Homogeneous Field Research Articles

Harmonic analysis and optimization for closed-loop superconducting shim coils of 7 T MRI magnet.

High-resolution brain imaging is crucial in clinical diagnosis and neuroscience, with ultra-high field strength MRI systems ( ) offering significant advantages for imaging neuronal microstructures. However, achieving magnetic field homogeneity is challenging due to engineering faults during the installation of superconducting strip windings and the primary magnet. This study aims to design and optimize active superconducting shim coils for a 7 T animal MRI system, focusing on the impact of safety margin, size, and adjustability of the second-order shim coils on the MRI system's optimization. The study employs a nonlinear optimization method to determine the parameters of the shim coils, considering the size of the coil, the level of undesired harmonics, and the whole number approximation of the turns in each coil. The study also conducts a thorough robustness analysis, examining the effects of coil winding accuracy, former processing accuracy, and assembly angle accuracy on the harmonic intensity of each coil. The optimization design results for the 7 T MRI system's shim coils show that the magnetic field changes are less than 0.5 %. After second-order shimming and the harmonic coupling an, the low-order harmonics are minimized, resulting in an improved magnetic field peak-to-peak uniformity from 254.47 to 8.970ppm. The study successfully demonstrates the creation of a set of second-order shim coils for a 7 T animal MRI system through numerical optimization. The design outputs provide essential technological support for the development of a human whole-body 7 T MRI system, ensuring high-quality imaging at the neuronal level. The project also highlights the importance of considering manufacturing and assembly flaws in the shim coil design process to achieve effective shimming in practical engineering scenarios.

Write error ballooning due to anisotropy variation within the free layer in perpendicular magnetic tunnel junctions

Abstract A finite-temperature micromagnetic investigation is conducted to analyze magnetization switching and write error rates in perpendicular spin-transfer torque random access memory (STT-RAM), including the influence of domain wall (DW) pinning arising from variations within the free layer (FL) magnet and stray field from reference layer assembly in the magnetic tunnel junction. The study reveals that uneven perpendicular magnetic anisotropy within the FL could lead to the formation of pinning sites that give rise to metastable states during switching. Such metastable states could result in a significant increase in the write error rates as compared to the ideal situation. Unlike the case with stray fields arising from reference layer assembly, where the DW propagation is hindered only for one switching direction, inhomogeneity within the FL causes the DW pinning to occur in both switching directions. Combined effects of FL inhomogeneity and stray field from the reference layer result in write error ballooning similar to those reported in recent experiments. The impact of free layer inhomogeneity could be further exacerbated by the stray field for one of the switching directions. Surprisingly, the impact of inhomogeneity is also observed to persist at smaller FL dimensions where quasi-coherent switching is expected. These findings provide deeper insights into the potential factors contributing to anomalous write errors in perpendicular STT-RAM.

Application of J-Integral to a Random Elastic Medium

Abstract This study investigates the use of the J-integral to compute the statistics of the energy release rate of a random elastic medium. The spatial variability of the elastic modulus is modeled as a homogeneous lognormal random field. Within the framework of Monte Carlo simulation, a modified contour integral is applied to evaluate the first and second statistical moments of the energy release rate. These results are compared with the energy release rate calculated from the potential energy function. The comparison shows that, if the random field of elastic modulus is homogenous in space, the path independence of the classical J-integral remains valid for calculating the mean energy release rate. However, this path independence does not extend to the higher order statistical moments. The simulation further reveals the effect of the correlation length of the spatially varying elastic modulus on the energy release rate of the specimen.

TEMPERATURE FIELD OPTIMIZATION AND TRIBOLOGICAL PERFORMANCE FOR BATCH PREPARATION OF DIAMOND-COATED MECHANICAL SEAL RINGS BY HFCVD METHOD

Diamond coatings are deposited on the end surfaces of mechanical seal rings by the hot filament chemical vapor deposition (HFCVD) method. To improve the homogeneity of batch preparation of HFCVD diamond-coated seal rings, the temperature distributions on the end surfaces of seal rings are simulated and optimized using the finite volume method (FVM). The influence mechanism of various setup parameters on coating deposition is demonstrated systematically with the orthogonal experimental method, including the filament diameter d, the filament temperature [Formula: see text], the separation between two adjacent filaments D and the filament height H. The optimal HFCVD setting parameters for batch preparation of diamond-coated seal rings are proposed based on the simulation results of orthogonal experiments. Afterwards, mass deposition experiments of seal rings are conducted using the optimal setting parameters, and the diamond coatings deposited on the end surfaces of seal rings are characterized. Furthermore, mechanical seal operation bench tests are carried out to evaluate the abrasion resistance of diamond-coated seal rings. The results indicate that the optimized setting parameters can produce a homogeneous temperature field, and diamond coatings with uniform thickness and excellent wear resistance can be obtained in the batch preparation of diamond-coated seal rings.

THE SYNERGIC ELECTROWEAK SYNCHROTRON RADIATION

The power spectral formula is derived for radiation of neutral electroweak bosons from an electron in a weak external homogeneous electromagnetic field. The calculation method is based on the source theory formulation of quantum field theory introduced by Schwinger. For ultra-relativistic electrons, it is derived that the Z0 emission rate is suppressed relative to photon emission. This implies that the decay of electron into W-boson and neutrino is also exponentially small under similar conditions. The article is written in the form of mathematical simplicity and the Schwinger pedagogical clarity.

Anode-Free Zinc-Bromine Batteries Enabled by a Simple Prenucleation Strategy.

The design of anode-free zinc (Zn) batteries with high reversibility at high areal capacity has received significant attention recently, which is quietly challenging yet. Here, a Zn alloyed interface through electroplating is introduced, providing homogeneous Zn prenucleation sites to stabilize subsequent Zn nucleation and plating. By employing Zn-Cu alloy as a module, the complementary simulations and characterizations confirm that the prenucleation alloyed interfaces achieve a homogeneous electric field distribution and greatly enhance the stability of the Zn anode. Accordingly, the Zn//Zn-Cu@Cu half-cells show a long cycle life of over 900h and an average Coulombic efficiency (CE) of 99.8% at an areal capacity of10 mAh cm-2. The assembled anode-free zinc-bromine (Zn-Br2) battery exhibits an attractive stable cycling of 11 000 cycles at 1 mAh cm-2, while over 1000 cycles at the higher areal capacity of 10 mAh cm-2. Excitingly, the Zn-Br2 pouch cell with a capacity of 1000 mAh operates stably over 50 cycles, and achieves successful integration with photovoltaic systems. This anode-free Zn-Br2 batteries constructed through a prenucleation strategy offer new insights into the potential for large-scale energy storage applications.

Ultra-high-resolution brain MRI at 0.55T: bSTAR and its application to magnetization transfer ratio imaging.

This study aims to evaluate the feasibility of structural sub-millimeter isotropic brain MRI at 0.55 T using a 3D half-radial dual-echo balanced steady-state free precession sequence, termed bSTAR and to assess its potential for high-resolution magnetization transfer imaging. Phantom and in-vivo imaging of three healthy volunteers was performed on a low-field 0.55 T MR-system with isotropic bSTAR resolution settings of 0.87 × 0.87 × 0.87 mm3 and 0.69 × 0.69 × 0.69 mm3. Furthermore, off-resonance mapping was performed using 3D double-echo spoiled gradient imaging. For magnetization transfer (MT) MRI, the RF pulse duration of the 0.87 mm bSTAR scan was modified. Data were reconstructed using a GPU-accelerated compressed sensing algorithm. Magnetization transfer ratio (MTR) maps were calculated from two bSTAR scans with and without RF pulse prolongation. The MTR scan took 5 minutes and the reproducibility was assessed through repeated scans. Off-resonance mapping revealed that bSSFP brain imaging with TR < 5ms is essentially free of off-resonance-related artifacts even near the nasal cavities. Phantom and in-vivo scans demonstrated the feasibility of sub-millimeter isotropic bSTAR imaging. MTR maps obtained with high isotropic resolution bSTAR showed contrast between white and gray matter in agreement with expectations from high-field studies. The MTR measurements were highly reproducible with an average inter-scan MTR peak value of 43.3 ± 0.3 percent units. This study demonstrated the potential of sub-millimeter and artifact-free morphologic brain imaging at 0.55 T using bSTAR leveraging the advantages of low-field MRI, such as reduced susceptibility artifacts and improved radio-frequency field homogeneity. Furthermore, MT-sensitized bSTAR brain MRI enabled whole-brain MTR assessment within clinically feasible times and with high reproducibility.

Study of the boundary migration of recrystallized grains under inhomogeneous deformation field by crystal plasticity – phase field simulation

ABSTRACT In this study, a grain boundary (GB) migration model based on crystal plasticity (CP) and phase field (PF) was constructed for recrystallization of magnesium alloys. The model introduced the stored energy field obtained by CP into the PF simulations, aiming to analyze the effect of the inhomogeneous stored energy generated under the plastic deformation on the GB migration. It was found that the curvature plays a more significant role in the inhomogeneous stored energy field than in the uniform deformation field. Although grains with hard orientation have a lower deformation storage energy, they are still partially swallowed due to mutual influence between GBs near the triple junction. The high energies in the original GBs increase the migration rate at the triple junctions locally for a short period of time, but it is not enough to make a significant difference over extended periods at mesoscopic scales. The simulation results in this study are helpful in explaining the previous experimental observations.

StretchView - A Multi-Axial Cell-Stretching Device for Long-Term Automated Videomicroscopy of Living Cells.

Incorporating mechanical stretching of cells in tissue culture is crucial for mimicking (patho)-physiological conditions and understanding the mechanobiological responses of cells, which can have significant implications in areas like tissue engineering and regenerative medicine. Despite the growing interest, most available cell-stretching devices are not compatible with automated live-cell imaging, indispensable for characterizing alterations in the dynamics of various important cellular processes. In this work, StretchView is presented, a multi-axial cell-stretching platform compatible with automated, time-resolved live-cell imaging. Using StretchView, long-term image acquisition of cells in the relaxed and stretched states is shown for the first time (experimental time of 12 h) without the need for human intervention. Homogeneous and stable strain fields are demonstrated for 18 h of cyclic stretching, highlighting the platform's versatility and robustness. As proof-of-principle, the effect of stretching on cell kinematics and spatiotemporal localization of the cell-cell adhesion protein E-cadherin is examined for MDCK cells in monolayer. First evidence of a monotonic increase in junctional E-cadherin localization upon exposure to stretch is presented using live-cell imaging data acquired during cyclic stretching, suggestive of an increase in barrier integrity of the monolayer. These findings highlight the potential of StretchView in providing insights into cell mechanobiology and beyond.

High-Resolution NMR of the Hydrogenation Reaction with Parahydrogen in an Inhomogeneous Magnetic Field.

Nuclear magnetic resonance is extremely attractive for operando studies of chemical reactors. However, the heterogeneous catalyst particles placed inside an NMR probe greatly affect the uniformity of the magnetic field. This problem is especially acute when studying heterogeneous hydrogenation processes using parahydrogen. Despite the increased sensitivity due to hyperpolarization, under conditions of a strong heterogeneity of the magnetic field, the antiphase nature of the NMR signals leads to a partial or even a complete loss of spectroscopic information due to significant NMR signal broadening. The use of intramolecular multiple-quantum coherences in 2D NMR allows one to circumvent this problem. We used the COSY pulse sequence to acquire 2D NMR spectra of the reaction mixture upon propene hydrogenation with parahydrogen. The selection of double-quantum coherences in the resulting 2D NMR spectrum allowed us to obtain a highly resolved NMR spectrum under conditions of severe inhomogeneity of the magnetic field caused by the presence of catalyst granules.

Exploring in vivo human brain metabolism at 10.5 T: Initial insights from MR spectroscopic imaging.

Ultra-high-field magnetic resonance (MR) systems (7 T and 9.4 T) offer the ability to probe human brain metabolism with enhanced precision. Here, we present the preliminary findings from 3D MR spectroscopic imaging (MRSI) of the human brain conducted with the world's first 10.5 T whole-body MR system. Employing a custom-built 16-channel transmit and 80-channel receive MR coil at 10.5 T, we conducted MRSI acquisitions in six healthy volunteers to map metabolic compounds in the human cerebrum in vivo. Three MRSI protocols with different matrix sizes and scan times (4.4 × 4.4 × 4.4 mm³: 10 min, 3.4 × 3.4 × 3.4 mm³: 15 min, and 2.75×2.75×2.75 mm³: 25 min) were tested. Concentric ring trajectories were utilized for time-efficient encoding of a spherical 3D k-space with ∼4 kHz spectral bandwidth. B0/B1 shimming was performed based on respective field mapping sequences and anatomical T1-weighted MRI were obtained. By combining the benefits of an ultra-high-field system with the advantages of free-induction-decay (FID-)MRSI, we present the first metabolic maps acquired at 10.5 T in the healthy human brain at both high (voxel size of 4.4³ mm³) and ultra-high (voxel size of 2.75³ mm³) isotropic spatial resolutions. Maps of 13 metabolic compounds (aspartate, choline compounds and creatine + phosphocreatine, γ-aminobutyric acid (GABA), glucose, glutamine, glutamate, glutathione, myo-inositol, scyllo-inositol, N-acetylaspartate (NAA), N-acetylaspartylglutamate (NAAG), taurine) and macromolecules were obtained individually. The spectral quality was outstanding in the parietal and occipital lobes, but lower in other brain regions such as the temporal and frontal lobes. The average total NAA (tNAA = NAA + NAAG) signal-to-noise ratio over the whole volume of interest was 12.1± 8.9 and the full width at half maximum of tNAA was 24.7± 9.6 Hz for the 2.75 × 2.75 × 2.75 mm³ resolution. The need for an increased spectral bandwidth in combination with spatio-spectral encoding imposed significant challenges on the gradient system, but the FID approach proved very robust to field inhomogeneities of ∆B0 = 45 ± 38 Hz (frequency offset ± spatial STD) and B1+ = 65 ± 11° within the MRSI volume of interest. These preliminary findings highlight the potential of 10.5 T MRSI as a powerful imaging tool for probing cerebral metabolism. By providing unprecedented spatial and spectral resolution, this technology could offer a unique view into the metabolic intricacies of the human brain, but further technical developments will be necessary to optimize data quality and fully leverage the capabilities of 10.5 T MRSI.

The Effect of Pacemakers and Defibrillators on Distortion in 2 Magnetic Resonance Imaging (MRI) Sequences Commonly Used in Radiation Oncology Practice—3D True Fast Imaging with Steady State Precession (TrueFISP) at 0.35T MR-Linear Accelerator (LINAC) and 3D T1 at 3T MR Simulator

Background: We aimed to measure the pacemaker- and defibrillator-induced distortion at 0.35T and 3.0T magnetic fields. Methods: The pacemaker/defibrillator was placed at the top center of a water-filled/MagPhan phantom, followed by a T1 scan at 3T and a TrueFISP scan at 0.35T. The extent of distortion (i.e., the distance from the device to the furthest signal loss/void/rings) in the water-filled phantom was measured in MIM. For geometrical distortion (i.e., dislocation of geometrical structures), the spheres in the MagPhan phantom were contoured and their distortion was calculated based on their manufacturing coordinate positions. Results: The maximum extent of distortion caused by the defibrillator was 18.8 cm at 0.35T and 5.8 cm at 3.0T. Similarly, the maximum extent of distortion caused by the pacemaker was 9.28 cm at 0.35T and 2.8 cm at 3.0T. Geometrical distortion measurements using the MagPhan phantom showed that the maximum distortion caused by the defibrillator was 12.8 mm at 0.35T and 13.2 mm at 3.0T. Likewise, the maximum distortion caused by the pacemaker was 8.7 mm at 0.35T and 6.0 mm at 3.0T. Conclusions: Defibrillators cause larger distortions/signal voids than pacemakers, and require careful consideration when performing MRI-based treatment planning. To minimize distortion, sequences with lower sensitivity to magnetic field inhomogeneity should be used.

Nuclear Spins in a Solid Exceeding 10-Hour Coherence Times for Ultra-Long-Term Quantum Storage

Long-term quantum storage for quantum communication applications will require optically accessible states with coherence times exceeding the desired storage time. Ground-state hyperfine levels in Eu3+:Y2SiO5 have previously shown coherence times as long as 6 h at 1.4 K when operated at a zero-first-order-Zeeman (ZEFOZ) point. Here, we have characterized the limiting decoherence mechanisms acting in this regime, which shows that the coherence time is limited by the presence of spin impurities. This dephasing effect has been suppressed by cooling the sample to 125 mK, resulting in an extension in the coherence time to over 13 h. In the absence of these impurities, the dominant source of dephasing is the field inhomogeneity of the ZEFOZ point arising from the spin inhomogeneous broadening. We have shown that coherence times over 18 h can be achieved by operating using a spectrally narrow subensemble selected from within the inhomogeneous line. The observed coherence time is longer than that of any other system suitable for an optical quantum memory. Furthermore, at a much higher temperature of 6 K, we have realized a 6-h coherence time, which has been obtainable using single-stage cryocoolers. These results imply advanced engineering feasibility in designing and operating future quantum information-storage devices for a wide range of applications, including in satellites. Published by the American Physical Society 2025

Suffusion characteristics of a heterogeneous dam foundation with a cut-off wall of stochastic defects

Natural alluvial foundations are inherently heterogeneous. To enhance seepage safety, a cut-off wall is commonly embedded in a dam foundation. However, walls can also have stochastic defects. The dual uncertainties arising from soil heterogeneity and wall defects pose significant challenges for seepage safety evaluation. In this study, systematic numerical simulations were conducted on an internally unstable dam foundation based on a four-constituent mixture framework. Soil heterogeneity was characterized by stochastic initial hydraulic conductivity and initial fines content. An erosion model, specifically incorporating the influence of overburden pressure, was employed to quantify suffusion. A probabilistic assessment utilizing Monte Carlo simulations revealed that suffusion in heterogeneous fields could be more severe than that in homogeneous fields. Various combinations of stochastic soil properties and defect locations can result in substantial disparities in seepage and erosion fields. The mean values of the total flux and the fines eroded ratio are insensitive to the spatial variation length, while their deviations increase with increasing spatial variation length, leading to larger uncertainties in the leakage channel morphology. For highly heterogeneous alluvial foundations with large spatial variations, conventional seepage and suffusion analyses that rely on homogeneous assumptions may considerably underestimate the internal erosion risk.

Tensile ductility of a duplex surface treated maraging steel produced by Laser Powder Bed Fusion: Interaction between the inhomogeneous microstructure and transformation induced plasticity

The tensile ductility of a surface-treated 18Ni300 maraging steel produced by Laser Powder Bed Fusion (L-PBF) was investigated in the present work. Both plasma nitriding and duplex treatments (plasma nitriding and Physical Vapour Deposition (PVD) coating) were considered using a direct aged material as a reference. The inhomogeneous microstructure of the surface-treated materials causes an early strain-induced austenite transformation, with a positive effect on the uniform plastic deformation. However, the brittle failure of the hardened surface and the coating promotes an inhomogeneous stress field and inhomogeneous deformation whose effect on tensile ductility prevails on the enhanced TRansformation Induced Plasticity (TRIP), resulting in a negative overall effect on ductility.The study's results suggest a metallurgical approach to optimizing the inhomogeneous microstructure of the surface-treated material, with the aim of preventing the decrease of ductility.

An insight into Fermat's principle via acoustic propagation in inhomogeneous air temperature field

An insight into Fermat's principle via acoustic propagation in inhomogeneous air temperature field

Calculating spectra by sequential filtering

We expand on the method of sequential filtering for calculating the spectra of inhomogeneous fields. Sadek and Aluie [Phys. Rev. Fluids 3, 124610 (2018)] showed that the filtering kernel has to have at least p vanishing moments to extract a power-law spectrum k−α with α&amp;lt;p+2 by low-pass filtering. Here, we show that sequential high-pass filtering allows for extracting steeper spectra with α&amp;lt;2p+3 using the same pth order kernel. For example, the spectrum of a field that is shallower than k−5 can be extracted by sequential high-pass filtering the field using any first-order kernel such as a Gaussian or top-hat. Finally, we demonstrate how the second-order structure function fails to capture spectral peaks because it cannot detect scaling that is too shallow.

Локальное изменение напряженного состояния в образцах горных пород перед разрушением по данным коэффициента Лоде–Надаи

In this paper, the relationship of the Lode–Nadai coefficient, which determines the shape of the stress ellipsoid, with the nature of brittle fracture in sandstone, pyrophyllite and dolomite samples for a rigid loading machine is investigated. The measurement of the inhomogeneous deformation field in the samples and the observation of the internal fracture process were performed using strain gauges and acoustic sensors. It is shown that the formation of the main crack is preceded by the evolution of local deformations in the sample, recorded by the strain gauge method. Special processing of strain gauge data, taking into account the free lateral surface of the samples, made it possible to calculate the principle deformations and three invariants of the strain tensor: maximum shear deformation, volume change deformation and the Lode–Nadai coefficient. It has been established that at the final stage of deformation, when a main crack is formed in local sections of the sample, the axes of the principal deformations are reindexed, which is expressed in the achievement of the maximum values of +1 and -1 by the Lode–Nadai coefficient. A comparison with field observations was made.

Relationship of Locally Inhomogeneous, Elastic and Magnetic Fields in Mn–Zn Ferrites

Using the methods of X-ray diffraction analysis, X-ray spectroscopy and theoretical physics, we studied the patterns of changes in the atomic, electronic and magnetic subsystems in ferrites of variable composition MnxZnyFezO4 associated with the formation of clusters differing in the composition of cations. Experimentally detected clusters, which appear in X-ray diffraction patterns as a halo, are characterized by a certain superposition of ion states, the magnetic moment of which depends not only on the spin of the electron, but also on its orbital moment and the spin of the nucleus. A phase transition was discovered in the mesoscopic cluster structure from manganese-containing clusters, caused by the interaction of trivalent manganese ions with oxygen ions, to clusters with a predominance of di- and trivalent manganese ions with oxygen ions. It is found that the clustered structure of manganese-zinc ferrites is responsible for the appearance of extreme magnetic properties; the maximum corresponds to a change in the dominant type of clusters. It is found that with an increase in mass density, a repopulation of energy states occurs as a decrease in the states of the low-energy electron group and an increase in the high-energy Fermi surface in the form of a saddle. It has been established that the peculiarities of condensation of the fundamental and soft modes of complexes (clusters) containing manganese and oxygen ions lead to changes in the physical parameters of the samples.

Estimation of the helicity of atmospheric disturbances caused by gravity field inhomogeneities

The helicity of atmospheric disturbances caused by inhomogeneities in the gravity field is theoretically estimated. For this purpose, a three-dimensional stationary analytical model of linear disturbances introduced by spatial inhomogeneities of the gravity field into the background geostrophic flow of a stratified rotating medium (atmosphere) was used. From the solution found, it follows that in highly anomalous regions the amplitude of the helicity of emerging disturbances can be noticeable.