Strong Potential Gradient Research Articles

Influence of the position relationship between the cathode and magnetic separatrix on the discharge process of a Hall thruster

Previous studies have shown that there is an obvious coupling relationship between the installation location of the external cathode and the magnetic separatrix in the plume region of a Hall thruster. In this paper, the particle-in-cell simulation method is used to compare the thruster discharge process under the conditions of different position relationships between the cathode and the magnetic separatrix. By comparing the distribution of electron conduction, potential, plasma density and other microscopic parameters, we try to explain the formation mechanism of the discharge difference. The simulation results show that the cathode inside and outside the magnetic separatrix has a significant effect on the distribution of potential and plasma density. When the cathode is located on the outer side of the magnetic separatrix, the potential above the plume region is relatively low, and there is a strong potential gradient above the plume region. This potential gradient is more conducive to the radial diffusion of ions above the plume, which is the main reason for the strong divergence of the plume. The distribution of ion density is also consistent with the distribution of potential. When the cathode is located on the outer side of the magnetic separatrix, the radial diffusion of ions in the plume region is enhanced. Meanwhile, by comparing the results of electron conduction, it is found that the trajectories of electrons emitted from the cathode are significantly different between the inner and outer sides of the magnetic separatrix. This is mainly because the electrons are affected by the magnetic mirror effect of the magnetic tip, which makes it difficult for the electrons to move across the magnetic separatrix. This is the main reason for the difference in potential distribution. In this paper, the simulation results of macroscopic parameters under several conditions are also compared, and they are consistent with the experimental results. The cathode is located on the inner side of the magnetic separatrix, which can effectively reduce the plume divergence angle and improve the thrust. In this paper, the cathode moves from R = 50 mm to R = 35 mm along the radial direction, the thrust increases by 3.6 mN and the plume divergence angle decreases by 23.77%. Combined with the comparison of the ionization region and the peak ion density, it is found that the main reason for the change in thrust is the change in the radial diffusion of ions in the plume region.

Enhanced acoustic streaming effects via sharp-edged 3D microstructures.

Acoustofluidic micromanipulation is an important tool for biomedical research, where acoustic forces offer the ability to manipulate fluids, cells, and particles in a rapid, biocompatible, and contact-free manner. Of particular interest is the investigation of acoustically driven sharp edges, where high tip velocity magnitudes and strong acoustic potential gradients drive rapid motion. Whereas prior devices utilizing 2D sharp edges have demonstrated promise for micromanipulation activities, taking advantage of 3D structures has the potential to increase their performance and the range of manipulation activities. In this work, we investigate high-magnitude acoustic streaming fields in the vicinity of sharp-edged, sub-wavelength 3D microstructures. We numerically model and experimentally demonstrate this in fabricating parametrically configured 3D microstructures whose tip-angle and geometry influence acoustic streaming velocities and the complexity of streaming vortices, finding that the simulated and realized velocities and streaming patterns are both tunable and a function of microstructure shape. These sharp-edge interfaces hold promise for biomedical studies benefiting from precise and targeted micromanipulation.

Surface acoustic wave induced instability and pattern formation in a liquid polymer film

Recently, standing surface acoustic waves (SAWs) have been demonstrated to stimulate wrinkles on polymer films. Here, we theoretically investigate the underlying mechanism of the acoustic-induced micro-structures on a thin liquid film. The standing SAWs leak into the liquid polymer and generate radiation potential gradients. The strong potential gradients produce acoustic radiation pressure and induce an instability that features the wrinkling structure. Surface stability of the liquid film is firstly investigated by a linear instability analysis and the critical conditions of instability are analytically derived. Theoretical prediction of the film thickness that is favorable for surface instability well agrees with the optimal value found in previous experiments. Balance between the radiation and gravity-capillary pressure at the fluid surface forms the equilibrium equation of the surface profiles. Analytic solution reveals that the component of radiation pressure varying spatially makes the wavelength of the generated surface patterns equal to half of the SAW wavelength, while the constant component of radiation pressure absorbs the film onto the substrate. The interplay between the acoustic waves and the surface morphology is also investigated. Effects of the surface deformation on the radiation pressure is negligible except for that the mean thickness of the liquid approaches the resonance mode where singularity of the radiation pressure occurs.

Interspecific variation in the timing and magnitude of hydraulic redistribution in a forest with distinct water sources

AimsTrees regulate water availability among their rooting strata through a nocturnal, passive transference of water known as hydraulic redistribution (HR). This study investigates differences in HR and groundwater use among common canopy species in longleaf pine (Pinus palustris Mill., Pinaceae) woodlands and explores environmental factors influencing HR.MethodsHR was estimated by sap flux of lateral roots and main stems of three mature canopy species (P. palustris, Quercus laevis Walter., Fagaceae and Quercus margarettae Ashe., Fagaceae). We used δ18O and δD of xylem water, soil water, and groundwater to determine water source. Finally, we related HR to environmental factors (Temperature, VWC, VPD) to better understand controls of HR dynamics.ResultsPinus palustris had higher water use than either Quercus species, and also redistributed significantly more water as a nocturnal subsidy. HR fluxes were inversely related with mean nightly temperature and independent of shallow soil moisture. Stable isotope mixing models, based on δ18O and δD, indicated that all species have access to groundwater, but utilized shallow soil water in differing amounts when available.ConclusionsIn systems with strong water potential gradients among soil strata, any species with access to a groundwater source is likely capable of HR; however, the magnitude of HR varies significantly by species, even among closely related taxa.

Strong Potential Gradients and Electron Confinement in ZnO Nanoparticle Films: Implications for Charge-Carrier Transport and Photocatalysis

Zinc oxide (ZnO) nanomaterials are promising components for chemical and biological sensors and photocatalytic conversion and operate as electron collectors in photovoltaic technologies. Many of th...

Selective Cocatalyst Deposition on ZnTiO3-x Ny Hollow Nanospheres with Efficient Charge Separation for Solar-Driven Overall Water Splitting.

Pt and RhOx cocatalysts are selectively deposited at inner and outer surface of ZnTiO3-x Ny hollow nanospheres, respectively. The resulting photocatalytic systems exhibit promising activity for photocatalytic overall water splitting with stoichiometric H2 /O2 ratio under simulated solar insolation. Selective deposition of Pt and RhOx cocatalysts at different surfaces not only mitigates back reactions of the products but also induces strong potential gradient within nanospheres that promotes efficient dissociation and fast migration of photocarriers to the surface.

Interplay of Topological States on TI/TCI Interfaces.

Based on first-principles calculations, we study electronic structure of interfaces between a topological insulator (TI) SnBiTe and a topological crystalline insulator (TCI) SnTe. We consider two interface models characterized by the different atomic structure on the contact of the SnTe(111) and SnBiTe(0001) slabs: the model when two materials are connected without intermixing (abrupt type of interface) and the interface model predicted to be realized at epitaxial immersion growth on topological insulator substrates (smooth interface). We find that a strong potential gradient at the abrupt interface leads to the redistribution of the topological states deeper from the interface plane which prevents the annihilation of the Dirac states, predicted earlier. In contrast, a smooth interface is characterized by minor charge transfer, which promotes the strong interplay between TI and TCI Dirac cones leading to their complete annihilation.The topologically protected Dirac state of SnTe(111) survives irrespective of the interface structure.

Analysis of Turbulent Characteristics of Sheared Convective Boundary Layer under Different Shear and Potential Temperature Gradients

Effects of shear and potential temperature gradient on the formation mechanism and turbulence characteristics of the sheared convective boundary layer are studied through Large-Eddy Simulation (LES) experiments. Four cases with different velocity difference ΔU of 1.5ms−1 and 3.0ms−1 and different potential temperature gradient Γθ of 0.006Km−1 and 0.015Km−1 were conducted in the horizontal homogeneous atmospheric convective boundary layer of 6000m × 6000m × 2000m. The results show that the formation of the entrainment layer is affected integratedly by the horizontal shear effect and the vertical buoyancy effect, in which turbulence shows strong anisotropy and intermittent characteristics, wherein the mixed layer is dominated by stable small eddies, and the entrainment layer by strip-shaped large eddies. Compared with the pure buoyancy boundary layer, the entrainment flux ratio Af is 0.176∼0.385, and the height of the entrainment layer zi is shifted to the side with bigger velocity (U2). ΔU promotes entrainment and Γθ inhibits, especially for the upper limit of the entrainment layer h2. Obviously, the shear-shelter entrainment phenomenon appears under the weak shear and strong potential temperature gradient.

E×B Flux Driven Detachment Bifurcation in the DIII-D Tokamak.

A bifurcative step transition from low-density, high-temperature, attached divertor conditions to high-density, low-temperature, detached divertor conditions is experimentally observed in DIII-D tokamak plasmas as density is increased. The step transition is only observed in the high confinement mode and only when the B×∇B drift is directed towards the divertor. This work reports for the first time a theoretical explanation and numerical simulations that qualitatively reproduce this bifurcation and its dependence on the toroidal field direction. According to the model, the bifurcation is primarily driven by the interdependence of the E×B-drift fluxes, divertor electric potential structure, and divertor conditions. In the attached conditions, strong potential gradients in the low field side (LFS) divertor drive E×B-drift flux towards the high field side divertor, reinforcing low density, high temperature conditions in the LFS divertor leg. At the onset of detachment, reduction in the potential gradients in the LFS divertor leg reduce the E×B-drift flux as well, such that the divertor plasma evolves nonlinearly to high density, strongly detached conditions. Experimental estimates of the E×B-drift fluxes, based on divertor Thomson scattering measurements, and their dependence on the divertor conditions are qualitatively consistent with the numerical predictions. The implications for divertor power exhaust and detachment control in the next step fusion devices are discussed.

Numerical Simulation of Sea Breeze Convergence over Antarctic Peninsula

The convergence zone induced by sea breeze systems over Antarctic Peninsula is analyzed for the summer season of 2013–2015. 59 days, selected by satellite images for the absence of major synoptic forcing, are simulated using the WRF model. Sea breeze convergence has been detected in 21 of these days, mostly during evening hours and under large-scale winds. Breeze events are associated with a cold anomaly at the peninsula with respect to the climatology. This condition favors the onset of the necessary horizontal thermal gradients to trigger the breeze circulation. At the same time, no anomaly of the average pressure at sea level is found, indicating that events are favored when the average synoptic flow is present. Case studies indicate that the convergence location over the peninsula is controlled by the synoptic wind. An average convergence over the peninsula happens from 14:00 to 22:30 UTC, with a maximum at 18:00 UTC. There is a strong potential temperature gradient between the surface of the peninsula and the sea, with the sea breeze circulation system extending up to 1.2 km or higher. The sensible heat flux reaches 80 W/m2at the top of mountains and 10 W/m2near the coast.

Multi-instability plasma dynamics during the route to fully developed turbulence in a helicon plasma

We report experimental studies demonstrating a controlled transition to fully developed broadband turbulence in an argon helicon plasma in a linear plasma device. We show the detailed dynamics during the transition from nonlinearly coupled but distinct eigenmodes at low magnetic fields to fully developed broadband turbulence at larger magnetic fields. As the magnetic field (B) is increased from B ∼ 40 mT, initially we observe slow smooth changes in the dynamics of the system (to B ∼ 140 mT), followed by a sharp transition (within ∼10 mT) to centrally peaked narrow density profiles, strong edge potential gradients and a pronounced bright, well-defined plasma core. At low magnetic fields, the plasma is dominated by drift waves. As the magnetic field is increased, a strong potential gradient at the edge introduces an E × B shear-driven instability. At the transition, another mode with signatures of a rotation-induced Rayleigh–Taylor instability appears at the central plasma region. Concurrently we also find large axial velocities in the plasma core. For larger magnetic fields, all the instabilities co-exist, leading to rich plasma dynamics and fully developed broadband turbulence at B ∼ 240 mT.

Misfit strain driven cation inter-diffusion across an epitaxial multiferroic thin film interface

Cation intermixing at functional oxide interfaces remains a highly controversial area directly relevant to interface-driven nanoelectronic device properties. Here, we systematically explore the cation intermixing in epitaxial (001) oriented multiferroic bismuth ferrite (BFO) grown on a (001) lanthanum aluminate (LAO) substrate. Aberration corrected dedicated scanning transmission electron microscopy and electron energy loss spectroscopy reveal that the interface is not chemically sharp, but with an intermixing of ∼2 nm. The driving force for this process is identified as misfit-driven elastic strain. Landau-Ginzburg-Devonshire-based phenomenological theory was combined with the Sheldon and Shenoy formula in order to understand the influence of boundary conditions and depolarizing fields arising from misfit strain between the LAO substrate and BFO film. The theory predicts the presence of a strong potential gradient at the interface, which decays on moving into the bulk of the film. This potential gradient is significant enough to drive the cation migration across the interface, thereby mitigating the misfit strain. Our results offer new insights on how chemical roughening at oxide interfaces can be effective in stabilizing the structural integrity of the interface without the need for misfit dislocations. These findings offer a general formalism for understanding cation intermixing at highly strained oxide interfaces that are used in nanoelectronic devices.

Band alignment in LaAlO3/SrTiO3oxide heterostructures inferred from hard x-ray photoelectron spectroscopy

We present a detailed study of the electronic structure and band alignment in LaAlO${}_{3}$/SrTiO${}_{3}$ oxide heterostructures by hard x-ray photoelectron spectroscopy. Our spectroscopic measurements find no evidence for the strong potential gradient within the polar LaAlO${}_{3}$ film predicted by band theory. Due to the high interface sensitivity of the method, we are further able to determine the valence band offset between the LaAlO${}_{3}$ film and the SrTiO${}_{3}$ substrate, which is found to be independent of the number of LAO overlayers. Finally, we discuss several explanations for the apparent absence of the built-in field in ex situ prepared LaAlO${}_{3}$/SrTiO${}_{3}$ heterostructures.

Benzene Dynamics and Biodegradation in Alluvial Aquifers Affected by River Fluctuations

The spatial distribution and temporal dynamics of a benzene plume in an alluvial aquifer strongly affected by river fluctuations was studied. Benzene concentrations, aquifer geochemistry datasets, past river morphology, and benzene degradation rates estimated in situ using stable carbon isotope enrichment were analyzed in concert with aquifer heterogeneity and river fluctuations. Geochemistry data demonstrated that benzene biodegradation was on-going under sulfate reducing conditions. Long-term monitoring of hydraulic heads and characterization of the alluvial aquifer formed the basis of a detailed modeled image of aquifer heterogeneity. Hydraulic conductivity was found to strongly correlate with benzene degradation, indicating that low hydraulic conductivity areas are capable of sustaining benzene anaerobic biodegradation provided the electron acceptor (SO4 (2-) ) does not become rate limiting. Modeling results demonstrated that the groundwater flux direction is reversed on annual basis when the river level rises up to 2 m, thereby forcing the infiltration of oxygenated surface water into the aquifer. The mobilization state of metal trace elements such as Zn, Cd, and As present in the aquifer predominantly depended on the strong potential gradient within the plume. However, infiltration of oxygenated water was found to trigger a change from strongly reducing to oxic conditions near the river, causing mobilization of previously immobile metal species and vice versa. MNA appears to be an appropriate remediation strategy in this type of dynamic environment provided that aquifer characterization and targeted monitoring of redox conditions are adequate and electron acceptors remain available until concentrations of toxic compounds reduce to acceptable levels.

Temperature dependent optical properties of pentacene films on zinc oxide

The optical transitions of pentacene films deposited on ZnO have been studied by absorption spectroscopy as a function of temperature in the range of room temperature down to 10 K. The pentacene films were prepared with thicknesses of 10 nm, 20 nm, and 100 nm on the ZnO-O(000-1) surface by molecular beam deposition. A unique temperature dependence has been observed for the two Davydov components of the excitons for different film thicknesses. At room temperature, the energetic positions of the respective absorption bands are the same for all films, whereas the positions differ more than 20 meV at 10 K caused by the very different expansion coefficients of pentacene and ZnO. Although the pentacene is just bonded via van der Waals interaction to the ZnO substrate, the very first pentacene monolayer (adlayer) is forced to keep the initial position on the ZnO surface and suffering, therefore, a substantial tensile strain. For all the subsequent pentacene monolayers, the strain is reduced step by step resulting electronically in a strong potential gradient at the interface.

The evolution of the plasma potential in a HiPIMS discharge and its relationship to deposition rate

An electron-emitting probe has been used to measure the temporal evolution of the plasma potential Vp along a line from target (Ti) to substrate above the racetrack in a high-power impulse magnetron sputtering discharge pulsed at 100 Hz. The 20 ns time-resolution of the probe allowed us to observe the highly dynamic nature of Vp as the discharge voltage waveform develops, with large negative Vp values (−210 V) and strong potential gradients existing in the magnetic trap region in the first 6 to 8 µs. After 55 to 60 µs, Vp is elevated towards ground potential (0 V) and the bulk electric field collapses. Outside the magnetic trap, i.e. on the open field lines, Vp reveals much smaller axial and temporal variations, similar to those observed in conventional pulsed dc magnetrons.At standard conditions (Ar pressure of 0.54 Pa and 650 W average power), the results show that for over 50% of the 100 µs plasma ‘on-time’ the spatial structure of Vp provides a large potential barrier for the sputtered post-ionized species so impeding their transport and deposition at the substrate. This barrier is reduced markedly (by 50%) through a small reduction in the magnetic field strength (33% at the target) so increasing the deposition rate by a factor of 6 at a typical position of the substrate (z = 100 mm). The structure of Vp is marginally sensitive to changes in pressure (over the range 0.54 to 1.08 Pa), but more strongly dependent on the applied power (over the range 650 to 950 W).

Growth of bultfonteinite and hydrogarnet in metasomatized basalt xenoliths in the B/K9 kimberlite, Damtshaa, Botswana: insights into hydrothermal metamorphism in kimberlite pipes

Metamorphic assemblages within Karoo basalt xenoliths, found within volcaniclastic kimberlite of the B/K9 pipe, Damtshaa, Botswana, constrain conditions of kimberlite alteration. Bultfonteinite and chlorite partially replace the original augite-plagioclase assemblage, driven by the serpentinisation of the kimberlite creating strong chemical potential gradients for Si and Mg. Hydrogarnet and serpentine replace these earlier metamorphic assemblages as the deposits cool. The bultfonteinite (ideally Ca2SiO2[OH,F]4) and hydrogarnet assemblages require a water-rich fluid containing F−, and imply hydrothermal alteration dominated by external fluids rather than autometamorphism from deuteric fluids. Bultfonteinite and hydrogarnet are estimated to form at temperatures of ca. 350–250°C, which are similar to those for serpentinisation. Alteration within the B/K9 kimberlite predominantly occurs between 250 and 400°C. We attribute these conditions to increased efficiency of mass transfer and chemical reactions below the critical point of water and a consequence of volume-increasing serpentinisation and metasomatic reactions that take place over this temperature range. A comparison of the B/K9 kimberlite with kimberlites from Venetia, South Africa suggests that the composition and mineralogy of included xenoliths affects the alteration assemblages within kimberlite deposits.

Stem‐mediated hydraulic redistribution in large roots on opposing sides of a Douglas‐fir tree following localized irrigation

*Increasing evidence about hydraulic redistribution and its ecological consequences is emerging. Hydraulic redistribution results from an interplay between competing plant and soil water potential gradients. In this work, stem-mediated hydraulic redistribution was studied in a 53-year-old Douglas-fir tree during a period of drought. *Sap flux density measurements using the heat field deformation method were performed at four locations: in two large opposing roots and on two sides of the tree stem. Hydraulic redistribution was induced by localized irrigation on one of the measured roots, creating heterogeneous soil water conditions. *Stem-mediated hydraulic redistribution was detected during night-time conditions when water was redistributed from the wet side of the tree to the nonirrigated dry side. In addition to stem-mediated hydraulic redistribution, bidirectional flow in the dry root was observed, indicating radial sectoring in the xylem. *It was observed that, through stem-mediated hydraulic redistribution, Douglas-fir was unable to increase its transpiration despite the fact that sufficient water was available to one part of the root system. This resulted from the strong water potential gradient created by the dry soil in contact with the nonirrigated part of the root system. A mechanism of stem-mediated hydraulic redistribution is proposed and its possible implications are discussed.

Surface Potential Temperature as an Analysis and Forecasting Tool

Abstract In the last decade, Fred Sanders was often critical of current surface analysis techniques. This led to his promoting the use of surface potential temperatures to distinguish between fronts, baroclinic troughs, and non-frontal baroclinic zones, and to the development of a climatology of surface baroclinic zones. In this paper, criticisms of current surface analysis techniques and the usefulness of surface potential temperature analyses are discussed. Case examples are used to compare potential temperature analyses and current National Centers for Environmental Prediction analyses. The 1-yr climatology of Sanders and Hoffman is reconstructed using a composite technique. Annual and seasonal mean potential temperature analyses over the continental United States, southern Canada, northern Mexico, and adjacent coastal waters are presented. In addition, gridpoint frequencies of moderate and strong potential temperature gradients are calculated. The results of the mean potential temperature analyses show that moderate and strong surface baroclinic zones are favored along the coastlines and the slopes of the North American cordillera. Additional subsynoptic details, not found in Sanders and Hoffman, are identified. The availability of the composite results allows for the calculation of potential temperature gradient anomalies. It is shown that these anomalies can be used to identify significant frontal baroclinic zones that are associated with weak potential temperature gradients. Together the results and reviews in this paper show that surface potential temperature analyses are a valuable forecasting and analysis tool allowing analysts to distinguish and identify fronts, baroclinic troughs, and nonfrontal baroclinic zones.

The Solid-State Neck Growth Mechanisms in Low Energy Laser Sintering of Gold Nanoparticles: A Molecular Dynamics Simulation Study

Molecular dynamics (MD) simulations were employed to investigate the mechanism and kinetics of the solid-state sintering of two crystalline gold nanoparticles (4.4–10.0nm) induced by low energy laser heating. At low temperature (300K), sintering can occur between two bare nanoparticles by elastic and plastic deformation driven by strong local potential gradients. This initial neck growth occurs very fast (<150ps), and is therefore essentially insensitive to laser irradiation. This paper focuses on the subsequent longer time scale intermediate neck growth process induced by laser heating. The classical diffusion based neck growth model is modified to predict the time resolved neck growth during continuous heating with the diffusion coefficients and surface tension extracted from MD simulation. The diffusion model underestimates the neck growth rate for smaller particles (5.4nm) while satisfactory agreement is obtained for larger particles (10nm). The deviation is due to the ultrafine size effect for particles below 10nm. Various possible mechanisms were identified and discussed.