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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

On the question of quark confinement in the Abelian U(1) QED gauge interaction

If we approximate light quarks as massless and apply the Schwinger confinement mechanism to light quarks, we will reach the conclusion that a light quark q and its antiquark ${\bar q}$ will be confined as a $q\bar q$ boson in the Abelian U(1) QED gauge interaction in (1+1)D, as in an open string. From the work of Coleman, Jackiw, and Susskind, we can infer further that the Schwinger confinement mechanism persists even for massive quarks in (1+ 1)D. Could such a QED-confined $q\bar q$ one-dimensional open string in (1 + 1) D be the idealization of a flux tube in the physical world in (3+l)D, similar to the case of QCD-confined $q\bar q$ open string? If so, the QED-confined $q\bar q$ bosons may show up as neutral QED mesons in the mass region of many tens of MeV [Phys. Rev. C 81, 064903 (2010) & J. High Energy Phys. 2020(8), 165 (2020)]. Is it ever possible that a quark and an antiquark be produced and interact in QED alone to form a confined QED meson? Is there any experimental evidence for the existence of a QED meson (or QED mesons)? The observations of the anomalous soft photons, the XI7 particle, and the E38 particle suggest that they may bear the experimental evidence for the existence of such QED mesons. Further confirmation and investigations on the XI7 and E38 particles will shed definitive light on the question of quark confinement in QED in (3+1)D. Finally, implications of quark confinement in the QED interaction are discussed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Elementary framework for cold field emission: Incorporation of quantum-confinement effects

Although the Fowler-Nordheim (FN) equation serves as the foundation of cold field emission theory, it may not be suitable for predicting the emitted current density (ECD) from emitters with a quantum-confined electron supply. This work presents an analytical framework for treating cold field emission from metals that includes the effects of a quantum-confined electron supply. Within the framework, quantum confinement in emitters is classified into transverse and normal quantum confinement based on the orientation of the confinement relative to the emission direction. The framework is used to generate equations predicting the ECD from rectangular and cylindrical emitter geometries comprised of electron supplies of reduced dimensionality. Transverse quantum confinement of the electron supply leads to a reduction in the total ECD as transverse emitter dimensions decrease and normal quantum confinement results in an oscillatory ECD as a function of the normal quantum well width. Incorporating a geometry-dependent field enhancement factor into the model reveals an optimal transverse well width for which quantum confinement of the electron supply and field enhancement equally affect the ECD and a maximum total ECD for the emitter geometry at a given applied field is obtained. As a result, the FN equation over-predicts the ECD from emitters with transverse dimensions under approximately 5 nm, and in those cases, geometry-specific ECD equations incorporating quantum-confinement effects should be employed instead.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Confinement-Driven Heterogeneous Benzene Crystallization in Silica Nanopores

Nanoconfinement alters the thermodynamics, dynamics, and kinetics of fluids hosted in nanoscale solid nanopores to an extent that depends on the characteristics of the confining space and the chemistry of the confined fluids. Confinement-induced alterations in the phase behavior of confined energetic fluids under high pressure or low temperature are highly relevant to subsurface and subsea phenomena such as fluid flow in porous media, hydrate formation and dissociation, and gas storage capacity. Although extensive efforts have been directed toward understanding the phase behavior of confined fluids, the role of solid-liquid interfaces in the phase transitions of organic liquids has not been resolved yet. Here, we explore the onset and growth of benzene crystallization confined in 6 nm sized SBA-15 silica nanopores in the temperature range 300-200 K using in situ extended range small-angle and wide-angle X-ray scattering (SAXS/WAXS) measurements and atomistic classical molecular dynamics (MD) simulations. The crystallization onset of confined benzene depresses to 265 K compared to the freezing point of bulk benzene ( ~278 K), followed by the continuous growth of the emerged crystals in the pore space with complete crystallization at 200 K. The orientation of the emerged benzene crystals is dominated by parallel (π-π stacking) and perpendicular (T-shape stacking) orientations along the cylindrical pore radius and pore length, respectively. The onset of benzene crystals occurs heterogeneously on the pore surface and grows continuously toward the pore center. Further, confined benzene undergoes a dynamical crossover from fragile to strong dynamics behavior, inferred from the rotational and translational diffusion. The insights provided by this study have significant implications for the phase transitions of confined organic liquids that are relevant to a wide range of applications in the biological, geological, environmental, and chemical fields.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Dynamic Reorganization and Confinement of Ti IV Active Sites Controls Olefin Epoxidation Catalysis on Two-Dimensional Zeotypes

The effect of dynamic reorganization and confinement of isolated Ti IV catalytic centers supported on silicates is investigated for olefin epoxidation. Active sites consist of grafted single-site calix[4]arene–Ti IV centers or their calcined counterparts. Their location is synthetically controlled to be either unconfined at terminal T-atom positions (denoted as type-(i)) or within confining 12-MR pockets (denoted as type-(ii); diameter ~7 Å, volume ~185 Å 3 ) composed of hemispherical cavities on the external surface of zeotypes with *-SVY topology. Electronic structure calculations (density functional theory) indicate that active sites consist of cooperative assemblies of TiIV centers and silanols. When active sites are located at unconfined type-(i) environments, the rate constants for cyclohexene epoxidation (323 K, 0.05 mM TiIV, 160 mM cyclohexene, 24 mM tert-butyl hydroperoxide) are 9 ± 2 M –2 s –1 ; whereas within confining type-(ii) 12-MR pockets, there is a ~5-fold enhancement to 48 ± 8 M –2 s –1 . When a mixture of both environments is initially present in the catalyst resting state, the rate constants reflect confining environments exclusively (40 ± 11 M–2 s–1), indicating that dynamic reorganization processes lead to the preferential location of active sites within 12-MR pockets. While activation enthalpies are ΔH ‡ app = 43 ± 1 kJ mol–1 irrespective of active site location, confining environments exhibit diminished entropic barriers (ΔS ‡ app = -68 J mol –1 K –1 for unconfined type-(i) vs -56 J mol –1 K –1 for confining type-(ii)), indicating that confinement leads to more facile association of reactants at active sites to form transition state structures (volume ~ 225 Å 3 ). These results open new opportunities for controlling reactivity on surfaces through partial confinement on shallow external-surface pockets, which are accessible to molecules that are too bulky to benefit from traditional confinement within micropores.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Water confinement in small polycyclic aromatic hydrocarbons

The confinement of water molecules is vital in fields from biology to nanotechnology. The conditions allowing confinement in small finite polycyclic aromatic hydrocarbons (PAHs) are unclear, yet are crucial for understanding confinement in larger systems. Here, we report a computational study of water cluster confinement within PAHs dimers. Our results serve as a model for larger carbon allotropes and for understanding molecular interactions in confined systems. We identified size and structural motifs allowing confinement and demonstrated the motifs in various PAHs systems. Here we show that optimal OH∙∙∙π interactions between water clusters and the PAH dimer permit optimal confinement to occur. However, the lack of such interactions leads to the formation of CH∙∙∙O interactions, resulting in less ideal confinement. Confinement of layered clusters is also possible, provided that the optimal OH∙∙∙π interactions are conserved.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Roothaan’s approach to solve the Hartree-Fock equations for atoms confined by soft walls: Basis set with correct asymptotic behavior

In this report, we use a new basis set for Hartree-Fock calculations related to many-electron atoms confined by soft walls. One- and two-electron integrals were programmed in a code based in parallel programming techniques. The results obtained with this proposal for hydrogen and helium atoms were contrasted with other proposals to study just one and two electron confined atoms, where we have reproduced or improved the results previously reported. Usually, an atom enclosed by hard walls has been used as a model to study confinement effects on orbital energies, the main conclusion reached by this model is that orbital energies always go up when the confinement radius is reduced. However, such an observation is not necessarily valid for atoms confined by penetrable walls. The main reason behind this result is that for atoms with large polarizability, like beryllium or potassium, external orbitals are delocalized when the confinement is imposed and consequently, the internal orbitals behave as if they were in an ionized atom. Naturally, the shell structure of these atoms is modified drastically when they are confined. The delocalization was an argument proposed for atoms confined by hard walls, but it was never verified. In this work, the confinement imposed by soft walls allows to analyze the delocalization concept in many-electron atoms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Origin of melting point depression for rare gas solids confined in carbon pores

To obtain insights into the mechanism of the melting-point depression of rare gas solids confined in crystalline carbon pores, we examined the freezing and melting behavior of Xe and Ar confined to the crystalline pores of ordered mesoporous carbons as well as compressed exfoliated graphite compared to the amorphous pores of ordered mesoporous silicas, by means of X-ray diffraction. For the Xe and Ar confined to the crystalline carbon pores, there was no appreciable thermal hysteresis between freezing and melting. Furthermore, the position of the main diffraction peak did not change appreciably on freezing and melting. This strongly suggests that the liquids confined in the carbon pores form a multilayered structure parallel to the smooth walls. For the Xe and Ar confined to the amorphous silica pores, on the other hand, the position of the main diffraction peak shifted into higher scattering angle on freezing suggested that the density of the confined solid is distinctly larger than for the confined liquid. Using compressed exfoliated graphite with carbon walls of higher crystallinity, we observed that three-dimensional (3D) microcrystals of Xe confined in the slit-shaped pores melted to leave the unmelted bilayers on the pore walls below the bulk triple point. The lattice spacing of the 3D microcrystals confined is larger by ∼0.7% than that of the bilayer next to the pore walls in the vicinity of the melting point.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

An Industrial Steerable Needle for Inspecting Extremely Confined Spaces in Nuclear Facilities - 18316

We present a novel, compliant mechanism inspired by steerable needles for endoscopic surgery that provides the capability to navigate extremely confined spaces for the purpose of infrastructure inspection. Extremely confined spaces are commonly encountered during infrastructure inspection of nuclear facilities. These spaces can include pipes and ventilation ducts. Often these infrastructure features go uninspected simply because there is no viable way to access their interior. In addition, it is not uncommon for extremely confined spaces to be laid-out in a spider-web fashion that must be selectively navigated in order to execute an inspection. Recently, millimeter-scale imagers have become commercially available. These imagers are small enough to fit inside an extremely confined space, but there is currently no means to bring the imager to the portion of the structure that must be inspected. In this work this problem is addressed by taking inspiration from the field of steerable needles used in endoscopic surgery. Steerable needles are long flexible shafts that are capable of being directed through the human body using control forces applied external to the body. Unfortunately, steerable needles cannot be directly applied to the problem of navigating maze-like arrangements of extremely confined spaces. Traditional steerable needles must operate within a solid medium that can be penetrated in order to be steered so they are not directly applicable to navigating extremely confined spaces. However surgical steerable needles do suggest an approach to the problem. Steerable needles are typically made from super-elastic nitinol. This material has the property that it can experience high strain without yielding. Furthermore, superelastic nitinol is stiff enough that it is possible to push the tube down a long, maze-like arrangement of a confined space without buckling. The stiffness properties of nitinol combined with its high yield strain makes it an attractive material for navigating a tube through a maze-like arrangement of extremely confined spaces. The ability to selectively navigate extremely confined spaces opens up new possibilities to use emerging miniature imaging technology for infrastructure inspection. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2021

Interfacial and Confinement-Mediated Organization of Gas Hydrates, Water, Organic Fluids, and Nanoparticles for the Utilization of Subsurface Energy and Geological Resources

Harnessing the subsurface geologic environments in an efficient and environmentally sustainable manner is challenged by uncertainties associated with predicting the fate of fluids and sustaining porosity and permeability in subsurface geologic environments. Some of these uncertainties arise from confined and interfacially induced structures of fluids in subsurface geologic environments. The formation of gas hydrates, phase transitions of confined fluids, assembly and deposition of heavy hydrocarbons, and the agglomeration and fate of nanoparticles in confined environments are summarized in this review. Nanoscale confinement contributes to anisotropic structures and dynamics of fluids, which is the basis for anomalous phase transition thermodynamics, reactivity, transport, and geomechanical behavior. In this review, we discuss the structures of confined fluids and deviation in observed properties from bulk fluids. The factors influencing the structures of confined fluids can be generally divided into two groups: (a) pore characteristics including pore size, pore surface chemistry, and pore geometry and (b) confined fluid/solid characteristics such as molecular structure, concentrations, charges, pore filling, and presence of additives. Scientific advancements and knowledge gaps in our understanding of the structures of confined fluids and the associated differences in observed properties compared to bulk fluids are discussed. Here, the phenomena discussed in this review are of particular relevance to our efforts in harnessing the subsurface environments for a low carbon future by increasing the utilization of geothermal energy, using CO 2 as a working fluid, and storing CO 2 in subsurface geologic environments.

42 ENGINEERING↗
DOE OSTI2015

Lubrication analysis of interacting rigid cylindrical particles in confined shear flow

Lubrication analysis is used to determine analytical expressions for the elements of the resistance matrix describing the interaction of two rigid cylindrical particles in two-dimensional shear flow in a symmetrically confined channel geometry. The developed model is valid for non-Brownian particles in a low-Reynolds-number flow between two sliding plates with thin gaps between the two particles and also between the particles and the walls. Using this analytical model, a comprehensive overview of the dynamics of interacting cylindrical particles in shear flow is presented. With only hydrodynamic interactions, rigid particles undergo a reversible interaction with no cross-streamline migration, irrespective of the confinement value. However, the interaction time of the particle pair substantially increases with confinement, and at the same time, the minimum distance between the particle surfaces during the interaction substantially decreases with confinement. By combining our purely hydrodynamic model with a simple on/off non-hydrodynamic attractive particle interaction force, the effects of confinement on particle aggregation are qualitatively mapped out in an aggregation diagram. The latter shows that the range of initial relative particle positions for which aggregation occurs is increased substantially due to geometrical confinement. The interacting particle pair exhibits tangential and normal lubrication forces on the sliding plates, which will contribute to the rheology of confined suspensions in shear flow. Due to the combined effects of the confining walls and the particle interaction, the particle velocities and resulting forces both tangential and perpendicular to the walls exhibit a non-monotonic evolution as a function of the orientation angle of the particle pair. However, by incorporating appropriate scalings of the forces, velocities, and doublet orientation angle with the minimum free fraction of the gap height and the plate speed, master curves for the forces versus orientation angle can be constructed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Real-time confinement regime detection in fusion plasmas with convolutional neural networks and high-bandwidth edge fluctuation measurements

Abstract A real-time detection of the plasma confinement regime can enable new advanced plasma control capabilities for both the access to and sustainment of enhanced confinement regimes in fusion devices. For example, a real-time indication of the confinement regime can facilitate transition to the high-performing wide-pedestal (WP) quiescent H-mode, or avoid unwanted transitions to lower confinement regimes that may induce plasma termination. To demonstrate real-time confinement regime detection, we use the 2D beam emission spectroscopy (BES) diagnostic system to capture localized density fluctuations of long wavelength turbulent modes in the edge region at a 1 MHz sampling rate. BES data from 330 discharges in either L-mode, H-mode, quiescent H (QH)-mode, or WP QH-mode were collected from the DIII-D tokamak and curated to develop a high-quality database to train a deep-learning classification model for real-time confinement detection. We utilize the 6×8 spatial configuration with a time window of 1024 µ s and recast the input to obtain spectral-like features via fast Fourier transform preprocessing. We employ a shallow 3D convolutional neural network for the multivariate time-series classification task and utilize a softmax in the final dense layer to retrieve a probability distribution over the different confinement regimes. Our model classifies the global confinement state on 44 unseen test discharges with an average F 1 score of 0.94, using only ∼1 ms snippets of BES data at a time. This activity demonstrates the feasibility for real-time data analysis of fluctuation diagnostics in future devices such as ITER, where the need for reliable and advanced plasma control is urgent.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Entropy, confinement, and chiral symmetry breaking

This paper studies the way in which confinement leads to chiral symmetry breaking (CSB) through a gap equation. We argue that a combination of entropic effects, related to fluctuations of Wilson loops with massless constituents, and an Abelian gauge invariance of the confinement action as expressed in terms of the usual confining effective propagator 8{pi}K{sub F{delta}{mu}{nu}}/k{sup 4}, in effect removes infrared singularities coming from use of this propagator in a standard gap equation (K{sub F} is the string tension). Beginning from an Abelian gauge-invariant description of CSB that differs from this standard gap equation, we show how to extract a corresponding gap equation that incorporates both entropic effects and Abelian gauge invariance by replacement of the confining propagator with 8{pi}K{sub F{delta}{mu}{nu}}/(k{sup 2}+m{sup 2}){sup 2}. Here the finite mass m turns out to be {approx_equal}M(0)[M(p{sup 2}) is the running quark mass], based on an extension of an old calculation of the author. This massive propagator gives semiquantitatively two critical properties of confinement: (1) a negative contribution to the confining potential coming from entropy; (2) an infrared cutoff required by Abelian gauge invariance. Entropic effects lead to a qq condensate and contribute a negative term {approx}-K{sub F}/M(0), essential for a massless pion, to the pion Hamiltonian. The resulting gap equation leads to M{sup 2}(0){approx_equal}K{sub F}/{pi}. We argue that one-gluon exchange is not strong enough in the IR to drive quark CSB, but in any case is necessary to get the correct renormalization-group ultraviolet behavior. We find the standard renormalization-group result with the improvement that the prefactor (related to <qq>) can be calculated from the confining solution. Finally, we briefly point out the Minkowski-space virtues of using a principal-part propagator to describe confinement.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Enhanced axial confinement in a monolithically integrated self-rolled-up SiN x vertical microring photonic coupler

We report an efficient method to introduce enhanced axial confinement in the self-rolled-up SiN x vertical microtube coupler by depositing a thin layer of high refractive index material strip within the coupling section and effectively forming a vertical microring. Three times wider mode spacing is observed in such a vertical microring coupler monolithically integrated with a silicon nitride ridge waveguide as compared to the one without such axial confinement. More importantly, single mode operation within the telecomm C-band and S-band is achieved. The self-rolled-up microtube has been demonstrated to be a versatile device platform, which has found application in a number of areas including nanophotonics,1–7 microelectronics,8 chemical and biological sensing,9–11 and intelligent synthetic neural circuits.12 Because the microtube is a three dimensional (3D) structure readily fabricated using two dimensional processing technology, it may serve naturally as a component in 3D-integrated devices. We had previously demonstrated a vertical microtube coupler (VμTC) monolithically integrated with planar ridge waveguides in silicon photonic platform to achieve 3D photonic coupling and observed greatly enhanced coupling between the microtube and the underlying ridge waveguide.1,13 A similar result was reported independently by another research group shortly thereafter.2 One limitation of the VμTC used in previous experiments is that it supports many modes within the telecommunication C-band because of poor axial confinement. For example, we found multiple resonant coupling peaks, which is not desirable for wavelength division multiplexing (WDM) switches. In order to increase the mode spacing of this 3D photonic coupler and to achieve single mode operation within the C-band, we propose efficient axial confinement in the microtube to suppress the higher order modes. Axial confinement in self-rolled-up microtubes had been studied both theoretically and experimentally by a couple of research groups.14–17 In all these previous cases, the axial confinement was introduced by a specially designed geometrical shape of a section of the microtube. After being rolled up, the microtube wall will have different thicknesses at different axial locations. The different wall thicknesses result in a slightly varying effective refractive index of the supported whispering gallery modes (WGMs) along the axial direction of the microtube. When a parabolic shape is used, the effective index of the WGM at the center of the parabola will be the highest and decrease adiabatically along both sides. However, the refractive index change due to different wall thicknesses is very small; hence, the axial confinement is rather weak. Here in this paper, we present our experimental endeavor to incorporate a different axial confinement scheme in the monolithically integrated vertical microtube (VμT) to form a vertical microring (VμR) resonator and demonstrate 3 times wider mode spacing than the microtube based vertical photonic coupler and the potential to achieve a higher Q-factor.

36 MATERIALS SCIENCE↗
DOE OSTI2011

A COMPARATIVE STUDY OF CONFINED AND ERUPTIVE FLARES IN NOAA AR 10720

We investigate the distinct properties of two types of flares: eruptive flares associated with coronal mass ejections (CMEs) and confined flares without CMEs. Our study sample includes nine M- and X-class flares, all from the same active region (AR), six of which are confined and three others which are eruptive. The confined flares tend to be more impulsive in the soft X-ray time profiles and show slenderer shapes in the Extreme-ultraviolet Imaging Telescope 195 A images, while the eruptive ones are long-duration events and show much more extended brightening regions. The location of the confined flares is closer to the center of the AR, while the eruptive flares are at the outskirts. This difference is quantified by the displacement parameter, which is the distance between the AR center and the flare location; the average displacement of the six confined flares is 16 Mm, while that of the eruptive ones is as large as 39 Mm. Further, through nonlinear force-free field extrapolation, we find that the decay index of the transverse magnetic field in the low corona ({approx}10 Mm) is larger for eruptive flares than for confined ones. In addition, the strength of the transverse magnetic field over the eruptive flare sites is weaker than it is over the confined ones. These results demonstrate that the strength and the decay index of the background magnetic field may determine whether or not a flare is eruptive or confined. The implication of these results on CME models is discussed in the context of torus instability of the flux rope.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

High-temperature and high-pressure NMR investigations of low viscous fluids confined in mesoporous systems

Abstract In this contribution, the relaxation and diffusional behaviors of low viscous fluids, water and methanol confined into mesoporous silica and controlled size pore glass were investigated. The engineered porous systems are relevant to geologically important subsurface energy materials. The engineered porous proxies were characterized by Brunauer–Emmett–Teller (BET) surface analyzer, nuclear magnetic resonance (NMR) spectroscopy, and electron microscopy (EM) to determine surface area, pore-wall protonation and morphology of these materials, respectively. The confined behavior of the low viscous fluids was studied by varying pore diameter, fluid-to-solid ratio, temperature, and pressure, and then compared to bulk liquid state. Both relaxation and diffusion behaviors for the confined fluids showed increasing deviation from pure bulk fluids as the fluid-to-solid ratio was decreased, and surface-to-volume ratio ( S / V ) was varied. Variable pressure deuteron NMR relaxation of confined D 2 O and confined methanol, deuterated at the hydroxyl or methyl positions, were performed to exploit the sensitivity of the deuteron quadrupole moment to molecular rotation. The methanol results demonstrated greater pressure dependence than those for water only in bulk. The deviations from bulk liquid behavior arise from different reasons such as confinement and the interactions between confined fluid and the nano-pore wall. The results of the present report give insight into the behavior of low viscosity fluid in nano-confined geometries under different state conditions.

Chemistry↗
DOE OSTI2021

Cylindrical Confinement of Nanocolloidal Cholesteric Liquid Crystal

The organization of nanocolloidal liquid crystals in constrained geometries has fundamental and practical importance, since under confinement, liquid crystals contain stable topological defects that can serve as templates for nanoparticle organization. Three-dimensional confinement of cholesteric (Ch) liquid crystals formed by cellulose nanocrystals (CNCs) have been extensively studied; however, their two-dimensional confinement remains underinvestigated. In this work, we report the results of systematic experimental studies of two-dimensional confinement of Ch–CNC liquid crystal in cylindrical capillaries with varying inner diameters. Confinement resulted in phase separation of the Ch–CNC liquid crystal into a Ch shell formed by concentric CNC pseudolayers with the helicoidal axis perpendicular to the inner surface of the capillary walls, and a micrometer-diameter isotropic core thread running parallel to the long axis of the capillary. The morphology of the confined Ch–CNC liquid crystal varied when progressively increasing the degree confinement. Finally, we show that phase separation of the Ch–CNC liquid crystal into a Ch shell and an isotropic core is preserved in flexible capillary tubing, suggesting the applicability of this system for the fabrication of flexible optical waveguides.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

A high-density and high-confinement tokamak plasma regime for fusion energy

The tokamak approach, utilizing a toroidal magnetic field configuration to confine a hot plasma, is one of the most promising designs for developing reactors that can exploit nuclear fusion to generate electrical energy. To reach the goal of an economical reactor, most tokamak reactor designs simultaneously require reaching a plasma line-averaged density above an empirical limit—the so-called Greenwald density and attaining an energy confinement quality better than the standard high-confinement mode. However, such an operating regime has never been verified in experiments. In addition, a long-standing challenge in the high-confinement mode has been the compatibility between a high-performance core and avoiding large, transient edge perturbations that can cause very high heat loads on the plasma-facing-components in tokamaks. Here we report the demonstration of stable tokamak plasmas with a line-averaged density approximately 20% above the Greenwald density and an energy confinement quality of approximately 50% better than the standard high-confinement mode, which was realized by taking advantage of the enhanced suppression of turbulent transport granted by high density-gradients in the high-poloidal-beta scenario. Furthermore, our experimental results show an integration of very low edge transient perturbations with the high normalized density and confinement core. The operating regime we report supports some critical requirements in many fusion reactor designs all over the world and opens a potential avenue to an operating point for producing economically attractive fusion energy.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Structure of ice confined in silica nanopores

Observed anomalous thermodynamic properties of confined water such as deviations in the melting point and freezing point motivate the determination of the structure of confined water as a function of pore size and temperature. Here, we investigate the dynamic evolution of the structure of confined ice in SBA-15 porous materials with pore diameters of 4 nm, 6 nm, and 8 nm at temperatures ranging from 183 K to 300 K using in operando Wide-Angle X-Ray Scattering (WAXS) measurements, X-Ray Partial Distribution Function (PDF) measurements, and classical Molecular Dynamics (MD) simulations. Formation of hexagonal ice structures is noted in all the three pore sizes. In silica nanopores with diameters of 4 nm, cubic ice formation is noted in addition to hexagonal ice. Longer lasting hydrogen bonds and longer residence times of the water molecules in the first coordination shell contribute to observed crystalline organization of ice in confinement. Self-diffusion coefficients of confined liquid water, predicted from classical MD simulations, are four orders of magnitude higher compared to ice formed in confinement. These experimental and simulation results provide comprehensive insights underlying the organization of confined water and ice in silica nanopores and the underlying physico-chemical interactions that contribute to the observed structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗