Coil Resistance Research Articles

Development of a Novel Hybrid Protection System for Superconducting MRI Magnets

In order to reliably protect the superconducting MRI magnets as soon as possible, a hybrid protection system is developed. The protection system, which combines the both merits of active and passive protection schemes, includes a quench detection controller, a protection superconducting switch, a capacitor bank, a strip heaters network and a protection circuitry with back to back diode sets and shunting resistance coils. The quench detection controller is employed to detect the start of a quench and to subsequently switch on the capacitor bank to quench the protection superconducting switch. The shunting current in the coil subdivision flows through the shunting resistance coil also to quench the protection superconducting switch. The voltage across the protection superconducting switch energizes the strip heaters. Once a quench occurs in a magnet, the protection system immediately initiates all superconducting coils to quench. The hybrid protection system is successfully applied to a 1.5 T whole-body MRI magnet. This paper describes the details of the hybrid protection system and presents the quench simulation results.

Development of a simultaneous Hugoniot and temperature measurement for preheated-metal shock experiments: Melting temperatures of Ta at pressures of 100 GPa

Equations of state of metals are important issues in earth science and planetary science. A major limitation of them is the lack of experimental data for determining pressure-volume and temperature of shocked metal simultaneously. By measuring them in a single experiment, a major source of systematic error is eliminated in determining from which shock pressure release pressure originates. Hence, a non-contact fast optical method was developed and demonstrated to simultaneously measure a Hugoniot pressure-volume (P(H)-V(H)) point and interfacial temperature T(R) on the release of Hugoniot pressure (P(R)) for preheated metals up to 1000 K. Experimental details in our investigation are (i) a Ni-Cr resistance coil field placed around the metal specimen to generate a controllable and stable heating source, (ii) a fiber-optic probe with an optical lens coupling system and optical pyrometer with ns time resolution to carry out non-contact fast optical measurements for determining P(H)-V(H) and T(R). The shock response of preheated tantalum (Ta) at 773 K was investigated in our work. Measured data for shock velocity versus particle velocity at an initial state of room temperature was in agreement with previous shock compression results, while the measured shock data between 248 and 307 GPa initially heated to 773 K were below the Hugoniot evaluation from its off-Hugoniot states. Obtained interfacial temperatures on release of Hugoniot pressures (100-170 GPa) were in agreement with shock-melting points at initial ambient condition and ab initio calculations of melting curve. It indicates a good consistency for shock melting data of Ta at different initial temperatures. Our combined diagnostics for Hugoniot and temperature provides an important approach for studying EOS and the temperature effect of shocked metals. In particular, our measured melting temperatures of Ta address the current controversy about the difference by more than a factor of 2 between the melting temperatures measured under shock and those measured in a laser-heated diamond anvil cell at ∼100 GPa.

Thickness measurement of moving nonmagnetic metal foil by impedance variation

AbstractWhen an AC‐excited coil faces a nonmagnetic moving metal foil, an eddy current is induced in the conductor and the coil impedance is influenced by the speed and thickness. By examining the speed and thickness characteristics, a coil can be used as the thickness sensor. In this paper, we derive rigid theoretical formulas for the magnetic field and coil impedance of a circular coil facing a stationary or moving foil conductor. We also present several characteristics with regard to the coil inductance and resistance based on calculations. The good agreement between the theoretical and measured values demonstrates the validity of the method. We found that the speed has no influence at a thickness in the tens of micrometers, and that the upper limit of the exciting frequency is 1 kHz. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 180(1): 15–23, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21269

Classical and lateral skin effect contributions estimation in strip MR coils

AbstractRadiofrequency (RF) transmitter and receiver coils in magnetic resonance imaging systems are used, respectively, to excite the nuclei in the object to be imaged and to pick up the signals emitted by the nuclei. The quality of obtained images strongly depends upon the correct choice of the coils geometry and typology. For low‐field magnetic resonance imaging systems, the dominant power dissipation mechanisms are mainly due to conductor losses and the coils performance are affected by cross‐sectional shape of RF coil conductors. This work proposes a method for the estimation of the different skin effects contribution to the total conductor resistance for coil built using strip conductor. The evaluation of coil resistance was performed at different tuning frequencies usually used in clinical scanner (21–128 MHz). © 2012 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 41B: 57–61, 2012

A new eddy current displacement measuring instrument independent of sample electromagnetic properties

Eddy current sensors, as one of nondestructive testing and evaluation instruments, have been widely used in displacement measurement as well as defect characterization. However, in eddy current displacement measurement, the test not only depends on the displacement between the coil and samples, but also relates to the electrical and magnetic properties (such as electrical conductivity and magnetic permeability) of samples. So, the eddy current sensor is limited to test one sample after one calibration. In order to make eddy current sensors compatible with more samples after one calibration without testing error, this paper designed a new eddy current displacement instrument extended from the previous theoretical analysis and experiment study investigating the relationship of the coil resistance and inductance. A series of experiments indicate that the designed new eddy current displacement instrument can eliminate the influence of the samples' electromagnetic properties effectively.

Genetic neural-based modeling of AC resistance of heating coil used for high-frequency inverter-fed induction cooker

The present paper deals with modeling of AC resistance of twisted litz wires used for high-frequency inverter-fed induction cooker. Several traditional approaches are available, most of which have concentrated in deriving the analytical relationships between the AC resistances with the parameters of the wire. However, it is very difficult to get the exact relationship, due to several reasons. An attempt is made in this paper to model the AC resistance using a three-layered feed-forward Neural Network. For this purpose, four inputs (wire type, number of strand, number of spiral turn and operating frequency) and one output as AC resistance have been considered. Since the performance of Neural Network alone might not be optimal; it is optimized using a binary-coded Genetic Algorithm. Performances of the proposed approach were compared with the method of AC resistance computation proposed by Ferreira. Genetic-neural system has given a very close accuracy, and the computational complexity was found to be very low. Thus, it is suitable for online implementations.

Sensitivity enhancement of remotely coupled NMR detectors using wirelessly powered parametric amplification

A completely wireless detection coil with an integrated parametric amplifier has been constructed to provide local amplification and transmission of MR signals. The sample coil is one element of a parametric amplifier using a zero-bias diode that mixes the weak MR signal with a strong pump signal that is obtained from an inductively coupled external loop. The NMR sample coil develops current gain via reduction in the effective coil resistance. Higher gain can be obtained by adjusting the level of the pumping power closer to the oscillation threshold, but the gain is ultimately constrained by the bandwidth requirement of MRI experiments. A feasibility study here shows that on a NaCl/D(2) O phantom, (23) Na signals with 20 dB of gain can be readily obtained with a concomitant bandwidth of 144 kHz. This gain is high enough that the integrated coil with parametric amplifier, which is coupled inductively to external loops, can provide sensitivity approaching that of direct wire connection.

Hyperpolarized 13C MRS Cardiac Metabolism Studies in Pigs: Comparison Between Surface and Volume Radiofrequency Coils

Cardiac metabolism assessment with hyperpolarized 13C magnetic resonance spectroscopy in pig models requires the design of dedicated coils capable of providing large field of view with high signal-to-noise ratio (SNR) data. This work presents a comparison between a commercial 13C quadrature birdcage coil and a homebuilt 13C circular coil both designed for hyperpolarized studies of pig heart with a clinical 3T scanner. In particular, the simulation of the two coils is described by developing an SNR model for coil performance prediction and comparison. While coil resistances were calculated from Ohm’s law, the magnetic field patterns and sample-induced resistances were calculated using a numerical finite-difference time-domain algorithm. After the numerical simulation of both coils, the results are presented as SNR-versus-depth profiles using experimental SNR extracted from the [1-13C]acetate phantom chemical shift image and with a comparison of metabolic maps acquired by hyperpolarized [1-13C]pyruvate injected in a pig. The accuracy of the developed SNR models was demonstrated by good agreement between the theoretical and experimental coil SNR-versus-depth profiles.

Simulation of chain of quenches on toroidal HTS-SMES taking account of thermal and electromagnetic characteristics

Abstract When a temperature rise occurs at a local area inside a coil of toroidal HTS-SMES by any reason, a temperature hotspot which results in a thermal runaway appears at the local area. Subsequently, after appearing the local normal zone in the HTS coil, the transport current of the HTS coil decrease since the resistance of HTS coil appears and the current partially flows into a parallel-connecting shunt resistance. However, if the transport current of the normal-transitioned HTS coil is hardly changed, the temperature on the hotspot would rise more and then the normal zone would spread rapidly. It may cause a serious accident due to high stored energy. Therefore, using the numerical simulation, we have investigated the behaviors of the coil current, the critical current, and the temperature in the superconducting element coils of HTS-SMES. Consequently, the temperature of the superconducting element coils rises up extremely when a large heat is generated at a certain area of one of them by any reason. Moreover, there is a possibility that the shunt resister hardly functions for protection since the coil is burned out due to high inductances and low resistance of the superconducting element coil.

A high-resolution NMR probe in which the coil and preamplifier are cooled with liquid helium

In a well designed NMR spectrometer, the noise originates predominantly from the resistance of the receiver coil. Significant improvements in sensitivity can be achieved by cooling the coil to cryogenic temperatures, provided that a preamplifier can be designed to match the coil’s performance. A probe is described in which the coil and preamplifier are cooled with liquid helium, but the sample is maintained at room temperature. Carbon-l3 spectra at 45 MHz demonstrate improved sensitivity over conventional probes at the same field.

Demagnetization of a Bi-2223 high-temperature superconducting coil in RT-1 through spontaneous temperature rise

The Ring Trap 1 (RT-1) device produces a magnetospheric configuration for the confinement of a high- β plasma with a Bi-2223 high-temperature superconducting magnet. Here we report the results of emergency demagnetization of the superconducting coil, where we could not connect current leads, temperature measurement connectors, and connectors for a persistent-current switch (PCS) heater to the coil. The spontaneous warming of the coil caused a rise in the flux-flow resistance of the superconducting coil, and the persistent current slowly decreased as coil resistance increased. Approximately 98% of the total stored magnetic energy was safely released before the quenching of the PCS, and there was no substantial damage to the superconducting coil.

Passive Electronic Componets Parameters Searching with Differential Evolutionary Algorithm

This paper describes computing of inductor parameters as passive electronic component with algorithm of differential evolution. Computing of parameters is based on substitute model of inductor which is described with three basic parameters such as resistance, inductance and capacitance. Measured frequency response of complex impedance of the real inductor is input parameter to the iterative loop driven by the differential evolution algorithm. There are three output parameters of the algorithm, where resistance value is considered to be a combination of DC coil resistance and skin effect influence, second value describes inductance of a real inductor and third parameter defines interturn parasitic capacitance of a real inductor.

Design of Pickup Coil Made of Litz Wire and Cooled at 77 K for High Sensitive Measurement of AC Magnetic Fields

We studied the performance of a copper pickup coil for high sensitive measurements of ac magnetic fields. The coil was made of Litz wire to prevent the skin effect, and was cooled at 77 K to reduce thermal noise. We first obtained an analytical expression for the coil resistance by analyzing the eddy current loss of the coil. The analytical result explains well the measured dependences of the coil resistance on the frequency, number of turns of the coil (N), and filament diameter of the Litz wire (df). Next, we show the design of the pickup coil. When appropriate values are selected for N and df, we can expect the magnetic field noise SB1/2 < 10 fT/Hz1/2 for f > 1 kHz and SB1/2 < 2 fT/Hz1/2 for f > 10 kHz for the pickup coil with diameter of 50 mm. The experimental results agreed with the designed values.

Development of the Energy Simulator for the Water Hydraulic System Under Flow Condition Changes

The new energy simulator we developed is based on a hydraulic servosystem dynamic flow model introducing flow coefficients determined by Reynolds number. One is the pipe flow coefficient flow determined by the Moody diagram and the other is the servovalve flow coefficient based on flow model experiments. The motor dynamic model is introduced to determine efficiency such as coil resistance or rotor viscosity loss. Leakage of the hydraulic servovalve was also determined by the leak model. The feasibility of the proposed simulator was verified using computer simulation and experiments, showing differences from conventional simulators that depend on manually set parameters such as flow coefficients. Simulation and experiment results agreed well, and the proposed simulator determines hydraulic servosystem energy consumption. New simulator concepts, calculation models, and experiment results are also discussed.

Multilayer high-aspect-ratio RF coil for NMR applications

Microcoils offer a high degree of mass sensitivity and high magnetic field gradient strength in magnetic resonance microscopy applications. This paper presents a novel multilayer high-aspect-ratio metal fabrication process that can be used to fabricate a nanoliter-volume radio frequency (RF) saddle coil with an embedded flow-through fluidic channel for nuclear magnetic resonance (NMR) applications. The fabrication process is based on repeated electroplating processes and structure release processes. The achieved aspect ratio of the developed RF saddle coil is 4 with a structure line width of 25 μm. The resistance of the RF coil and the 1H spectrum line width have been measured and are found to be 0.7 Ω and 350 Hz, respectively. Our results indicate that this novel fabrication process for RF microcoils is feasible for NMR applications.

Performance of HTS SQUID using resonant coupling of cooled Cu pickup coil

We designed and tested an HTS SQUID magnetometer using resonant coupling of a copper pickup coil cooled at T=77K. The pickup coil was made of a twisted multi-filamentary wire, the so-called Litz wire, to prevent the increase in coil resistance at high frequencies. First, we measured the coil characteristics in the frequency range from DC to 100kHz when the diameter df of the elementary filament of the Litz wire was varied between 0.04 and 0.2mm. We showed that the coil characteristics at high frequencies can be improved when the diameter df is reduced. Next, we constructed a magnetometer using a pickup coil with an average diameter D=45mm and number of turns N=150. The measured magnetic field noise of the magnetometer was 3.3fT/Hz1/2 at a resonant frequency of fr=10.15kHz. The Q value of the resonant circuit was Q=153. The experimental results agreed well with the designed value. The obtained high sensitivity of the magnetometer is expected to be useful for its application to low-field NMR.

Design and Construction of a Rogowski Coil for Measuring Wide Pulsed Current

In this paper, the design, installation, and performance of a Rogowski coil are presented. Based on the lumped-element model of Rogowski coil, the low-frequency distortion of wide pulsed current measurement is studied. The optimal damping resistance of the external-integrating Rogowski coil is obtained under the sinusoidal steady-state excitation. The amplitude-frequency characteristics of measuring systems (MSs) based on Rogowski coils with an RC integrator and an active integrator, respectively, are compared, and the actual bandwidths of the two MSs are also determined by the measurement accuracy. As a result of using an active integrator, the measuring bandwidth is broadened and the transient performance of the MS is improved compared with that of the MS with an RC integrator. Some measures of resisting electromagnetic interference caused by the Marx generator and spark gap switch are described. In the end, the experimental results of the MS based on the Rogowski coil in SG-III are presented.

Induction Magnetometer with a Metal Shielded Pickup Coil for MHz Range Operation

This paper presents an induction magnetometer with a metal shielded pickup coil for MHz range operation. The validity of two simple equivalent circuit models for voltage detection and current detection is also confirmed by experimental results. From the measured frequency response for a calibrated magnetic field of 10 nT, we successfully demonstrate the ability of the magnetometer. The experimental results reveal the quantitative data of the equivalent resistance and inductance. When the resistance of the pickup coil is quite low, the equivalent resistance can be regarded as the resistance of the BNC connector (50 Ω). With the equivalent resistance, the absolute error of the output voltage between measurement and estimation is less than ±1 dBμV between 0.3 MHz and 2 MHz. In contrast, it is found that the existence of the metal shielded layer affects the estimated equivalent inductance. If the pickup coil has no metal shielded layer, the estimated value of the equivalent inductance is 56.3 times the inductance of the pickup coil. This value is verified by the estimated and measured results. However, this phenomenon is not found if the pickup coil has the metal shielded layer.

Selection of Power Semiconductor Switches – a Tool to Reduce Switching &amp; Conduction Losses of High Frequency Hybrid Resonant Inverter fed Induction Cooker

An exhaustive method for the selection of different power semiconductor switches for high frequency hybrid resonant inverter fed induction heater is presented here. Heating coil of the induction heater is made of litz wire which minimizes the skin effect and proximity effect at high operating frequency. With the calculated optimum values of inductance and resistance of heating coil at a particular operating frequency, high frequency hybrid resonant inverter topology has been simulated using PSPICE software as well as constructing the experimental model of same rating. Obtained the voltage waveforms across heating coil and current waveforms through it, have been taken for further analysis. From this analysis, selection of suitable power semiconductor switches like IGBT, GTO and MOSFET are made. Waveforms have been shown to justify the feasibility for real implementation of high frequency hybrid resonant inverter fed induction heater in domestic applications.

Performance Improvement of a Coil Launcher

In order to improve the performance of a coil launcher, various cores and shells are considered in the structure design in this paper. Different geometry shapes and materials are loaded on the cores and the shells. A single-stage coaxial induction coil launcher is analyzed for simplicity based on Ansoft 2-D simulations. The conclusion can be summarized as follows. A magnetic core will improve the performance of a coil launcher greatly. Coil current and resistance loss of the system will be reduced while kinetic energy conversion rate and efficiency will be increased. A conductive core will reduce the performance because of eddy-current effect. A magnetic conductive core combines magnetization and eddy-current effect. Geometry of a core will also influence the performance of a coil launcher. In general, the greater the cross-sectional area and the length is, the better the performance. A shell takes little effect on the performance of a coil launcher, no matter what material and geometry is used. A magnetic nonconductive core with large cross-sectional area and enough length is suggested to be used in a coil launcher design. A detailed study will be presented in this paper.