Radial Gradient Research Articles

Scanning WAXS microscopy of regenerated cellulose fibers at mesoscopic resolution.

In this work, regenerated cellulose textile fibers, Ioncell-F, dry-wet spun with different draw ratios, have been investigated by scanning wide-angle X-ray scattering (WAXS) using a mesoscopic X-ray beam. The fibers were found to be homogeneous on the 500 nm length scale. Analysis of the azimuthal angular dependence of a crystalline Bragg spot intensity revealed a radial dependence of the degree of orientation of crystallites that was found to increase with the distance from the center of the fiber. We attribute this to radial velocity gradients during the extrusion of the spin dope and the early stage of drawing. On the other hand, the fiber crystallinity was found to be essentially homogeneous over the fiber cross section.

High-Z impurity neoclassical transport in the tokamak plasmas due to the single-null divertor configuration

A new mechanism for the high-Z impurity neoclassical particle transport in the tokamak plasmas due to the single-null divertor configuration is discovered for the first time. It will play an important role in the tokamak plasma pedestal region with the coexisting of the strong bulk ion radial gradients and the up/down asymmetry of the poloidal magnetic field. The outward (inward) high-Z impurity neoclassical particle transport will be driven with the B×∇B drift towards (away from) the X-point. The new finding indicates that the International Thermonuclear Experimental Reactor lower single-null divertor configuration will be beneficial for screening and flushing out the high-Z impurity in the pedestal region with the toroidal magnetic field in the normal direction ( B×∇B drift towards the X-point).

Dynamics near the inner dead-zone edges in a proprotoplanetary disk

Abstract We perform three-dimensional global non-ideal magnetohydrodynamic simulations of a protoplanetary disk containing the inner dead-zone edge. We take into account realistic diffusion coefficients of the Ohmic resistivity and ambipolar diffusion based on detailed chemical reactions with single-size dust grains. We found that the conventional dead zone identified by the Elsässer numbers of the Ohmic resistivity and ambipolar diffusion is divided into two regions: “the transition zone” and “the coherent zone.” The coherent zone has the same properties as the conventional dead zone, and extends outside of the transition zone in the radial direction. Between the active and coherent zones, we discover the transition zone, the inner edge of which is identical to that of the conventional dead zone. The transition zone extends out over the regions where thermal ionization determines diffusion coefficients. The transition zone has completely different physical properties than the conventional dead zone, the so-called undead zone, and the zombie zone. The combination of amplification of the radial magnetic field owing to the ambipolar diffusion and a steep radial gradient of the Ohmic diffusivity causes the efficient evacuation of the net vertical magnetic flux from the transition zone within several rotations. Surface gas accretion occurs in the coherent zone but not in the transition zone. The presence of the transition zone prohibits mass and magnetic flux transport from the coherent zone to the active zone. Mass accumulation occurs at both edges of the transition zone as a result of mass supply from the active and coherent zones.

Soft X-ray wavefront sensing at an ellipsoidal mirror shell

A reliable `in situ' method for wavefront sensing in the soft X-ray domain is reported, developed for the characterization of rotationally symmetric optical elements, like an ellipsoidal mirror shell. In a laboratory setup, the mirror sample is irradiated by an electron-excited (4.4 keV), micrometre-sized (∼2 µm) fluorescence source (carbon K α, 277 eV). Substantially, the three-dimensional intensity distribution I(r) is recorded by a CCD camera (2048 × 512 pixels of 13.5 µm) at two positions along the optical axis, symmetrically displaced by ±21–25% from the focus. The transport-of-intensity equation is interpreted in a geometrical sense from plane to plane and implemented as a ray tracing code, to retrieve the phase Φ(r) from the radial intensity gradient on a sub-pixel scale. For reasons of statistical reliability, five intra-/extra-focal CCD image pairs are evaluated and averaged to an annular two-dimensional map of the wavefront error {\cal W}. In units of the test wavelength (C K α), an r.m.s. value \sigma_{\cal{W}} = ±10.9λ0 and a peak-to-valley amplitude of ±31.3λ0 are obtained. By means of the wavefront, the focus is first reconstructed with a result for its diameter of 38.4 µm, close to the direct experimental observation of 39.4 µm (FWHM). Secondly, figure and slope errors of the ellipsoid are characterized with an average of ±1.14 µm and ±8.8 arcsec (r.m.s.), respectively, the latter in reasonable agreement with the measured focal intensity distribution. The findings enable, amongst others, the precise alignment of axisymmetric X-ray mirrors or the design of a wavefront corrector for high-resolution X-ray science.

Three-dimensional Interaction between a Planet and an Isothermal Gaseous Disk. III. Locally Isothermal Cases

We performed linear calculations to determine the Type I planetary migration rate for 3D locally isothermal disks with radial temperature gradients. For 3D disks with radial temperature gradients, the linear wave equation has a divergent term of the third pole, which makes corotation a nonremoval singularity. We suppressed the divergence with the Landau prescription to obtain the wave solutions. Despite the singularity at corotation, we derived a definite torque on the planet because the divergent term amplifies the waves only in the neighborhood of corotation and has little effect on the planetary torque. Consequently, we derived the formulas for the total, Lindblad, and corotation torques for locally isothermal disks. The resulting torque term due to the disk temperature gradient agrees well with the results of previous 3D hydrodynamical simulations for locally isothermal disks. Our linear calculation also provides the 3D horseshoe torque, which is close to the results of previous 3D hydrodynamical simulations.

Validation of the thermo-chemical approach to modelling of CO2 conversion in sub-atmospheric pressure microwave gas discharges

The CO 2→ CO + 12 O2 conversion experiment [D’Isa et al 2020 Plasma Sources Sci. Technol. 29 105009] has been compared with thermo-chemical calculations. The experiment is a 2.45 GHz plasma torch with a straight channel in the effluent. The 1.5D model of the CO2/CO/O2/O/C mixture without turbulent transport has been applied with plasma acting only as a prescribed heat source. The parameter range covered is the specific energy input (SEI) 0.3–5 eV/molecule at pressure p = 0.9 bar, and SEI = 0.6–2 eV/molecule at p = 0.5, 0.2 bar. The calculated conversion χ is always close to the experimental values. At the same time, the calculated temperatures T deviate significantly from the experiment, especially for p = 0.2 bar. The calculated T were also found to be sensitive with respect to the uncertain model parameters, but χ is not sensitive. The net conversion in the model is driven to a large extent by the radial diffusion of CO and O from the hot core towards the wall and steep radial temperature gradients. The main factor that reduces the energy efficiency is the re-oxidation of CO at the edge of the hot plasma region and downstream. The 1.5D approximation applied has the principle limitation that the impact of a realistic bulk flow field on the chemical process cannot be studied. Hence, the results must be considered preliminary and have to be confirmed with a more elaborate and accurate model of the vortex-stabilized flow inside the reactor.

Morphological and chemical changes in nuclear graphite target under vacuum and high-temperature conditions

Nuclear-grade graphite is a high-efficiency material, widely used for vacuum applications in nuclear reactors and accelerators as targets facing particle beams. In these contexts, graphite is often exposed to extreme thermal stresses altering its physical and chemical properties. The thermal-induced release of volatile contaminants from targets and the damage of structural components are critical issues that can affect the safety and operation efficiency of beamline facilities. Here, we provide for the first time a detailed picture of the chemical and morphological changes occurring in a nuclear-grade graphite target, obtained through Electrical Discharge Machining (EDM), when exposed in vacuum to high temperatures. The radial temperature gradient induced by the impact of a pulsed energetic (MeV- GeV range) focused particle beams was reproduced by cyclically heating, in the 1300–1800 K temperature range, a disc-shaped graphite target in a vacuum setup. An accurate surface and in-depth chemical analysis of the graphite target was obtained thanks to the high sensitivity (ppm/ppb) of the Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) technique. The chemical maps clearly show the presence of several metal oxides and impurities in the surface and subsurface regions of the untreated sample. Such contaminants were removed because of the thermal treatment in vacuum more or less efficiently, as demonstrated by Thermogravimetric analysis (TGA), X-ray Photoelectron Spectroscopy (XPS), and ToF-SIMS. However, Raman spectroscopy and SEM-EDS revealed that the high-temperature treatment induces a decrease in the crystallite size of the graphite as well as changes in the target surface porosity with the appearance of microvoids, leading the graphite target to be more prone to the breakage.

Experimental study on radial suction flow and its effect in water vortex unit

A water vortex unit is a non-contact adsorption unit that utilizes water as a flowing medium. Negative pressure is generated on the adsorbed surface through the high-velocity rotation of the fluid, resulting in suction force. In comparison to the air vortex unit, the water vortex unit exhibits a substantial increase in suction force and has the capability to deliver greater power. However, the presence of the central bubble in the water vortex unit weakens the suction force. To address the impact of the central bubble, a suction hole was introduced on the vortex unit to remove them. Experimental results indicate that solely removing the central bubble has almost no effect on the radial pressure gradients in the gap flow path and chamber center of the water vortex unit. However, in the cylindrical neighborhood, the radial pressure gradient increases, leading to a certain degree of improvement in suction force. Further increasing the suction flow rate induces inward radial flow within the vortex chamber. This flow field further enhances the suction force. To analyze the reasons for this pressure distribution, we indirectly obtained the radial velocity distribution by examining the relationship between the pressure gradient and tangential velocity. The results of the tangential velocity distribution indicate that the radially inward suction flow significantly increases the rotational velocity of the fluid in the chamber center, enhancing the centrifugal inertial effect. This further reduces the negative pressure, resulting in a significant improvement in the suction of the vortex unit.

The CO-to-H2 Conversion Factor in the Barred Spiral Galaxy M83

We analyze the CO-to-H2 conversion factor (α CO) in the nearby barred spiral galaxy M83. We present new H i observations from the VLA and single-dish GBT in the disk of the galaxy, and combine them with maps of CO(1-0) integrated intensity and dust surface density from the literature. α CO and the gas-to-dust ratio (δ GDR) are simultaneously derived in annuli of 2 kpc width from R = 1–7 kpc. We find that α CO and δ GDR both increase radially, by a factor of ∼2–3 from the center to the outskirts of the disk. The luminosity-weighted averages over the disk are α CO = 3.14 (2.06, 4.96) M⊙pc−2[Kkms−1]−1 and δ GDR = 137 (111, 182) at the 68% (1σ) confidence level. These are consistent with the α CO and δ GDR values measured in the Milky Way. In addition to possible variations of α CO due to the radial metallicity gradient, we test the possibility of variations in α CO due to changes in the underlying cloud populations, as a function of galactic radius. Using a truncated power-law molecular cloud CO luminosity function and an empirical power-law relation for cloud mass and luminosity, we show that the changes in the underlying cloud population may account for a factor of ∼1.5–2.0 radial change in α CO.

Microstructure-dependent deformation mechanisms and fracture modes of gradient porous NiTi alloys

Gradient porous structures based on triply periodic minimal surfaces offer exceptional specific strength and multi-functionality. However, strain heterogeneity complicates their deformation mechanisms and fracture modes. In this study, we fabricated a series of gradient porous NiTi alloys based on gyroid (G), diamond (D), and I-WP (I) unit cells, with porosities ranging from 50% to 70%. Surprisingly, the I structure exhibited an abnormally high compressive strength of nearly 600 MPa at around 50% porosity, more than twice that of the G and D structures. This performance gap was attributed to the activation of mixed deformation mechanisms and distinct fracture modes. While the G structure showed a uniform radial strain distribution, the D and I structures displayed pronounced radial strain gradients with concentrated central strains. The D structure deformed through relative sliding of oblique struts, while the I structure maintained macrostructural stability, with thick edge struts providing continuous strain hardening until monolithic fracture. This unique deformation behavior results from the synergistic effects of its intrinsic macrostructural stability, significant radial strain gradients, and sufficiently thicker edge struts. The integration of metallurgical and architectural design principles represents a novel approach to tailoring the mechanical properties of gradient porous metals for advanced applications.

Impact of Reverse Buoyancy on Heat and Mass Transfer in MHD Flow of PbLi Within a Poloidal Duct in a Typical DCLL Liquid Breeder Blanket

The typical dual-coolant lead-lithium (PbLi) design of a liquid breeder blanket in a magnetic confinement fusion reactor involves the utilization of PbLi as the working fluid to effectively remove neutron heat. However, the nonuniform heating of neutrons with a significant radial gradient induces a buoyancy effect, resulting in the formation of vortexes ices within the downward flow duct. These vortexes have an adverse impact on the heat and mass transfer characteristics of the magnetohydrodynamic (MHD) flow of PbLi. The simulations in this work employed a MHD buoyant mixed-convection solver to resolve the characteristics of PbLi flow and a one-way coupled Lagrangian method to analyze the qualitative characteristics of tritium transport in PbLi flow. The results indicate that buoyant reverse flow can create vortexes that contain hot spots in the PbLi fluid, which can significantly impede heat transport. Additionally, the vortex causes tritium recirculation in the flow field and retention, resulting in adverse effects on tritium transport.

In-plane and out-of-plane compressive properties of regular and graded cellular cores of sandwich panels fabricated by additive manufacturing

Cellular materials with a gradient of properties become appealing as cores of the sandwich panels due to the possibility of improving strength and absorbed energy in lightweight components. 2D cellular structures designated by honeycombs have an anisotropic behaviour when loaded under in- and out-plane. Thus, when proposing new designs, it is essential to analyse how the in-plane arrangement with a gradient in cell wall thickness affects in-plane and out-of-plane mechanical properties. This work aims to study graded cellular structures in comparison with regular hexagonal honeycombs. Structures were manufactured by laser powder bed fusion using an aluminium alloy. Regular arrangements were formed with cells with the same thickness, while graded structures possessed a radial gradient of cell thickness. Three types of innovative gradients, where cell length varies radially along concentric layers, were analysed. The compressive properties of regular and graded structures were evaluated when loaded both under in-plane and out-of-plane conditions. Compression behaviour was assessed, both experimentally and by numerical modelling. Even though there is a mismatch between numerical and experimental results, they exhibit the same trends. All graded samples showed an increased mechanical performance when loaded under out-of-plane conditions in comparison with the results from tests under in-plane loading with values, for example, of stiffness four hundred times larger, absorbed energy around thirty times higher and with yield stress four times larger. The results showed that the graded samples attain higher values of strength, stiffness and absorbed energy in comparison with regular hexagonal honeycombs, for the same relative density.

Compressive properties and energy absorption of selective laser melting formed Ti-6Al-4V porous radial gradient scaffold

Uniform porous structures are widely considered as the biological implant materials, but they are difficult to meet the needs of complex bone transplantation. The radial gradient porous structure can match the mechanical properties of different levels of natural femur by adjusting the internal and external porosity. Based on Gyroid unit cells, a parameterized model was employed to create a biomimetic bone structure. Uniform porous structure with an average porosity of 50% and radial gradient porous structure were prepared by selective laser melting (SLM).The mechanical properties, deformation behavior and energy absorption properties of uniform and radially graded porous structures were studied by compression test and finite element method (FE).The results show that the compression test curves of all porous structure exhibit elastic stage, platform stage and densification stage. In the platform stage, uniform porous structures are prone to early shear failure and poor energy absorption performance, whereas radially gradient porous structures exhibit higher elastic modulus and yield strength. When the strain is 0.3, uniform porous structures and porous structures with an inner porosity of 80% mainly exhibit 45° flexural-shear failures, while radially gradient porous structures with inner porosity of 62.5% and 70% experience V-shaped shear failures. Porous structures with an inner porosity of 70% have good energy absorption (107.99 MJ/m3), maximum energy absorption efficiency (46.94%), elastic modulus (5.2 GPa), and yield strength (296.02 MPa), matching the demands of cortical bone.

Thermal Field Simulation and Optimization of PbF2 Single Crystal Growth by the Bridgman Method

PbF2 single crystals are usually grown in the temperature gradient region by the Bridgman–Stockbarger method. Temperature distribution during the growth process is particularly important for the preparation of high-quality crystals. In this study, the temperature field during the growth of the PbF2 single crystals was simulated based on the finite element method. The temperature distribution and temperature gradient changes in the crucible were investigated at different growth stages, including the seeding, shouldering, and iso-diameters stages. The calculated results show that as the crucible position continues downward during the growth process, the axial temperature gradient increases and then decreases from the bottom to the top of the crucible, with almost flat isotherms near the solid–liquid interface where the axial temperature gradient is larger. At the shoulder below the crucible, the solid–liquid interface was improved by adjusting the tilt angle. Furthermore, based on a novel design of the heat-insulating baffle, the concave solid–liquid interface in the iso-diameter stage can be effectively adjusted to realize a lower radial temperature gradient. This study provides theoretical guidance for the optimization of the growth of high-quality PbF2 crystals by the Bridgman method.

Laser-driven pointed acceleration of electrons with preformed plasma lens

The simultaneous laser-driven acceleration and angular manipulation of the fast electron beam are experimentally demonstrated. The bunch of multi-MeV energy charged particles is generated during the propagation of the femtosecond laser pulse through the near-critical plasma slab accompanied by plasma channeling. Plasma is formed by the controlled breakdown of a thin-tape target by a powerful nanosecond prepulse. The electron beam pointing approach is based on the refraction of a laser pulse in the presence of a strong radial density gradient in the breakdown of the tape with a small displacement of the femtosecond laser beam relative to the breakdown symmetry axis. A shift of several micrometers makes it possible to achieve beam deflection by an angle up to 10° with acceptable beam charge and spectrum conservation. This opens up opportunities for applications for scanning objects with an electron beam and the multistage electron beam energy gain in consecutive laser accelerators without bulk magnetic optics for particles. Experimental findings are supported by numerical particle-in-cell calculations of laser-plasma acceleration and hydrodynamic simulations. Published by the American Physical Society 2024

Laser chemical vapor deposition of TiC fibers and tubes

Laser Chemical Vapor Deposition (LCVD) is a process in which fibers are deposited from a gaseous precursor to a solid phase under a traversing laser focal point. Here, we grow titanium carbide (TiC) fibers and tubes dependent upon a hyperbaric hydrogen-rich, hydrogen-balanced, and hydrogen-lean environment referenced to a titanium tetrachloride and ethylene gas mixture under three different laser intensities. X-ray diffraction confirms the deposition of TiC with energy dispersive spectroscopy noting a radial compositional gradient of titanium and carbon in the diameter of the deposit. Depending on the hydrogen concentration, the TiC encased a core carbon-rich fiber or formed a hollow tube, with these formations explained by thermophoresis. Furthermore, depending on the gas mixture and laser intensity, the carbon-rich interior and TiC outer shell exhibited different morphologies that are discussed in terms of growth rate regimes. The collective outcomes demonstrate the complex processing space in forming ceramic materials by LCVD.

The original composition of the gas forming first-generation stars in clusters: Insights from HST and JWST

Globular cluster (GC) stars composed of pristine material, also known as first-generation (1G) stars, are not chemically homogeneous as they exhibit extended sequences in the chromosome map (ChM). Recent studies characterized 1G stars within the center of 55 Galactic GCs, revealing metallicity variations. Despite this progress, several unanswered questions persist, particularly concerning the link between the 1G metallicity spread and factors such as the radial distance from the cluster center or the host GC parameters. Additionally, it remains unclear whether the extended 1G sequence phenomenon is exclusive to old Galactic GCs with multiple populations. This work addresses these open issues, examining 1G stars in different environments. First, we combine Hubble Space Telescope (HST) and James Webb Space Telescope photometry of the GC 47\,Tucanae to study 1G stars at increasing distances from the cluster center. We find that metal-rich 1G stars are more centrally concentrated than metal-poor ones, suggesting a metallicity radial gradient. Additionally, the two groups of 1G stars share similar kinematics. Since our analysis focuses on giant stars in the cluster center and M dwarfs in external fields, we discuss the possibility that the metallicity distribution depends on stellar mass. Subsequently, we analyze HST multi-band photometry of two simple-population clusters, NGC\,6791 and NGC\,1783, revealing elongated sequences in the ChM associated with metallicity variations. Finally, we investigate the 1G color distribution in 51 GCs, finding no connections with the host cluster parameters. These results shed light on the complex nature of 1G stars, providing insights into the GC formation environment.

Radial IR-GRIN lens prepared by multi-temperature fields manipulated gradient crystallization within chalcogenide glass

Chalcogenide glass has achieved great success in manufacturing axial-type infrared gradient refractive index (IR-GRIN) lenses. However, studies on radial-type IR-GRIN lenses, which are more ideal for optical design, remain rare. The present study introduces what we believe to be a new method for preparing radial IR-GRIN lens by creating high refractive index (n) In2S3 nanocrystals within a 65GeS2-25In2S3-10CsCl (GIC, in molar percentage) glass matrix. Upon introduction of multi-temperature field manipulation, we have successfully achieved central crystallization and simultaneous gradient attenuation spreading toward the edge within GIC glass, providing a radial GRIN profile with Δn over 0.1 while maintaining excellent IR transparency. In addition, the optical and structural properties of the GIC GRIN samples were characterized. The relationship between Raman intensity and the n of glass ceramics at different heat treatment temperatures was investigated, thereby enabling the indirect confirmation of the presence of radial gradient crystallization within the prepared GIC GRIN samples through Raman intensity. Multiple experimental results have shown that this approach has excellent reproducibility and potential for large-scale productions.

Crashworthiness Study of Functional Gradient Lattice-Reinforced Thin-Walled Tubes under Impact Loading.

Creating lightweight and impact-resistant box structures has been an enduring pursuit among researchers. A new energy-absorbing structure consisting of a bionic gradient lattice-enhanced thin-walled tube is presented in this article. The gradient lattice and thin-walled tube were prepared using selective laser melting (SLM) and wire-cutting techniques, respectively. To analyze the effects of gradient pattern, mass ratio, diameter range and impact speed on structural crashworthiness, low-speed impact at 4 m/s and finite element simulation experiments were conducted. The study demonstrates that the design of inward radial gradient lattice-reinforced thin-walled tubes can effectively enhance structure's energy-absorption efficiency and provide a more stable mode of deformation. It also shows a 17.44% specific energy-absorption advantage over the uniformly lattice-reinforced thin-walled tubes, with no significant overall gain in peak crushing force. A complex scale evaluation method was used to determine the optimum structure and the structure type with the best crashworthiness was found to be a gradient lattice-filled tube with a thickness of 0.9 mm and a slope index of 10. The gradient lattice-reinforced thin-walled tube suggested in this investigation offers guidance for designing a more efficient thin-walled energy-absorption structure.

Super-resolution radial fluctuations microscopy for optimal resolution and fidelity.

Fluorescence fluctuation super-resolution microscopy (FF-SRM) has emerged as a promising method for the fast, low-cost, and uncomplicated imaging of biological specimens beyond the diffraction limit. Among FF-SRM techniques, super-resolution radial fluctuation (SRRF) microscopy is a popular technique but is prone to artifacts, resulting in low fidelity, especially under conditions of high-density fluorophores. In this Letter, we developed a novel, to the best of our knowledge, combinatory computational super-resolution microscopy method, namely VeSRRF, that demonstrated superior performance in SRRF microscopy. VeSRRF combined intensity and gradient variance reweighted radial fluctuations (VRRF) and enhanced-SRRF (eSRRF) algorithms, leveraging the enhanced resolution achieved through intensity and gradient variance analysis in VRRF and the improved fidelity obtained from the radial gradient convergence transform in eSRRF. Our method was validated using microtubules in mammalian cells as a standard biological model system. Our results demonstrated that VeSRRF consistently achieved the highest resolution and exceptional fidelity compared to those obtained from other algorithms in both single-molecule localization microscopy (SMLM) and FF-SRM. Moreover, we developed the VeSRRF software package that is freely available on the open-source ImageJ/Fiji software platform to facilitate the use of VeSRRF in the broader community of biomedical researchers. VeSRRF is an exemplary method in which complementary microscopy techniques are integrated holistically, creating superior imaging performance and capabilities.