Local Magnetic Field Variations Research Articles

MR based magnetic susceptibility measurements of 3D printing materials at 3 Tesla

Commercial availability, ease of printing and cost effectiveness have rendered 3D printing an essential part of magnetic resonance (MR) experimental design. However, the magnetic properties of several materials contemporarily used for 3D printing are lacking in literature to some extent. A database of the magnetic susceptibilities of several commonly used 3D printing materials is provided, which may aid MR experiment design. Here, we exploit the capability of magnetic resonance imaging (MRI) to map the local magnetic field variations caused by these materials when placed in the scanner's B0 field. Exact analytical solutions of the magnetic flux density distribution for a cylindrical geometry are utilized to fit experimentally obtained data with theory in order to quantify the magnetic susceptibilities. A detailed explanation of the data processing and fitting procedure is presented and validated by measuring the susceptibility of air along with high resolution MR measurements. Furthermore, an initiative is taken to address the need for a comprehensive database comprising of not only the magnetic susceptibilities of 3D printing materials, but also information on the 3D printing parameters, the printers used, and other information available for the materials that may also influence the measured magnetic properties. An open platform with the magnetic susceptibilities of materials reported in this work besides existing literature values is provided here, with the aim to invite researchers to enable further extension and development towards an open database to characterize commonly used 3D printing materials based on their magnetic properties.

Dynamics of Current-Sharing Within a REBCO Tape-Stack Cable

Current redistribution in a non-insulating REBCO tape-stack cable may enable the use of such a cable in a high field dipole without provision for transposition of the constituent tapes. As current is increased in the REBCO cable a dynamic resistance arises in the superconducting layer near operation at critical current. The critical current of each tape in the stack is determined by the local magnetic field variation within the cable. The superconducting-normal transition in REBCO occurs over a working range of current and, within that range, the longitudinal Ohmic electric field produces a transverse electric field between adjacent tapes with different critical currents. Circuit models indicate that, as a given tape approaches critical operation, current will naturally redistribute to neighboring tapes with higher current carrying capacity due to the dynamic rise in resistivity thus preventing premature quench. A multi-scale model is being developed to study the dynamics of current-sharing, the limits of stability, and the impact of fluctuations in critical-current density along each tape.

Radio Frequency-Based Diagnostics for Superconducting Magnets

Operation of high-field superconducting magnets relies on diagnostic instrumentation for measuring strain, temperature, and magnetic field variations, detecting quenching, and identifying performance problems. Discrete sensor implementation is costly, requires multi-channel data acquisition systems, and sensor density is often insufficient to resolve problematic locations spatially. Distributed sensing is a viable alternative approach providing location-specific diagnostic information over single terminal output. In particular, it can be the key to quench protection of high-temperature superconductor (HTS)-based magnets where hot spots are known to form and persist before a quench. Fiber-optic technology offers distributed sensing solution for magnets but suffers from drawbacks such as fiber fragility, high costs of optical interrogators, and difficulty in differentiating between physical quantities such as temperature and strain. We propose an alternative, robust, and easily integrable way of implementing distributed sensing in magnets using radio-frequency (RF) technologies. RF Time Domain Reflectometry (TDR) technique has been around for nearly 60 years, and it is an essential diagnostic tool used in multiple areas of technology and applied research. We discuss operational principles and practical implementation of RF TDR sensors capable of detecting local variations of strain, temperature, and magnetic field through changes in RF impedance and wave propagation time. Results of cryogenic testing of our TDR sensors with HTS tape conductors are presented. The perspective of enabling a new diagnostics paradigm for high-energy physics and fusion energy applications based on distributed RF sensing is discussed.

A Low Complexity Persistent Reconnaissance Algorithm for FANET

In recent years, with the rapid progress of unmanned aerial vehicle (UAV) technology, UAV-based systems have been widely used in both civilian and military applications. Researchers have proposed various network architectures and routing protocols to address the network connectivity problems associated with the high mobility of UAVs, and have achieved considerable results in a flying ad hoc network (FANET). Although scholars have noted various threats to UAVs in practical applications, such as local magnetic field variation, acoustic interference, and radio signal hijacking, few studies have taken into account the dynamic nature of these threat factors. Moreover, the UAVs' high mobility combined with dynamic threats makes it more challenging to ensure connectivity while adapting to ever-changing scenarios. In this context, this paper introduces the concept of threat probability density function (threat PDF) and proposes a particle swarm optimization (PSO)-based threat avoidance and reconnaissance FANET construction algorithm (TARFC), which enables UAVs to dynamically adapt to avoid high-risk areas while maintaining FANET connectivity. Inspired by the graph editing distance, the total edit distance (TED) is defined to describe the alterations of the FANET and threat factors over time. Based on TED, a dynamic threat avoidance and continuous reconnaissance FANET operation algorithm (TA&CRFO) is proposed to realize semi-distributed control of the network. Simulation results show that both TARFC and TA&CRFO are effective in maintaining network connectivity and avoiding threats in dynamic scenarios. The average threat value of UAVs using TARFC and TA&CRFO is reduced by 3.99~27.51% and 3.07~26.63%, respectively, compared with the PSO algorithm. In addition, with limited distributed moderation, the complexity of the TA&CRFO algorithm is only 20.08% of that of TARFC.

Magnetic molecules as local sensors of topological hysteresis of superconductors

Superconductors and magnetic materials, including molecules, are key ingredients for quantum computing and spintronics. However, only a little is known about how these materials interact in multilayer nanostructures like the hybrid architectures nowadays under development for such advanced applications. Here, we show that a single layer of magnetic molecules, Terbium(III) bis-phthalocyaninato (TbPc2) complexes, deposited under controlled UHV conditions on a superconducting Pb(111) surface is sensitive to the topology of the intermediate state of the superconductor, namely to the presence and evolution of superconducting and normal domains due to screening and penetration of an external magnetic field. The topological hysteresis of the superconducting substrate imprints a local evolution of the magnetisation of the TbPc2 molecules in the monolayer. Element and surface selective detection is achieved by recording the X-ray magnetic circular dichroism of the Tb atoms. This study reveals the impressive potential of magnetic molecules for sensing local magnetic field variations in molecular/superconductor hybrid devices, including spin resonators or spin injecting and spin filtering components for spintronics applications.

Effect of Zeolite Addition on Partial Discharge and Dielectric Behavior of Thermally Aged Synthetic Ester Fluid Under External Magnetic Field

In recent times, ester based liquids have met high standards and are being considered as potential alternatives over mineral oil transformer insulation. This paper investigates the effect of adding micro-porous zeolite to the thermally aged synthetic ester oil (SEO) to evaluate its electrical performance under the influence of external local magnetic field variation from 0 mT to 165 mT. It is observed that the Corona Inception Voltage (CIV) and Particle Levitation Voltage (PLV) have a declining tendency with ageing, in the presence of magnetic field. Under the influence of magnetic flux density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\simeq 160$ </tex-math></inline-formula> mT), the detected UHF signal was dominant at low frequencies (400 MHz - 900 MHz). The UHF voltage signals peak-to-peak magnitude demonstrates that the discharge magnitude increases with increased ageing duration. Further, it was observed that the addition of zeolite in the synthetic ester liquid tends to improve the electrical performance of the aged oil. The fluorescence spectroscopic analysis shows a shift of the fluorescence maxima together with increased intensity in samples with zeolite as compared to the samples without zeolite addition, which shows a relatively lower concentration of the polar contamination. The frequency dielectric spectroscopy also confirms that synthetic ester specimens showed improved dielectric properties after the zeolite treatment.

Magnetic filaments for anisotropic composite polymers

The use of contactless magnetic forces meets numerous needs in microelectromechanical systems (MEMS) or microfluidic devices. In this view, heterogeneous materials integrating magnetic nanostructures within a non-magnetic matrix such as polymer can produce local variations of magnetic field, at the sub-micrometer scale. Here we report on the synthesis of magnetic composites using electrospun nanofilaments and a polydimethylsiloxane (PDMS) matrix. Varying the precursor nature and heat treatment conditions, we obtained single phase filaments of Fe, FeNi, and MFe2O4 (M = Co, Fe, Ni). Thanks to a fine investigation of their structure and morphology, it was possible to measure from magnetically-soft (µ0HC ⩽ 5 mT) to relatively hard (µ0HC up to 93 mT, MR/MS up to 0.5) behaviors. The common one-dimensional shape of these filaments leads to an anisotropic magnetic response. This can be exploited to achieve self-organization of the filaments in arrays within the non-magnetic matrix. We show the first step towards the development of magnetically anisotropic membranes of PDMS with 0.23 wt% Fe filaments. These composite materials are promising for implementing magnetic functions in microsystems while circumventing complex micro-fabrication steps.

The influence of iron oxidation state on quantitative MRI parameters in post mortem human brain

A variety of Magnetic Resonance Imaging (MRI) techniques are known to be sensitive to brain iron content. In principle, iron sensitive MRI techniques are based on local magnetic field variations caused by iron particles in tissue. The purpose of this study was to investigate the sensitivity of MR relaxation and magnetization transfer parameters to changes in iron oxidation state compared to changes in iron concentration. Therefore, quantitative MRI parameters including R1, R2, R2∗, quantitative susceptibility maps (QSM) and magnetization transfer ratio (MTR) of post mortem human brain tissue were acquired prior and after chemical iron reduction to change the iron oxidation state and chemical iron extraction to decrease the total iron concentration. All assessed parameters were shown to be sensitive to changes in iron concentration whereas only R2, R2∗ and QSM were also sensitive to changes in iron oxidation state. Mass spectrometry confirmed that iron accumulated in the extraction solution but not in the reduction solution. R2∗ and QSM are often used as markers for iron content. Changes in these parameters do not necessarily reflect variations in iron content but may also be a result of changes in the iron’s oxygenation state from ferric towards more ferrous iron or vice versa.

A low-cost device for measuring local magnetic anomalies in volcanic terrain

Abstract. Reconstructions of the past behavior of the geomagnetic field critically depend on the magnetic signal stored in extrusive igneous rocks. These rocks record the Earth's magnetic field when they cool and retain this magnetization on geological timescales. In rugged volcanic terrain, however, the magnetic signal arising from the underlying flows may influence the ambient magnetic field as recorded by newly formed flows on top. To measure these local anomalies in the Earth's magnetic field directly we developed a low-cost field magnetometer based on a fluxgate sensor. To improve the accuracy of the obtained paleomagnetic vector and user-friendliness of the device, we combined this fluxgate sensor with tilt and GPS sensors to rotate the measured magnetic vector to true north, east, and down. The data acquisition is done using a ruggedized laptop, and data are immediately available for first-order interpretation. The first measurements done on Mt. Etna show local variations in the ambient magnetic field that are larger than expected and illustrate both the accuracy (certainly &lt;0.5∘ in paleomagnetic direction) and potential of our new device.

Spread-spectrum magnetic resonance imaging.

A novel method for the acceleration of MRI acquisition is proposed that relies on the local modulation of magnetic fields. These local modulations provide additional spatial information for image reconstruction that is used to accelerate image acquisition. In experiments and simulations, eight local coils connected to current amplifiers were used for rapid local magnetic field variation. Acquired and simulated data were reconstructed to quantify reconstruction errors as a function of the acceleration factor and applied modulation frequency and strength. Experimental results demonstrate a possible acceleration factor of 2 to 4. Simulations demonstrate the challenges and limits of this method in terms of required magnetic field modulation strengths and frequencies. A normalized mean squared error of below 10% can be achieved for acceleration factors of up to 8 using modulation field strengths comparable to the readout gradient strength at modulation frequencies in the range of 5 to 20 kHz. Spread-spectrum MRI represents a new approach to accelerate image acquisition, and it can be independently combined with traditional parallel imaging techniques based on local receive coil sensitivities.

Improvement of reproducibility in quantitative susceptibility mapping (QSM) and transverse relaxation rates ( ) after physiological noise correction.

Numerous studies have suggested that quantitative susceptibility mapping (QSM) and transverse relaxation rates ( ) are useful to monitor neurological diseases. For clinical use of QSM and , reproducibility is an important feature. However, respiration-induced local magnetic field variation makes artifacts in gradient echo-based images and reduces the reproducibility of QSM and . To investigate the improvement of reproducibility of QSM and after the correction of respiration-induced field variation, and to assess the effect of varying types of the region of interest (ROI) analysis on reproducibility. Reproducibility study. Ten controls. 3T/multiecho gradient echo sequence. Intrascan reproducibility of QSM and was investigated in ROIs before and after the respiration correction. Reproducibility was obtained by the square of voxel-wise correlation coefficients between scans. A paired t-test was performed for comparison between before and after the respiration correction and between QSM and . Based on the ROI analysis, reproducibility increased after the respiration correction. Reproducibility in the white matter (11.89% increased in QSM and 23.38% in , P = 0.009 and 0.024, respectively) and deep gray matter (5.50% increased in QSM and 13.96% in , P = 0.024 and 0.019, respectively) increased significantly after the respiration correction. Reproducibility of was higher than that of QSM in the whole brain and cortical gray matter, while QSM maps showed higher reproducibility than in the white matter and deep gray matter. Respiration-induced error correction significantly improved reproducibility in QSM and mapping. QSM and mapping showed a different level of reproducibility depending on the types of ROI analysis. 4 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018.

Probing the origins of inhomogeneous broadening in nitrogen-vacancy centers with Doppler-free-type spectroscopy

In order to better understand the underlying fundamental physical processes in nitrogen-vacancy centers in diamond, and to realize improvements in the use of this system for technological applications, it is imperative to gain new insight into the origins of the apparent inhomogeneous broadening. In this work we make use of a novel type of spectroscopy developed specifically for this task. The pump-probe spectroscopy closely follows Doppler-free spectroscopy used in atomic vapor. We show that the origin of inhomogeneous broadening comes from local magnetic field variations in the diamond lattice.

Influence of a large-scale field on energy dissipation in magnetohydrodynamic turbulence

In magnetohydrodynamic (MHD) turbulence, the large-scale magnetic field sets a preferred local direction for the small-scale dynamics, altering the statistics of turbulence from the isotropic case. This happens even in the absence of a total magnetic flux, since MHD turbulence forms randomly oriented large-scale domains of strong magnetic field. It is therefore customary to study small-scale magnetic plasma turbulence by assuming a strong background magnetic field relative to the turbulent fluctuations. This is done, for example, in reduced models of plasmas, such as reduced MHD, reduced-dimension kinetic models, gyrokinetics, etc., which make theoretical calculations easier and numerical computations cheaper. Recently, however, it has become clear that the turbulent energy dissipation is concentrated in the regions of strong magnetic field variations. A significant fraction of the energy dissipation may be localized in very small volumes corresponding to the boundaries between strongly magnetized domains. In these regions the reduced models are not applicable. This has important implications for studies of particle heating and acceleration in magnetic plasma turbulence. The goal of this work is to systematically investigate the relationship between local magnetic field variations and magnetic energy dissipation, and to understand its implications for modeling energy dissipation in realistic turbulent plasmas.

Electromagneticwaves caused by tectonic earth movement

Prior to large earthquakes the Earth sends out transient signals, sometimes strong, more often subtle and fleeting. These signals may consist of local magnetic field variations, electromagnetic emissions over a wide range of frequencies, a variety of atmospheric and ionospheric phenomena. Great uncertainty exists as to the nature of the processes that could produce such signals, both inside the Earth’s crust and at the surface..When rocks are stressed, peroxy links break, releasing electronic charge carriers, h_, known as positive holes. The positive holes are highly mobile and can flow out of the stressed subvolume. The situation is similar to that in a battery. The h_ outflow is possible when the battery circuit closes. The h_ outflow constitutes an electric current, which generates magnetic field variations and low frequency EM emissions. The study of chemical processes based on a model equation for electromagnetic waves is presented. Keywords : Electromagnetic waves; pre earthquake; tectonic.

Electromagneticwaves caused by tectonic earth movement

Prior to large earthquakes the Earth sends out transient signals, sometimes strong, more often subtle and fleeting. These signals may consist of local magnetic field variations, electromagnetic emissions over a wide range of frequencies, a variety of atmospheric and ionospheric phenomena. Great uncertainty exists as to the nature of the processes that could produce such signals, both inside the Earth’s crust and at the surface..When rocks are stressed, peroxy links break, releasing electronic charge carriers, h_, known as positive holes. The positive holes are highly mobile and can flow out of the stressed subvolume. The situation is similar to that in a battery. The h_ outflow is possible when the battery circuit closes. The h_ outflow constitutes an electric current, which generates magnetic field variations and low frequency EM emissions. The study of chemical processes based on a model equation for electromagnetic waves is presented.

Innovative NDT Technique Based on Ferrofluids for Detection of Surface Cracks

An innovative NDT technique is proposed for surface inspection of materials not necessarily magnetic or conductive, based on local magnetic field variations due to ferrofluid deposited in the cracks. The feasibility of the technique is assessed preliminarily, based on signal detectability without applied external magnetic field, and under applied DC fields. The signals (local magnetic flux density variations) are quantified analytically, experimentally and numerically. The model agrees well with the tests, showing that the signal increases with the applied field strength, up to the saturation magnetization of the ferrofluid, and decreases with the distance to the crack longitudinal axis, and thus it can provide useful estimations of the signal. The proposed technique, requiring application of external fields to magnetize the ferrofluid to enhance the signal, seems promising: the model suggests that signals associated to cracks significantly smaller than surface cracks in a target application like aircraft skin panel inspection NASA STD-5009 are easily detectable with commercial magnetometers.

Dynamic loss analysis for 4.5 MJ SMES

Dynamic or transient loss inside the proposed sector toroidal coil cryostat has been evaluated for the indigenous development of 4.5 MJ/1 MW superconducting magnetic energy storage (SMES) at VEC Centre, Kolkata. The frequent charging/discharging mode of operation in SMES coil results in dynamic loss that needs to be addressed for safe and reliable operation of the coil. The dynamic losses are derived considering local bi-directional magnetic field variation on the conductor for constant voltage output across the SMES coil through DC link. This loss will eventually determine the stability of Rutherford type cable inside the bath cooled cryostat. The result of dynamic loss analysis and subsequent additional cryogen load on plant is presented here.

2D harmonic filtering of MR phase images in multicenter clinical setting: Toward a magnetic signature of cerebral microbleeds

Cerebral microbleeds (CMBs) have emerged as a new imaging marker of small vessel disease. Composed of hemosiderin, CMBs are paramagnetic and can be detected with MRI sequences sensitive to magnetic susceptibility (typically, gradient recalled echo T2* weighted images). Nevertheless, their identification remains challenging on T2* magnitude images because of confounding structures and lesions. In this context, T2* phase image may play a key role in better characterizing CMBs because of its direct relationship with local magnetic field variations due to magnetic susceptibility difference. To address this issue, susceptibility-based imaging techniques were proposed, such as Susceptibility Weighted Imaging (SWI) and Quantitative Susceptibility Mapping (QSM). But these techniques have not yet been validated for 2D clinical data in multicenter settings. Here, we introduce 2DHF, a fast 2D phase processing technique embedding both unwrapping and harmonic filtering designed for data acquired in 2D, even with slice-to-slice inconsistencies. This method results in internal field maps which reveal local field details due to magnetic inhomogeneity within the region of interest only. This technique is based on the physical properties of the induced magnetic field and should yield consistent results. A synthetic phantom was created for numerical simulations. It simulates paramagnetic and diamagnetic lesions within a ‘brain-like’ tissue, within a background. The method was evaluated on both this synthetic phantom and multicenter 2D datasets acquired in standardized clinical setting, and compared with two state-of-the-art methods. It proved to yield consistent results on synthetic images and to be applicable and robust on patient data. As a proof-of-concept, we finally illustrate that it is possible to find a magnetic signature of CMBs and CMCs on internal field maps generated with 2DHF on 2D clinical datasets that give consistent results with CT-scans in a subsample of 10 subjects acquired with both modalities.

Pre-earthquake signals: Underlying physical processes

Prior to large earthquakes the Earth sends out transient signals, sometimes strong, more often subtle and fleeting. These signals may consist of local magnetic field variations, electromagnetic emissions over a wide range of frequencies, a variety of atmospheric and ionospheric phenomena. Great uncertainty exists as to the nature of the processes that could produce such signals, both inside the Earth’s crust and at the surface. The absence of a comprehensive physical mechanism has led to a patchwork of explanations, which are not internally consistent. The recognition that most crustal rocks contain dormant electronic charge carriers in the form of peroxy defects, O 3 Si / OO ⧹ SiO 3 , holds the key to a deeper understanding of these pre-earthquake signals from a solid state physics perspective. When rocks are stressed, peroxy links break, releasing electronic charge carriers, h ·, known as positive holes. The positive holes are highly mobile and can flow out of the stressed subvolume. The situation is similar to that in a battery. The h · outflow is possible when the battery circuit closes. The h · outflow constitutes an electric current, which generates magnetic field variations and low frequency EM emissions. When the positive holes arrive at the Earth’s surface, they lead to ionization of air at the ground–air interface. Under certain conditions corona discharges occur, which cause RF emission. The upward expansion of ionized air may be the reason for perturbations in the ionosphere. Recombination of h · charge carriers at the surface leads to a spectroscopically distinct, non-thermal IR emission.

Identification of breast calcification using magnetic resonance imaging

MRI phase and magnitude images provide information about local magnetic field variation (DeltaB0), which can consequently be used to understand tissue properties. Often, phase information is discarded. However, corrected phase images are able to produce contrast as a result of magnetic susceptibility differences and local field inhomogeneities due to the presence of diamagnetic and paramagnetic substances. Three-dimensional (3D) susceptibility weighted imaging (SWI) can be used to probe changes in MRI phase evolution and, subsequently, result in an alternate form of contrast between tissues. For example, SWI has been useful in the assessment of negative phase induced DeltaB0 modulation due to the presence of paramagnetic substances such as iron. Very little, however, has been done to assess positive phase induced contrast changes resulting from the presence of diamagnetic substances such as precipitated calcium. As ductal carcinoma in situ, which is the precursor of invasive ductal cancer, is often associated with breast microcalcification, the authors proposed using SWI as a possible visualization technique. In this study, breast phantoms containing calcifications (0.4-1.5 mm) were imaged using mammography, computed tomography (CT), and SWI. Corrected phase and magnitude images acquired using SWI allowed identification and correlation of all calcifications seen on CT. As the approach is a 3D technique, it could potentially allow for more accurate localization and biopsy and maybe even reduce the use of gadolinium contrast. Furthermore, the approach may be beneficial to women with dense breast tissue where the ability to detect microcalcification with mammography is reduced.