Coil Conductor Research Articles

Optimal design of spiral coil electromagnetic acoustic transducers considering lift-off sensitivity operating on non-ferromagnetic media

The generation efficiency of electromagnetic acoustic transducers (EMATs) dramatically reduces with increasing lift-off, thus restricting their applications. This paper aims to provide methods to decrease lift-off sensitivity. The process of generating a shear wave in a spiral coil EMAT is established based on a 2-D axisymmetric model using the finite element method. An equivalent excitation circuit, including an impedance-matching network and a limited power source, is presented for calculating the excitation current. Such an equivalent circuit has not been given adequate attention in previous publications in this field, but it has proven to be of great importance in determining the efficiency of ultrasonic generation. The predictions of static magnetic field, eddy current density and excitation current are validated by experimental results. The conversion efficiency and lift-off sensitivity of the EMAT are analysed with respect to impedance matching parameters, magnet dimensions, the presence of a copper backplate between the magnet and the coil, and backplate-to-coil distance. The results indicate that impedance matching parameters, magnet diameter and plate-to-coil distance have obvious effects on lift-off sensitivity. The backplate can be used to decrease lift-off sensitivity, and it can rearrange the source current density across the cross-sectional area of the coil conductor. Furthermore, compared with the original EMAT, the lift-off sensitivity of the optimised EMAT can be decreased by 7.9 dB at 2 mm lift-off.

An improved asymmetric gradient coil design for high-resolution MRI head imaging

For head magnetic resonance imaging, local gradient coils are often used to achieve high solution images. To accommodate the human head and shoulder, the head gradient coils are usually designed in an asymmetric configuration, allowing the region-of-uniformity (ROU) close to the coil’s patient end. However, the asymmetric configuration leads to technical difficulties in maintaining a high gradient performance for the insertable head coil with very limited space. In this work, we present a practical design configuration of an asymmetric insertable gradient head coil offering an improved performance. In the proposed design, at the patient end, the primary and secondary coils are connected using an additional radial surface, thus allowing the coil conductors distributed on the flange to ensure an improvement in the coil performance. At the service end, the primary and shielding coils are not connected, to permit access to shim trays, cooling system piping, cabling, and so on. The new designs are compared with conventional coil configurations and the simulation results show that, with a similar field quality in the ROU, the proposed coil pattern has improved construction characteristics (open service end, well-distributed wire pattern) and offers a better coil performance (lower inductance, higher efficiency, etc) than conventional head coil configurations.

Bending properties of different REBCO coated conductor tapes and Roebel cables at T = 77 K

Application of REBCO coated conductors in coils or cables involves deformation of the conductor in different modes, such as in-plane bending, out-of-plane bending and torsion. For example, the dipole magnet designs in the EuCARD-2 project require bending radii as low as 7.5 mm, inducing significant bending strain in the REBCO layer. In this paper, we investigate the effect of out-of-plane bending on the current-carrying properties of coated conductors from different manufacturers. The samples are manipulated by means of a Goldacker-type bending rig, which allows continuous bending at T = 77 K. By reversal to after each bending step, the reversible strain effect is separated from irreversible degradation. All tested conductors are found to tolerate compressive bending to a radius of 6 mm with less than 5% irreversible degradation of the critical current. The magnitude of the reversible strain effect shows a large variation among the samples. The effect of out-of-plane bending on Roebel cables is investigated as well, and the results are compared to the bending characteristic of single conductors. The results show no detrimental effect of the cable assembly on the bending properties within the constraints of the test.

Finite element method‐based approach for radiofrequency magnetic resonance coil losses estimation

AbstractThe simulation and the design of radiofrequency (RF) coils are fundamental tasks to maximize Signal‐to‐Noise Ratio (SNR) in Magnetic Resonance (MR) applications. The estimation of coil resistance, that is, the losses within the coil conductors, which depends on tuning frequency, allows the prediction of coil performance and data SNR. At RF, the conductor resistance is increased due to the skin effect, which distributes the current primarily near the conductor surface instead of uniformly over the cross section. Moreover, the radiative losses estimation as a function of tuning frequency permits a total coil performance characterization, especially for high‐frequency tuned coils when this loss mechanism could be the dominant one. In this work we compared Finite Element Method (FEM) simulations with analytical calculations performed in wire loop RF coils for MR applications. Our results showed that FEM can predict the losses within the coil conductors at 5.7 MHz with a relative difference of <3% compared to analytical calculation, while the relative difference increased to 58% at 127.8 MHz. Concerning the radiative losses, the relative difference between analytical formulation and FEM was lower than 3% at 5.7 MHz, and increasing to 44% at 127.8 MHz. Experimental measurements on a circular coil prototype were also performed at 85.2 MHz and 127.8 MHz, showing a better agreement with FEM simulations than with analytical calculations.

Thermal pattern reconstruction of surface condition on freeform-surface using eddy current pulsed thermography

Measuring and evaluating surface conditions of a freeform surface are difficult means of active thermography due to the varied curvatures. In this paper, we propose a method for obtaining the thermal pattern of the entire freeform surface of a propeller using eddy current pulsed thermography (ECPT). To understand the electromagnetic field and heating distributions on propeller, the induction fields of two induction coil types (line conductor and Helmholtz coil), one in air space and the other with a propeller placed are simulated and analysed through multiphysics-based models. It shows that the relatively uniform electromagnetic field induced between Helmholtz coil pair in air space becomes non-uniform with a propeller existing, as it is distorted by propeller freeform surface. Therefore, it is difficult to obtain precise thermal pattern to profile the surface condition. To overcome these challenges, segment heating is applied using a line conductor type induction coil and the thermal image stitching is proposed for showing the entire thermal pattern on propeller surface. Both finite element simulation and experimental study are implemented. The results show that more clearly thermal distributions on propeller freeform surface can be obtained using the proposed method.

Quasi-Equilibrium Model for Separating the Plasma and the Conductor in the Galatea Device

In order to study the plasma distribution around the conductor coil in Galatea device, a 1-D plasma dynamic model of the linear topological Galatea device is established in this paper, and the static equilibrium analytical solution of this model is derived. The analytical results show that in the vicinity of the conductor coil, the closer to the conductor coil, the greater the plasma pressure is, which causes collision losses due to the plasma concentration on the surface of conductor coil. Therefore, absolute static balance does not exist near the conductor coil. Based on this fact, this paper introduces a quasi-equilibrium model with the plasma separated from the conductor coil by changing its carrying current form. The proposed method is verified by a simulation model established in COMSOL, and the simulation results show that the plasma can be separated from the conductor coil when an increasing current passes through the conductor coil. At the end of this paper, the effects on the separation efficiency of time-varying current with different increasing rate are also given. The duration of the quasi-equilibrium is also analyzed.

Selection of parameters of the ferrite element for pulse UWB receiving antenna

Thetheoretical model of pulse ultrawideband (UWB) receiving antenna consisting of a ferrite element surrounded by a coil of conductor is investigated. The magnetodielectric cylinder of infinite length is selected as the ferrite element. The excitationUWBsignal represents a pulse of electromagnetic field with the envelope in the form of a Gaussian function with high-frequency filling signal. The peculiarities of amplitude-time dependence have been considered for the pulse of induction electromotive force occurring in the closed conducting loop, which surrounds the cylinder, under the assumption of absence of its influence on the scattered fields inside and outside the cylinder. The connection between the electromagnetic parameters of the ferrite element and the frequency-time parameters of the excitation pulse electromagnetic field, which are optimal from the point of view of effective undistorted signal reception, has been determined.

A multidisciplinary design approach for electromagnetic brakes

The design of an electromagnetic brake requires a multidisciplinary approach. Its performance is a field of interest having implications for mechanical, electrical, magnetic and material engineering. In this study, a comprehensive design procedure for the electromagnetic brake is presented. Simple and effective mechanical and thermal models are derived analytically and numerically. The analytical model is developed to optimize the coil turns, conductor radius and airgap length with the combination of input current and voltage. Candidate magnetic materials are discussed in terms of minimizing the cost while meeting electromagnetic performance characteristics. Simulation and experimental tests are conducted on a prototype of brake to verify the design. Good agreement between the test results and the prototype is achieved. Some differences are found and their root causes are investigated. Symptoms about parallelism in brake can be directly understood from the current waveform which enables to measure tolerances and to identify the problems of the brake at an early phase.

A modified circuit topology for inductive pulsed power supply based on HTSPPTs

High temperature superconducting pulsed power transformer (HTSPPT) provides an efficient method for inductive energy storage and current multiplication. The primary inductor of HTSPPT used for energy storage is made of high temperature superconducting coils, and the secondary inductor used for current pulse generation is made of normal conductor coils. In the initial circuit, the secondary inductor generates current pulse by switching out the coupled primary superconducting inductor. However, during the switching period, the leakage flux caused by imperfect coupling and the sudden change in primary current induce a voltage across the opening switch which exceeds the affordability of modern solid-state switches. In previous studies, a half-cycle oscillatory discharge circuit is proposed to mitigate these problems by using a capacitor to recapture the energy in the leakage flux and to slow down the turnoff of current in the primary. However, there are still some problems should be settled. For example, the output pulse cannot be adjusted, the residual energy cannot be recovered and the capacitor branch circuit may have an impact on the charging process. In the paper, a modified discharge circuit topology is introduced to solve these problems. A multi-module system comprising of several HTSPPTs charging in series connection and discharging in parallel is also designed and simulated. This system can be used to power an electromagnetic emission device.

Influence of conductor eccentricity on Faraday phase shift errors of portable fiber-optic current transformer

A portable fiber-optic current transformer (P-FOCT) with flexible sensing coil was proposed in this paper. The model has been established to describe the Faraday phase shift errors, which are influenced by the non-closed error of flexible sensing coil and eccentric position of current conductor. It has been obtained that the Faraday phase shift relative error is mainly influenced by the non-closed angle of sensing coil and can be decreased by increasing number of turns, the matching installation region of current conductor was found, the theoretical model was verified by the finite element method (FEM). It provides an important reference for the installation of P-FOCT to satisfy the accuracy requirement.

Modeling of screening currents in coated conductor magnets containing up to 40000 turns

Screening currents caused by varying magnetic fields degrade the homogeneity and stability of the magnetic fields created by REBCO coated conductor coils. They are responsible for the AC loss; which is also important for other power applications containing windings, such as transformers, motors and generators. Since real magnets contain coils exceeding 10000 turns, accurate modeling tools for this number of turns or above are necessary for magnet design. This article presents a fast numerical method to model coils with no loss of accuracy. We model a 10400-turn coil for its real number of turns and coils of up to 40000 turns with continuous approximation, which introduces negligible errors. The screening currents, the screening current induced field (SCIF) and the AC loss is analyzed in detail. The SCIF is at a maximum at the remnant state with a considerably large value. The instantaneous AC loss for an anisotropic magnetic-field dependent Jc is qualitatively different than for a constant Jc, although the loss per cycle is similar. Saturation of the magnetization currents at the end pancakes causes the maximum AC loss at the first ramp to increase with Jc. The presented modeling tool can accurately calculate the SCIF and AC loss in practical computing times for coils with any number of turns used in real windings, enabling parameter optimization.

Mathematical model to determine the dimensions of superconducting cylindrical coils with a given central field – the case study for MgB2 conductors with isotropic Ic(B) characteristic

In this work, we present a mathematical model which enables to design cylindrical coils with a given central field, made of the superconducting conductor with isotropic Ic(B) characteristic. The model results in a computer code that enables to find out the coil dimensions, and to calculate the coil parameters such as critical current, maximum field in the winding and field non-uniformity on the coil axis. The Ic(B) characteristic of the conductor is represented by the set of data measured in discrete points. This approach allows us to express the Ic(B) as a function linearized in parts. Then, it is possible to involve the central field of the coil, coil dimensions, and parameters of the conductor, including its Ic(B) characteristic, in one equation which can be solved using ordinary numerical non-linear methods. Since the coil dimensions and conductor parameters are mutually linked in one equation with respect to a given coil central field, it is possible to analyze an influence of one parameter on the other one. The model was applied to three commercially available MgB2/Ni/Cu conductors produced by Columbus Superconductors. The results of simulations with the Ic(B) data at 20K illustrate that there exists a set of winding geometries that generate a required central field, changing from a disc shape to long thin solenoid. Further, we analyze how the thickness of stabilizing copper influences the coil dimensions, overall conductor length, coil critical current, maximum field in the winding. An influence of the safety coefficient in operating current on coil dimensions and other above mentioned parameters is studied as well. Finally, we compare the coil dimensions, overall conductor length as well as coil critical current and maximum field in the winding if the value of required central field changes between 1 and 3 T.

Modelization of the thermal coupling between the ITER TF coil conductor and the structure cooling circuit

The ITER Toroidal Field (TF) coils are required not to quench during the most demanding event: a plasma disruption followed by a fast discharge of the Central Solenoid (CS), the Poloidal Field (PF) coils and the Correction Coils (CC). This event creates large heat deposition in the ITER magnet stainless steel structures in addition to the conductor AC losses. In order to prevent quench occurring in the TF conductor, cooling channels, implemented in the TF coil structure (TFCS), have to remove a large fraction of the heat deposited. The first integrated TF and structure mock-up has been manufactured and then tested in the HELIOS cryogenic test facility (CEA Grenoble) to determine the thermal coupling between the TFCS and the TF conductor, both actively cooled by supercritical helium at 4.4K and 5bar. It consists in a stainless steel casing, a cooling pipe glued with resin in the casing groove, winding pack (WP) ground insulation, a radial plate and a copper dummy cable-in-conduit-conductor (CICC). Steady state as well as transient thermal characterizations have been completed in May 2015. Simulation results by thermal hydraulic codes (VENECIA/SuperMagnet) and some of the experimental data are presented and discussed. The thermal coupling between the helium in the cooling tube and the TF coil structure is then modelled as an equivalent heat transfer coefficient in order to simplify the thermal hydraulic (TH) models. Comparison between simplified coupling and detailed coupling is presented.

Electric-Field-Assisted Extraction of Garlic Polysaccharides via Experimental Transformer Device

Using the mutual conversion between electric energy and magnetic energy, we designed and built an experimental device based on the o-core transformer structure for extraction of garlic polysaccharides. That is, an acidic solvent acted as the secondary coil conductor under the action of alternating magnetic flux and then induced an electric field in the biochemical medium (the sample and extractant) at 20–60 kHz. The polysaccharide yield improved, in comparison to the conventional heating method, when the excitation voltage applied on the primary coil was increased. Low pH helps to improve the polysaccharide yield due to the enhancement of the ionic conduction resulting from the induced electric field. However, the polysaccharide yield decreases with the increase of partial frequency due to the resulting decrease in the input power of the system. The orthogonal experiments showed that the excitation voltage, frequency, pH, and temperature all significantly affected the polysaccharide yield. The garlic polysaccharide extracted by using the induced electric field method had a DPPH radical scavenging ability and a ferric-reducing power. This novel system is a potential alternative for the extraction of garlic polysaccharides without the use of powered electrodes inserted into solutions, unlike other electric-field-assisted techniques.

Least squares reconstruction of non-linear RF phase encoded MR data

PurposeThe numerical feasibility of reconstructing MRI signals generated by RF coils that produce B1 fields with a non-linearly varying spatial phase is explored. TheoryA global linear spatial phase variation of B1 is difficult to produce from current confined to RF coils. Here we use regularized least squares inversion, in place of the usual Fourier transform, to reconstruct signals generated in B1 fields with non-linear phase variation. MethodsRF encoded signals were simulated for three RF coil configurations: ideal linear, parallel conductors and, circular coil pairs. The simulated signals were reconstructed by Fourier transform and by regularized least squares. ResultsThe Fourier reconstruction of simulated RF encoded signals from the parallel conductor coil set showed minor distortions over the reconstruction of signals from the ideal linear coil set but the Fourier reconstruction of signals from the circular coil set produced severe geometric distortion. Least squares inversion in all cases produced reconstruction errors comparable to the Fourier reconstruction of the simulated signal from the ideal linear coil set. ConclusionMRI signals encoded in B1 fields with non-linearly varying spatial phase may be accurately reconstructed using regularized least squares thus pointing the way to the use of simple RF coil designs for RF encoded MRI.

Comparison of three formulations for eddy-current problems in a spiral coil electromagnetic acoustic transducer

Three differential equations based on different definitions of current density are compared. Formulation I is based on an incomplete equation for total current density (TCD). Formulations II and III are based on incomplete and complete equations for source current density (SCD), respectively. Using the weak form of finite element method (FEM), three formulations were applied in a spiral coil electromagnetic acoustic transducer (EMAT) example to solve magnetic vector potential (MVP). The input impedances calculated by Formulation III are in excellent agreement with the experimental measurements. Results show that the errors for Formulations I & II vary with coil diameter, coil spacing, lift-off distance and external excitation frequency, for the existence of eddy-current and skin & proximity effects. And the current distribution across the coil conductor also follows the same trend. It is better to choose Formulation I instead of Formulation III to solve MVP when the coil diameter is less than twice the skin depth for Formulation I is a low cost and high efficiency calculation method.

New Progress of Nb3Sn Strand Production for ITER in WST

ITER toroidal field (TF) systems consist of 18 independent coils that are around the torus, whose primary function is to confine the plasma particles. The TF coil conductor is a cable-in-conduit conductor (CICC) made up of superconducting, Nb 3 Sn-based strands mixed with pure copper strands. As the only supplier in China, Western Superconducting Technologies Company, Ltd. (WST) will supply TF Nb 3 Sn strands using internal tin route for ITER, and over 6,000 km of Nb 3 Sn strands have been produced in the past four years. Main performance of Nb 3 Sn strands, including critical current, n value, wire diameter, Cu/non-Cu ratio, hysteresis loss and RRR are reported and analyzed in this paper.

Analysis of -Parameters in Magnetic Resonance WPT Using Superconducting Coils

After Prof. Marin Soljacic of Massachusetts Institute of Technology proposed the magnetic resonance wireless power transfer (WPT) in 2007, magnetic-resonance-type WPT has been tried to be applied to various charging equipment. The need for research of magnetic resonance WPT technology is increasing owing to the expectation on its application to electric vehicle and electric railway. In this paper, we used the superconducting coil in order to improve the transmission efficiency of the magnetic resonance WPT system. We found that the superconducting coil indicated higher transmission efficiency and $Q$ -factor bigger than that of the normal conductor coil through the analysis of the $S$ -parameters between the superconductor and normal conductor coils. If this research will be continued, it is expected to be applicable to high-power transmission systems.

Analysis of transmission efficiency of the superconducting resonance coil according the materials of cooling system

The wireless power transfer (WPT) system using a magnetic resonance was based on magnetic resonance coupling of the transmission and the receiver coils. In these system, it is important to maintain a high quality-factor (Q- factor) to increase the transmission efficiency of WPT system. Our research team used a superconducting coil to increase the Q-factor of the magnetic resonance coil in WPT system. When the superconductor is applied in these system, we confirmed that transmission efficiency of WPT system was higher than normal conductor coil through a preceding study. The efficiency of the transmission and the receiver coil is affected by the magnetic shielding effect of materials around the coils. The magnetic shielding effect is dependent on the type, thickness, frequency, distance, shape of materials. Therefore, it is necessary to study the WPT system on the basis of these conditions. In this paper, the magnetic shield properties of the cooling system were analyzed using the High-Frequency Structure Simulation (HFSS, Ansys) program. We have used the shielding materials such as plastic, aluminum and iron, etc. As a result, when we applied the fiber reinforced polymer (FRP), the transmission efficiency of WPT was not affected because electromagnetic waves went through the FRP. On the other hand, in case of a iron and aluminum, transmission efficiency was decreased because of their electromagnetic shielding effect. Based on these results, the research to improve the transmission efficiency and reliability of WPT system is continuously necessary.

Developments of non-destructive test method of jacket section for ITER Poloidal Field (PF) coils

The ITER magnet system is one of the largest and most complex superconducting installations ever undertaken. Therefore, all subcomponents have very demanding requirements, including the conductors which are of cable-in-conduit conductor type. Based on the structural analysis and fatigue life requirements of the coil systems, the maximum acceptable defect sizes inside the jacket material were defined. This paper presents the nondestructive test (NDT) method developed for the poloidal field (PF) coil conductor jackets made of 316L stainless steel. The difficulty in the development of the PF jacket NDT arises from the circular-in-square shape, i.e., extruded bars of a square cross section with an inner circular bore. Furthermore, in addition to volumetric defects, the inspection procedure has to detect planar defects oriented longitudinal (i.e., along the jacket) and transversal on the surfaces and inside the jacket material. After a series of experiments and optimization, the phased array ultrasonic test (PAUT) combined with eddy current testing (ECT) was developed for the PF jacket inspection. The outer surface and volume are inspected by PAUT consisting of 11 separate runs per jacket surface and using four different probes. The inner bore surface up to a depth of 2–3 mm is inspected by multifrequency ECT. The characteristics and limits of all inspections runs are discussed, particularly for the most difficult one with detecting transversal planar defects.