System for coil simulation measurements of losses and instabilities in superconducting wires

A new system for measuring ac losses in superconductors under coil simulation conditions is described. It is aimed to investigate instabilities in local points of a coil. Currents up to 150 A and magnetic fields up to 0.4 T can be applied simultaneously and can be varied independently. The magnetic and electric fields on both sides of the sample are measured by means of pick-up coilds and potential leads, respectively. By this method the instantaneous values of the power introduced through each of both sample surfaces can be measured separately. The validity of the method and the performance of the system are demonstrated by measuring the losses of a copper-clad single-core Nb---Ti---Zr wire for several values of the ratio between the current and the magnetic field, which are inphase.     

Low magnetic field measurement with a reversed-fieldelectron-spin-resonance magnetometer

An effective method of measuring low magnetic fields is presented. In this method, the magnetic field direction is reversed periodically so that the effects of external magnetic fields, such as the earth's magnetism, are removed. The usefulness of the method is investigated by using a split coil magnetic field source and an electron-spin-resonance magnetometer. A magnetic field of 10 mT can be measured with an uncertainty of less than 0.2 μT, i.e., 20 ppm     

On the Measurement of Tooth-Ripple Losses in Laminated Poles

This paper presents the results of an experimental investigation into the problem of tooth-ripple losses in the laminated poles of rotating machines. The losses are measured by measuring the Poynting vector in the air-gap region of the machine between the surface of the poles and the slotted armature. Specially designed probes are used to measure the electric and magnetic fields and the signals from these probes are processed to give the required Poynting vector. This method also enables the direct measurement of the loss due to each individual harmonic under actual running conditions with all the harmonics simultaneously present. The experimental results are quite interesting in themselves and can also serve as a basis for comparing the various methods of calculating these losses. A comparison between the measured losses and those calculated on the basis of linear electromagnetic theory is also given.     

A calibrating device for large direct current instruments up to 320kiloampere-turns

A device for calibrating large dc instruments ranging up to 320 kA has been designed. The calibrating coil simulates the actual busbar and its turns can readily be changed to meet various required measurement ranges. The calibrating coil is arranged in the shape of a cross so that we can determine the influence of the external magnetic field. The accuracies are 6×10^-6 when a differential current measuring method is adopted and 200×10^-6 when a differential voltage measuring method is adopted. The stability of the power supply is 0.02% per hour. The error due to the current fluctuation is very small and thus can be neglected     

Comparison of AC Losses of YBCO Circular Pancake Coils and Infinitely Long Stack Approximation

High-temperature superconducting (HTS) coils are key parts of many large-scale AC applications, such as transformers, superconducting magnetic energy storage, and motors. The estimation of AC losses of pancake coils is necessary for optimizing the design of HTS devices and cryogenic systems. To speed up the computation of AC losses, the numerical model of an infinitely long HTS stack is often utilized. An HTS stack is a good approximation of the circular coil only if the coil radius is sufficiently large, since AC losses will exhibit a stronger asymmetry along the radial direction for small values of coil radii. To assess the validation of an infinitely long stack approximation, the comparisons of AC losses between infinitely long stacks and circular coils with different radii are presented. The turn number varies from 10 to 80. We find that the AC losses of HTS circular coils will gradually increase to the same value as an infinitely long stack with increasing coil radii. A new parameter is proposed to quantitatively describe the correctness of infinitely long stack approximation. Finally, a method of AC loss estimation of HTS pancake coils is proposed.     

AC loss measurements of twisted and untwisted BSCCO multifilamentary tapes

AC losses in twisted and untwisted BSCCO multifilamentary superconducting tapes with Ag matrix developed in DAPAS program were measured by an electrical method. Magnetization and transport losses were measured by a pick-up coil and by a voltage taps. Total AC loss during simultaneous application of AC transport current and an AC transverse magnetic field was given by the sum of the magnetization and transport losses measured during this simultaneous application. The magnetization loss without transport current of untwisted and twisted tapes was measured first to evaluate the effect of twisting to decouple filaments. Then, the total AC loss of the twisted tape was measured in transverse magnetic fields with various amplitudes and orientations, while the amplitude of the transport current was fixed. The measured total AC loss in a parallel transverse magnetic field was compared with some theoretical models to study the detailed characteristics of the measured total AC loss of the sample.     

A field-theoretical approach to magnetic induction heating of thin circular plates

A field-theoretical approach is used to analyze the subject of magnetic induction heating of thin circular plates by planar coils. Closed-form solutions for the electric and magnetic fields are found to the basic field problem of a single circular loop carrying current at a frequency ω in the presence of a plate characterized by a permeability μ and conductivity σ. By using these fields, expressions are derived for the complex Poynting vector at the surface of the plate, and for the induced EMF in the coil. The theory is extended to include multiturn coils and a field-dependent permeability, and a specific multiturn coil and plate combination is chosen as an example. The complex amplitude of the magnetic field and the Poynting vector are calculated along the surface of the plate using iterative methods to assure self-consistency with the field dependent permeability of the plate. By using Fourier transform techniques, the transient coil current and coil voltage waveforms are calculated under the experimental conditions used to take data on the sample coil and plate. The absorbed power is calculated from these waveforms and is found to be within 10 percent of the measured power absorption for all levels of operation from 50 to 2000 W. The calculated coil current waveform is compared with the measured waveform and is found to be in very good agreement in both shape and period.     

An improved measurement of the absolute efficiency of magnetic heads by saturating the gap field

A method that can be used to measure the absolute efficiency of narrow-track magnetic heads such as video heads (gap area <10³(μm)²) is described. The method is based on the detection of the gap flux by means of a subsidiary coil wound around the gap. Usually a considerable amount of leakage flux is detected together with the gap flux, which obviously would impair the accuracy of the results. Therefore, measures were taken by raising the current through the main coil progressively and thus determining the saturation value of the gap flux. In this way it was possible to distinguish between the gap flux and leakage flux components. With this method the efficiency can be measured with an accuracy of better than 10 percent at frequencies of up to 10 MHz. As a second result, the gap area can thus be determined magnetically as well. A comparison of the magnetic and optical gap area offers a check on the reliability of the efficiency measurement. The merits of this measuring method are discussed with the aid of results obtained for a typical ferrite video head.     

Physical scale modeling to verify energy storage inductor parameters

In the pulse charging and discharging of energy storage inductors, the influence of I²R heating, magnetic diffusion skin depth, and eddy current losses must be evaluated. These effects are not easily treated analytically. To address these problems, the system can be constructed according to governing scaling laws. The performance of the model can then be evaluated and scaled up to predict actual system performance. The scaling laws as applied to a 1O-MJ homopolar generator charging a 2-MJ cryogenic aluminum Brooks coil are presented in this paper. System parameters measured on the scale model are compared to parameters subsequently measured on the full-size equipment.     

Absolute Measurements for Small Loop Antennas for RF Magnetic Field Standards

A method to determine accurately the strength of magnetic fields produced by a transmitting small loop antenna for the RF field standards is presented. The field strength can be determined by the magnetic dipole moment of the loop. A loop antenna factor is introduced to express the magnetic dipole moment of a transmitting small loop antenna in terms of the incident power to the antenna input port. The emergent power from the output port of a receiving small loop antenna can also be expressed in terms of the magnetic field to be detected and of the loop antenna factor. The insertion loss method (or the three antenna method) is applied to measuring the loop antenna factor. Small loop antennas with diameters 10 cm were designed, and loop antenna factors were measured by the insertion loss method over frequencies up to 30 MHz with a systematic error of 0.08 dB.     

Measurement of flux distribution in toroidal and multiaperture cores

Recently, magnetic devices utilizing a partial switching have been used in many fields. From a point of core design, a flux distribution must be known. It is possible to calculate them under some assumptions, but it is difficult to measure them experimentally. This paper describes a technique of determining a flux distribution in toroidal and multiaperture ferrite cores. The patterns of flux distribution are expressed by a flux of local zone. The flux level is determined by measuring a second-harmonic voltages induced in a pick up coil under an application of a small ac magnetic field (10 kHz) orthogonal to the plane of core. Small holes for sensing are drilled through the core, and the pick up coil encloses cross-sectional areas between each of the holes. It was found that a flux reversal boundary in a toroidal core travels from inner edge to outer edge when it is switched partially by a dc field, but a pulse field switching produces a complex flux distribution, and that in both cases, the flux reversal boundary is not sharp. By a similar method, the flux distribution in the transfluxor is shown for the "unblocked" and "blocked" state before and after the interrogation. The transfluxor in the unblocked state after the interrogration showed a "kidney shaped" pattern. Readout characteristics obtained from the measured flux distributions agree with those obtained by ordinary induced voltage methods. The accuracy of the readout flux is about 8.7 percent.     

Analysis of a novel laminated coil using eddy currents for AC highmagnetic field

The analysis and performance of an eddy current type laminated coil for a high AC magnetic fields are described. It is usually difficult to obtain a high AC magnetic field using an air-gap coil because of eddy currents. The present coil circumvents this limitation by making use of the magnetic shielding effect of eddy currents. Two different realizations of the coil are proposed to provide design flexibility. The field distributions are analyzed by the two-dimensional finite-element method. The coils can also be applied to an induction electromagnetic pump     

Method for estimation of magnetic viscosity of ferromagnetics

A method providing quantitative estimation of the degree of magnetic viscosity of ferromagnetic materials by measurement of the phase of the signal from the measuring coil at field excitation by a sinusoidal signal in longitudinally extended samples with constant cross section is described. It is demonstrated that, in estimation of magnetic viscosity, it is necessary to take into account the sample size and shape, as well as the frequency of measurement. Additional information obtained by determination of the decrement of the signal from the measuring coil can be used together with the magnetic viscosity for determination of steel grade. Corresponding plots are presented.     

Simultaneous measurement of electric and magnetic properties of aspherical sample

A method using a solenoid coil for simultaneously estimating the electric and magnetic properties of a spherical conductor was studied. These properties are estimated by finding the difference in the complex impedance of the coil with and without a sample to find out the measuring value that best coincides with the theoretical value. A new formula applicable to a nonmagnetic, a magnetic or a superconductive spherical sample was derived. The conductivities a and permeabilities μ measured by this method and by the conventional methods were compared using various samples. The deviations were no larger than 3% for a nonmagnetic samples, and 1.5% for μ of magnetic samples     

First experience with the new Coupling Loss Induced Quench system

New-generation high-field superconducting magnets pose a challenge relating to the protection of the coil winding pack in the case of a quench. The high stored energy per unit volume calls for a very efficient quench detection and fast quench propagation in order to avoid damage due to overheating.A new protection system called Coupling-Loss Induced Quench (CLIQ) was recently developed and tested at CERN. This method provokes a fast change in the magnet transport current by means of a capacitive discharge. The resulting change in the local magnetic field induces inter-filament and inter-strand coupling losses which heat up the superconductor and eventually initiate a quench in a large fraction of the coil winding pack.The method is extensively tested on a Nb–Ti single-wire test solenoid magnet in the CERN Cryogenic Laboratory in order to assess its performance, optimize its operating parameters, and study new electrical configurations. Each parameter is thoroughly analyzed and its impact on the quench efficiency highlighted.Furthermore, an alternative method is also considered, based on a CLIQ discharge through a resistive coil magnetically coupled with the solenoid but external to it. Due to the strong coupling between the external coil and the magnet, the oscillating current in the external coil changes the magnetic field in the solenoid strands and thus generates coupling losses in the strands. Although for a given charging voltage this configuration usually yields poorer quench performance than a standard CLIQ discharge, it has the advantage of being electrically insulated from the solenoid coil, and thus it can work with much higher voltage.     

Magnetic Nondestructive Test for Resistance Spot Welds Using Magnetic Flux Penetration and Eddy Current Methods

Resistance spot welding technologies are widely used in industry. A highly reliable monitoring method is needed to effectively weld and create a robust structure. We developed a combined technique using magnetic flux penetration and an eddy current test (ECT). The magnetic measuring system consists of a pair of magnetic probes having an induction coil and detection coil, a lock-in amplifier, a current source, and a personal computer. The magnetic flux penetration through both surfaces at the weld was measured at low frequency. The ECT was performed at each surface with multiple frequencies. The magnetic flux penetration method showed good correlation with the destructive shear test because of the change in permeability due to the formation of the nugget. The ECT method reflected the depth profile of the nugget and was effective for determining a defective product.     

Resistance thermometry in magnetic fields 1. Thermistors and platinum thermometers at 77 K

The effects of static magnetic field up to 16 kG on seven midget disc thermistors and three miniature platinum resistance thermometers were measured at 77 K. Corrections for the magnetoresistance of the electrical leads and solder junctions were made from data taken on a copper wire coil. A virtual temperature rise of the order of millikelvins occurs for both temperature sensors; the lead wire correction is less than a few microkelvins. The thermistors have larger magnetoresistance effects than similar types recommended for use at lower temperatures. A figure of merit comparison suggests that they are slightly less desirable than platinum thermometers in magnetic fields at liquid nitrogen temperatures.     

Apparatus for calorimetric measurements of losses in MgB2 superconductors exposed to alternating currents and external magnetic fields

Inexpensive superconducting wires with low AC losses would open up for a large superconductor market in AC electrical power applications. One candidate for this market is the MgB₂ conductor. In the development of an AC optimized superconductor, high-quality measurements of the AC losses under application-like conditions must be available. This article describes an apparatus built for this purpose. The measurement method is calorimetric. The temperature increase of the superconductor sample is measured and compared to the temperature increase due to a heater with known heat input. The system is designed for measurements of losses due to magnetic fields combined with transport currents. Results from tests verifying the capabilities of the system are given, as well as from initial AC loss measurements on a tape-shaped MgB₂ superconductor.     

A Faraday Magnetometer for Studies of Monolayer 3He Films on Graphite

No Heading A Faraday-type magnetometer has been developed to measure the nuclear magnetization of monolayer ³He films adsorbed on Grafoil at temperatures down to 0.1 mK under high magnetic fields up to 10 T. The magnetic force is measured capacitively by monitoring the displacement of a wire-suspended copper plate. A double gradient coil system is employed to produce opposite field gradients at two regions the same distance apart from the center. At each region, exfoliated and non-exfoliated graphite foils are put one by one to eliminate a large background signal from Grafoil and copper.PACS numbers: 67.70.+n, 75.10.Jm     

Application of the differential calorimeter in magnetic measurements

Conclusions The calorimetric method of measuring losses in ferromagnetic materials can be used as a control method for checking the accuracy of measuring devices and for checking magnetic characteristics of materials at high frequencies under laboratory conditions (at plants producing magnetic materials) and in research institutes. This method is the most reliable when the voltage or current waveform are distorted, since the measurement results are almost completely independent of frequency.The differential calorimetric system provides measurements of small losses in ferromagnetic materials at high frequencies with great precision. Measurements of a power greater than 5 mw in a stable state condition and a completely balanced system can be made with an accuracy of the order of ± 1%. The sensitivity at the same time can be sufficiently high (the threshold of sensitivity in our case amounts to approximately 0.005 mv).