Surface Current Model Research Articles

Stochastic evaluation of quasi-zero stiffness magnetic spring using a reluctance-corrected analytical model

—In state of the art high-precision motion systems, which utilize magnetic levitation, a quasi-zero stiffness gravity compensation is one of the essentials to improve dynamic performance, eliminate position dependency and thus simplify controller design. Special configurations of permanent magnets, such as Halbach magnet arrays, can realize magnetic levitation as a form of magnetic spring. However, unlike conventional permanent magnet machines with large stiffness, the quasi-zero stiffness magnetic spring requires higher modeling accuracy for force estimation, which is affected by the nonlinear reluctance effect of permanent magnets. In this study, we propose an accurate magnetic modeling method for the quasi-zero stiffness magnetic spring. By correcting the reluctance effect of magnets, the proposed magnetic model achieves superior accuracy estimating levitation force and stiffness to the conventional surface current model. Using the reluctance-corrected magnetic model, tolerance analysis was performed to identify dominant geometric parameters affecting the uncertainty of the Halbach array magnetic spring performance. A Monte-Carlo simulation was used to estimate the overall tolerance of magnetic forces and stiffness. The experimental results fit in the tolerances.

TWO-DIMENSIONAL FLOW PATTERN MODEL IN THE WATERS OF BAKAU BAY BINTAN DISTRICT

Ocean currents are the movement of sea water masses in a certain direction caused by several factors, namely wind, tides and bathymetry. Hydrodynamic modeling aims to determine the speed and direction of ocean currents in the waters of Bakau Bay through a two-dimensional model. The current pattern of the north season to the east season is influenced by sea water masses with an average current speed ranging from 0.22-0.24 m/s, while the south season to West season currents with an average speed of 0.04-0.26 m/s, this shows the meeting of different water masses so that the speed and direction of the current results in random or rotating currents. From the calculations of the normality test and anova test, it is known that the field current and model simulation values are normally distributed and there is no significant difference in values in the two data, so based on the RMSE value, it shows that the difference in values between the model simulation results and direct data measurements is relatively small. This also shows that the ocean surface current model created is sufficient to describe actual conditions.

Study of Coherent Smith–Purcell Radiation in the Terahertz Region Using Ultra-Short Electron Bunches

Smith–Purcell radiation (SPR) can be generated nondestructively, providing valuable applications in light sources and beam monitors. Coherent SPR is expected to enable single-shot measurements of very short bunch lengths on the fs scale. Since the reconstruction of the longitudinal bunch shape from the coherent SPR is based on the reliable SPR spectrum, a more detailed understanding of the properties of the radiation is important in this context. Employing a 100 fs ultrashort electron bunch at the t-ACTS test accelerator, the spectrum, angular distribution, and polarization of the produced coherent SPR were measured in the terahertz frequency region and compared with a model calculation. In addition to the widely known surface current model evaluation, the effect of the geometrical shading effect on induced currents on metal surfaces was evaluated using 3D numerical calculations. The obtained SPR characteristics are also presented. In the evaluation of the grating with a shallow blaze angle, it was found that the shading effect has a non-negligible effect on the generated SPR intensity; the measured angular distribution and polarization results were in good agreement with this result.

Research on Air Gap Magnetic Field Characteristics of Trapezoidal Halbach Permanent Magnet Linear Synchronous Motor Based on Improved Equivalent Surface Current Method

Accurate analysis of the air gap magnetic field is the focus of research in the field of precision permanent magnet linear synchronous motors. In this paper, the two-dimensional air gap magnetic field of a secondary trapezoidal Halbach permanent magnet array coreless permanent magnet linear synchronous motor (PMLSM) was taken as our research object. On the basis of the equivalent surface current method, we proposed an improved equivalent analytical algorithm with a trapezoidal side length unit. The equivalent analytical model of the magnetic induction vector of the two-dimensional air gap was established, and the air gap magnetic field of the trapezoidal Halbach array coreless PMLSM was calculated. At the same time, we analyzed the influence of the bottom angle α of a trapezoidal permanent magnet equivalent width coefficient αw, pole height coefficient αh, and air gap height coefficient αg on the amplitude (Bpeak) and total harmonic distortion (THDB) of the central magnetic field in the air gap. The results show that α and αw have a significant influence on the Bpeak and THDB of the central magnetic field air gap. With the synergy of α and αw, we identified the “flux convergence” effect, which makes the maximum range of Bpeak α > 90° and αw < 0.5. We also found the “equilateral” effect, which causes the minimum region of THDB to change linearly. The calculation results of the improved equivalent surface current analytical model established in this paper agree with those verified by the finite element method. The calculation is convenient, and the accuracy of the result is high. This research provides a new method for analyzing the air gap magnetic field of a permanent magnet with a nonrectangular cross-section and lays a theoretical foundation for optimizing the PMLSM pole model.

Comparative analysis of two models for the optical response of a thin slab: a film of finite thickness versus a surface current sheet.

An accurate description of the electromagnetic properties of materials is fundamental to optical and electric devices. As a current research hotspot, thin slabs generally are modeled as a film of finite thickness with a dielectric function. However, inspired by two-dimensional materials, thin slabs can be regarded as surface current sheets with conductivity. Due to the convenience of the latter in simplifying the calculations, it becomes increasingly significant to determine the equivalent conditions of the two models. In this work, we compare the differences between the thin film and surface current models in calculating the transmissivity, reflectivity, and absorptivity of a SiC film. For normal incidence, the difference between the calculations of the two models is only non-negligible when the thickness is large (500 nm), because of the invalidation of surface current models and the excitation of Fabry-Perot resonance. In particular, we derive analytical formulas for the relative error in transmittance versus phase difference, which can be used to predict the difference between the two models as a function of film thickness. For oblique incidence, the two models have significant differences in the vicinity of epsilon-near-zero (ENZ) frequency. The excitation of the Berreman leaky mode in a thin film model causes a narrow blank absorption peak close to the ENZ frequency. However, we found that the surface current model is unable to form this resonance mode and further demonstrate it theoretically. In addition, it is found that the two models are equivalent in the case of a transverse electric wave even though the incidence is oblique. This work can enhance the awareness of the light-matter interaction and open unprecedented avenues for designing ultrathin optical devices.

Loaded Slot Cavity Induced Sensing Enhancement and Transparency Based on Plasmonic Structure

Biosensors have proved immensely useful in numerous vital areas for detection and testing. Here, a novel plasmonic nanosensor, based on loaded slot cavity, is proposed and studied. We conduct a detailed analysis of the influence of the load’s parameters on the transmission characteristics based on the finite element method and surface charge and current model. Simulation results reveal that the existence of the load can cause the resonant wavelength to have a linear or nonlinear red shift, and yield an enhanced plasmonic nanosensor with sensitivity about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}={2900}$ </tex-math></inline-formula> nm/RIU and a detection limit about 1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times \,\,10^{\text {-3}}$ </tex-math></inline-formula> . In addition, the proposed structure is well used in actual biosensing for blood plasma concentration, glucose concentration, ethanol temperature and diseased cell detection with high sensitivity. Finally, an extended structure with detuned loaded slot cavities is proposed to realize PIT/multi-PIT and slow light effect. The special features of our proposed structure are applicable in the realization of various integrated components for the development of high-performance plasmonic biosensor.

Limitations of Surface Current Model of Magnetic Field and a Remedy

It is well known that any realistic static magnetic field in a source-free volume can be generated by electric currents on a surface enclosing the volume. The magnetic field generated by surface currents is equivalent to that from magnetic dipoles distributed on the surface and along the surface normal directions. We demonstrate the limitations of the surface current model for instrumentation, and show vertical current loops corresponding to magnetic dipoles along the surface tangential directions may be employed to mitigate these limitations. We consider loop coil elements distributed on a cylindrical surface. A closed-surface array has loops on the entire surface, and an open-surface array does not have loops on the surface of both ends. A coil array may contain only flat elements (each loop is inside the cylindrical surface) or composite elements consisting of three orthogonal loops (one flat loop and two vertical loops) instead of just one flat loop at the location of each element. With computer simulations, the array coils were tested for generating the spatial field pattern of a vertical loop or a linear field gradient. The simulations showed that (1) an open-surface composite loop array produces less errors than a similar flat loop array in generating linear field patterns; the percentage root mean square error (RMSE) of the composite coil loops was 8.46% compared to 21.5% for the similar flat loops; (2) A composite loop array consumes less power than a corresponding flat array. For the open-surface array with a similar number of loops, the power consumption of the composite loop array was 73.9 versus 184 (arbitrary power units) for the flat loop array.

Implementing the MULTI-VP coronal model in EUHFORIA: Test case results and comparisons with the WSA coronal model

Context.In this study, we focus on improving EUHFORIA (European Heliospheric Forecasting Information Asset), a recently developed 3D magnetohydrodynamics space weather prediction tool. The EUHFORIA model consists of two parts covering two spatial domains: the solar corona and the inner heliosphere. For the first part, the semiempirical Wang-Sheeley-Arge (WSA) model is used by default; this model employs the potential field source surface and Schatten current sheet models to provide the necessary solar wind plasma and magnetic conditions above the solar surface, at 0.1 AU, which serve as boundary conditions for the inner heliospheric part. Herein, we present the first results of the implementation of an alternative coronal model in EUHFORIA, the so-called MULTI-VP model.Aims.After we replace the default EUHFORIA coronal setup with the MULTI-VP model, we compare their outputs both at 0.1 AU and 1 AU, for test cases involving high speed wind streams (HSSs). We select two distinct cases in which the standard EUHFORIA setup failed to reproduce the HSS plasma and magnetic signatures at Earth to test the performance of MULTI-VP coupled with EUHFORIA-heliosphere.Methods.To understand the quality of modeling with MULTI-VP in comparison with the default coronal model in EUHFORIA, we considered one HSS case during a period of low solar activity and another one during a period of high solar activity. Moreover, the modeling of the two HSSs was performed by employing magnetograms from different providers: one from the Global Oscillation Network Group (GONG) and the second from theWilcoxSpace Observatory (WSO). This way, we were able to distinguish differences arising not only because of the different models but also because of different magnetograms.Results.The results indicate that when employing a GONG magnetogram, the combination MULTI-VP+EUHFORIA-heliosphere reproduces the majority of HSS plasma and magnetic signatures measured at L1. On the contrary, the standard WSA+EUHFORIA-heliosphere combination does not capture the arrival of the HSS cases at L1. When employing WSO magnetograms, MULTI-VP+EUHFORIA-heliosphere reproduces the HSS that occurred during the period of high solar activity. However, it is unclear if it models the HSS during the period of low solar activity. For the same magnetogram and periods of time, WSA+EUHFORIA-heliosphere is not able to capture the HSSs of interest.Conclusions.The results show that the accuracy of the simulation output at Earth depends on the choice of both the coronal model and input magnetogram. Nevertheless, a more extensive statistical analysis is necessary to determine how precisely these choices affect the quality of the solar wind predictions.

Interaction of Tearing Modes With Passive Structures in a Tokamak

In this work, a surface current plasma model is coupled to a volume integral formulation for studying the interaction between tokamak tearing modes (TMs) and the machine metallic structures surrounding the plasma. By evaluating the passive response on a set of pickup probes, TMs' amplitude is estimated.

Improved terahertz Smith–Purcell radiation using side-opening cylindrical gratings

The Smith–Purcell radiation (SPR) is a promising way for generating terahertz emission, which has tremendous application prospects. Yet, the conventional SPR schemes generally utilize the Gaussian electron beam/bunch with round cross section (also called a pencil beam) to interact with a planar grating. The shape mismatching between the electron beam and the grating essentially restricts the interaction efficiency together with the radiation intensity. To counteract this effect, we propose to use side-opening cylindrical gratings (SOCGs) to replace planar gratings. Detailed theoretical analyses using the surface-current model and mode-matching method are performed for the first time. A train of premodulated electron bunches is proposed to drive the SOCG, on which the electron bunches are further micro-bunched by interacting with the surface waves. The multi-bunched electron beam then generates multi-color coherent SPR at the superimposed harmonics of the train of bunches. Compared with that from a planar grating, the radiation intensity from the SOCG can be significantly enhanced, indicating that it is a promising way of developing high-power terahertz wave sources.

Torque Calculation of Permanent-Magnet Spherical Motor Based on Permanent-Magnet Surface Current and Lorentz Force

Due to the complex spatial magnetic field distribution and the variable motion mode, the torque analysis of spherical motors is usually carried out by the finite-element method (FEM) with a large amount of calculation and time consumption. In order to improve analysis efficiency, this article proposes a spherical motor torque calculation method based on the permanent-magnet surface current model and Lorentz force, aiming to replace the finite-element calculation. First, the equivalent surface current analytical expression of the rectangular permanent-magnet magnetic field is derived. Then, this expression is used to establish the 3-D magnetic field analytical model of Halbach array permanent magnets in the rotor. Based on this magnetic field, the Lorentz force of the energized stator coil is calculated. Then, the electromagnetic torque generated by the rotor is analyzed. Finally, the analytical results of the magnetic field and the motor torque are compared with those by the FEM and experiments, verified that, under the premise of ensuring the calculation accuracy, the proposed method effectively shortens the calculation time and has higher stability.

Complex Optical Conductivity of Two-Dimensional MoS2: A Striking Layer Dependency.

The complex optical conductivities of two-dimensaionl (2D) materials are fundamental for extended applications of related optoelectronic devices. Here, we systematically investigate the layer-dependent evolutions in the complex optical conductivities of 1-6 layer 2D MoS2 over an ultrawide spectral range (0.73-6.42 eV) by spectroscopic ellipsometry. We identify five feature peaks (A-E) in the optical conductivity spectra, which present interesting layer dependencies due to the scaling effect. Results suggest that the center energies of peaks A and B are nearly layer-independent, while those of peaks C and D exhibit redshifts as the layer increases. We interpret these layer-dependent evolutions as the competition between the decreasing exciton effect and the prominent band shrinkage with the increasing layer number. Additionally, the applicability of the classical slab model and the surface current model in evaluating the optical conductivities of 2D MoS2 with different layers is discussed from an experimental perspective.

The given surface current distribution model of a rectangular patch antenna with metamaterial-like substrate

Model of a rectangular patch antenna on composite substrate is analytically developed in this study. The proposed metamaterial is a layer of periodically situated non-magnetic metal cylinders of circular cross section symmetrically imbedded into a dielectric substrate. The model is based on the long-wave approximation of appropriate boundary-value problems stated for Helmholtz equation.The commercial finite-difference time-domain (FDTD) software is used in the study for validating the model. It has been shown that the return loss, efficiency, power gain have improved due to the usage of the metamaterial layer in the substrate.

Submesoscale Surface Tidal, Vortical, and Residual Circulations in a Semienclosed Bay

AbstractWe scrutinize three different components of submesoscale surface tidal, vortical, and residual circulations in a semienclosed bay using (1) observations of high‐frequency radar‐derived surface current maps and (2) numerical model simulations run under realistic vertical stratification and boundary conditions at O(1)‐km spatial and 1‐hourly temporal resolutions over a 2‐year period (2013‐2014). The tidal circulation is characterized by (1) the tidal ellipses and spectral contents having more baroclinic motions at the M2 and S2 frequencies and more barotropic motions at the K1 frequency and (2) the temporal and spatial variability in the tidal fronts appearing as tidal ellipses with clustered shapes and opposite rotational directions. The regional vortical circulation is presented with the submesoscale eddies at diameters ranging from 3 to 12 km and normalized vorticity magnitudes of 0.2 to 2 for both clockwise and counterclockwise rotations. Based on the spatial statistics of the identified submesoscale eddies and eddy kinetic energy budget analysis, the submesoscale eddies are primarily generated by the detachment of shoreline‐following tidal currents at the coastal boundaries, persist for less than 1.5 days, and are dissipated dominantly via vertical buoyancy fluxes associated with bottom bathymetric interactions of the tidal currents. The residual circulation is clearly shown with the nontidal geostrophic currents associated with the pressure gradients generated by wind‐driven Ekman transports against the coast and the ageostrophic low‐frequency currents.

Search and Rescue Applications: On the Need to Improve Ocean Observing Data Systems in Offshore or Remote Locations

Search and rescue (SAR) in remote maritime locations is a difficult mission. One of the limitations in these isolated regions is the low density of available oceanographic data for model validation. In order to examine the state of remote search and rescue a review of maritime search theory and advances was conducted. This included basic drift theory, leeway, available environmental data, and the current methods used by the United States Coast Guard for SAR operations. In particular the U.S. Coast Guard’s fourteenth district’s SAR case history was examined and it was found that 60% of SAR cases fall outside of areas that have high-resolution wind and current data, with only global scale model forecasts available. In addition, 2% of cases occurred in offshore waters ( > 12nm from land) and exceeded 36 hours in asset response time. Three SAR simulations were run off the coast of Oahu, Hawaii using the same wind data, but different surface current models. These simulations had extremely large (up to 12,000 km2) search areas, highlighting the need for solutions that narrow these expected areas.

Research on air suspension with novel dampers based on glowworm swarm optimization proportional–integral–derivative algorithm

In order to enhance automotive ride performance, permanent magnets and magnetic valves were introduced in magnetorheological dampers and hence permanent magnets and magnetic valves introduced in magnetorheological damper’s air suspension was studied. First, on the basic principle of providing magnetic field by permanent magnets and adjusting damping force by magnetic valves, a novel permanent magnets and magnetic valves introduced in magnetorheological damper was designed. On the basis of equivalent surface current model, magnetic induction intensity activated by permanent magnets in damping channel was calculated, and damping force model was built based on hydromechanics theory. Damping force–displacement experiments and damping force–velocity experiments were carried out, and the theoretical model of permanent magnets and magnetic valves introduced in magnetorheological damper was verified. Then, physical and mathematical models of 4 degree-of-freedom permanent magnets and magnetic valves introduced in magnetorheological damper air suspension model were built. In order to enhance the controlling effects of the studied air suspension, optional moving operation, moving variation operation and operation of step size changing with fluorescein proportionality were introduced into glowworm swarm optimization algorithm, and glowworm swarm optimization proportional–integral–derivative controlling algorithm was designed. Finally, three experiments with input of impact road surface, sine wave road surface, and white noise road surface were carried out, and the experimental results verified that the working performance of permanent magnets and magnetic valves introduced in magnetorheological damper air suspension based on glowworm swarm optimization proportional–integral–derivative controlling algorithm was good.

Rare long‐distance dispersal of a marine angiosperm across the Pacific Ocean

AbstractAimLong‐distance dispersal (LDD) events occur rarely but play a fundamental role in shaping species biogeography. Lying at the heart of island biogeography theory, LDD relies on unusual events to facilitate colonization of new habitats and range expansion. Despite the importance of LDD, it is inherently difficult to quantify due to the rarity of such events. We estimate the probability of LDD of the seagrass Heterozostera nigricaulis, a common Australian species, across the Pacific Ocean to colonize South America.LocationCoastal Chile, Australia and the Pacific Ocean.MethodsGenetic analyses of H. nigricaulis collected from Chile and Australia were used to assess the relationship between the populations and levels of clonality. Ocean surface current models were used to predict the probability of propagules dispersing from south‐east Australia to central Chile and shipping data used to determine the likelihood of anthropogenic dispersal.ResultsOur study infers that the seagrass H. nigricaulis dispersed from Australia across the entire width of the Pacific (c. 14,000 km) to colonize South America on two occasions. Genetic analyses reveal that these events led to two large isolated clones, one of which covers a combined area of 3.47 km2. Oceanographic models estimate the arrival probability of a dispersal propagule within 3 years to be at most 0.00264%. Early shipping provides a potential alternative dispersal vector, yet few ships sailed from SE Australia to Chile prior to the first recording of H. nigricaulis and the lack of more recent and ongoing introductions demonstrate the rarity of such dispersal.Main conclusionsThese findings demonstrate LDD does occur over extreme distances despite very low probabilities. The large number of propagules (100s of millions) produced over 100s of years suggests that the arrival of propagules in Chile was inevitable and confirms the importance of LDD for species distributions and community ecology.

Multi-band planar miniaturised negative-index metamaterials

In this paper, a miniaturised negative-index metamaterial (NIM) based on planar concentric split ring resonators was designed, simulated, fabricated and tested that can maintain NIM properties. The offered metamaterial structure showed miniaturisation factor compared with traditional metamaterial unit cells and conformed better effective medium ratio (EMR). Finite-difference time-domain method-based computer simulation technology for the numerical analyses and a vector network analyser (N5227A) was used for the measurements. The proposed metamaterial exhibited multi-band response in conjunction with the NIM property over the certain frequency bands in the microwave spectra and the EMR considerably improved compared to previously reported metamaterials. The measured results revealed good agreement with the simulated results. The distribution of surface current, electric field and equivalent circuit model were verified for the proposed structure. The size, scattering parameters and EMR parameters of the proposed miniaturised NIM is appropriate for multi-band applications.

High-Order-Harmonic Generation from a Relativistic Circularly Polarized Laser Interacting with Over-Dense Plasma Grating**Supported by the National Natural Science Foundation of China under Grant Nos 11375265, 11475259 and 11675264, the National Basic Research Program of China under Grant No 2013CBA01504, and the Science Challenge Project under Grant No JCKY2016212A505.

The simple surface current model is extended to study the generation of high-order harmonics for a relativistic circularly polarized laser pulse interacting with a plasma grating surface. Both exact relativistic electron dynamics and optical interference of surface periodic structure are considered. It is found that high order harmonics in the specular direction are obviously suppressed whereas the harmonics of the grating periodicity are strongly enhanced and folded into small solid angles with respect to the surface direction. The conversion efficiency of certain harmonics is five orders of magnitude higher than that of the planar target cases. It provides an effective approach to generate a coherent radiation within the so-called ‘water window’ while maintaining high conversion efficiency and narrow angle spread.

Dual-band tunable infrared metamaterial absorber with VO[formula omitted] conformal resonators

A dual-band tunable infrared metamaterial absorber with VO2 is proposed. This tunable metamaterial absorber is constructed of three individual layers of conformal resonating layer, dielectric layer, and metallic background. The resonator is composed of gold and VO2. The electromagnetic (EM) wave absorption is numerically calculated and the working mechanism is explained by the surface current and LC-circuit model. Due to the fact that the resonator frequency depends on the resonating gold structure, it is found that by partially replacing the gold structure with VO2, the absorption frequency and intensity can be tuned through changing the temperature of VO2 resonate structure. In addition, the proposed method can be applicable to other resonate shapes.