations in amplitudes and frequencies even during an observing season may be interpreted as beatings among densely distributed r-mode frequencies. Making the r-mode frequency distribution of a model to be consistent (by adjusting the rotation frequency) with the observed pulsation frequencies, we obtain the best-fit rotation rate for each case. The rotation periods thus obtained for 17 accreting white dwarfs lie between 4 and 10 minutes. Pulsation frequencies observed sometime after a dwarf-nova outburst tend to be higher than the pre-outburst values, so that the best-fit rotation rate for the post-outburst frequencies is also higher. This indicates a spin-up of the hydrogen-rich layers to occur by an enhanced accretion at the outburst. We also discuss the relation between the rotation frequencies of pulsating accreting white dwarfs and the orbital periods.

Spectral models for binary products: Unifying subdwarfs and Wolf-Rayet stars as a sequence of stripped-envelope stars

Stars stripped of their hydrogen-rich envelope through interaction with a binary companion are generally not considered when accounting for ionizing radiation from stellar populations, despite the expectation that stripped stars emit hard ionizing radiation, form frequently, and live 10–100 times longer than single massive stars. We compute the first grid of evolutionary and spectral models specially made for stars stripped in binaries for a range of progenitor masses (2–20M⊙) and metallicities ranging from solar to values representative for pop II stars. For stripped stars with masses in the range 0.3–7M⊙, we find consistently high effective temperatures (20 000–100 000 K, increasing with mass), small radii (0.2–1R⊙), and high bolometric luminosities, comparable to that of their progenitor before stripping. The spectra show a continuous sequence that naturally bridges subdwarf-type stars at the low-mass end and Wolf-Rayet-like spectra at the high-mass end. For intermediate masses we find hybrid spectral classes showing a mixture of absorption and emission lines. These appear for stars with mass-loss rates of 10−8−10−6M⊙yr−1, which have semi-transparent atmospheres. At low metallicity, substantial hydrogen-rich layers are left at the surface and we predict spectra that resemble O-type stars instead. We obtain spectra undistinguishable from subdwarfs for stripped stars with masses up to 1.7M⊙, which questions whether the widely adopted canonical value of 0.47M⊙is uniformly valid. Only a handful of stripped stars of intermediate mass have currently been identified observationally. Increasing this sample will provide necessary tests for the physics of interaction, internal mixing, and stellar winds. We use our model spectra to investigate the feasibility to detect stripped stars next to an optically bright companion and recommend systematic searches for their UV excess and possible emission lines, most notably HeIIλ4686 in the optical and HeIIλ1640 in the UV. Our models are publicly available for further investigations or inclusion in spectral synthesis simulations.

Cross-sectional investigation of radiation damage of 2 MeV proton-irradiated silicon carbide

Cross-sectional investigation is an important method to study ion irradiation effects in the depth direction. In this study, 2 MeV H+ was implanted in 6H-SiC single crystals to investigate the effects of light ion irradiation on SiC. Raman spectroscopy and scanning electronic microscopy (SEM) were carried out on cross-sectional samples to reveal the in-depth damage states and dopant behavior. The most damaged region is a little shallower than that predicted by the SRIM procedure, owing to the uncertainty in SRIM simulations. Layered structures representing zones of varying damage after 2 MeV H ion irradiation are clearly observed. Two bands are observed in SEM images, of which on band corresponds to the damage peak, while the other band at the end of the H ion-affected area is probably a result of H diffusion propelled by a hydrogen-rich layer during irradiation. A charge accumulation effect related with conductivity on the sample surfaces during SEM tests is observed in the H-implanted area. A model is proposed to explain these phenomena.

Recombination activity of grain boundaries in high-performance multicrystalline Si during solar cell processing

In this work, we applied internal quantum efficiency mapping to study the recombination activity of grain boundaries in High Performance Multicrystalline Silicon under different processing conditions. Wafers were divided into groups and underwent different thermal processing, consisting of phosphorus diffusion gettering and surface passivation with hydrogen rich layers. After these thermal treatments, wafers were processed into heterojunction with intrinsic thin layer solar cells. Light Beam Induced Current and Electron Backscatter Diffraction were applied to analyse the influence of thermal treatment during standard solar cell processing on different types of grain boundaries. The results show that after cell processing, most random-angle grain boundaries in the material are well passivated, but small-angle grain boundaries are not well passivated. Special cases of coincidence site lattice grain boundaries with high recombination activity are also found. Based on micro-X-ray fluorescence measurements, a change in the contamination level is suggested as the reason behind their increased activity.

Imaging the Hydrogen Absorption Dynamics of Individual Grains in Polycrystalline Palladium Thin Films in 3D.

Defects such as dislocations and grain boundaries often control the properties of polycrystalline materials. In nanocrystalline materials, investigating this structure-function relationship while preserving the sample remains challenging because of the short length scales and buried interfaces involved. Here we use Bragg coherent diffractive imaging to investigate the role of structural inhomogeneity on the hydriding phase transformation dynamics of individual Pd grains in polycrystalline films in three-dimensional detail. In contrast to previous reports on single- and polycrystalline nanoparticles, we observe no evidence of a hydrogen-rich surface layer and consequently no size dependence in the hydriding phase transformation pressure over a 125-325 nm size range. We do observe interesting grain boundary dynamics, including reversible rotations of grain lattices while the material remains in the hydrogen-poor phase. The mobility of the grain boundaries, combined with the lack of a hydrogen-rich surface layer, suggests that the grain boundaries are acting as fast diffusion sites for the hydrogen atoms. Such hydrogen-enhanced plasticity in the hydrogen-poor phase provides insight into the switch from the size-dependent behavior of single-crystal nanoparticles to the lower transformation pressures of polycrystalline materials and may play a role in hydrogen embrittlement.

Parametric study on density stratification erosion caused by a horizontal steam jet interacting with a vertical plate obstruction

During postulated severe accident scenarios in nuclear power plants, a hydrogen-rich layer might form at the top of the reactor containment. Various flow patterns resulting from the release of steam from the primary circuit might break the layer and redistribute hydrogen in the containment. The prediction of the gas transport during the accident requires detailed modeling of the processes involved. Advanced lumped parameter codes or computational fluid dynamics codes are used for this purpose. These codes need to be validated against experimental data obtained in large scale experimental facilities, where scale distortions are reduced. In order to obtain the required data with high spatial and temporal resolution, experiments were carried out in the PANDA facility in Switzerland as a part of OECD/HYMERES (HYdrogen Mitigation Experiments for Reactor Safety) project. The present experiments address the breakup of a layer rich in helium (used as simulant for hydrogen), under steam environment and its redistribution in two interconnected vessels (total volume of 183.3m3) under the action of a diffused flow resulting from the interaction of a horizontal steam jet with a vertical plate obstruction. The influence of the distance between the jet exit and the obstruction on the flow pattern was investigated. Spatial and temporal distribution of the gas concentration, the temperature and local gas velocity field were measured. It was found that a small change in the geometric configuration lead to a large change in the flow pattern. Reducing the jet-obstruction distance slowed down the helium-layer erosion process by a factor of two. Additionally, the creation of a concentration stratification in the adjacent vessel connected by an interconnecting pipe was observed.