Measurement Of Magnetic Field Distribution Research Articles

Active shimming for a 25 T NMR superconducting magnet by spectrum convergence method

In the design of ultrahigh field nuclear magnetic resonance (NMR) superconducting magnets, it typically requires a high homogeneous magnetic field in the diameter of spherical volume (DSV) to obtain high spectrum resolution. However, shimming technique presents challenges due to the magnet bore space limitations, as accurate measurement of magnetic field distribution is very difficult, especially for customized micro-bore magnets. In this study, we introduced an active shimming method that utilized iterative adjustment of shim coil currents to improve the magnetic field homogeneity based on the full width at half maximum (FWHM) of the spectrum. The proposed method can determine the optimal set of currents for shim coils, effectively enhancing spatial field homogeneity by converging the FWHM. Experimental validation on a 25 T NMR superconducting magnet demonstrated the efficacy of the proposed method. Specifically, the active shimming method improved the field homogeneity of a 10 mm DSV from 7.09 ppm to 2.27 ppm with only four shim coils, providing a superior magnetic field environment for solid NMR and further magnetic resonance imaging (MRI) experiment. Furthermore, the proposed method can be promoted to more customized micro-bore magnets that require high magnetic field homogeneity.

Small scale magnetic field source detection using recessed atomic vapor cell

With the development of high spatial resolution spin image and magnetic field distribution measurement in atomic vapor cell, one can localize the position and calculate the magnetic moment of the field source around the cell. However, traditional cubic or spherical vapor cell can only measure the magnetic field distribution on one side of the field source, which limits the precision of the field inversion results. Here, we use a recessed atomic vapor cell to obtain field distribution around the source, which is positioned at the center of the cell. The magnetic field distributions around five sides of the field source are measured using movable stages and digital micro-mirror device. We combine the Levenberg–Marquardt algorithm with a genetic algorithm as the magnetic source localization algorithm to realize a quick global search as well as a precise local extreme point search. We obtain a spatial resolution of 219.2 × 387.0 × 451.5 μ m 3 and a magnetic field sensitivity of 0.06 nT / Hz 1 / 2 in a volume pixel of 0.0383 mm 3. The error in the localization of the magnetic source is 1.295 mm in the x direction, 185 μ m in the y direction, and 40 μ m in the z direction. The field distribution measurement method using recessed atomic vapor cell and related inversion algorithm demonstrated here have great potential applications in small scale field source detection of biology and materials science.

Frequency and modeshape evaluation of steam turbine blades using the metal magnetic memory method and vibration wave propagation

Identifying frequency and modeshape of vibration to which a given mechanical structure is subjected most of its operational time is a key issue in diagnostics of the technical condition of the machine. By evaluating these modal parameters, the sources of vibration, and thus the possible sources of damage can be identified. The current paper presents a new diagnostic technique based on the metal memory method (MMM), which can be used for identification of operational frequency and modeshape of steam turbine blades. The MMM method includes the measurement and analysis of blade's residual magnetic field (RMF) distribution, which reflects directions of main stresses resulting from working loads. It is demonstrated that based on the results of RMF measurement, vibration frequency and modeshape, to which the blade is subjected most of the time, can be evaluated. To calculate these modal parameters, stress waves propagating along the blade, and the resulting standing wave have been considered. Clear correlation between blade’s magnetic field distribution, stress distribution, and FEM results of frequency and modeshape calculation has been observed. It has been confirmed that the frequency and modeshape at which the blade vibrates during its normal operation is close to the blade's 10th natural frequency, which is about 1014 Hz.

Modelling and Measurement of Magnetic Field Distributions of Diametrical Magnetized Annular Permanent Magnet for Capsule Robot Driving

Modelling and Measurement of Magnetic Field Distributions of Diametrical Magnetized Annular Permanent Magnet for Capsule Robot Driving

Misalignment of magnetic field in DIII-D assessed by post-mortem analysis of divertor targets

We assess the toroidal magnetic field B t asymmetry in DIII-D due to a misalignment of the toroidal field coils with respect to the poloidal magnetic field coils and vacuum vessel. The peak-to-peak variation of the divertor strike point (SP) radius is measured to be 1 cm, with an n = 1 toroidal pattern. We use the centre of a narrow carbon deposition band on tungsten-coated divertor tiles just inside the outer strike point (OSP) as a proxy for the divertor SP location. The band occurred in a series of reverse B t discharges with the OSP positioned on the divertor inserts due to strong E × B drift transport of C from the inner to the outer SP through the private flux region. The variation in band radius (and hence the magnetic SP) is a (4.89 ± 0.31) mm shift toward (310 ± 4)° toroidal direction. These measurements agree well with previous measurements of the 3D magnetic field distribution (Luxon 2003 Nucl. Fusion 43 1813), simulations performed by the mafot field line integration code, and recent Langmuir probe measurements in the small-angle-slot (SAS) divertor (Watkins et al 2019 Nucl. Mater. Energy 18 46). Comparison of these measurements in the SAS divertor also indicates that there is the possibility of a tilt (in conjunction with the shift) of the B t coil field of (0.04 ± 0.07)° towards the toroidal angle of (215 ± 25)°. Previous measurements suggested a field misalignment of (4.6 ± 0.3) mm in the 270° toroidal direction, and a tilt of (0.06 ± 0.02)° toward the 114° toroidal direction, which is similar to the results reported here. These studies will be important for better understanding the radial variation of the toroidal strike line in DIII-D, for designing the new generation of SAS divertor, and for developing an understanding of the impact of error fields on tokamaks with tightly baffled slot divertors.

Development of Magnetic Field Mapping System for the MuSEUM Experiment With High Precision Using CW-NMR Probes

The MuSEUM experiment is planned at J-PARC to measure the hyperfine structure of a muonium at 1.7 T. The aim is to test the standard theory by precisely measuring the hyperfine structure of a muonium in the ground state. An important parameter is the magnetic field uniformity, which is required to be smaller than 0.2 ppm (peak-to-peak) in a rotating ellipsoid region with a diameter of 20 cm and a major axis length of 30 cm. To perform magnetic field shimming and verify the uniformity of the magnetic field, We are developing a magnetic field distribution measurement system using a CW (Continuous Wave)-NMR probe with a resolution of smaller than 10 ppb. In the present design, twenty-four CW-NMR probes are arranged in a semicircular circumference for quick measurement. Because each probe is located at a short distance, the effect of a crosstalk among probes should be minimised. Parameters of each probe has been determined and operation with multiple channels has been tested. Moreover, an influence of iron shims has been measured accurately with a eight-channel system.

3-D Visualization of Magnetic Field Using In-Line Holographic Microscopy for Micro-Magnetofluidic Applications

The increasing attentions paid to magnetofluidic devices with their applications in cell manipulation, drug delivery, immunoassay, etc., call for higher requirements for precise detection of the magnetic field parameters at micro-scale for more flexible manipulations in precision. Nevertheless, conventional magnetometers can only measure magnetic field strength or flux off the device, while it still remains a challenge to determine the real field distribution within the microchips with high precision. Herein, a digital holography-based technique capable of on- chip measurement and visualization of magnetic field distribution is presented to solve this problem. The interference pattern of the ferromagnetic chain is firstly recorded by an in-line holographic microscopy system with the assistance of the magnetic particles which can be aligned and form a chain along the field direction. Each part digitally reconstructed in different depths is then auto-focalized in an extended focus image with corresponding colors. The detection angle of the ferromagnetic chain is presented in three dimensions, covering a range from -40 to 40 degrees and allowing for an automatic recording and visualization of the magnetic field direction. Our on- chip measuring approach can pronouncedly enhance the magnetic control for magnetofluidic devices with complex 3-D architectures, facilitating their effective applications in the near future.

Kinetic-scale Flux Ropes: Observations and Applications of Kinetic Equilibrium Models

Abstract Magnetic flux ropes with helical field lines and a strong core field are ubiquitous structures in space plasmas. Recently, kinetic-scale flux ropes have been identified by high-resolution observations from the Magnetospheric Multiscale (MMS) spacecraft in the magnetosheath, which have drawn a lot of attention because of their nonideal behavior and internal structures. Detailed investigation of flux rope structure and dynamics requires the development of realistic kinetic models. In this paper, we generalize an equilibrium model to reconstruct a kinetic-scale flux rope previously reported via MMS observations. The key features in the magnetic field and electron pitch-angle distribution measurements of all four satellites are simultaneously reproduced in this reconstruction. Besides validating the model, our results also indicate that the anisotropic features previously attributed to asymmetric magnetic topologies in the magnetosheath can be alternatively explained by the spacecraft motion in the flux rope rest frame.

Measurement of magnetic field distribution produced by high-current pulse using Zeeman splitting of Na emission distributed by laser ablation.

Measurement of the magnetic field distribution in Z-pinch experiments remains an ongoing challenge. We present a method of measuring the radial distribution of the magnetic field around a copper rod using Zeeman splitting of sodium (Na) emission lines, in which an Na layer is formed by the laser ablation of NaCl crystals on a load surface. The load consists of a copper rod of 2mm diameter and is pre-covered on its surface by the NaCl crystals. An 8ns pulsed laser with an energy of 1J and wavelength of 532nm is focused on the crystals. The Na plasma is produced and expands from the surface of the copper rod into a vacuum. After applying a pulsed current with a peak value of 375 kA to the load, the Na 3s-3p doublet displays significant Zeeman splitting patterns. The self-luminosity of the Na plasma is recorded by a spectrometer coupled with an intensified charge-coupled device camera from an end-on view to eliminate the effects of different observing angles and Doppler shifts. We determine the magnetic field by fitting the measured spectra with the calculated results of the Voigt profile. The measurable range of radial position is 5-7mm, and the corresponding magnetic field is 5-15T. The averaged error of curve fitting is less than 12%.

Local measurements of the spatial magnetic field distribution in a z-pinch plasma during and near stagnation using polarization spectroscopy

We present here the detailed measurements of radial distribution of the magnetic field in a gas-puff z-pinch plasma at the final stages of the implosion phase and at stagnation. While the measurements are chordal, the radial distribution of different charge states was utilized to measure the magnetic field locally for certain radii, so that unlike chordal measurements in general, the magnetic field radial distribution was obtained with no need for the Abel inversion of the data. The distribution was measured using the Zeeman effect via a novel spectroscopic technique, at several axial locations, and demonstrates striking features such as the peak field remaining at a radius much larger than the stagnation radius at all times. Furthermore, while the distribution observed is sometimes monotonic with respect to the radius, it is often not, a behavior that can be linked to 2D features in the plasma column resulting from the Rayleigh–Taylor instability. The current flowing through the stagnating plasma was found to be a small fraction of the total current, resulting in clearly insufficient magnetic pressure to balance the plasma pressure at stagnation. The magnetic field data, taken over several axial positions, are used to obtain the true inductance in the imploding plasma for the first time; it is found that the data cannot explain the current turnover at stagnation. A simulation with the MACH2-Tabular Collisional-Radiative Equilibrium magnetohydrodynamics code in the r–z plane shows that the peak of the magnetic field pinches to a much smaller radius than is observed in the spectroscopic data. Furthermore, the depth of the computed current turnover at stagnation is smaller than the measured one. The two observed features of a radially extended magnetic field at stagnation together with a deep current turnover are a challenge to match in simulations. Various calculations and estimates of the inductive and resistive load voltages are examined to ascertain if they are responsible for the observed current notch. The results demonstrate that the knowledge of the true inductance in the driven load requires such magnetic-field-distribution measurements and that imaging data or electrical measurements are insufficient.

Measurement of the spatial magnetic field distribution in a single large spin-exchange relaxation-free vapor cell

This article presents a method to determine the magnetic field distribution within the vapor cell of a spin-exchange relaxation-free (SERF) atomic magnetometer with a sensitivity of the order of 10 femtoTesla and a bandwidth of DC to 100 Hz, in the presence of an uncompensated ambient magnetic field of up to several nanoTesla. The method is based on the analysis of the atomic polarization in a multichannel pump–probe configuration, in which a spatially selective optical pumping enables to probe specific layers of the atomic vapor contained in a gas-buffered cell. An SERF magnetometer is inherently sensitive to one component of the magnetic field, orthogonal to the pump and probe laser beams. The sensor’s performance can be drastically degraded by the other uncompensated components of the magnetic field. Typically, SERF magnetometry requires very good magnetic shielding and active compensation of residual magnetic field to properly function; this is commonly achieved by applying a complex design of Helmholtz coils in a sophisticated compensation procedure. The method suggested in this article eliminates the influence of non-uniform residual magnetic fields on the accuracy of measurements without precise compensation of the interfering field components. This procedure is used to simplify the measurements of magnetic field distribution in a large vapor cell with required accuracy. This is critically important for precise multichannel magnetic field mapping used for localization of the magnetic dipole-field source.

Magneto-optical imaging plate with backlight for quantitative measurement of magnetic field distribution

We develop a light source, a polarizing film, and a highly bismuth-substituted neodymium iron garnet film which exhibits a large Faraday rotation angle of 11 deg., prepared on a ϕ150 mm diameter glass substrate by the metal–organic decomposition method. To measure the magnetic field distribution quantitatively, the 45° polarizer method was carried out. The magnetic field distribution of a bar magnet was quantitatively measured. Furthermore, we also carried out the polarization modulation method using a white light for quantitative magneto-optical (MO) imaging.

Accuracy enhancement of magnetic field distribution measurements within a large cell spin-exchange relaxation-free magnetometer

The factorial design technique is implemented to achieve greater accuracy in the determination of magnetic field distribution within a single cell of spin-exchange relaxation-free atomic magnetometer. Three-dimensional magnetic field distribution within a single vapor cell can be found by consecutively pumping, layer by layer, all the cell volumes perpendicular to the probe laser beam, detected by a photodiode array. Thus each element of the array collects information about the magnetic field in the small volume (voxel) which forms when the corresponding part of the probe beam and optically pumped layer cross. One of the most effective ways to enhance measurement accuracy is repeated pumping of the layers and averaging the measured results. However, the measurement time is multiplied several times due to the repeated scanning of the cell volume. The suggested technique enables increased measurement accuracy of each voxel while preserving the number of measurements. Magnetic field distribution is determined by the illumination of the cell layers one by one or simultaneously, according to a special algorithm, with subsequent multifactorial analysis of the obtained results.

Measurements of the spatial magnetic field distribution in a z-pinch plasma throughout the stagnation process

We report on measurements, made for the first time for an imploding plasma at its stagnation, of the magnetic field spatial distribution. Utilized for the measurements is a spectroscopic technique based on simultaneous recording of each of the left- and right-handed circularly polarized Zeeman emissions. While this method allows for overcoming the Stark- and Doppler-broadenings that obscure the Zeeman splitting, it requires a line of sight that is parallel to the magnetic field lines. To this end, the radial charge-state distribution in the stagnating z-pinch plasma was measured, and the method was employed for emission lines of several selected charge states located in various radial locations. This also allowed for measuring the time-resolved magnetic field radial distribution in a single discharge, which is advantageous for high-energy-density plasma experiments. These distributions were measured at various locations along the pinch axis.

RETRACTED: A new perspective on remote Saddle Sine and Cosine coils technique for determination of tokamak plasma equilibrium status

In this contribution we have presented a new perspective for determination of tokamak plasma column shift based on remote multipole moments technique. First, we presented analytical details for using this technique. Then principle of different models based on this technique for design and fabrication of a six coils will be presented: four modified Rogowski coil (two Cosine coils and two Sine coils) and two Saddle coils (Saddle Sine coil (SSC) and Saddle Cosine coil (SCC)). Also, in order to comparison of results the flux loops technique is used. Because of continuous measurements of magnetic field distribution around the tokamak plasma using multipole coils, this technique give us more reliable information about the plasma current displacement.

Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease.

Quantitative magnetic resonance imaging (qMRI) describes the development and use of MRI to quantify physical, chemical, and/or biological properties of living systems. Neuromuscular diseases often exhibit a temporally varying, spatially heterogeneous, and multi-faceted pathology. The goal of this protocol is to characterize this pathology using qMRI methods. The MRI acquisition protocol begins with localizer images (used to locate the position of the body and tissue of interest within the MRI system), quality control measurements of relevant magnetic field distributions, and structural imaging for general anatomical characterization. The qMRI portion of the protocol includes measurements of the longitudinal and transverse relaxation time constants (T1 and T2, respectively). Also acquired are diffusion-tensor MRI data, in which water diffusivity is measured and used to infer pathological processes such as edema. Quantitative magnetization transfer imaging is used to characterize the relative tissue content of macromolecular and free water protons. Lastly, fat-water MRI methods are used to characterize fibro-adipose tissue replacement of muscle. In addition to describing the data acquisition and analysis procedures, this paper also discusses the potential problems associated with these methods, the analysis and interpretation of the data, MRI safety, and strategies for artifact reduction and protocol optimization.

Measurements of Magnetic Field Distributions With an Optically Pumped K-Rb Hybrid Atomic Magnetometer

We have developed a K-Rb hybrid optically pumped atomic magnetometer (OPAM) for biomagnetic measurements. With this hybrid OPAM, we used a linear photodiode array and a charge-coupled device (CCD) sensor to instantaneously measure 1-D and 2-D magnetic field distributions generated by a test coil, respectively. The measured distributions were compared with those calculated using the Biot-Savart law and showed good agreement for the linear photodiode array; however, in the case of the CCD sensor in the area away from the test coil, the intensity of the detected magnetic field was slightly different from the calculated one, possibly because of the diffusion of the spin-polarized atoms as well as smear and blooming in the CCD sensor. The sensitivity of the OPAM was 5-6 pT/Hz 1/2 using the linear photodiode array and ~10 pT/Hz 1/2 using the CCD sensor. In future experiments, we plan to fabricate a sensor cell with a high density of probe atoms and reduce the noise generated in the electronic circuitry of the detectors in order to increase sensitivity.

Homogeneous Magnetic Field Source For Attenuated Total Reflection

Abstract The paper is focused on the study of two-dimensional magnetic field distribution used for an analysis of samples containing magnetically active films by means of the Attenuated Total Reflection (ATR) method. The design of a proposed electromagnet and the magnetic field model computation are presented together with the results obtained from magnetic field distribution measurement. The ATR method can provide information about a thin film thickness, refractive index, and attenuation in addition to the perfunctory coupling of an optical wave into and off a waveguide [1, 2]. The prism coupling conditions are determined for magnetic structures with induced anisotropy. The prism - a film coupler is located in the central cavity of a magnetic yoke. By current switching in the coils, we can change the amplitude and magnetic field direction in order to modulate the induced anisotropy in a thin film with magnetic ordering. By the in-plane modulation of the magnetization direction in the samples, we can change the rotation and elasticity of outgoing light.

Measurement of GHz range magnetic field distribution near a coplanar waveguide using a beating field-type magnetic force microscope

This paper describes the measurement of microscopic radio frequency (RF) field distribution with a magnetic force microscope (MFM) tip exploring the beat signal from a coplanar waveguide (CPW) with a signal line as fine as 5 μm and a ground line of 50 μm. To produce a beating field in close proximity to the CPW, two RF currents with slightly different frequencies are supplied, where one is the signal current with a fixed frequency of 1.1 GHz and the other is the reference. The reference is adjusted to produce a beat with a frequency near the cantilever mechanical resonance frequency. Thus, the mechanical resonance of the cantilever excited by the beat field includes information about the 1.1 GHz field distribution from the CPW signal current. Detection of a beating field with a MFM tip can provide very high resolutions of the RF field distribution in the proximity of RF circuit component.

Comparison of Magnetic Field Parameters Obtained from 2D and 3D Finite Element Analysis for an Active Magnetic Bearing

The paper presents numerical modeling of 8-pole radial active magnetic bearing based on 2-dimensional and 3-dimensional magnetic field computation with nonlinear characteristic of magnetic material. In this work has been used numerical models based on finite element methods. In paper has been specified differences between two and three dimensional models. Results of numerical calculation have been verified by measurement of magnetic field distribution and inductances of windings.