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A simple method combining Output‐Error (OE) model and Finite‐Difference Time‐Domain (FDTD) method is proposed to reconstruct lightning current in this paper. First, the FDTD method is adopted to calculate electromagnetic fields of any desired point, and the transfer function between the obtained fields and channel base current is built by using OE model. Then, the lightning current is reconstructed by putting the actual measured electromagnetic field into the corresponding transfer function. The validity of the proposed method is demonstrated through the measured data given in the literature. Moreover, three influence factors on the reconstruction performance of this method are analyzed as well, including the effect of distance from field observation point to lightning channel, ground conductivity and the velocity of return stroke. Key Points A simple method combining Output‐Error model and Finite‐Difference Time‐Domain method is proposed to reconstruct lightning current in this paper The validity of the proposed method is demonstrated through the measured data given in the literature The return stroke speed plays an important role in the reconstruction method

Lightning electromagnetic fields in the presence of conducting (grounded) structure having a height of 60 m and a square cross-section of 40 m × 40 m within about 100 m of the observation point are analyzed using the 3D finite-difference time-domain (FDTD) method. Influence of the conducting structure on the two orthogonal components of magnetic field is analyzed, and resultant errors in the estimated lightning azimuth are evaluated. Influences of ground conductivity and lightning current waveshape parameters are also examined. When the azimuth vector passes through the center of conducting structure diagonally (e.g., azimuth angle is 45°) or parallel to its walls (e.g., azimuth angle is 0°), the presence of conducting structure equally influences Hx and Hy, so that Hx/Hy is the same as in the absence of structure. Therefore, no azimuth error occurs in those configurations. When the conducting structure is not located on the azimuth vector, the structure influences Hx and Hy differently, with the resultant direction finding error being greater when the structure is located closer to the observation point.

The division of focal plane (DoFP) polarimeter is a vital tool for polarization imaging due to its compact structure and stable performance. However, its detection accuracy is significantly influenced by fabrication and integration errors of the micro-polarizer array (MPA). To address this, we establish a clear relationship between the accuracy of DoFP polarimeter and error sources, including the integration alignment, integration distance, integration angle, transmission axis angles, and extinction ratio of the MPA. Using a novel mathematical model based on the finite difference time domain method, we quantitatively analyze the impact of these errors on polarization detection accuracy. Our results demonstrate that as the detection accuracy improves from 10 − 1 to 10 − 2 and 10 − 3 , the required fabrication accuracy of MPA’s transmission axis angles and the integration accuracy both increase by approximately one order of magnitude. Additionally, to achieve same accuracy improvements, the extinction ratio of the MPA exhibits nonlinear growth, increasing by about 2.5 times and 20 times, respectively. These findings provide a critical foundation for error control and quantitative performance assessment in DoFP polarimeters, advancing their application in various fields.

The subgridding finite-difference time-domain (FDTD) method has a great attraction in ground penetrating radar (GPR) modeling. The challenge is that the interpolation of the field unknowns at the multiscale grid interfaces will aggravate the asymmetry of the numerical system which results in its instability. In this paper, an explicit unconditionally stable technique for a lossy object is introduced into the subgridding FDTD method. It removes the eigenmodes of the coefficient matrix which make the algorithm unstable. Therefore, the proposed approach not only maintains the advantages of simple implementation of the traditional FDTD method but also adopts a relatively large time step in both coarse and fine grid, which breaks through the restriction of the Courant-Friedrichs-Lewy (CFL) stability condition. The proposed method is applied in simulating the transverse magnetic (TM) wave backscattering of the two-dimensional buried objects in lossy media. Its accuracy and efficiency are examined by comparison with conventional FDTD and subgridding FDTD approaches.

Corona discharge is a typical discharge in gas-insulated equipment; however, the correlation between microscopic discharge process and macroscopic electromagnetic (EM) wave signals excited by discharge remains unclear. Therefore, this study innovatively employs the space current pulse as a bridge to reveal their relationship through the multi-scale simulation. First, the needle-plate discharge process in SF 6 gas is simulated based on a fluid dynamics model. Then, the effects of voltage, temperature, and the curvature of needle tip on the space current pulse are investigated. Lastly, the current pulses generated under varying conditions serve as excitation sources, and the finite-difference time-domain (FDTD) method is utilized to establish correlations between the corona discharge stages and discharge conditions and the amplitude-frequency characteristics of excited EM waves. The simulation results indicate that in the rising and falling stages of current pulse, the spectral energy is predominantly concentrated in the high frequency band (2.3–3.0 GHz) of the ultra-high-frequency (UHF) range, whereas the spectral energy constitutes the highest proportion within the mid-high frequency band (1.6–2.3 GHz) in the stabilization stage. As voltage, temperature, or the curvature of needle tip increases, there is a corresponding rise in the proportion of EM energy within both the low frequency band (0.2–0.9 GHz) and the mid-low frequency band (0.9–1.6 GHz), as well as in the mid-high frequency band; conversely, the proportion of energy within the high frequency band diminishes. The proposed multi-scale simulation method provides a novel way to obtain the characteristics of EM waves induced by partial discharge (PD) in gas.

A plasmonic temperature-sensing structure, based on a metal-insulator-metal (MIM) waveguide with dual side-coupled hexagonal cavities, is proposed and numerically investigated by using the finite-difference time-domain (FDTD) method in this paper. The numerical simulation results show that a resonance dip appears in the transmission spectrum. Moreover, the full width of half maximum (FWHM) of the resonance dip can be narrowed down, and the extinction ratio can reach a maximum value by tuning the coupling distance between the waveguide and two cavities. Based on a linear relationship between the resonance dip and environment temperature, the temperature-sensing characteristics are discussed. The temperature sensitivity is influenced by the side length and the coupling distance. Furthermore, for the first time, two concepts—optical spectrum interference (OSI) and misjudge rate (MR)—are introduced to study the temperature-sensing resolution based on spectral interrogation. This work has some significance in the design of nanoscale optical sensors with high temperature sensitivity and a high sensing resolution.

A new design and analysis of a wide-band double-negative metamaterial, considering a frequency range of 0.5 to 7 GHz, is presented in this paper. Four different unit cells with varying design parameters are analyzed to evaluate the effects of the unit-cell size on the resonance frequencies of the metamaterial. Moreover, open and interconnected 2 × 2 array structures of unit cells are analyzed. The finite-difference time-domain (FDTD) method, based on the Computer Simulation Technology (CST) Microwave Studio, is utilized in the majority of this investigation. The experimental portion of the study was performed in a semi-anechoic chamber. Good agreement is observed between the simulated and measured S parameters of the developed unit cell and array. The designed unit cell exhibits negative permittivity and permeability simultaneously at S-band (2.95 GHz to 4.00 GHz) microwave frequencies. In addition, the designed unit cell can also operate as a double-negative medium throughout the C band (4.00 GHz to 4.95 GHz and 5.00 GHz to 5.57 GHz). At a number of other frequencies, it exhibits a single negative value. The two array configurations cause a slight shift in the resonance frequencies of the metamaterial and hence lead to a slight shift of the single- and double-negative frequency ranges of the metamaterial.

Mobile communication has achieved enormous technology innovations over many generations of progression. New cellular technology, including 5G cellular systems, is being deployed and making use of higher frequencies, including the Millimetre Wave (MMW) range (30–300 GHz) of the electromagnetic spectrum. Numerical computational techniques such as the Finite Difference Time Domain (FDTD) method have been used extensively as an effective approach for assessing electromagnetic fields’ biological impacts. This study demonstrates the variation of the accuracy of the FDTD computational simulation system when different meshing sizes are used, by using the interaction of the critically sensitive human cornea with EM in the 30 to 100 GHz range. Different approaches of base cell size specifications were compared. The accuracy of the computation is determined by applying planar sensors showing the detail of electric field distribution as well as the absolute values of electric field collected by point sensors. It was found that manually defining the base cell sizes reduces the model size as well as the computation time. However, the accuracy of the computation decreases in an unpredictable way. The results indicated that using a cloud computing capacity plays a crucial role in minimizing the computation time.

The external-quantum efficiency (EQE) of AlGaN-based ultraviolet-B light-emitting diodes (UVB LEDs) has achieved a world record value of 9.6% on wafers but suffers from a low light extraction efficiency (LEE) of < 15%, notably lower than that of the LEE of InGaN blue LEDs (> 89%). This study employed the finite-difference time-domain (FDTD) method to explore how micro-patterned c-plane Sapphire substrates (microPSS) or nano-patterned c-plane Sapphire substrates (nanoPSSs) and reflecting photonic crystals (R-PhCs) influence light scattering in flip-chipped AlGaN-based UVB LEDs, with or without an Al-side reflector. First, various microPSS and nanoPSS shapes (Pillar-like and Hole-like) were analysed by the FDTD to optimise the pitch (a), diameter (d), height (h), and diffraction order (m) under Bragg’s condition. The nanoPSS were found most effective for UVB LEDs at an emission peak of 304 nm with cylindrical Hole-like nanoPSS (m = 10, d = 596 nm, a = 746 nm, h = 500 nm, R/a = 0.38), (R is the radius of the holes of the nanoPSS or PhC) improving LEE enhancement to the maximum possible value of approximately 18%. Next, an Al-side reflector was introduced to evaluate the combined impact of optimised nanoPSS and R-PhC (Hole-like) on theoretical light extraction. Parameters (m = 3; h = 150 nm; R/a = 0.40) applied in p-GaN or p-AlGaN contact layers boosted light extraction to approximately 148% or 150% (with an Al-side reflector) and approximately 120% (without an Al-side reflector), marking significant theoretical and experimental advancements in AlGaN UVB LED efficiency.

Analyzing the electromagnetic (EM) scattering properties of high-speed moving objects is a hot research topic in recent years. However, EM calculations for high-speed moving targets always involve challenges of high computational complexity and low computational efficiency. In this paper, we integrate the Message Passing Interface (MPI) based parallel finite difference time domain (FDTD) method and Lorentz transformation to calculate the EM scattering of a moving metal sphere coated with time-varying plasma. Subsequently, by comparing the outcomes of the proposed Parallel FDTD approach with the serial computing results, the validity of the Parallel FDTD method is validated. Additionally, for a moving and time-varying plasma sheath coated object, the impacts of the time-varying parameters and plasma parameters on the EM scattering properties are investigated using the Parallel FDTD approach. The results indicated that the MPI-Based Parallel FDTD approach displays almost identical precision as the serial approach. Furthermore, the Parallel FDTD approach can enhance computation speed and significantly reduce the computation time.