Measurement Of Magnetic Field Distribution Research Articles

Measurement of AC magnetic field distribution using magnetic resonance imaging.

Electric currents are applied to body in numerous applications in medicine such as electrical impedance tomography, cardiac defibrillation, electrocautery, and physiotherapy. If the magnetic field within a region is measured, the currents generating these fields can be calculated using the curl operator. In this study, magnetic fields generated within a phantom by currents passing through an external wire is measured using a magnetic resonance imaging (MRI) system. A pulse sequence that is originally designed for mapping static magnetic field inhomogeneity is adapted. AC current in the form of a burst sine wave is applied synchronously with the pulse sequence. The frequency of the applied current is in the audio range with an amplitude of 175-mA rms. It is shown that each voxel value of sequential images obtained by the proposed pulse sequence is modulated similar to a single tone broadband frequency modulated (FM) waveform with the ac magnetic field strength determining the modulation index. An algorithm is developed to calculate the ac magnetic field intensity at each voxel using the frequency spectrum of the voxel signal. Experimental results show that the proposed algorithm can be used to calculate ac magnetic field distribution within a conducting sample that is placed in an MRI system.

Integrated LTS gradiometer SQUIDs for measurements of magnetic field distributions in an unshielded environment

We have developed various types of planar LTS dc gradiometer SQUIDs for measurements of magnetic field distributions in an unshielded environment, for instance washer type or a SQUID with integrated antenna and flux transformer. They were fabricated in Nb/AIO~ / Nb technology. The high level of external disturbances limits unshielded measurements of magnetic field distributions with SQUID magnetometers. It is better to use gradiometers because they are only sensitive to magnetic field gradients. Due to this a high balance of these gradiometers of first order is necessary. To evaluate the balance one has to determine the effective area A~r and the parasitic areas of the SQUID. The parasitic areas represent the sensitivity of the gradiometer in homogenious magnetic fields produced by large Helmholtz coils. Fig. 1 shows the definition of the parasitic areas corresponding to the direction of the homogenious magnetic field.

Magnetic field distribution measurements in a low-pressure inductive discharge

The magnitude and relative phase of the rf magnetic flux density has been measured as a function of radial position using a magnetic (B-dot) probe in an inductively coupled low pressure argon discharge driven at 13.56 MHz. The spatial variation of the rf electric field and the discharge current density were determined from the probe measurements and are compared at gas pressures of 3, 30, and 300 mTorr for a fixed discharge power of 50 W and at discharge powers of 21, 50, and 103 W for a fixed gas pressure of 30 mTorr. In the active zone near the induction coil where the rf field is strong, the electron rf drift velocity derived from the B-dot data and Langmuir probe measurement was found to be considerably less than the electron thermal velocity even at the lowest argon pressure. For an axially symmetric inductive discharge, as is found in many applications, a two-dimensional magnetic probe measurement with space and phase resolution is necessary to correctly infer the rf electric field and the discharge current density distribution.

Flux trapping in superconducting thin films in weak magnetic fields

Magnetic-field distribution measurements over a patterned superconducting strip line sample were conducted using a superconducting quantum interference device pickup coil, showing that, in the range of 500 μG–50 mG of perpendicular magnetic field B⊥,i, the superconducting films record previous magnetic histories precisely. The magnetic-field distribution with a field B⊥,i applied at all times is identical to one with no field applied at any time. A calculation based on the flux trapping model explains these results indicating that all the magnetic fluxes penetrate the superconducting thin films.

Influence of a protective coating on the aging of epitaxial YBa2Cu3O7-x films

The influence of aging and temperature cycling effects on thin epitaxial YBa2Cu3O7-x films on (100) SrTiO2 has been investigated over a period of 15 months using measurements of the spatial magnetic field distribution. Two of the four investigated samples have an amorphous YBaCuO cover layer which appears to protect the samples against degrading.

Detection of trapped flux quanta in superconductors by a hydrodynamically controlled SQUID pick-up coil

The detection of the flux quanta trapped in a large area superconductor by scanning a SQUID pickup coil is discussed. For this purpose a gas floating technique has been developed. In this technique, the supplied gas flow supports the coil while the coil scans the superconductor surface in liquid helium. The applicability of the gas floating technique is examined, and it is shown that this technique can be utilized even in liquid nitrogen or liquid helium. Measurements of magnetic field distribution over several superconductors in liquid helium are reported. By scanning the pickup coil over the superconductor, changes in the magnetic field, which come from the flux trapped in the superconductor, are observed. Furthermore, the background variation during sample rotation and displacement is reduced to about 2.4 Phi /sub 0/. The minimum detectable flux is shown to be less than Phi /sub 0/. These results show the possibility of using this technique in the detection of trapped flux quanta in superconductors.

Magnetic Field Distribution Measurement by the Modified FLASH Method

Abstract Magnetic field inhomogeneities cause blurring and distortion of images gained by nuclear magnetic resonance. In order to adjust the magnetic coils finely, a precise and rapid method for measuring the magnetic field is needed. We describe a gradient echo technique of the FLASH version for mapping static magnetic fields.

Magnetic detection for underground pipes (abstract)

Magnetic detection has been investigated for use in detecting the location of underground pipelines, such as gas and water lines, by inputting a signal current into a pipeline and detecting magnetic field (≲100 μG) generated by that current. Measurement factors necessary for accurate detection have been experimentally determined and are discussed. From measurements made on signal current distribution (the current flowing in pipes and leaking from them to ground), it has been concluded that detection error is caused by the leakage current, namely, the current to ground. The detection error is due to the noncenter symmetric distribution of the ground current flowing around a pipe. Even though the ground current is inevitable when flowing a signal current into pipes, it is possible to reduce the error by reducing the nonsymmetry by lowering the signal frequency below 10 kHz in this work. The ground current distribution was obtained by measuring the potential difference between two electrodes, both of which were rod shaped and inserted in the ground to the same depth. At low frequencies below 10 kHz, the ground current distribution in a cross section perpendicular to the pipe showed a rather center symmetry around the pipe, while at high frequencies the distribution changed to a noncenter symmetry with a significant current increase in a region near the ground surface. The drastic change in the distribution of the ground current was also confirmed by magnetic field distribution measurements conducted on the ground using three orthogonally arranged magnetic sensors, each with a sensitivity of 1 μG.

Measurement of magnetic field distribution on a metal and power density in induction heating

Electrical Engineering in JapanVolume 105, Issue 5 p. 110-118 Article Measurement of magnetic field distribution on a metal and power density in induction heating Kazuhisa Ishibashi, Kazuhisa Ishibashi Toyo Seikan GroupSearch for more papers by this author Kazuhisa Ishibashi, Kazuhisa Ishibashi Toyo Seikan GroupSearch for more papers by this author First published: 1985 https://doi.org/10.1002/eej.4391050514AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Volume105, Issue51985Pages 110-118 RelatedInformation

Magnetic field measurement by NMR imaging

Measurement of magnetic field distributions is performed using a method based on the nuclear magnetic resonance (NMR) technique of Fourier imaging. By selecting objects which limit the spatial distribution of the NMR-observable nuclei, images can be obtained which allow easy interpretation of the magnetic field distribution and facilitate rapid shimming of magnets. Results are presented showing the effect of a shim coil set of a superconducting solenoid magnet of the type use for NMR imaging. The application of this type of measurement to the rapid shimming of similar magnet designs is analysed. The effects of steel plates close to the magnet are also observed.

Direct measurement of stray magnetic field distributions from a magnetic bubble domain in yttrium orthoferrite with a micro Hall element

The stray magnetic field distributions from an isolated magnetic bubble domain of about 150-µm diameter in YFeO 3 are measured by a micro Hall element made of a single crystal InSb with active dimensions of 5 × 5 × 2.6 µm3. Experimental results for an isolated bubble domain show fair agreement with the theoretical ones for a cylindrical bubble domain.

Measurement of Internal Magnetic Field Distribution in Axially Magnetized YIG Rods Based on Magnetoelastic Resonance Absorption

A new method for measuring internal magnetic field in ferromagnetic rods is described. Magnetoelastic resonance is observed in longitudinally magnetized YIG rods. The separation ΔH of adjacent resonance peaks is varied with signal frequency and applied magnetic field He. It is shown the internal magnetic field is calculated from He-ΔH curves and the results agree well with the theoretical predictions previously published. The shape of axial field distribution depends on the applied external field. In the low external field region the shape is smooth, in the medium field region it becomes square and in the high field region it again becomes rather smooth, approaching Sommerfeld field.

Dual Magnetic Probe System for Phase Measurements in Thermal Induction Plasmas

Previously reported measurements of rf magnetic field distributions in thermal induction plasmas by a water-cooled coil probe have been extended to phase measurements through the use of a second identical probe placed at a fixed position near the plasma boundary. From measurements of the vector sum and/or difference in addition to the individual signals from both probes, data for the variation of the phase angle with radius were obtained. The reliability of the method was checked with an electrolyte of known conductivity. Tests conducted at 2.6 MHz in a 14-cm-wide argon-plasma column at ambient pressure yielded a change in phase angle of 285 deg over the radius. The variations in phase and magnitude of the magnetic field were used to derive those of the induced electric field, the current density, the electrical conductivity, and the real part of the Poynting vector. From the latter a specific power input of about 1500 W/cm was obtained. A correlation of the conductivity data with temperatures derived from absolute line and continuum intensities is in good agreement with predictions from an equilibrium theory.

Measurement of the rf Magnetic Field Distribution in a Thermal Induction Plasma

The radial distribution of the rf magnetic field in the equator section of a 25-kW argon induction flame maintained at 2.6 MHz in a 155-mm-wide tube is measured with a 3-mm-diam water-cooled search coil. It is found that in the annular space between plasma flame and inductor coil the field is increased by about 10% from an unperturbed value of 15 A/cm whereas in the flame interior it drops to a value of 0.8 A/cm at the axis. The penetration depth is of the order of 1 cm from which a mean value for the plasma conductivity of 10 mho/cm is estimated. From the field distribution a variation in magnetic pressure over the plasma radius of 16 dyn/cm2 is calculated which is in order-of-magnitude agreement with values quoted in the literature. Probe errors caused by signal averaging, current obstruction, and plasma cooling are discussed and found to be of minor consequences under the test conditions employed.

Experimental investigations on a toroidal screw pinch

High-β Deuterium plasmas in a temperature range of 0.1–1 keV were produced in the fast toroidal screw pinch experiment ISAR IV (Bmax≈33 kG,Iz max≈100–150 kA, coil diameter 12 cm, aspect ratio 5). The experimentally determined growth rates of the appearing instabilities—especially them=1 mode—are compared with two simple models and a more exact numerical calculation. This comparison and direct measurements of the magnetic field distribution show that the constant pitch model of Bobeldijk1 is a suitable first approximation for the normal screw pinch. Using a delay of thez and θ-bank as a first simple method of field programming this constant pitch profil could be varied in different directions. The results are discussed in more detail and confirm the theoretical picture of stability behaviour of the high-β screw pinch.

Construction of the Guide Field for a Low Inductance Air-Core Plasma Betatron

The guide field for a low inductance (55×10−9 H) air-core plasma betatron is constructed by placing flux-concentrators within the interior of a single-turn solenoid. The betatron field coil is energized by the discharge of a 75 μF, 20 kV capacitor bank. The quarter period of the field-coil capacitor bank system is 4 μsec and the peak magnetic and electric fields at the equilibrium orbit (r0=4.83 cm) are ≈3250 G and 60 V/cm, respectively. Techniques used to make accurate (±0.1%) measurements of transient magnetic field distributions are also described.