Neutral Transport Research Articles

(Invited) Well-Defined Electrocatalysis

Electrochemical energy conversion hydrogen technologies, such as fuel cells and electrolyzers have evolved as a prevailing option in achieving environmentally neutral energy and transportation sectors. Global deployment calls for improved performance of employed materials that are mainly based on scarce elements. Research aimed towards the design and synthesis of materials with advanced electrochemical properties, while diminishing the need for rare constituents, will be presented. Emphasis will be placed on the fundamental understanding of well-defined electrified interfaces and resolving their functionality at atomic and molecular scale. The research strategy is built on insights at the atomic/molecular level that define events controlled by electrode potential, which triggers electron and ion transfers across the electrochemical interfaces. The role of structure, spatial arrangement and nature of the surface atoms will be discussed for different reactive species, spectators, and impurities.

From CAD model to WAM component: implementation of complex geometries using the example of a hydrogen tank.

Abstract Hydrogen is expected to play a decisive role on the way to climate neutral transportation. This paper explores the innovative use of wire arc Directed Energy Deposition (waDED) in the manufacturing of a double walled, vacuum insulated liquid hydrogen storage tank. The study demonstrates the feasibility of the waDED process for the manufacturing of tank shells and furthermore the integration of heat exchanger channels into tank shells. Key findings include the development of a suitable process chain with integration of multiple features to ease manufacturing. However, challenges such as thermal shrinkage of the shells as well as geometrical deviations of the outer tank heat exchanger are encountered. Future research will focus on improvements in the process reliability with regards to geometric accuracy as well as in-depth testing under liquid hydrogen condition.

Global particle buildup simulations with gas puff scan: application to WEST discharge

This paper deals with the distribution of sources, transport, and exhaust of particles in a tokamak. Knowledge and understanding of all the physical phenomena involved in the global particle buildup are necessary to study and predict density regimes and subsequently to develop optimized scenarios for tokamak operation in order to control heat and particle exhaust. Neutral particles and their interactions with plasma are central in this perspective. This paper discusses the impact of varying the intensity of particle fueling in 2D transport simulations of a WEST discharge. Simulations are performed with an updated version of SOLEDGE-HDG that allows a more realistic transport of neutrals using a self-consistent diffusive model based on charge exchange and ionization processes. New code capabilities allow the entire WEST poloidal cross section to be simulated in a realistic configuration for both geometry and the range of control parameters. A gas puff scan illustrates the main features of the sheath-limited, high-recycling, and detached regimes, such as the buildup of the temperature gradient and the pressure drop in the scrape-off layer (SOL), the target temperature falling to 1 eV, and the ionization source moving away from the targets, as well as the particle flux rollover. A crude estimate of wall erosion is also provided, showing the respective role of each plasma wall component in each of these regimes.

Hybrid simulation of a capacitive Ar/SiH4 discharge driven by electrically asymmetric voltage waveforms

Voltage waveforms associated with the electrical asymmetry effect (EAE) have the potential to be used in the deposition of the silicon-based film, since they are expected to decouple ion energy and flux at the wafer surface, and further facilitate control of the process. In this study, a one-dimensional fluid/electron Monte Carlo hybrid model is employed to examine the EAE in a capacitively coupled argon-silane discharge, encompassing both amplitude asymmetry effect (AAE) and slope asymmetry effect (SAE). In the case of AAE, with the increasing pressure, the discharge electronegativity gradually intensifies, in conjunction with a transition of the electron heating mode from α to drift-ambipolar, a reduction of the absolute value of the DC self-bias voltage, and a decrease in Ar+ content, with an increase in SiH3 + content. For SAE, the trend in the discharge characteristics with the increasing pressure is similar to that for AAE, but the details are different. In SAE, the electronegativity and bulk electric field are much enhanced, resulting in higher content of high-energy electrons and Ar+ in the bulk. In addition, the absolute value of the self-bias is lower, but shows a fewer decline with the increasing pressure. The deposition rate is lower in SAE, due to the lower electron heating efficiency. However, larger voltage drop difference between two sheaths leads to a wider range of ion energy modulation at higher pressures. This study systematically investigates and compares Ar/SiH4 discharges driven by two electrically asymmetric voltage waveforms across various parameters including electron dynamics, ion and neutral transport properties, and deposition rates, with the aim of providing valuable insights and a reference for industrial applications.

Vacuum‐ultraviolet‐photoionization chamber for the investigation of ion‐based surface treatment and thin film deposition at atmospheric pressure

AbstractThe vacuum‐ultraviolet‐photoionization chamber was constructed to allow controlled ion generation and well‐defined surface treatment with these ions at atmospheric pressure. It utilizes atmospheric helium plasma as a photon source and separates the ion generation from the plasma by an aerodynamic window. The ions are guided to the grounded substrate by a weak electric field, while the diffusive transport of neutrals is reduced by a helium gas flow along the substrate. Ionic species and the absolute ion flux were determined in the substrate region. Ion‐based thin film deposition using a precursor was investigated as a model process. The proposed system enables future investigation of e.g. the isolated interaction of ions with biological substrates.

Reconstruction and interpretation of ionization asymmetry in magnetic confinement via synthetic diagnostics

Strong poloidal refueling asymmetry in the DIII-D tokamak is inferred from line radiation measurements. Synthetic diagnostics in neutral transport modeling coupled to gyrokinetic simulations illuminate implications for the plasma flow profile in the scrape-off layer of single-null beam-driven discharges. Recycling occurs primarily either on the inner or outer divertor legs, depending on the toroidal magnetic field direction. By reversing the toroidal magnetic field, the observed line radiation asymmetry is nearly eliminated or reversed. It is determined that, while relatively simple physics can describe the observed ionization asymmetry, predicting the overall brightness of the hydrogenic Lyman-α signal requires detailed simulation of the plasma and resulting turbulence. To this end, kinetic plasma simulations fully coupled to comprehensive neutral transport calculations—a novel capability—provide first-principles reproduction of Lyman-α observations on DIII-D.

Manifesto for a “Made in Europe” Transition Towards Carbon Neutral Transport

Manifesto for a “Made in Europe” Transition Towards Carbon Neutral Transport

On the Application of the Nyström Method to Two-Dimensional Neutral Particle Transport Problems in Heterogeneous Media

Over time, several methods were developed to deal with neutral particle transport problems. The interest in these problems is related to their wide range of applications, from neutron transport and heat transfer in nuclear reactors to radiative transfer in atmospheric clouds. Unlike the discrete ordinates or discrete ordinates–like methods, integral methods do not require discretization of angular variables. Instead, angular variables are completely eliminated by an integration procedure over the solid angle, which allows elimination of the ray effect. That said, this paper presents a new approach to estimate the scalar flux in two-dimensional fixed-source neutron transport problems in a heterogeneous medium, considering isotropic scattering and vacuum and reflective boundary conditions. Here, the Nyström method is combined with the singularity-subtraction technique to present an integral formulation for the scalar flux in a mesh grid over all regions of the domain. The iterative method of the Neumann series is used as an alternative to direct methods to solve the resulting system of equations generated from the domain discretization. Numerical results are given to verify the offered method.

Intrinsic rotation modulation by diffusive neutral particles in tokamaks

The modulated transport model, a model kinetic ion transport equation for the pedestal and scrape-off layer (SOL), is generalized to self-consistently include effects of a single neutral particle species. The neutrals contribute additional transport terms, modifying the v∥ -dependent orbit-averaged ion diffusivities of the original work and the resulting predicted intrinsic rotation of the ions. After making simplifying assumptions of the neutral transport, in particular taking the continuous transport limit via a short charge-exchange step expansion, we derive relatively simple analytic expressions that capture the diffusive neutral physics. Within the scope of the model’s validity, the neutral-driven intrinsic rotation can compete with the turbulence-driven intrinsic rotation. However, for physically motivated parameters, the neutral-driven intrinsic rotation appears negligible, either on a term-by-term basis or due to a strong cancellation between the neutral-driven momentum diffusion and pinch terms. It appears that a treatment containing finite charge-exchange steps is necessary to capture neutral transport of strong flow momentum into the confined region from the SOL.

MaCaw: Domain-Decomposed Monte Carlo Neutral Particle Transport on Unstructured Mesh in MOOSE

MaCaw is a Multiphysics Object-Oriented Simulation Environment (MOOSE)–based application that enables domain-decomposed neutral particle transport calculations in MOOSE. It leverages MOOSE’s ray-tracing module for unstructured mesh particle tracking and OpenMC for collision physics. Additionally, the OpenMC implementation of several calculation steps (e.g. initialization and normalization) needed in a Monte Carlo particle transport eigenvalue calculation were adapted for domain decomposition. This paper reports on MaCaw’s implementation and several limitations, a single verification case, and early single-node scaling studies. This paper also serves as an announcement of the public release of MaCaw on the Idaho National Laboratory GitHub at https://github.com/idaholab/macaw.

A variance deconvolution estimator for efficient uncertainty quantification in Monte Carlo radiation transport applications

Monte Carlo simulations are at the heart of many high-fidelity simulations and analyses for radiation transport systems. As is the case with any complex computational model, it is important to propagate sources of input uncertainty and characterize how they affect model output. Unfortunately, uncertainty quantification (UQ) is made difficult by the stochastic variability that Monte Carlo transport solvers introduce. The standard method to avoid corrupting the UQ statistics with the transport solver noise is to increase the number of particle histories, resulting in very high computational costs. In this contribution, we propose and analyze a sampling estimator based on the law of total variance to compute UQ variance even in the presence of residual noise from Monte Carlo transport calculations. We rigorously derive the statistical properties of the new variance estimator, compare its performance to that of the standard method, and demonstrate its use on neutral particle transport model problems involving both attenuation and scattering physics. We illustrate, both analytically and numerically, the estimator’s statistical performance as a function of available computational budget and the distribution of that budget between UQ samples and particle histories. We show analytically and corroborate numerically that the new estimator is unbiased, unlike the standard approach, and is more accurate and precise than the standard estimator for the same computational budget.

A direct Monte Carlo approach for the modeling of neutrals at the plasma edge and its self-consistent coupling with the 2D fluid plasma edge turbulence model HESEL

This paper presents a novel coupling of a kinetic description of neutrals with a fluid description of a fusion plasma. The code, plasma interacting super-atoms and molecules (PISAM), employs a grid-free Cartesian geometry and a direct simulation Monte Carlo approach to solve the kinetic equations of deuterium atoms and molecules. The grid-free geometry and the parallel nature of the neutral dynamics, in the absence of neutral–neutral interactions, allow for an unlimited and work-efficient parallelization of PISAM that always ensures a balanced workload. The highly optimized Python implementation obtains good performance while securing easy accessibility to new users. The coupling of PISAM with the edge turbulence model HESEL is outlined with emphasis on the technical aspects of coupling Message Passing Interface-parallelized Python and C++ codes. Furthermore, the paper presents and analyzes simulation results from running the coupled HESEL-PISAM model. These results demonstrate the impact of radial neutral transport and plasma–neutral dynamics perpendicular to the magnetic field. Specifically, they illustrate how the inward flow of neutral kinetic energy and the inhibition of radial electric shear, resulting from poloidal momentum transfer between atoms and ions, can affect the energy containment time. By comparing the results of the HESEL-PISAM model with those obtained from coupling HESEL with a diffusive-fluid-neutral model, the capabilities of diffusion models in predicting neutral transport in the plasma edge and scrape-off layer are elucidated.

Development of Monte Carlo neutral transport code and the application to methane seeding plasma detachment

AbstractThe collisions between neutral particles and plasma in the divertor region determine divertor detachment and heat flux to the target. However, the diagnostic of neutral particle information in the tokamak experiment is very lacking. To study the behavior of neutral particles in the boundary region flexibly and comprehensively, a two‐dimensional kinetic neutral transport code based on Monte Carlo method is developed in this work. The code is applied to simulate the neutral particles in the boundary of EAST device. Firstly, the simulation results are compared with EIRENE using an identical plasma background, and good agreement is obtained. Secondly, the transport of methane and the corresponding hydrocarbon compounds is simulated using the developed code. The methane injection case is compared with the injection of carbon and deuterium molecule mixture gas (ratio 1:2) to investigate their impact on detachment and fueling. The simulation reveals that, compared to the direct carbon atom injection case, charge‐exchange collision is the dominant collision of neutral methane (hydrocarbons), which suppresses divertor power radiation and plasma detachment while promoting the improvement of fueling efficiency.

Performance Portable Monte Carlo Particle Transport on Intel, NVIDIA, and AMD GPUs

OpenMC is an open source Monte Carlo neutral particle transport application that has recently been ported to GPU using the OpenMP target offloading model. We examine the performance of OpenMC at scale on the Frontier, Polaris, and Aurora supercomputers, demonstrating that performance portability has been achieved by OpenMC across all three major GPU vendors (AMD, NVIDIA, and Intel). OpenMC’s GPU performance is compared to both the traditional CPU-based version of OpenMC as well as several other state-of-the-art CPU-based Monte Carlo particle transport applications. We also provide historical context by analyzing OpenMC’s performance on several legacy GPU and CPU architectures. This work includes some of the first published results for a scientific simulation application at scale on a supercomputer featuring Intel’s Max series “Ponte Vecchio” GPUs. It is also one of the first demonstrations of a large scientific production application using the OpenMP target offloading model to achieve high performance on all three major GPU platforms.

Exponential Response Matrix Spectral Nodal Method for the Discrete Ordinates Neutral Particle Transport Model

In this work, the response matrix‐exponential nodal method is presented to solve fixed source neutral particle transport problems with isotropic scattering and discrete ordinates formulation in two‐dimensional Cartesian geometry. The method is based on the response matrix scheme by using the general solution of the transverse integrated SN nodal equations with exponential approximations for the leakage terms. This approach eliminates the need for additional auxiliary equations found in the spectral Green’s function (SGF) schemes, streamlining internodal computation to a matrix‐vector multiplication. The formulation includes a lambda parameter possible determined by the medium characteristics, e.g., the scattering ratio coefficient and/or empirical methods. Two model problems with low absorption rates were investigated to analyze the performance of the proposed method. The numerical solutions exhibited superior accuracy for the coarsest spatial grid configurations in both homogeneous and heterogeneous study cases. However, refining the spatial grid revealed a relative loss in accuracy compared to the RM‐CN method, potentially due to increased truncation error propagation. Nevertheless, all numerical solutions converge to the reference solution when refining the spatial discretization grid. Comparison of both response matrix schemes demonstrated similar convergence rates and computational efficiency in terms of iterations and CPU consumption.

Integrated modelling of vacuum flashover development: Above‐surface processes and breakdown threshold analyses

AbstractThe flashover in vacuum is a rapid interfacial discharge across the insulator surface when subjected to high applied voltage. Here a theoretical model covering above‐surface processes is introduced. The model calculates the flashover threshold in vacuum, with revised secondary electron emission avalanche theory and improved desorbed neutral transport model. The model serves as the first part of an integrated flashover model, aiming for consistent treatment of both above‐surface and subsurface processes during the entire flashover development. The flashover threshold is obtained by combining the desorbed neutral pressure at given applied voltage and the gas breakdown criterion dictated by the Paschen's law. An analytical formula for threshold estimation containing physical parameters, and a simplified formula consisting of empirical coefficients are introduced, catering for both conceptual understanding and practical application. The derived formulae are validated by experimental data for a variety of insulator materials. The theory generalisation for non‐uniform electric field distribution is further discussed.

A rigorous model reduction for the anisotropic-scattering transport process

The mesoscopic particle transport mechanics is bridged to the macroscopic fluid dynamics through the asymptotic theory. The construction of a concise model in the asymptotic regime is a major objective of kinetic theory. In this work, we propose a reduced-order model to bridge the particle transport mechanics and the macroscopic fluid dynamics in the highly scattered regime. A rigorous mathematical derivation and a concise physical interpretation are presented for an anisotropic-scattering transport process with arbitrary order of scattering kernel. The prediction of the theoretical model perfectly agrees with the numerical experiments. A clear picture of the diffusion physics is revealed for the neutral particle transport in the asymptotic optically thick regime.

Diffusion of HTO, 36Cl and 22Na in the Mesozoic rocks of Northern Switzerland: III. Cross-lab comparison of diffusion measurements on argillaceous twin samples

In the past decades, there has been a high interest in evaluations of argillaceous rock formations to host repositories for spent fuel or radioactive waste material. One of the main reasons for this interest is that argillaceous rocks are dominated by clay minerals, resulting in a fine-grained sediment matrix with micropores, low pore connectivity and thus low permeability, and good self-sealing properties. This entails that diffusion is the prime mechanism for transport of gas, fission products and radionuclides in clay-rich rocks. The determination of diffusion parameters is therefore key to evaluate the quality of the rock from long-term safety perspectives. Cross-lab comparison of diffusion data is however often challenging as various methods, concepts and models are utilised in the different laboratories across the globe. Here, a direct cross-lab comparison study of through-diffusion experiments was performed to compare and assess the effect of experimental method (through-diffusion) and modelling uncertainties of the parameters by comparing results obtained by two independent laboratories. The R&D group ‘Disposal’ at SCK CEN (Belgium) and the PSI-LES group (Switzerland) both performed through-diffusion experiments on (nearly) the same sample material using their in-house experimental and modelling methodologies for through-diffusion experiments. Adjacent (twin) samples at five depths between 870 and 940 m in the Trüllikon1-1 borehole (Switzerland) were selected and each lab subjected these five clay rock samples to diffusion in synthetic pore water with three different tracers, HTO, 36Cl− and 22Na+, representative for neutral, anionic and cationic transport behaviour. The two labs used a similar design of diffusion cell and worked with similar experimental conditions, but there were small differences in the experimental set-up/conditions and in the modelling approach. The independently determined diffusion parameters from SCK CEN and PSI-LES for all three tracers confirmed previously observed uncertainties. For all three radionuclides, the variability of the effective diffusion coefficients estimated independently by both institutes was less than a factor 2 and in general much lower (deviations ranging between 0 and 73%). Besides, the parameter estimations of the capacity factor (or accessible porosity in case of HTO and 36Cl−) agreed well. Moreover, the experimental datasets of HTO and 36Cl− were also cross-fitted. The evaluation revealed that the minor deviations can be attributed predominantly to variations in temperature (experimental conditions) and to a lesser extent to minor distinctions in the modelling approach. It is important to acknowledge that local heterogeneity might also contribute to these differences.

The Random Ray Method Versus Multigroup Monte Carlo: The Method of Characteristics in OpenMC and SCONE

The Random Ray Method (TRRM) is a recently developed approach to solving neutral particle transport problems based on the Method of Characteristics. While the method previously has been implemented only in closed-source or limited-functionality codes, this work describes its implementation in two open-source Monte Carlo codes: OpenMC and SCONE. The random ray implementations required small modifications to the existing Multigroup Monte Carlo (MGMC) solvers, offering a rare venue for redundant, fine-grained, “apples-to-apples” speed and accuracy comparisons between transport methods. To this end, TRRM and MGMC solvers are evaluated against each other using each code’s native capabilities on reactor eigenvalue problems with different degrees of energy discretization. On the C5G7 benchmark (featuring only seven energy groups), TRRM achieves a maximum pin power error comparable to or lower than that of MGMC for a given run time. On a problem with 69 energy groups, MGMC is found to scale more efficiently, obtaining a lower pin power error for a given run time. However, the defining difference between the two transport methods is found to be their vastly different uncertainty distributions. Specifically, TRRM is found to maintain similar levels of accuracy and uncertainty throughout the simulation domain whereas MGMC can exhibit orders-of-magnitude greater errors in areas of the problem that feature low neutron flux. For instance, TRRM provided an up to 373 times speed advantage compared with MGMC for computing the flux in low-flux regions in the moderator surrounding the C5G7 core.

Transitioning to battery electric vehicles in Japan: Impact of promotion policy, battery performance and carbon neutrality on greenhouse gas emissions reduction

This paper examines the life-cycle greenhouse gas (GHG) emissions reduction and associated marginal costs (MC) in Japan when the government enhances the transition to battery electric passenger vehicles (BEVs) by implementing a rapid investment regime for charging stations and fossil-fueled vehicle bans. Important factors affecting BEV emissions are battery performance, battery production technology, electricity generation mix and battery minimum state of charge (SOCmin). We find that in addition to a rapid investment regime, a fossil-fueled vehicles ban policy is necessary to significantly increase the uptake of BEVs and achieves the deepest reduction in life cycle GHG emissions. Under this policy, with carbon neutral electricity by 2050 and a low SOCmin charging regime, life-cycle GHG emissions are reduced by 48% and the MC by 53% in 2060. A carbon neutral light transport fleet in 2050 will require carbon capture and storage to offset 24 million tonnes-CO2 equivalent of emissions even if electricity is carbon neutral by 2050 and users adhere to a low SOCmin charging regime.