Electromagnetic Response Characteristics Research Articles

Frequency insensitive electromagnetic absorption core-shell sandwich structure with excellent electromagnetic damage tolerance

The compatibility of broadband electromagnetic absorption and electromagnetic damage tolerance poses challenges to the regulation of electromagnetic response characteristics, which are typically restricted by the intrinsic dispersion of materials and strong resonant features of unit cell structures. In this work, triply-periodic-minimal-surfaces (TPMS) based gradient core-shell sandwich structure is proposed to address this challenge for its mathematical defined unique conductive pore structure. The reflection loss-frequency curve is less than −10 dB in 2–18 GHz frequency band, accompanied by two separate resonant absorption peaks at 2.4 GHz and 17.5 GHz. The reflection loss curve is insensitive to frequency in a wide frequency band of 4–14 GHz, using merely three kinds of absorbing materials. When the damage proportion is less than 40 %, effective electromagnetic absorption can be maintained in panel damage, core damage and penetrating damage modes, thanks to the extraordinary conduction-dissipation effect. Our study provides valuable insights for the design of damage tolerant electromagnetic absorption structures.

An ultrathin ultralight electromagnetic absorber based on shortcut glass-coated amorphous magnetic Fiber/Salisbury-like screen

A structural design methodology is proposed for an ultrathin, ultralight, and absorption-adjustable electromagnetic absorber. The proposed absorber (SFSL) consists of an absorbing layer with shortcut glass-coated amorphous magnetic fiber and a substrate layer with transmitting material. This absorber features a Salisbury-like screen structure and incorporates multiple loss mechanisms. By investigating the influence of fiber distribution, length, content, and substrate layer thickness on absorption performance, it has been determined that the weight per square meter and thickness of a single-layer SFSL can be lowered within 50 g/m2 and 1.5 mm respectively. Furthermore, the absorption intensity and bandwidth can be adjusted by manipulating these parameters. The SFSL exhibits resonant behavior similar to that of a metamaterial absorber; however, SFSL with randomly distributed fibers demonstrates broader and stronger absorption characteristics in the frequency range from 2 to18 GHz. Additionally, the thicknesses of the substrate layer and surface covering affect the electromagnetic response characteristics. This work provides a simple strategy for constructing an ultrathin and ultralight composite to achieve efficient absorption of electromagnetic waves.

High-precision imaging of small voids in tunnel lining based on reverse time migration

Abstract Diseases such as voids behind the initial support of the tunnel will lead to problems such as lining rupture and concrete damage in the tunnel structure, which seriously affects the driving safety in the tunnel. In tunnel construction, ground penetrating radar is frequently employed as a method for detecting hidden defects. However, when the cavity size is small, there will be a large error when the original radar data is directly interpreted, which cannot meet the needs of practical engineering. To enhance the precision in tunnel detection, the Finite-Difference Time-Domain method is used to simulate various small cavities of different shapes located behind the primary support lining of the tunnel, and the study involves examining the electromagnetic response characteristics of different kinds of holes. The migration techniques of KIRCHHOFF, F-K, and reverse time are employed to reconstruct the hole target. Furthermore, digital morphology is employed to enhance the clarity of the image. Finally, the edge detection technology is used to further extract the hole features. After that, small cavities of different shapes are buried in the established outdoor concrete model box and radar detection is carried out. The detection position of the reconstructed radar image and the actual position of the cavity in the model box are verified. The results show that compared with KIRCHHOFF migration and F-K migration, the image processed by reverse time migration is clearer and more intuitive, and can restore the contour of small holes well. The research findings can serve as a reference for the interpretation of radar data of the cavity behind the initial support of the tunnel.

Rapid-Transfer Matrix Method for Analysis of Electromagnetic Properties of Uniaxial/Biaxial Biaxially Anisotropic Media

The employment of uniaxial/biaxial bianisotropic materials in the field of optical device manufacturing is widespread due to the distinctive electromagnetic response characteristics exhibited by such materials. To effectively analyze the electromagnetic properties of uniaxial/biaxial bianisotropic materials, rapid-transfer matrix method (R-TMM) is proposed to investigate the propagation process of plane waves in the media. Starting from the Maxwell’s equations in the time domain, the homogeneous differential equation about the electric field is constructed by processing the matrix containing dielectric and magnetic conductivity, electric and magnetic loss, tellegen and chirality carrier parameters, and the complex matrix operation is applied to that equation to get the booker quartic equation, and then the formulae method are utilized to obtain the eigenvalues in the uniaxial/biaxial bianisotropic media. Subsequently, the tangential continuity of layered media at the interface is employed to establish a transfer matrix for single-layered media. In the case of multi-layered media, the transfer matrix of plane waves propagating in multi-layered uniaxial/biaxial bianisotropic media can be obtained by means of a continuous iteration process based on the transfer matrix of single-layered media. The formula for calculating the propagation coefficient of uniaxial/biaxial bianisotropic materials can be derived based on the different upward and downward waves in the reflection/transmission region. Finally, the reliability and efficiency of R-TMM is verified from two numerical experiments with the plane waves incident at different angles on a uniaxial/biaxial bianisotropic media. The first experiment is designed as a single-layered biaxial bianisotropic model with more general electromagnetic parameters, and the second experiment is designed as a double-layered uniaxial and biaxial bianisotropic model consisting of common optical materials, which are composed of two non-magnetic materials, lithium niobate (LiNbLO<sub>3</sub>) and cadmium sulfide (CdS). The experimental results demonstrate that, in comparison with the conventional conventional-transfer matrix method (C-TMM), the R-TMM reduces the computational memory and CPU time required for calculating the reflection and transmission coefficients of the uniaxial/ biaxial bianisotropic model by over 98%, while maintaining the accuracy of the reflection and transmission coefficients calculation. Therefore, R-TMM provides an efficient and dependable approach for the design of complex optical devices and the analysis for uniaxial/biaxial bianisotropic propagation characteristics.

Metasurfaces based terahertz and mid- and long-wave infrared high-efficiency beam splitting devices

The multi-mode composite imaging technology integrates the advantages of different sensors, and thus has the advantages of high image quality, strong information acquisition capability, high target detection and recognition ability, strong adaptability to complex environments, and high stability and robustness of the system. Among them, the terahertz and infrared composite imaging technology combines the characteristics of terahertz band and infrared band, has the advantages of wide spectrum coverage, high resolution and strong penetration, and has broad application prospects. As one of the key components of the common aperture composite imaging system, the efficient optical splitters in terahertz and infrared band are still lacking at present, and their performance needs to be improved urgently. In this paper, a kind of dichroic metasurface with a simple structure and high performance is proposed by combining simulation experiment and theoretical explanation. When used as a spectroscopic device at an incident angle of 45°, it achieves a transmission coefficient greater than 97% near the center frequency of 1.1 THz, and a reflection coefficient greater than 98% in a wavelength range of 3–5 μm for medium-wave infrared and 8–14 μm for long-wave infrared. The design has good robustness to structural mismatches and machining errors such as structural misalignment, structural fillet, small magnification scaling, and polarization insensitivity. When the incident angle changes in a range of 0–60°, the device still maintains excellent spectral characteristics. In this paper, based on Babinet theorem and equivalent circuit model, the electromagnetic response characteristics of the metasurface are analyzed theoretically, and the analysis results are in agreement with the simulation results. The results of this study prove the feasibility of metasurface as a spectral device in the multiwavelength composite imaging system of terahertz and infrared bands, and provide support for future studying new composite imaging detection technology. In addition, the metasurface structure described in this paper has broad application prospects in many fields such as multi-band infrared stealth, laser and pump light separation in lasers, and provides a valuable reference for designing terahertz and infrared spectroscopy in various scenarios. In the following figure, for S wave and P wave at an incident angle of 45°, panel (a) shows the reflection coefficients varying with the wavelength of metasurface and panel (b) displays terahertz transmission coefficient changing with frequency.

Bound states in continuum domain of double resonant ring metal metasurfaces

Metasurfaces have found extensive applications in microwave, terahertz, and optical range, serving different purposes such as filters, sensors, slow light devices, and nonlinear devices due to their distinctive electromagnetic response characteristics. Recent development requires metasurface devices to exhibit enhanced monochromaticity and stronger light interaction. Consequently, there is a growing interest in designing metasurfaces with high-quality factor (<i>Q</i>-factor) resonances, considering their crucial role in achieving sharp resonances through constructing bound states in the continuum (BIC) mode. The utilization of BIC has emerged as a prominent method of designing metasurfaces with high <i>Q</i>-factor resonances. Due to the fact that the changes in the structural parameters of metasurfaces can simultaneously affect the resonance of two components of q-BICs, it is difficult to achieve on-demand design of operating frequency, bandwidth, and <i>Q</i>-factor. In this work, we investigate a novel THz metasurfaces supporting q-BIC resonance. We optimize the geometric parameters of two split ring resonators (SRRs) to tailor the operating frequencies of intrinsic resonance, and tune the coupling between different resonance modes to form the q-BIC mode resonance. The dominant modes are demonstrated by the results of multipolar decomposition calculations of the electromagnetic field distributions and scattered power at different resonant operating frequencies. In <i>x</i>-polarized and <i>y</i>-polarized incident electromagnetic wave, the normalized coupling strength ratio between the two modes are calculated by Jaynes-Cummings model to be 0.54% (<i>x-</i>polarized) and 4.42% (<i>y-</i>polarized) respectively, which explains the law that the resonant frequency of different modes changes with the structural parameters of SRRs device. In order to analyze the refractive index sensing capabilities of our designed metasurfaces under the incident electromagnetic waves with different polarizations, we investigate the variations of the transmitted spectrum of the metasurface with refractive index of matters. The calculated results show that the sensitivity of the metasurface is 151 GHz/RIU when the incident wave is <i>y</i>-polarized and 108 GHz/RIU when the incident wave is <i>x</i>-polarized. We realize the effective control of the operating frequency, bandwidth, and <i>Q</i>-factor of the q-BIC mode resonance in the transmission spectrum of the metasurface, which provides a new idea for the practical designing of terahertz metasurfaces with high <i>Q</i>-factor.

Crack Propagation and AE/EMR Response Characteristics of Pre-Holed Coal Specimens under Uniaxial Compression

Drainage boreholes in soft coal seams are prone to deformation and failure under the action of in situ stress and mining stress, which has a significant impact on gas drainage in coal mines. To simulate the development and propagation of cracks around the shaft wall caused by in situ stress, the crack propagation of coals with different diameters and strengths during the failure process, and the acoustic emission (AE) and electromagnetic radiation (EMR) law and response characteristics are explored. The results show that: The failure process of coal with pores is divided into four stages: initial compaction stage (OA), elastic deformation stage (AB), yield deformation stage (BC), and macroscopic crack development stage (CD). The crack propagation develops significantly in the post-load peak stage, the coal body damage is aggravated, and the coal body is unstable and fractured. For the pre-holed coal specimens with the same diameter, as the coal becomes softer, the peak stress decreases significantly (from 15.73 to 10.05 MPa). The cumulative value of AE counts of hard coal samples increased from 2.3 × 105 to 3.6 × 105 with increasing diameters. The Digital Image Correlation system (DIC) strain cloud diagram found that there are ‘I’-type cracks around the axial direction of the prefabricated holes. Coal samples with smaller hole have shorter cracks, indicating that the diameter of the holes significantly changes the axial loading limit. The research results have a certain reference significance for understanding the crack propagation of coal under static loads and evaluating the deformation characteristic and spatiotemporal stability of gas drainage in soft coal seams.

Tailoring electromagnetic response of three-dimensional waffle-like metacomposite based on arrangement angle of ferromagnetic microwires

Three-dimensional waffle-like metacomposite (3DWM) composed of ferromagnetic microwires is proposed with the aim of integrating structure and microwave absorption performance. The metamaterial units were formed by weaving the ferromagnetic microwires into certain pattern along with the quartz fiber substrate, making it a layer-by-layer integral structure without delamination. Based on the significance of angle arrangement of ferromagnetic microwires on the electromagnetic response during the rotation of metacomposite in practical application, this study conducted experiment and simulation considering different angle arrangements ranging from 90° to 165° to reveal their electromagnetic response characteristics and microwave absorption mechanism within X and Ku bands. It has been found that the arrangement angle of ferromagnetic microwire can adjust the complex permittivity. The dielectric loss of 3DWM was mainly originated from multiple Debye relaxation process and the primary magnetic loss came from eddy current loss and exchange resonance. The threshold angle was found to be 135°, within which the improvement of impedance matching led to the enhancement of microwave absorption. In addition, the angle arrangement of ferromagnetic microwire affected the direction of electric and magnetic fields, which determines the dielectric loss and magnetic loss mechanisms. The 3DWM proposed in this study is promising in adjusting electromagnetic response of microwaves as a radar absorbing material in engineering applications.

Electrical Insulation and Radar-Wave Absorption Performances of Nanoferrite/Liquid-Silicone-Rubber Composites.

Novel radar-wave absorption nanocomposites are developed by filling the nanoscaled ferrites of strontium ferroxide (SrFe12O19) and carbonyl iron (CIP) individually into the highly flexible liquid silicone rubber (LSR) considered as dielectric matrix. Nanofiller dispersivities in SrFe12O19/LSR and CIP/LSR nanocomposites are characterized by scanning electronic microscopy, and the mechanical properties, electric conductivity, and DC dielectric-breakdown strength are tested to evaluate electrical insulation performances. Radar-wave absorption performances of SrFe12O19/LSR and CIP/LSR nanocomposites are investigated by measuring electromagnetic response characteristics and radar-wave reflectivity, indicating the high radar-wave absorption is dominantly derived from magnetic losses. Compared with pure LSR, the SrFe12O19/LSR and CIP/LSR nanocomposites represent acceptable reductions in mechanical tensile and dielectric-breakdown strengths, while rendering a substantial nonlinearity of electric conductivity under high electric fields. SrFe12O19/LSR nanocomposites provide high radar-wave absorption in the frequency band of 11~18 GHz, achieving a minimum reflection loss of −33 dB at 11 GHz with an effective absorption bandwidth of 10 GHz. In comparison, CIP/LSR nanocomposites realize a minimum reflection loss of −22 dB at 7 GHz and a remarkably larger effective absorption bandwidth of 3.9 GHz in the lower frequency range of 2~8 GHz. Radar-wave transmissions through SrFe12O19/LSR and CIP/LSR nanocomposites in single- and double-layered structures are analyzed with CST electromagnetic-field simulation software to calculate radar reflectivity for various absorbing-layer thicknesses. Dual-layer absorbing structures are modeled by specifying SrFe12O19/LSR and CIP/LSR nanocomposites, respectively, as match and loss layers, which are predicted to acquire a significant improvement in radar-wave absorption when the thicknesses of match and loss layers approach 1.75 mm and 0.25 mm, respectively.

Simulation research and application on response characteristics of detecting water-filled goaf by transient electromagnetic method

Water inrush disasters poses a great threat to the safe exploitation of coal resources. To solve this problem, the transient electromagnetic method (TEM) was proposed to accurately detect the water accumulation in the goaf. The electromagnetic response characteristics of different water-filled goaves were studied by electromagnetic field theory, numerical simulation and field verification. Through the models of 100% water accumulation, 50% water accumulation, 0% water accumulation, 100% water accumulation with collapsed rock, 50% water accumulation with collapsed rock and 0% water accumulation with collapsed rock goaf, the characteristics of induced voltage attenuation curves were studied. Meanwhile, the relationship between the attenuation voltage value and area of the transmitting coil, the depth of the goaf, the background resistivity, and the delay time were also simulated. The results illustrate that the attenuation curve of induced voltage presented a regular exponential decay form in the 0% water accumulation model but existed abnormal exaltation for voltage in water-filled model. Through the linear fitting curve, it can be seen that the abnormal intensity of the induced voltage becomes stronger as the distance between the measuring point and the center of the target decrement. Moreover, the abnormal amplitude of the induced voltage increases with the rise of the water accumulation and collapsed rock will weakly reduce the low-resistivity anomalous effect on the water-accumulated goaf. In addition, the response value of the attenuation voltage increased as the area of the transmitting coil increases, but decreased with increasing delay time and increasing background resistivity and depth of the target body. The field detection results of the Majiliang coal mine also confirmed the theoretical analysis and the numerical simulation.

Comparison of nodal and edge basis functions for the forward modelling of three‐dimensional frequency‐domain wire source electromagnetic data using a potentials formulation

ABSTRACTAn investigation of forward modelling 3‐D wire source electromagnetic data in the frequency domain using nodal and edge finite‐element basis functions is presented. The equations solved are those for the coupled secondary vector–scalar potentials (i.e. magnetic vector potential A and electric scalar potential φ), which give rise to a less ill‐conditioned system of equations to be solved. The effectiveness of these two methods is validated with two 3‐D examples, i.e. multi‐blocks models and topography models, which considered the computation of the A–φ potentials and the electromagnetic fields due to a grounded electric line source by inter‐comparing the numerical solutions computed from each method and by comparing them against the equivalent solutions for the same model computed using other methods. Based on the multiple‐blocks models, we compare the relative contributions of the inductive and galvanic parts to the E‐field for different frequencies. Results show that for the type of source considered in this paper, the galvanic parts dominate for a relatively low frequency (10 Hz), but that for a relatively high frequency (10 kHz), the inductive parts also contribute. The extent to which the Coulomb gauge condition is satisfied when using nodal basis functions for A is also investigated, with results showing that the divergence of the numerically calculated A is almost zero as desired. Based on the topography models, we analyse the electromagnetic response characteristics at different survey planes, with results of this investigation indicating that the electromagnetic responses measured over a plane close to the target will carry more accurate geometry information (such as position, shape and size) of the target, which is expected, and less distortion due to topography compared to those measured at the earth's surface. This suggests that when conducting electromagnetic prospecting in an area with complex topography, it is possible to obtain better anomaly detection affected less by topography if the electromagnetic measurement is carried out underground (such as in a borehole).

Highly-anisotropic plasmons in two-dimensional hyperbolic copper borides.

Hyperbolic materials have wide application prospects, such as all-angle negative refraction, sub-diffraction imaging and nano-sensing, owning to the unusual electromagnetic response characteristics. Compared with artificial hyperbolic metamaterials, natural hyperbolic materials have many advantages. Anisotropic two-dimensional (2D) materials show great potential in the field of optoelectronics due to the intrinsic in-plane anisotropy. Here, the electronic and optical properties of two hyperbolic 2D materials, monolayer CuB6 and CuB3, are investigated using first-principles calculations. They are predicted to have multiple broadband hyperbolic windows with low loss and highly-anisotropic plasmon excitation from infrared to ultraviolet regions. Remarkably, plasmon propagation along the x-direction is almost forbidden in CuB3 monolayer. The hyperbolic windows and plasmonic properties of these 2D copper borides can be effectively regulated by electron (or hole) doping, which offers a promising strategy for tuning the optical properties of the materials.

Dynamically tunable terahertz metamaterial sensor based on metal–graphene hybrid structural unit

By verifying the electromagnetic response characteristics of graphene in the low terahertz (THz) band, a terahertz metamaterial sensor is proposed. The unit cell of the metamaterial sensor is a split ring resonator nested square ring resonator. The split ring resonator with four gaps is made of lossy metal, and the square ring resonator is formed by graphene. This structure can produce two high-performance resonant valleys in the transmission spectrum of 0.1–1.9 THz. The quantum interference between metal–graphene hybrid units also produces a reverse electromagnetically induced transparency (EIT)-like resonant peak between the two resonant valleys. Compared with the bimetallic ring resonator having the same shape and size, the sensor can dynamically adjust the position of the lower frequency resonant valley, thus, realizing the active tuning of the bandwidth and amplitude of the EIT-like resonant peak. The results demonstrate that the proposed sensor has a better sensing performance and can improve the detection precision by tuning itself to avoid the interference of environmental factors and the properties of samples. Combined with the advantages of convenience, rapidity, and non-damage of terahertz spectrum detection, the sensor has a good application potential to improve the unlabeled trace matter detection.

Electromagnetic response characteristics of lightweight hierarchical 2D nitrogen-doped graphene@amorphous carbon

Two dimensional (2D) nitrogen-doped graphene@amorphous carbon was constructed by in-situ polymerization of sandwich-like structured polyaniline/graphene oxide composites then carbonization at 650 °C in vacuum. The results revealed that the as-prepared 2D N-doped graphene@amorphous carbon and N-doped amorphous carbon nanorods have almost the same decomposition degree and crystal structure. However larger defective degree and more polar bonds are conducive to enhance the polarization loss capacity, and larger dielectric constant can contribute to prominent conductive loss potential. The standing-wave ratio at 13.325 GHz of 2D N-doped graphene@amorphous carbon is 1.08426 approaching to 1, almost perfect impedance-matching level achieving. HFSS simulations certify phase mutation, group delay and the resonance response between N-doped graphene@amorphous carbon and testing microwave, as well as the stronger microwave loss capacity and better impedance matching. Both of the above make 2D N-doped graphene@amorphous carbon a promising microwave absorbing material. The minimum reflection loss (RL) of nitrogen-doped graphene@amorphous carbon can reach −29.5 dB and the effective absorbing bandwidth (EAB, RL < −10 dB) can reach 5.355 GHz ranging from 12.56 to 17.915 GHz only at a thickness of 2.25 mm.

Controllable modification of helical carbon nanotubes for high-performance microwave absorption

Abstract Helical carbon nanotubes (HCNTs) are a kind of potential microwave absorption (MA) material due to their chiral and dielectric properties. However, the inert surface property makes HCNTs with poor polarization loss ability and impedance matching characteristic, which impedes its ability in attenuating microwaves. Herein, the HCNTs were modified with defects and functional groups on the surface to optimize their electromagnetic response characteristics and achieve an enhanced MA performance. The experimental results show that the modified HCNTs (F-HCNTs) exhibit a significant enhancement in MA performance when compared with HCNTs. The minimum reflection (RLmin) loss of F-HCNTs reaches −45.4 dB at 17.5 GHz at a thickness of 2.4 mm and the bandwidth of RL &lt; −10 dB is 3.6 GHz (from 14.4 to 18.0 GHz). Further analysis demonstrates that proper modification of HCNTs leads to enhanced dielectric loss ability and optimized impedance matching characteristics, both of which are beneficial to the MA performance of HCNTs.

Simulation Study on Electromagnetic Response Characteristics of Unfavorable Geological Body Under Complex Conditions

The second phase of the Chongqing Rail Transit Line project has poor rock and soil stability, slow construction progress, complex and variable geological conditions, and development of groundwater, which seriously threatens the safety of life and property. Therefore, this paper takes this as the engineering background to carry out a simulation research of the electromagnetic response characteristics of the unfavorable geological body under complex geological conditions. The results show that the maximum eddy current is formed in the poor geological body, indicating that the transient electromagnetic method is more sensitive to the low resistance body. The closer the tunnel face, the greater the thickness, and the larger the scale, the stronger the electromagnetic response. The induced electromotive force curve of the B value and the Z axis component of the electromagnetic induction intensity shows that the mutual influence of the two bad geological bodies is small. When there is a low-resistance karst body behind the tunnel face, it will have less impact on the detection of bad geological bodies directly in front of the tunnel face. When the metal body is close to the tunnel face, the detection curve of the induced electromotive force in front of the tunnel face is different from when there is no metal body. When the metal body is more than 30 m away from the tunnel face, the impact on the detection result is limited.

Electromagnetic Noise Reduction in GREATEM Signal Using Singular Value Decomposition in a Junkyard

The grounded electrical source airborne transient electromagnetic system (GREATEM) is a popular geophysical method in recent years [1], has advantages such as convenience, high efficiency, large detection range, high signal-to-noise ratio, and good spatial resolution. However, since the GREATEM transient electromagnetic secondary field electromagnetic response belongs to a wide-band signal and a large attenuation amplitude, late-stage data is easily contaminated by multiple noises, which will seriously affect the results of the inversion interpretation of the data. Therefore, it is still meaningful to study the de-noising methods for GREATEM’s data. According to the characteristics of the secondary field electromagnetic response, the singular value decomposition method is used to process the theoretical synthesized signal containing white Gaussian noise. The singular value decomposition and reconstruction of the effective singular value and its corresponding vector are performed, and the analog signal is restored for the purpose of denoising. Then process the measured data of a landfill site. Whether it is viewed from the transient electromagnetic secondary field attenuation curve or the electromagnetic induction profile curve, it can be seen that the singular value decomposition is very useful in denoising. Finally, comparing the effect of singular value decomposition method on the inversion results, the study found that this method can effectively improve the quality of inversion imaging.

Study on montoring simulation of urban space Environmental Science grouting by transient electromagnetic field

Grouting is one of the most effective methods to control underground space engineering disasters. It can effectively achieve a series of governance goals such as reinforcement of broken ground, reduction of settlement, and prevention and control of water and mud inrush. The form of the grouting process, the diversity of the grout, and the complexity of the stratum being injected are the key issues that restrict the development of grouting monitoring. Traditional grouting monitoring is mainly to control the grouting flow and pressure to achieve the grouting process and the state of grout diffusion. Routine testing The method can only perform spot inspection on the space or judge the overall space density, and it is difficult to monitor the spread of the slurry during the grouting process and the effective detection of the effect after the grouting. Based on the characteristics of transient electromagnetic sensitivity to low-resistance bodies, simple construction methods, and large detection depth, this paper uses theoretical analysis and numerical simulation as the main research methods to carry out forward simulation research on the diffusion process of grout in the formation. Through the establishment of numerical models for the different content of slurry in the middle layer of the same thickness layered formation, the possibility of using the electromagnetic field to explain the slurry diffusion in the grouting process is explored. Under ideal conditions, the transient electromagnetic response characteristics of the middle layer with different slurry conditions are analyzed. It is proved that the change of the field value of the transient electromagnetic secondary field before and after grouting can effectively judge the intrusion effect of the grout during the grouting process.

Effects of stainless steel thickness and winding tension on electrical and thermal characteristics of metal insulation racetrack coils for 10-MW-class HTS wind generator

Experimental results on the electrical and thermal characteristics of GdBCO racetrack coils co-wound with stainless steel (SS) tapes of various thicknesses and winding tensions are presented in this paper. The SS thickness and winding tension are identified as the factors affecting the charging time and thermal stability of metal insulation (MI) coils. To verify these characteristics, three types of MI-SS racetrack coils were fabricated with various SS thicknesses and winding tensions: 100-μm SS thickness and 10-kgf winding tension; 100-μm SS thickness and 5-kgf winding tension; and 50-μm SS thickness and 10-kgf winding tension. Further, three MI-SS racetrack coils were characterized by sudden discharging, charging, and overcurrent tests. The sudden discharging and charging tests were performed in a steady state to investigate the decay time constant of the center magnetic field discharging the coils and the delay time of the magnetic field charging the test coils, respectively. To estimate the thermal stability of the three MI-SS coils, overcurrent tests were conducted in a transient state at 1.1 times the critical current (Ic) and 1.05 Ic. Based on the experimental results of three small-scale test coils, the total contact resistance between the layers of the test coils was calculated and applied to design a field winding using a second-generation high-temperature superconducting (HTS) tape for a 10-MW-class HTS generator used in an offshore wind turbine environment. Subsequently, an equivalent electrical circuit model was proposed to analyze the electromagnetic response and thermal stability characteristics of three HTS field coils of a 10-MW-class HTS generator with the key parameters of the HTS field coils, which were obtained using electromagnetic finite-element analysis. Finally, the charging delay time and thermal stability of the three HTS field coils were compared and analyzed in detail.

Nanoscale electromagnetic boundary conditions based on Maxwell’s equations

The electromagnetic boundary conditions have great important applications in many physical branchs. Here, the nanoscale electromagnetic boundary conditions are derived by using the integral Maxwell’s equations through constructing the dielectric transition layer across the interface between the two materials. The two interface response functions are obtained to reflect the electromagnetic field response characteristics of the interface. Based on the Maxwell’s equations, the physical meanings of the interface response functions are given as the position of the equivalent interfacial polarization charge and the gradient position of interfacial polarization current density, respectively. The influence of the dielectric constant of the medium, the transition line shape of the electric field and the frequency on the interface response functions are analyzed. When the material scale is large, the interface response function can be ignored, and the nanoscale electromagnetic boundary conditions degenerate to the classical boundary conditions given by the abrupt junction. On this basis, the interface electric dipole moment, the equivalent interfacial polarization charge area density, the equivalent interfacial polarization current density and the equivalent interfacial magnetic current density are introduced, leading to three forms of nanoscale electromagnetic boundary conditions. The results provide a clear physical picture and necessary theoretical basis for nanoscale electromagnetism and interface optics.