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At least 631 records · Page 35
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DOE OSTI2020

Driving a magnetized domain wall in an antiferromagnet by magnons

We theoretically study the interaction of magnons, quanta of spin waves, and a domain wall in a one-dimensional easy-axis antiferromagnet in the presence of an external magnetic field applied along the easy axis. To this end, we begin by obtaining the exact solution for spin waves in the background of a domain wall magnetized by an external field. The finite magnetization inside the domain wall is shown to give rise to reflection of magnons scattering off the domain wall, deviating from the well-known result of reflection-free magnons in the absence of a magnetic field. For practical applications of the predicted reflection of magnons, we show that the magnon reflection contributes to the thermally driven domain-wall motion. Our work leads us to envision that inducing a finite magnetization in antiferromagnetic solitons such as vortices and skyrmions can be used to engender phenomena that do not occur in the absence of magnetization.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Intrinsic and extrinsic conduction contributions at nominally neutral domain walls in hexagonal manganites

Conductive and electrostatic atomic force microscopy (cAFM and EFM) are used to investigate the electric conduction at nominally neutral domain walls in hexagonal manganites. The EFM measurements reveal a propensity of mobile charge carriers to accumulate at the nominally neutral domain walls in ErMnO3, which is corroborated by cAFM scans showing locally enhanced direct current conductance. Our findings are explained based on the established segregation enthalpy profiles for oxygen vacancies and interstitials, providing a microscopic model for previous, seemingly disconnected observations ranging from insulating to conducting behavior. In addition, we observe variations in conductance between different nominally neutral walls that we attribute to deviations from the ideal charge-neutral structure within the bulk, leading to a superposition of extrinsic and intrinsic contributions. Our study clarifies the complex transport properties at nominally neutral domain walls in hexagonal manganites and establishes the possibility to tune their electronic response based on oxidation conditions, opening the door for domain-wall-based sensor technology.

Schultheiß, J. (ORCID:0000000173891295)↗
DOE OSTI2023

Theoretical investigation of braking of tearing mode rotation by resistive walls in ITER

The locking of the 2/1 tearing mode to the resistive wall in the ITER tokamak (15 MA inductive scenario 2) is investigated theoretically using a cylindrical asymptotic matching model. The model takes into account the fact that ITER plasmas will effectively be surrounded by two walls; the inner blanket module layer with a time constant of about 23 ms, and the outer vacuum vessel with a time constant of about 380 ms. The model also takes cognizance of the fact that neither the blanket module layer nor the vacuum vessel can be accurately described as “thin” walls (in the ordinarily accepted sense). The model incorporates changes in both the plasma poloidal and the toroidal angular velocity profiles, in response to the electromagnetic braking torque that develops at the rational surface, because it turns out that neoclassical poloidal flow-damping is not strong enough to completely suppress changes in the poloidal velocity. Finally, the model accurately calculates changes in the poloidal and toroidal plasma angular velocity profiles by evolving the full angular equations of motion, taking the electromagnetic braking torque, plasma inertia, plasma viscosity, and poloidal flow-damping into account. The time required for the 2/1 tearing mode to grow from a small amplitude to a sufficient one to lock to the walls is found to be about 3.5 s. As a result, the critical full radial island width at which wall locking is triggered is found to be about 9% of the plasma minor radius.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Modeling of the spray-induced wall stress acting on the ignition assistance device

This research introduces a novel wall-stress model called the Spray-Induced Wall Stress (SIWS) model, which considers the effects of spray-wall impingement and the resulting formation of wall stress within the Lagrangian spray modeling framework. The primary objective of this paper is to provide a mathematical description of the fundamental physics underlying the model. Subsequently, the proposed model is validated using existing experimental data. Here, the remainder of the study focuses on the practical application of the model to an ignition assistance device. Specifically, this device is installed in a compression ignition engine and designed to enhance ignition in aviation-fueled high-altitude aircraft propulsion systems. The research sheds light on the mechanical impulse caused by the high-speed impact of the spray jet, leading to the accumulation of mechanical stress on the rigid body of the ignition assistance device. Previous studies on fluid-structure interaction have only considered the interaction between the gas phase and the solid wall. However, the SIWS model incorporates the additional impact of the impinging liquid spray jet. Consequently, the simulated stress distribution on the ignition assistance device can be estimated by considering both the gas-phase-induced term and the spray-induced term simultaneously.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2019

Spatially-periodic microplasmas in complex microchannel networks: wall-plasma interactions and dynamic behavior

Wall-plasma interactions have been observed for spatially-periodic microplasmas generated in 300–700 µm wide channels fabricated in nanoporous alumina. Examination of Ne microplasma discs produced in a standing-wave pattern in Al 2 O 3 channels illustrates the competition between electron production at the sheath-wall interface and loss by recombination in an atmospheric pressure background. Two topologies of the microplasma arrays are observed. For channel widths (d) less than 450 µm, the microplasmas are generally centered in the channel and sustained by electron generation at both plasma sheath/channel wall interfaces, presumably including the release of charge residing in the hexagonal alumina pores. As d is progressively increased from 300 to 450 µm, the microchannel plasma cross-section is gradually transformed from circular to elliptical, and its surface area declines by as much as 50% so as to minimize e - losses to the background gas. Increasing d above ~450 µm abruptly switches the topology to one in which plasmas having a triangular cross-section attach to one of the channel walls in a pattern that alternates along the channel axis. Microfabricating trapezoidal cross-section channels into complex geometries, including the Cornu spiral and intersecting linear arrays, also reveals dynamic behavior in the propagation of microplasmas. For a common spiral structure, observations of plasma expansion show the wavefront propagates over the corrugated surface or within the channel with radial and azimuthal velocities of 3 ± 1 km s -1 and 8 ± 1.5 km s -1 , respectively. Plasma formation is initiated in each ring of the spiral through electron seeding by streamers propagating radially outward at velocities approaching 200 km s -1 . In addition to electrostatic charge-mediated variations in the mean separation between adjacent microplasmas, the time-dependent interference between two 1D microplasma arrays has been observed during plasma expansion. Reproducible ignition of a microplasma ensemble along ridges micromachined into channels near the intersections of two linear arrays has also been realized. The results reported here demonstrate that microchannels with the walls overcoated with any of a variety of materials provide a promising platform for examining in detail the interaction of low temperature plasma with a surface of arbitrary composition and topography.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Evolution of thick domain walls in de Sitter universe

We consider thick domain walls in a de Sitter universe following paper by Basu and Vilenkin. However, we are interested not only in stationary solutions found therein, but also investigate the general case of domain wall evolution with time. When the wall thickness parameter, δ{sub 0}, is smaller than H {sup −1}/√2, where H is the Hubble parameter in de Sitter space-time, then the stationary solutions exist, and initial field configurations tend with time to the stationary ones. However, there are no stationary solutions for δ{sub 0} ≥ H {sup −1}/√2. We have calculated numerically the rate of the wall expansion in this case and have found that the width of the wall grows exponentially fast for δ{sub 0} >> H {sup −1}. An explanation for the critical value δ{sub 0} {sub c} = H {sup −1}/√2 is also proposed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Overexpression of REDUCED WALL ACETYLATION C increases xylan acetylation and biomass recalcitrance in Populus

Abstract Plant lignocellulosic biomass, i.e. secondary cell walls of plants, is a vital alternative source for bioenergy. However, the acetylation of xylan in secondary cell walls impedes the conversion of biomass to biofuels. Previous studies have shown that REDUCED WALL ACETYLATION (RWA) proteins are directly involved in the acetylation of xylan but the regulatory mechanism of RWAs is not fully understood. In this study, we demonstrate that overexpression of a Populus trichocarpa PtRWA-C gene increases the level of xylan acetylation and increases the lignin content and S/G ratio, ultimately yielding poplar woody biomass with reduced saccharification efficiency. Furthermore, through gene coexpression network and expression quantitative trait loci (eQTL) analysis, we found that PtRWA-C was regulated not only by the secondary cell wall hierarchical regulatory network but also by an AP2 family transcription factor HARDY (HRD). Specifically, HRD activates PtRWA-C expression by directly binding to the PtRWA-C promoter, which is also the cis-eQTL for PtRWA-C. Taken together, our findings provide insights into the functional roles of PtRWA-C in xylan acetylation and consequently saccharification and shed light on synthetic biology approaches to manipulate this gene and alter cell wall properties. These findings have substantial implications for genetic engineering of woody species, which could be used as a sustainable source of biofuels, valuable biochemicals, and biomaterials.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Temporal slow-growth formulation for direct numerical simulation of compressible wall-bounded flows

A slow-growth formulation for DNS of wall-bounded turbulent flow is developed and demonstrated to enable extension of slow-growth modeling concepts to wall-bounded flows with complex physics. As in previous slow-growth approaches, the formulation assumes scale separation between the fast scales of turbulence and the slow evolution of statistics such as the mean flow. This separation enables the development of approaches where the fast scales of turbulence are directly simulated while the forcing provided by the slow evolution is modeled. The resulting model admits periodic boundary conditions in the streamwise direction, which avoids the need for extremely long domains and complex inflow conditions that typically accompany spatially developing simulations. Further, it enables the use of efficient Fourier numerics. Unlike previous approaches [Guarini, Moser, Shariff, and Wray, J. Fluid Mech. 414, 1 (2000); Maeder, Adams, and Kleiser, J. Fluid Mech. 429, 187 (2001); Spalart, J. Fluid Mech. 187, 61 (1988)], the present approach is based on a temporally evolving boundary layer and is specifically tailored to give results for calibration and validation of Reynolds-averaged Navier–Stokes (RANS) turbulence models. The use of a temporal homogenization simplifies the modeling, enabling straightforward extension to flows with complicating features, including cold and blowing walls. To generate data useful for calibration and validation of RANS models, special care is taken to ensure that the mean slow-growth forcing is closed in terms of the mean and other quantities that appear in standard RANS models, ensuring that there is no confounding between typical RANS closures and additional closures required for the slow-growth problem. The performance of the method is demonstrated on two problems: an essentially incompressible, zero-pressure-gradient boundary layer and a transonic boundary layer over a cooled, transpiring wall. The results show that the approach produces flows that are qualitatively similar to other slow-growth methods as well as spatially developing simulations and that the method can be a useful tool in investigating wall-bounded flows with complex physics.

42 ENGINEERING↗
DOE OSTI2019

Chirality switching of an antiferromagnetic spiral wall and its effect on magnetic anisotropy

An antiferromagnetic NiO spiral wall in Fe / NiO / C o 0.5 N i 0.5 O / vicinal Ag(001) was created by rotating Fe magnetization and investigated using x-ray magnetic linear dichroism (XMLD). Different from the Mauri's 180° spiral wall, we find that the NiO spiral wall always switches its chirality at ~ 90 ° rotation of the Fe magnetization, and unwinds the spiral wall back to a single domain with a further rotation of the Fe magnetization from 90° to 180°. The effect of this chirality switching on the magnetic anisotropy was studied using rotational magneto-optic Kerr effect (ROTMOKE) on Py / NiO / C o 0.5 N i 0.5 O / vicinal Ag(001). We find that the original Mauri's model has to be corrected by an energy folding due to the chirality switching, which consequently converts the exchange bias from the Mauri's 180° spiral wall into a uniaxial anisotropy and a negative fourfold anisotropy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

First Wall Design of a Tokamak Pilot Plant Using a Monte Carlo Model for 3-D Heat Flux Deposition

We present a method for calculating the heat fluxes deposited on nonaxisymmetric tokamak first wall components, allowing for a first-of-its-kind model for power handling in the tokamak far scrape-off layer (SOL). The DIV3D Monte Carlo model features strict global power conservation and can calculate the finite cross-field plasma transport into magnetically-shadowed regions, which is significant when dealing with meter-scale shadows introduced by components such as poloidal limiters or antennas. As a case study, we apply the DIV3D model to inform the distribution of first wall poloidal limiters in an ARC-class reactor device. We demonstrate that discrete protection limiters can efficiently reduce peak heat fluxes on recessed breeder wall components in the presence of significant far-SOL plasma fluxes. By varying the toroidal periodicity and radial standoff depth of the limiters, we demonstrate one of the tradeoffs that must be considered in first wall design: more limiters provide greater protection, but at the cost of reduced breeding performance. We also present the impact that radial misalignments between limiters would have on first wall power loading.

Monte Carlo methods↗
DOE OSTI2018

Secondary cell wall biosynthesis

Secondary walls are synthesized in specialized cells, such as tracheary elements and fibers, and their remarkable strength and rigidity provide strong mechanical support to the cells and the plant body. The main components of secondary walls are cellulose, xylan, glucomannan and lignin. Biochemical, molecular and genetic studies have led to the discovery of most of the genes involved in the biosynthesis of secondary wall components. Cellulose is synthesized by cellulose synthase complexes in the plasma membrane and the recent success of in vitro synthesis of cellulose microfibrils by a single recombinant cellulose synthase isoform reconstituted into proteoliposomes opens new doors to further investigate the structure and functions of cellulose synthase complexes. Most genes involved in the glycosyl backbone synthesis, glycosyl substitutions and acetylation of xylan and glucomannan have been genetically characterized and the biochemical properties of some of their encoded enzymes have been investigated. The genes and their encoded enzymes participating in monolignol biosynthesis and modification have been extensively studied both genetically and biochemically. A full understanding of how secondary wall components are synthesized will ultimately allow us to produce plants with custom-designed secondary wall composition tailored to diverse applications.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

PHOTO‐SENSITIVE LEAF ROLLING 1 encodes a polygalacturonase that modifies cell wall structure and drought tolerance in rice

Summary The biosynthesis and modification of cell wall composition and structure are controlled by hundreds of enzymes and have a direct consequence on plant growth and development. However, the majority of these enzymes has not been functionally characterised. Rice mutants with leaf‐rolling phenotypes were screened in a field. Phenotypic analysis under controlled conditions was performed for the selected mutant and the relevant gene was identified by map‐based cloning. Cell wall composition was analysed by glycome profiling assay. We identified a photo‐sensitive leaf rolling 1 ( psl1 ) mutant with ‘napping’ (midday depression of photosynthesis) phenotype and reduced growth. The PSL1 gene encodes a cell wall‐localised polygalacturonase (PG), a pectin‐degrading enzyme. psl1 with a 260‐bp deletion in its gene displayed leaf rolling in response to high light intensity and/or low humidity. Biochemical assays revealed PG activity of recombinant PSL1 protein. Significant modifications to cell wall composition in the psl1 mutant compared with the wild‐type plants were identified. Such modifications enhanced drought tolerance of the mutant plants by reducing water loss under osmotic stress and drought conditions. Taken together, PSL1 functions as a PG that modifies cell wall biosynthesis, plant development and drought tolerance in rice.

Zhang, Guangheng↗
DOE OSTI2018

Downregulation of p- COUMAROYL ESTER 3-HYDROXYLASE in rice leads to altered cell wall structures and improves biomass saccharification

p-Coumaroyl ester 3-hydroxylase (C3'H) is a key enzyme involved in the biosynthesis of lignin, a phenylpropanoid polymer that is the major constituent in vascular plants’ secondary cell walls. Although the crucial role of C3'H in lignification and its manipulation to upgrade lignocellulose have been investigated in eudicots, limited information is available in monocotyledonous grass species despite their potential as biomass feedstocks. Here in this paper we address the pronounced impacts of C3'H-deficiency on the structure and properties of grass cell walls. C3'H-knockdown lines generated via RNAi-mediated gene silencing, with ~0.5% of residual expression levels, reached maturity and set seeds, whereas, in contrast, C3'H-knockout rice mutants generated via CRISPR/Cas9-mediated mutagenesis were severely dwarfed and sterile. Cell wall analysis of the mature C3'H-knockdown RNAi lines revealed that their lignins were largely enriched in p-hydroxyphenyl (H) units while substantially reduced in the normally dominant guaiacyl (G) and syringyl (S) units. Interestingly, however, the enrichment of H units was limited to being within the non-acylated lignin units, with grass-specific γ-p-coumaroylated lignin units remaining apparently unchanged. Suppression of C3'H also resulted in relative augmentation in tricin residues in lignin as well as substantial reduction in wall cross-linking ferulates. Collectively, our data demonstrate that C3'H expression is an important determinant, not only of lignin content and composition, but also of the degree of cell wall cross-linking. We also demonstrated that C3'H-suppressed rice displays enhanced biomass saccharification.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2019

Kinetic Theory of the Wall Sheath for Arbitrary Conditions in a Gas-Discharge Plasma

The self-consistent problem of the structure of a perturbed wall sheath in a dc gas-discharge plasma near a flat surface under negative potential relative to the plasma has been solved for an arbitrary relation between the Debye radius and the ion mean free path. The solution has been obtained without artificial separation of this layer into the quasi-neutral “presheath” and the wall sheath in which quasi-neutrality is violated substantially. The actual ion distribution function in the unperturbed plasma, the dependence of the charge-exchange cross section on the ion energy, and the nonzero electric field in the unperturbed plasma have been considered. It is shown that when the average electron energy is conserved the structure of the perturbed wall sheath weakly depends on the form of the electron distribution function. It has been established that the mean energy of ions in the unperturbed plasma substantially affects the structure of the quasi-neutral presheath as well as the structure of a part of the wall sheath in which quasi-neutrality is not observed even under the assumption that the mean electron energy is much higher than the mean energy of ions. The calculations of ion flow parameters and the structure of the perturbed wall sheath are in conformity with experimental data obtained by other authors, which could not be adequately interpreted earlier.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Toward predictive and computationally affordable Lagrangian–Eulerian modeling of spray–wall interaction

The work presented in this article focuses on improving the understanding of spray–wall interaction phenomena and the predictivity of computational models for modern internal combustion engine–like conditions. Previous work from the authors highlighted some of the limitations of the currently available spray–wall interaction sub-models, especially those based on experimental observations carried out on single droplet impingement of non-hydrocarbon fluids. Previous experimental observations and direct numerical simulations provided unique qualitative and quantitative details that were leveraged to improve Lagrangian–Eulerian simulations of spray–wall interaction. In this work, Yarin and Weiss’ theory for droplet train impingements, as interpreted by Stanton and Rutland, was implemented in the CONVERGE software. A novel and mathematically rigorous calculation of the impingement frequency for Lagrangian parcels was proposed to replace the zero-droplet-spacing assumption present in the original model and improve the prediction of droplet splash. Preliminary testing and validation of the new formulation against experiments carried out in a constant volume vessel showed encouraging results. The feedback loop between experiments, Lagrangian–Eulerian simulations, and direct numerical simulations also motivated the investigation of the surface roughness model available in the Lagrangian–Eulerian framework. This part of the study pointed out the importance of correctly predicting the interaction between liquid and surrounding gas in the near-wall region. Overall, the results shown indicate that correctly capturing the pre-impingement physics and liquid–gas interaction, together with improving splash predictions, was key for improving the accuracy of Lagrangian–Eulerian computational fluid dynamics simulations of spray–wall impingement.

Torelli, Roberto↗
DOE OSTI2022

Low-abundance populations distinguish microbiome performance in plant cell wall deconstruction

Abstract Background Plant cell walls are interwoven structures recalcitrant to degradation. Native and adapted microbiomes can be particularly effective at plant cell wall deconstruction. Although most understanding of biological cell wall deconstruction has been obtained from isolates, cultivated microbiomes that break down cell walls have emerged as new sources for biotechnologically relevant microbes and enzymes. These microbiomes provide a unique resource to identify key interacting functional microbial groups and to guide the design of specialized synthetic microbial communities. Results To establish a system assessing comparative microbiome performance, parallel microbiomes were cultivated on sorghum ( Sorghum bicolor L. Moench) from compost inocula. Biomass loss and biochemical assays indicated that these microbiomes diverged in their ability to deconstruct biomass. Network reconstructions from gene expression dynamics identified key groups and potential interactions within the adapted sorghum-degrading communities, including Actinotalea , Filomicrobium , and Gemmatimonadetes populations. Functional analysis demonstrated that the microbiomes proceeded through successive stages that are linked to enzymes that deconstruct plant cell wall polymers. The combination of network and functional analysis highlighted the importance of cellulose-degrading Actinobacteria in differentiating the performance of these microbiomes. Conclusions The two-tier cultivation of compost-derived microbiomes on sorghum led to the establishment of microbiomes for which community structure and performance could be assessed. The work reinforces the observation that subtle differences in community composition and the genomic content of strains may lead to significant differences in community performance.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2019

The effects of a finite beam pipe on resistive wall instabilities

The effect of resistive wall instabilities on beam break-up is investigated for beam pipes with finite and infinite thickness walls. Analytic solutions for the electric field along the beam axis are determined for both finite and infinite thickness walls using complex analysis. The longitudinal and transverse impedances are also determined; variations from the infinite wall case are observed when the wall thickness is of the order of the skin depth of the beam pipe material.

43 PARTICLE ACCELERATORS↗
DOE OSTI2021

The Impact of Wall Control Performance on Connected Lighting Systems

Wall controls form the only touchpoint for individual room users and that room’s lighting control system. As such, these interfaces strongly influence user understanding and utilization of the system, as well as overall acceptance. The Next Generation Lighting Systems (NGLS) program began an evaluation of the wall controls in 14 connected lighting systems installed in various rooms of the NGLS Living Lab at Parsons School of Design in New York City. These systems demonstrate the wide range of approaches to wall control setup and functionality taken by manufacturers. Characterizing the 14 controls across six attributes, only four, or 29%, were identical. Requirements for wall control design were not specified explicitly by NGLS. Rather, wall controls were required to provide system performance according to three criteria: vacancy control (manual on/auto off) of two zones; manual continuous dimming of the same two zones; and an AV presentation mode in one of the two zones. This report examines the classification of entries, the assessment method used to evaluate the systems from installation through day-to-day use, and the system performance observed to date for each.

42 ENGINEERING↗