Presence Of Eddy Currents Research Articles

Influence of porous media and substrate rotation on AlN growth in MNVPE reactors based on CFD simulations

AlN is an important III-V wide-bandwidth semiconductor material. Its maximum (ultra-wide) direct bandgap of up to 6.2 eV at room temperature and excellent physical properties make it have great application prospects in the fields of high-power high-frequency electronics and high-efficiency optoelectronic devices. In this paper, based on Computational Fluid Dynamics (CFD), the finite volume method was employed to investigate the process of growing AlN thin films by MNVPE using CFD simulation software for three-dimensional numerical simulation. By optimizing the chamber structure, it was devoted to improving the growth rate and uniformity of AlN thin films. Different thicknesses of porous media were placed in front of the substrate respectively, and the flow field analysis showed that the presence of eddy currents can be effectively suppressed to reduce the pre-reaction, and the best growth effect was achieved when the thickness reached 7 cm. With the increasing of porosity, more gas reached the substrate surface and improved the growth rate of AlN films. Secondly, for the 90° substrate, substrate rotation created a pumping effect and the growth rate was higher with the increase of the rotation speed. At 160 rpm, the growth rate reached 11.29 μm/h and the coefficient of variation decreased to less than 5 %.

Efficient Numerical Solution of Coupled Axisymmetric Plasma Equilibrium and Eddy Current Problems

This paper presents an efficient numerical tool for solving the problem of transient nonlinear evolution of plasma equilibrium in presence of eddy currents. The code, named FRIDA-TD (FRIDA-Time Domain), and based on the existing FRIDA (FRee-boundary Integro-Differential Axisymmetric) code, exploits two different approaches for solving the elated nonlinear system of Partial Differential Equations (PDE): (i) Finite Element Method - Boundary Element Method (FEM-BEM) for plasma equilibrium, and (ii) Axisymmetric Volume Integral (AVI) formulation for eddy currents. The resulting FRIDA-TD is a fast and flexible tool suitable for engineering-oriented purposes, such as the design of Tokamak devices, set up of plasma operations, prediction of performance scenarios, and design of feedback control systems. FRIDA-TD is successfully validated at first with the well-established CarMa0NL code, and then against experimental data of the RFX-mod device.

Phantom based quality assurance for long-term stability and inter-scanner compatibility in T1, T2 and T2* measurements

Abstract Funding Acknowledgements Type of funding sources: None. Background The quantitative measurement of T1, T2 and T2* relaxation parameters is prone to numerous physical variables related to scanner performance and installation environment. These variables include the presence of eddy currents, main magnetic field inhomogeneity and gradient field performance. Thus, it is important to verify that the measured relaxation parameters are inter-scanner compatible and there are no drift in the measured values over time. Purpose In this study, we produced a phantom for the relaxation parameter measurements. With the phantom, long-time stability and inter-scanner comparison of parameters were studied with preliminary quality assurance protocol. Methods A cylindrical quality assurance phantom with 16 inserts filled with ammonium iron(II) ((NH4)2Fe(SO4)2 · 6H2O) doped gel was produced. The (NH4)2Fe(SO4)2 concentrations were from 0,0119 mmol/g to 0,5498 mmol/g, corresponding to approximate T1 values of 10–1030 ms, T2 values of 16–78 ms and T2* values of 5–61 ms. The phantom was measured quarterly for a one-year period with three scanners from two different vendors to follow the scanner stability and inter-scanner compatibility. The used imaging sequences included T1 MOLLI -based sequence, T2 prep -based sequence, and multi-echo gradient echo sequence for T2* calculation. Simulated ECG signal with 60 beats per minute was used in the scanning to mimic a typical rest heart rate of an adult patient. Results The range of variation between scanners from average relaxation coefficient in T1 was -2% to 4% and in T2 and T2* measurements -7% to 5% and -32% to 18%, respectively. In long-term stability test, the range of variation from average relaxation coefficient in T1 was -1% to 2% and in T2 and T2* measurements -1% to 1% and -15% to 16%, respectively. Conclusion(s) There were significant differences in the measured T1, T2, and T2* relaxation coefficients in both inter-scanner and long-term stability. However, the detected variations were acceptable compared to the typical physiological uncertainties of the measured parameters but have to be taken into account when the reliability of the study is assessed. Abstract Figure. Results and phantom image

Improving energy efficiency of magnetic CO2 methanation by modifying coil design, heating agents, and by using eddy currents as the complementary heating source

The Sabatier reaction activated by high-frequency magnetic fields is a promising approach for the power-to-gas process because of expected high energy efficiencies and fast switch-on times. Recent progresses have been achieved by combining nanoparticles displaying both a high heating power and a good catalytic activity. Here, we alternatively use iron microparticles associated with our own-designed Ni/CeO2 catalyst. The heating agent is cheap and abundant, and we demonstrate that the presence of eddy currents in the system improves its heating performance. The contribution of eddy currents to global heating is successfully determined by an original protocol consisting in comparing a calorimetric and a high-frequency hysteresis loop-based method to measure heating power. In addition, the optimization of the catalyst bed using SiC-spacers limits sintering and thus improves the durability of the catalyst. The energy efficiency of the catalysis process, calculated as a function of coil consumption and gas flow, is clearly improved by the use of an air-cooled Litz wire coil. These improvements are a step forward toward the development of a cheap and efficient process for chemical energy storage.

Повышение коммутационной устойчивости коллекторных тяговых электродвигателей электровозов за счет снижения пульсации тока возбуждения

Statistics of equipment failures of electric locomotives assigned to the operational depots of the East Siberian Traction Directorate shows that about 25% of the total number of equipment failures of electric locomotives are traction electric motors (TEM); most often the collector-brush apparatus fails. The main reason is the problem of TEM low switching stability, associated with the increase in unbalanced electromotive forces (EMF) due to non-sinusoidal values of EMF variables and large phase shifts between their main harmonics. To clarify the processes related to EMF variable components, a vector diagram of their main harmonics is constructed, which determines the qualitative nature of the phenomena. It is revealed that the presence of eddy currents in the massive elements of the TEM magnetic core and the resulting armature reaction lead to significant displacement of the vector of the switching EMF, which forms the unbalanced EMF, causing violation of the TEM switching stability. In this regard, an improved system for attenuating the excitation of traction motors based on IGBT transistors is proposed, which reduces pulsation of the excitation current by at least 80% increasing the TEM life resource.

The influence of eddy currents on the excitation winding impedance of solid and laminated salient pole synchronous machines

This work investigates the establishment of steady-state eddy currents in solid and laminated salient poles and rotor rim of synchronous machines due to a periodic excitation voltage. It shows that the presence of eddy currents in the rotor magnetic circuit has the double effect of increasing the excitation winding AC-resistance and decreasing its magnetizing AC-inductance. According to that a simple analytical model is presented in here which allows a rapid rough estimation of the excitation winding AC-resistance when little information is available about the machine geometry and its electric/magnetic materials properties. The model is then verified by reproducing in frequency the excitation winding AC-resistance and the related power loss measured in two synchronous generators. Finally, the limits of reliability and applicability of the model are discussed. The model has implications for periodic field winding current control and voltage regulation in synchronous machines.

Hybrid Additive Manufacturing Technology: Induction Heating Cold Spray—Part I: Fundamentals of Deposition Process

The prevention of delamination of soft and ductile materials from hard substrates is a challenge when using the cold spray process. As a potential solution to this issue, a tunable hybrid additive manufacturing process was developed by coupling induction heating and cold spray processes. Pure aluminum was used as the soft feedstock powder, while Ti-6Al-4V was used for the hard substrate. The effects of induction heating and of the substrate initial temperature on the deposition process were evaluated. Single-particle impact tests suggested metallic bond formation at the particle/substrate region as the bonding mechanism. Deposition efficiency, coating thickness and porosity levels were evaluated to characterize the induction heating and initial substrate temperature effects. The deposition efficiency and the coating thickness were doubled with the hybrid additive manufacturing process compared to the traditional cold spray process. Moreover, the presence of eddy currents in the coating (in situ induction heating of the coating) was confirmed by an electromagnetic analysis which modeled the interaction between the coating and the electromagnetic field. Findings demonstrated that substrate preheating has little influence on the overall deposition process and coating quality compared to in situ induction heating effects.

Dynamic Model of MR Dampers Based on a Hysteretic Magnetic Circuit

As a key to understand dynamic performances of MR dampers, a comprehensive dynamic magnetic circuit model is proposed in this work on the basis of Ampere’s and Gauss’s laws. It takes into account not only the magnetic saturation, which many existing studies have focused on, but also the magnetic hysteresis and eddy currents in a MR damper. The hysteresis of steel parts of MR dampers is described by Jiles‐Atherton (J‐A) models, and the eddy current is included based on the field separation. Compared with the FEM results, the proposed model is validated in low‐ and high‐frequency studies for the predictions of the magnetic saturation, the hysteresis, and the effect of eddy currents. A simple multiphysics model is developed to demonstrate how to combine the proposed magnetic circuit model with the commonly used Bingham fluid model. The damping force in the high‐frequency case obviously lags behind the coil current, which exhibits a hysteresis loop in the current‐force plane. The lag of damping force even exists in a low‐frequency varying magnetic field and becomes more severe in the presence of eddy currents.

The use of modified scaling factors in the design of high-power, non-linear, transmitting rod-core antennas

In this paper, we develop a technique for designing high-power, non-linear, transmitting rod-core antennas by using simple modified scale factors rather than running labor-intensive numerical models. By using modified scale factors, a designer can predict changes in magnetic moment, inductance, core series loss resistance, etc. We define modified scale factors as the case when all physical dimensions of the rod antenna are scaled by p, except for the cross-sectional area of the individual wires or strips that are used to construct the core. This allows one to make measurements on a scaled-down version of the rod antenna using the same core material that will be used in the final antenna design. The modified scale factors were derived from prolate spheroidal analytical expressions for a finite-length rod antenna and were verified with experimental results. The modified scaling factors can only be used if the magnetic flux densities within the two scaled cores are the same. With the magnetic flux density constant, the two scaled cores will operate with the same complex permeability, thus changing the non-linear problem to a quasi-linear problem. We also demonstrate that by holding the number of turns times the drive current constant, while changing the number of turns, the inductance and core series loss resistance change by the number of turns squared. Experimental measurements were made on rod cores made from varying diameters of black oxide, low carbon steel wires and different widths of Metglas foil. Furthermore, we demonstrate that the modified scale factors work even in the presence of eddy currents within the core material.

Improved half RF slice selectivity in the presence of eddy currents with out‐of‐slice saturation

Ultrashort echo time imaging with half RF pulse excitation is sensitive to eddy currents induced by the slice-select gradient that distorts the half pulse slice profile. This work demonstrates improvements in the half pulse profile by using spatial saturation on both sides of the imaged slice to suppress the out-of-slice magnetization. This effectively improves the selectivity of the half pulse excitation profile. A quadratic phase RF pulse with high bandwidth and selectivity was used to achieve a wide saturation band with sharp edges. Experimental results demonstrate substantially improved slice selectivity and R(2)* quantitation accuracy obtained with the out-of-slice saturation. This approach is effective in making short T(2) imaging and quantitation with half pulses less sensitive to eddy currents.

Axisymmetric eddy current problems using a special hybrid FE/BE method

This paper is a contribution to the open boundary axisymmetric quasi-static magnetic field problems in the presence of eddy currents. A hybrid method is developed combining the finite-element method inside an inner region with a special boundary-element method for the outer region based on an analytical treatment. Spherical coordinates become appropriate due to the consideration of spherical surfaces separating the inner and outer regions.

Reduced vector potential formulations for FI2TD schemes

Finite integration implicit time domain (FI2TD) schemes for transient magneto-quasistatic problems have so far been developed for a magnetic field description based on a modified vector potential formulation. A reduced vector potential formulation for these schemes is introduced, where the magnetic field is additively decomposed into a source part originating from the exciting coil currents, which thus can be modelled independently of the computational grid, and a part resulting from the presence of eddy currents due to electric conductivity and/or the nonlinear material behaviour. Numerical tests for 3-D transient problems show an improved convergence behaviour of the iterative CG-algorithm with respect to the new formulation.

An approach to the plasma magnetic contour identification in presence of eddy currents

The research in the thermonuclear fusion area has entered the design phase of ITER, the first experimental Tokamak reactor. In the ITER device, the plasma shape and position are controlled by an active system on the basis of a real-time estimate of the plasma magnetic boundary. This identification process must be prompt and accurate enough to satisfy the tight control requirements. In this paper, a brief discussion about the reconstruction of the plasma boundary by means of the equivalent (EC) method, in presence of conducting structures, will be presented. Some hints about a possible way to include the differential equations describing the vessel eddy currents will be illustrated.

Time evolution of tokamak plasmas in the presence of 3D conducting structures

This paper presents a method for the simulation of the time evolution of fusion plasmas in the presence of eddy currents induced in nonmagnetic 3D structures. The plasma is assumed to be 2D axisymmetric. The interaction between the plasma and the external currents is assumed to occur via the magnetic field averaged through the toroidal angle. The average poloidal field is computed by means of a 2D axisymmetric differential FEM formulation in terms of the poloidal magnetic flux. The 3D induced currents are calculated via an integral edge-element formulation in terms of a two-component current density vector potential. The method is also applied to derive a linearized plasma response model for the control of the plasma current position and shape in the presence of 3D conducting structures.

High speed diffusion imaging in the presence of eddy currents

This paper describes two schemes for fast diffusion imaging on a 1.5T MRI unit with unshielded gradients and 10-mT/m gradient system. The prepared longitudinal magnetization is sampled by a turbo fast low-angle shot (FLASH) sequence. The first scheme involves modifications to a diffusion-weighted driven-equilibrium preparation sequence that yields quantitative diffusion imaging. The second scheme consists of four 90 degree pulses that form the preparatory period with diffusion gradients applied between the first and last two 90 degrees pulses. Sufficient time is allowed between the diffusion gradients and the beginning of the echo readout to allow eddy currents to decay to an acceptable level. Phantom studies show agreement between diffusion coefficients in the literature with those determined using the above schemes. The potential application of these sequences in monitoring temperature is demonstrated in phantoms. Artifact-free quantitative diffusion-weighted images of the brain using the second scheme are presented.

An Analysis of the Effects of Eddy Currents on L-Band EPR Spectra

Recently, impressive results have been obtained with EPR studies of living animals and perfused organs using low-frequency EPR. In many instances in such studies, however, there are apparent distortions of the spectra. The shapes of these spectra and theoretical considerations indicate that these effects are due to eddy currents in the moderately conductive biological materials. Therefore the effects of eddy currents produced in biological samples under typical conditions being employed for in vivo EPR studies have been systematically studied in order to determine the extent of these effects and to develop methods for compensating for these effects. The presence of eddy currents was found to decrease the Q factor and distort the shapes of the EPR spectra. The distortion of the spectra led to linewidth broadening, changes in peak heights, and shifts of the apparent center of the lines. These effects could be corrected instrumentally, but this was effective only when signals have a high S/ N. The use of appropriate computer simulations, based on linear combinations of the expected absorption and dispersion signals, can effectively correct for the effects of eddy currents and provide accurate data on the parameters of EPR spectra that are needed for most or all of the analyses used in biological studies of animals and tissues at low frequencies.

A PFG NMR experiment for accurate diffusion and flow studies in the presence of eddy currents

A pulsed-field-gradient NMR experiment that permits accurate measurements of diffusion and flow rates in the presence of eddy currents is presented. The experiment employs short transverse evolution times and is thus especially useful for the study of systems exhibiting fast transverse relaxation or homonuclear scalar coupling. The insensitivity of the experiment to the presence of eddy currents is demonstrated with experiments on 5% polyacrylamide gels.

In vivo proton spectroscopy in presence of eddy currents

Spatially localized methods in spectroscopy often operate with magnetic field gradients for volume selection. The eddy currents induced by these gradients produce time-dependent shifts of the resonance frequency in the selected volume, which results in a distortion of the spectrum after Fourier transformation. In whole-body systems the complete compensation of eddy currents is a difficult procedure. To avoid this, a correction method is proposed for proton spectroscopy, which uses the signal of prominent water protons as a reference for the water-suppressed signal. The correction is performed in the time domain, dividing the water-suppressed signal by the phase factor of the water signal for each data point. The corrected spectra have a good resolution as shown by phantom measurements and brain and muscle spectra of volunteers.

An Eigenexpansion Technique for Modelling Plasma Start-Up

An algorithm has been developed and implemented in a computer program that allows the estimation of PF coil voltages required to start-up an axisymmetric plasma in a tokamak in the presence of eddy currents in toroidally continuous conducting structures. The algorithm makes use of an eigen-expansion technique to solve the lumped parameter circuit loop voltage equations associated with the PF coils and passive (conducting) structures. An example of start-up for CIT (Compact Ignition Tokamak) is included.

Dependence of the mechanical properties of steel 32G2 drill pipe of strength group D on chemical compsoition

The scale on the product is subjected to thermal decomposition by electromagnetic mixing of the melt in the presence of eddy currents formed from two oppositely directed electromagnetic fluxes. Thanks to high electrochemical activity, the productivity of the unit is twice as great as comparable models. The ferromagnetic properties of the induction coil make it possible to use its material as a magnetic circuit. The coil, made of magnet steel, is relatively lightweight and inexpensive and is very efficient.