Magnetic Flux Measurements Research Articles

Experimental results for 2D magnetic resonance electrical impedance tomography (MR-EIT) using magnetic flux density in one direction

Magnetic resonance electrical impedance tomography (MR-EIT) is an emerging imaging technique that reconstructs conductivity images using magnetic flux density measurements acquired employing MRI together with conventional EIT measurements. In this study, experimental MR-EIT images from phantoms with conducting and insulator objects are presented. The technique is implemented using the 0.15 T Middle East Technical University MRI system. The dc current method used in magnetic resonance current density imaging is adopted. A reconstruction algorithm based on the sensitivity matrix relation between conductivity and only one component of magnetic flux distribution is used. Therefore, the requirement for object rotation is eliminated. Once the relative conductivity distribution is found, it is scaled using the peripheral voltage measurements to obtain the absolute conductivity distribution. Images of several insulator and conductor objects in saline filled phantoms are reconstructed. The L2 norm of relative error in conductivity values is found to be 13%, 17% and 14% for three different conductivity distributions.

Demonstration of a new method for magnetic flux measurement in the interior of a magnetic material

A method is proposed for magnetic flux measurement in localised regions inside a magnetic material using a novel surface probe sensor. The principle of operation is described and its potential use in steel products is experimentally verified.

Magnetic suspension force of Ag/sub 2/O doped BiPbSrCaCuO superconductor

Magnetic flux measurements of a toroidal magnet revealed a concave shaped field distribution with a single minimum value and a null field along the axis of the torus at the point where the field was reversed. The nonlinear magnetic field of the toroidal magnet perpendicular to the Ag/sub 2/O doped superconducting disk sample with the trapped magnetic flux distorted the field line distribution. As a result, the interaction force between the magnet and sample exhibited regions of repulsive, null, attractive, null, and finally repulsive force. The asymmetrical concave shaped force pattern along the axis with two null force points indicates that the magnetic force exerted form the sample changed direction which resulted in the transition from repulsive force to attractive force at the null force point, and the force becomes repulsive again beyond the second null force point as the distance along the axis increases. The lateral stability of the suspended sample under the toroidal magnet is provided by the characteristics of the symmetrical nature of the field line with respect to the axis of the magnet. The magnetic moment of an undoped and 2% Ag/sub 2/O doped sample was shown to be m=0.043 emu and 0.06 emu, respectively. The measured suspension force exerted form the doped sample agreed well with the suspension force calculated from magnetostatic force distribution.

Needle-probe techniques for local magnetic flux measurements

Numerical two-dimensional field investigations in the cross section of laminated nonoriented electrical steel sheet are performed in order to verify the possibilities and limitations of the two needle-probe technique for the measurement of local magnetic flux near the cutting edge. If one of the needles approaches the lamination edge or when inhomogeneities are present in the flux distribution along the strip width due to magnetic properties altered by the cutting procedure, then the signal from the needle probes no longer represents the flux through a rectangular surface under the needles because of E-field components perpendicular to the lamination surface. The flux calculated from the needles is compared to the actual flux passing through the rectangular surface under the needles and corresponding correction factors as a function of the needle position away from the cutting edge are computed. Depending on the positioning method, the needle probes can give an under- or overestimation of the actual flux. Corrections from 15% to 80% are necessary if distances of about the lamination thickness are considered. This work suggests an iterative numerical procedure for the interpretation of experimental data obtained from local flux needle-probe measurements.

Current constrained voltage scaled reconstruction (CCVSR) algorithm for MR-EIT and its performance with different probing current patterns

Conventional injected-current electrical impedance tomography (EIT) and magnetic resonance imaging (MRI) techniques can be combined to reconstruct high resolution true conductivity images. The magnetic flux density distribution generated by the internal current density distribution is extracted from MR phase images. This information is used to form a fine detailed conductivity image using an Ohm's law based update equation. The reconstructed conductivity image is assumed to differ from the true image by a scale factor. EIT surface potential measurements are then used to scale the reconstructed image in order to find the true conductivity values. This process is iterated until a stopping criterion is met. Several simulations are carried out for opposite and cosine current injection patterns to select the best current injection pattern for a 2D thorax model. The contrast resolution and accuracy of the proposed algorithm are also studied. In all simulation studies, realistic noise models for voltage and magnetic flux density measurements are used. It is shown that, in contrast to the conventional EIT techniques, the proposed method has the capability of reconstructing conductivity images with uniform and high spatial resolution. The spatial resolution is limited by the larger element size of the finite element mesh and twice the magnetic resonance image pixel size.

Measurement of magnetic helicity flux into the solar corona

We developed a new methodology which can determine magnetic helicity flux across the photosphere based on the magnetograph observation, In order to derive the helicity flux, first the velocity tangential to the solar surface is constructed by applying a correlation tracking technique on the magnetic observation, and secondly the velocity component across the photosphere is derived from the condition that the magnetic evolution must be consistent with the induction equation, Through this procedure, we can determine the helicity flow across the photosphere as a function of time and space, Based on this new method, we analyzed magnetic helicity of the active regions NOAA 9026 and 9077. using the data taken by SOHO/MDI and the vector magnetograph at NAOJ/Tokyo. As a result, it was revealed that positive and negative helicity is supplied in each region simultaneously. In particular, temporary activation of the helicity injection of the both signs was observed prior to X-class flare events in the GOES classification. The investigation on the helicity distribution suggests some relationship between flare onset and the increase of complexity in the helicity structure.

Suspension of a field-cooled BiPbSrCaCuO high-Tc superconductor under a toroidal permanent magnet

Magnetic flux measurements of a toroidal magnet revealed a concave-shaped field distribution with a single minimum and a null field along the axis of the torus at the point where the field reversed. The non-linear magnetic field of the toroidal magnet perpendicular to the Ag2O-doped superconducting disc sample with trapped magnetic flux distorted the field line distribution. As a result, the interaction force between the magnet and the sample exhibited regions of repulsive, null, attractive, null and finally repulsive force. The asymmetrical concave-shaped force pattern along the axis with two null force points indicates that the force exerted on the sample changes direction, the transition from repulsive to attractive at the null force point, and the force becomes repulsive again beyond the second null force point as the distance along the axis increases. The magnetic field simulation using the Poisson numerical code for the toroidal magnet of 46 mm OD, 12 mm ID and 10 mm thickness was in close agreement with the force measurements. The lateral stability of a suspended sample under the toroidal magnet is provided by the characteristic symmetrical nature of the field line with respect to the axis of the magnet. The concave-shaped magnetic field forms a vertical magnetic wall around a suspended superconducting sample as demonstrated with lead, a type 1 superconductor. Thus, the toroidal magnet with a concave-shaped magnetic field distribution with respect to the mid-plane of the magnet provides much improved lateral stability for the magnetic bearing. Furthermore, this arrangement provides a loss-free shielding current in the absence of viscous drag from the environment or eddy current or hysteresis. The magnetic moment of an undoped and 2% Ag2O-doped samples was shown to be m = 0.043 emu and 0.06 emu, respectively. The measured suspension force exerted on the doped sample agreed well with that calculated from the magnetostatic force distribution.

ANALYSIS OF A TWO-DIMENSIONAL PERMANENT-MAGNETIC PASSIVE DAMPER

A linear model is presented by a simple form for a two-dimensional magnetic seesaw damper using permanent magnets, which was proposed by Murakami and Satoh. The magnetic system is simplified with an equivalent magnetic circuit from which the incremental magnetic flux is described by the displacement of the moving core. Since the magnetic reluctance of magnet cores varies with frequency, the expression is formulated in the frequency domain. A numerical analysis follows with the measurement of the incremental magnetic flux.

Differences in the Sun’s Radiative Output in Cycles 22 and 23

Analysis of the current solar cycle 23 shows a greater increase in total solar irradiance (TSI) for the early phase of this cycle than expected from measurements of the total magnetic flux and traditional solar activity indices, which indicate that cycle 23 is weaker than cycle 22. In contrast, space observations of TSI from the Solar and Heliospheric Observatory/VIRGO and the Upper Atmospheric Research Satellite/ACRIMII show an increase in TSI of about 0.8-1.0 W m-2 from solar minimum in 1996 to the end of 1999. This is comparable to the TSI increase measured by Nimbus 7/ERB from 1986 to 1989 during the previous cycle. Thus, solar radiative output near the maximum of the 11 yr cycle has been relatively constant despite a factor of 2 smaller amplitude increase for cycle 23 in sunspot and facular areas determined from ground-based observations. As a result, empirical models of TSI based on sunspot deficit and facular/network excess in cycle 22 underestimate the TSI measurements in 1999. This suggests either a problem in the observations or a change in the sources of radiative variability on the Sun.

549 マルテンサイト変態を応用した劣化・損傷の非破壊的検出に関する研究(OS7-7 微視的挙動・効果II)(OS7 微視構造を有する材料の力学)

The present study has attempted to establish a new method of non-destructive inspection method for material degradation by using martensitic transformation. Martensitic phase transformation occurs in an austenitic stainless steel SUS304 and thus resultant magnetization is induced due to severe deformation or to high applied stress. Peaks of vertical magnetic flux density B_z were measured in the plates of SUS304 stainless steel with fatigue cracks by flux gate sensor. The distance 2l between the minimum and the maximum peaks of the B_z distribution shows a linear correlation with real fatigue crack length 2a. The values of minimum and maximum peaks also have a linear relationship with the applied stress intensity factor range ΔK, a kind of a fatigue damage parameter imposed. These results imply that the measurement of the vertical magnetic flux density above a fatigue crack can estimate real crack length 2a and the amount of damage the specimens has suffered.

Detection of Plastic Deformation and Fatigue Damage in Pressure Vessel Steel by Leakage Magnetic Flux Sensors

In order to detect material degradations in pressure vessel steel of A533B employed as a component material in light water reactors (LWRs), the leakage magnetic flux distribution on plate-type A533B specimens has been examined using a magnetic sensor of 120μm square GaAs Hall element. As reference data, the residual stress distribution has also been obtained by X-ray diffraction method where an irradiation spot was 1mm in diameter. The data of the leakage magnetic flux normal to the specimen surface (Bz) measured by the Hall sensor were converted into the form of the first derivative dBz/dx by calculating the gradient of the Bz along the axial direction of the specimen (x-axis), and the data of the axial residual stress (σx) obtained by the X-ray diffraction method were also transformed into the differential form dσx/dx. It was found that there existed a correlation between the absolute value of the dBz/dx and the dσx/dx for the tensile specimens (correlation coefficient r=0.282). Full width at half maximum (FWHM) for the frequency distribution profile of the dBz/dx was calculated. The FWHM of the tensile specimens increased remarkably at the beginning stage of the plastic deformation where the Lüder's band was generated; The FWHM of the fatigued specimens increased with stress cycling. It was suggested that the leakage magnetic flux measurement by the GaAs Hall sensor was a promising non-destructive evaluation (NDE) method to detect the amount of plastic deformation and fatigue damage.

EMF exposure assessment in the Finnish garment industry: evaluation of proposed EMF exposure metrics.

Recently published studies indicate that having worked in occupations that involve moderate to high electromagnetic field (EMF) exposure is a risk factor for neurodegenerative diseases, including Alzheimer's disease. In these studies, the occupational groups most over-represented for EMF exposure comprised seamstresses, dressmakers, and tailors. Future epidemiologic studies designed to evaluate the possibility of a causal relationship between exposure to EMF and a neuro degenerative disease endpoint such as incidence of Alzheimer's disease, will benefit from the measurement of electromagnetic field metrics with potential biological relevance. Data collection methodology in such studies would be highly dependent upon how the metrics are defined. In this research the authors developed and demonstrated (1) protocols for collecting EMF exposure data suitable for estimating a variety of exposure metrics that may have biological relevance, and (2) analytical methods for calculation of these metrics. The authors show how exposure might be estimated under each of the three prominent EMF health-effects mechanism theories and evaluate the assertion that relative exposure ranking is dependent on which mechanism is assumed. The authors also performed AC RMS magnetic flux density measurements, confirming previously reported findings. The results indicate that seamstresses, as an occupational group, should be considered for study of the possible health effects of long-term EMF exposure.

The physical characteristics of neodymium iron boron magnets for tooth extrusion.

Impaction and non-eruption of teeth is a common problem encountered in orthodontics and many techniques have been proposed for the management of this condition. It has been advocated that a system utilizing magnets would supply a continuous, directionally sensitive, extrusive force, through closed mucosa and thus provide not only a physiological sound basis for successful treatment, but also reduce the need for patient compliance and appliance adjustment. This ex vivo investigation examined in detail the physical characteristics of neodymium iron boron magnets employed in attraction in order to assess their usefulness in the clinical situation. Attractive force and magnetic flux density measurements were recorded for nine sets of magnet pairs with differing morphologies. The effect of spatial relationship on force was assessed by varying vertical, transverse and horizontal positions of the magnets relative to each other, and by altering the pole face angles. The data obtained suggest that magnets with larger pole face areas and longer magnetic axes provide the best performance with respect to clinical usefulness. It was possible to formulate a specific relationship between force and flux density for each magnet pair. This relationship can be used in the clinical management of unerupted teeth to predict the force between the magnets by measuring the magnetic flux density present at mucosal level. The results indicate that magnetic systems may, indeed, have a place in the treatment of unerupted teeth.

Josephson-flux dynamics from the initial depinning up to complete grain decoupling at the irreversibility line in YBa 2Cu 3O 7/Ag nonrandom composites

We report on initial depinning of magnetic flux, magnetic irreversibility and resistivity measurements of granular YBa 2Cu 3O 7/Ag nonrandom superconducting composites, with 0.0, 3.6, 8.6, 16.1 and 20.7 wt.% Ag, under low fields, up to 120 Oe. We show that the flux depinning as well as the irreversibility line, in the H× T diagram, are only weakly affected by the silver mixing up to 16.1 wt.% Ag, but for 20 wt.% Ag depinning starts at considerably lower temperatures. In granular superconductors the magnetoresistance is known to vanish when about 15% of the superconducting grains are coupled by weak links, where the superconducting grain chains become virtually infinite. Although this zero resistivity line, T c0( H) being situated well below the irreversibility line, in our samples, it still lies tens of degrees above the initial depinning temperature. The vanishing of magnetoresistance under conditions, in which the flux mobility is already considerable, is only possible if this flux is of the Josephson type, which is restricted to the intergrain spaces. Our H– T diagram shows that the Josephson-flux depinning dominates the flux dynamics up to or closely to the irreversibility line T irr( H), where the grain decoupling is completed.

Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities

Abstract. Giant pulsations are nearly monochromatic ULF-pulsations of the Earth's magnetic field with periods of about 100 s and amplitudes of up to 40 nT. For one such event ground-magnetic observations as well as simultaneous GEOS-2 magnetic and electric field data and proton flux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic field indicate the excitation of an odd-mode type fundamental field line oscillation. A clear correlation between variations of the proton flux in the energy range 30-90 keV with the giant pulsation event observed at the ground is found. Furthermore, the proton phase space density exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce resonance instability as a possible generation mechanism, the azimuthal wave number of the pulsation wave field may be determined using a generalized resonance condition. The value determined in this way, m = - 21 ± 4, is in accord with the value m = - 27 ± 6 determined from ground-magnetic measurements. A more detailed examination of the observed ring current plasma distribution function f shows that odd-mode type eigenoscillations are expected for the case ∂f / ∂W > 0, much as observed. This result is different from previous theoretical studies as we not only consider local gradients of the distribution function in real space, but also in velocity space. It is therefore concluded that the observed giant pulsation is the result of a drift-bounce resonance instability of the ring current plasma coupling to an odd-mode fundamental standing wave. The generation of the bump-on-the-tail distribution causing ∂f / ∂W > 0 can be explained due to velocity dispersion of protons injected into the ring current. Both this velocity dispersion and the necessary substorm activity causing the injection of protons into the nightside magnetosphere are observed.Key words. Magnetospheric physics (energetic particles , trapped; MHD waves and instabilities) · Space plasma physics (wave-particle interactions).

Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities

Giant pulsations are nearly monochromatic ULF-pulsations of the Earth's magnetic field with periods of about 100 s and amplitudes of up to 40 nT. For one such event ground-magnetic observations as well as simultaneous GEOS-2 magnetic and electric field data and proton flux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic field indicate the excitation of an odd-mode type fundamental field line oscillation. A clear correlation between variations of the proton flux in the energy range 30-90 keV with the giant pulsation event observed at the ground is found. Furthermore, the proton phase space density exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce resonance instability as a possible generation mechanism, the azimuthal wave number of the pulsation wave field may be determined using a generalized resonance condition. The value determined in this way, m = - 21 ± 4, is in accord with the value m = - 27 ± 6 determined from ground-magnetic measurements. A more detailed examination of the observed ring current plasma distribution function f shows that odd-mode type eigenoscillations are expected for the case ∂f / ∂W > 0, much as observed. This result is different from previous theoretical studies as we not only consider local gradients of the distribution function in real space, but also in velocity space. It is therefore concluded that the observed giant pulsation is the result of a drift-bounce resonance instability of the ring current plasma coupling to an odd-mode fundamental standing wave. The generation of the bump-on-the-tail distribution causing ∂f / ∂W > 0 can be explained due to velocity dispersion of protons injected into the ring current. Both this velocity dispersion and the necessary substorm activity causing the injection of protons into the nightside magnetosphere are observed.Key words. Magnetospheric physics (energetic particles , trapped; MHD waves and instabilities) · Space plasma physics (wave-particle interactions).

Tunneling in rf-SQUIDs and Tests of Quantum Mechanics at the Macroscopic Level

Superconducting devices such as rf-SQUIDs have been proposed to test the validity of quantum mechanics by means of Bell-like inequalities involving different-time correlation probabilities for measurements of magnetic flux. We calculate the quantum mechanical violations to such temporal Bell inequalities taking into account the effect of the measurement process on the probability distribution for the outcomes. We define a general criterion quantifying the observability of the violations and show that it is not fulfilled for the various experimental configurations proposed so far.

Integrated HTS dc-SQUID magnetometers

Integrated magnetometers based on superconducting quantum interference devices (SQUIDs) are employed for the very sensitive measurement of magnetic flux and magnetic flux density. Today, most superconducting magnetometers made from high-temperature superconductors (HTS) are based on YB a2 Cu 3 O 7 and are optimized for a temperature of 77 K. Depending on the application, the size of the magnetometers is restricted. Many applications in the non-destructive evaluation (NDE) of materials allow the use of large chips with up to 5 cm diameter. The need for integrated, multilayer magnetometers arises, when high sensitivity has to be combined with high spatial resolution with areas below 1 cm 2 . This applies to the sensors in multichannel systems for biomagnetism which have to be adapted to the size of the sources of magnetic signals in the human heart or brain. Here, a survey on the current status of integrated, thin-film YB a2 Cu 3 O 7 magnetometers is given.

Eddy-current nondestructive measurements with different HTS-SQUID spatial orientations

Superconducting QUantum Interference Devices (SQUIDs) are extremety sensitive detectors for the measurement of magnetic flux. In particular the high critical temperature SQUIDs, which can be operated at liquid nitrogen temperature with easy cryogenic requirement, are well suited for nondestructive evaluation (NDE) of cracks and corrosion which result from the machining process or aging of electrically conductive structures. The main characteristics of SQUIDs as eddy-current NDE sensors are: high sensitivity even in unshielded environments, a large frequency range of operation which allows the detection of surface and subsurface flaws and high spatial resolution. We performed magnetic field measurements on the eddy-current double-D inducing coil. used in many NDE experimental systems. and on 200 x 100 x 4 mm 3 Al plates with a 6mm surface-breaking hole using high critical temperature SQUIDs in unshielded environments. The sensitivity to the anomalous magnetic field related to the defect along the three spatial directions is discussed in terms of unperturbed magnetic field distributions due to the eddy-current source.

Theoretical analysis of localized magnetic flux measurement by needle probe

Analytical study of the needle probe measurement of magnetic flux was performed on the basis of the electromagnetic theory to provide an advanced theoretical backbone for the measuring method. Provided that the flux change in the specimen is homogeneous, the electromotive force induced between the needles represents exactly the flux change in the area between the needles. The measuring error in the needle probe measurement is caused by lack of homogeneity in the flux change postulated in the theory. Among several possible causes of flux non-homogeneity, the existence of domain structure is most intrinsic in Iocalized area measurements, so that in this study the measuring error due to this cause was calculated theoretically in detaiL It can be concluded that even in the cases of large domain size often occurring in oriented core materials, the localized measurements by the needle probe provide fairly reliable measure of the flux change in the area.