Inhomogeneous Magnetic Field Research Articles

To an analytical theory of higher-order magnetic skyrmions in nonuniform magnetic fields

It is shown that axially symmetric inhomogeneous magnetic fields can lead to stabilization of higher-order magnetic skyrmions with topological charge |Q|>1 due to orbital effects. We developed the analytical theory on the energy, size, and domain wall width of the such skyrmions in a power-law inhomogeneous fields, with a parameters corresponding to strongly correlated electron systems. The results obtained may have applications in describing the formation of nontrivial magnetic structures in inhomogeneous fields of superconducting vortices in heterostructures superconductor --- chiral magnet of the type [Ir1Fe0.5Co0.5Pt1]10/MgO/Nb. Keywords: magnetic skyrmions, nonuniform magnetic fields.

Usefulness of Breath-hold DWI Focused on the Hepatic Dome in EOB-MRI

Respiratory-triggered-diffusion-weighted imaging (R-DWI) of the liver often results in poor image quality under the diaphragmatic dome on the cephalic side of the liver (hepatic dome) secondary to magnetic field inhomogeneity in liver magnetic resonance imaging (MRI). Hence, the usefulness of additional breath-hold-DWI (B-DWI) focusing on the hepatic dome was investigated. A total of 22 patients (14 men and 8 women; mean age 69.0±11.7 years) who underwent ethoxybenzyl (EOB)-MRI at our hospital between July and August, 2022 using a 3.0 T MRI system were included. One radiologist and three radiology technologists visually assessed the visibility of R-DWI and B-DWI in the hepatic dome on a 4-point scale (1 to 4). Additionally, the apparent diffusion coefficient (ADC) values of the hepatic parenchyma on each DWI were compared. B-DWI improved visibility in the hepatic dome compared to R-DWI (2.67±0.71 vs. 3.25±0.43, p<0.05). No significant difference was found in the ADC values for each DWI. B-DWI has excellent visibility in the hepatic dome and is expected to complement R-DWI. Therefore, B-DWI is very useful as an additional imaging in EOB-MRI.

Design of dipole magnets for Siam Photon Source II

Two types of dipole magnets are designed for the storage ring of Siam Photon Source II, the second synchrotron light source in Thailand. Magnetic field at the magnet center is 0.87 T, which bends a 3.0 GeV electron beam with the bending radius of 11.532 m and consequently the synchrotron radiation is generated. The first type of dipole magnet is a standard dipole with the magnet gap of 36 mm. The other type is designed for Infrared (IR) beamline with the larger magnet gap of 59 mm to accommodate a wide opening angle of IR radiation. The magnets are designed such that the operating current is the same while the turn number of magnet coils and the magnet gap are allowed to be different. Therefore, a single power supply can be used to excite all dipole magnets in the storage ring which will be connected in series. Magnet modelling and magnetic field calculation are performed in Opera-3D. The magnet poles are shimmed near the edges to improve magnetic field homogeneity to be within the requirement of 2×10−4 within the good field region. Mechanical analysis of magnet structure is performed in SOLIDWORKS and ANSYS where the maximum deformation of 18 - 23 µm is found at the magnet pole and the first-mode natural frequency is higher than 200 Hz.

About the Propagation of RSDN‐20 “Alpha” Signals in the Earth‐Ionosphere Waveguide During Geomagnetic Disturbances

AbstractThe paper presents the results of the modeling of VLF radio waves propagation in horizontally inhomogeneous region of the high‐latitude Earth‐ionosphere waveguide during different types of the geomagnetic disturbances. The frequencies of the model source of signal correspond to the frequencies of the long‐distance navigation radio system RSDN‐20 “Alpha” constantly broadcasting in the VLF range in the Russian Federation. It is considered the geomagnetic storm on 24 January 2012 and geomagnetic substorm on 11 December 2015. Electron density profiles and collision rates of electrons with neutrals are taken from the measurements of EISCAT radar in Tromso and EISCAT 42m radar on Svalbard. The numerical experiments were conducted with a highly horizontally inhomogeneous electron density in the ionosphere. It is shown that the amplitudes and phases of the VLF signals at these frequencies show a noticeable reaction to changes in the ionospheric parameters during the considered geomagnetic disturbances. Calculations show that changes in amplitude of RSDN‐20 “Alpha” signals caused by the action of the Earth's magnetic field and horizontal inhomogeneities depend on the direction of signal propagation relative to the Earth's magnetic field vector and cannot be reversed if the direction of propagation changes to the opposite. The results of the numerical experiment were compared with the corresponding characteristics of the RSDN‐20 signals calculated from the observations of electromagnetic signals at the Lovozero observatory for the case on 11 December 2015.

Formation and Behaviour of Active Droplets and Bubbles in a Magnetic Fluid in an Inhomogeneous Magnetic Field

This work proposes a new technique for creating active bubbles and droplets with a non-magnetic core and a coating formed by a magnetic fluid. The procedure consists of the injection of a non-magnetic phase into a magnetic one that is supported by the presence of an inhomogeneous magnetic field from the source, which combines an annular magnet and an electromagnet. We explored various modes leading to different active bubbles and drops as well as the influence of the magnetic field on the size, velocity, and acceleration of the formed active droplets. It is shown that active bubbles change their trajectory under the action of a constant magnetic field and also disintegrate under the action of a pulsed one. This provides a new mechanism for controlling the absorption of droplets and bubbles using a magnetic field. Therefore, these results can be applied to create droplet-based microfluidics systems, in which an inhomogeneous magnetic field can be used for focusing droplet and bubble flows in a magnetic fluid.

Toroidal magnetic field ripple in the presence of misaligned toroidal field coils on the COMPASS-U tokamak

Homogeneity of the toroidal magnetic field is a critical aspect of the tokamak fusion device. It is affected by the design of toroidal magnetic field coils and their manufacturing and assembly misalignments. The magnetic field of the COMPASS-U project is analysed using toroidal field ripple quantity. ANSYS modelling of bitter-plate-designed toroidal field coils is performed to prepare an accurate simplistic representation of the coils using a virtual current line. Toroidal field ripple is analysed using a Gaussian distribution of possible misalignments. The worst-case toroidal field ripple corresponding to the technical drawings only slightly exceeds the chosen limit of 0.5%.

Deep learning-based Accelerated and Noise-Suppressed Estimation (DANSE) of quantitative Gradient-Recalled Echo (qGRE) magnetic resonance imaging metrics associated with human brain neuronal structure and hemodynamic properties.

The purpose of the current study was to introduce a Deep learning-based Accelerated and Noise-Suppressed Estimation (DANSE) method for reconstructing quantitative maps of biological tissue cellular-specific, R2t*, and hemodynamic-specific, R2', metrics of quantitative gradient-recalled echo (qGRE) MRI. The DANSE method adapts a supervised learning paradigm to train a convolutional neural network for robust estimation of R2t* and R2' maps with significantly reduced sensitivity to noise and the adverse effects of macroscopic (B0 ) magnetic field inhomogeneities directly from the gradient-recalled echo (GRE) magnitude images. The R2t* and R2' maps for training were generated by means of a voxel-by-voxel fitting of a previously developed biophysical quantitative qGRE model accounting for tissue, hemodynamic, and B0 -inhomogeneities contributions to multigradient-echo GRE signal using a nonlinear least squares (NLLS) algorithm. We show that the DANSE model efficiently estimates the aforementioned qGRE maps and preserves all the features of the NLLS approach with significant improvements including noise suppression and computation speed (from many hours to seconds). The noise-suppression feature of DANSE is especially prominent for data with low signal-to-noise ratio (SNR ~ 50-100), where DANSE-generated R2t* and R2' maps had up to three times smaller errors than that of the NLLS method. The DANSE method enables fast reconstruction of qGRE maps with significantly reduced sensitivity to noise and magnetic field inhomogeneities. The DANSE method does not require any information about field inhomogeneities during application. It exploits spatial and gradient echo time-dependent patterns in the GRE data and previously gained knowledge from the biophysical model, thus producing high quality qGRE maps, even in environments with high noise levels. These features along with fast computational speed can lead to broad qGRE clinical and research applications.

Off-resonance artifact correction for MRI: A review.

In magnetic resonance imaging (MRI), inhomogeneity in the main magnetic field used for imaging, referred to as off-resonance, can lead to image artifacts ranging from mild to severe depending on the application. Off-resonance artifacts, such as signal loss, geometric distortions, and blurring, can compromise the clinical and scientific utility of MR images. In this review, we describe sources of off-resonance in MRI, how off-resonance affects images, and strategies to prevent and correct for off-resonance. Given recent advances and the great potential of low-field and/or portable MRI, we also highlight the advantages and challenges of imaging at low field with respect to off-resonance.

New challenges and opportunities for low-field MRI

In this manuscript we deal with recent advances in low-field Magnetic Resonance Imaging (MRI). The development of low-cost MRI solutions allowing portability and trustable diagnosis is a hot topic worldwide by these days. We analyze basic technical issues of recent examples of fixed-field instruments operating at low-field. Then we discuss pros and cons of the pre-polarized approach, from both physical and technical perspectives. Permanent magnet and electromagnet technology are confronted. Finally, magnetic field-cycling is introduced as an alternative MRI technique, where field-dependent experiments can be exploded for the development of new contrast mechanisms that are not feasible for fixed-field MRI instruments. As field cycled machines usually deals with switched currents in electromagnets, magnetic field instability and inhomogeneity are the main limiting factors affecting image quality. We finalize this manuscript discussing how it turns possible to overcome these limitations, thus opening new possibilities for the development of cost effective MRI technology.

Blood in the Brain on Susceptibility-Weighted Imaging.

Intraparenchymal brain hemorrhage is not uncommon and results from a wide variety of causes ranging from trauma to tumor. Many a time, it is not possible to determine the exact cause of non-traumatic hemorrhage on conventional magnetic resonance imaging (MRI). Susceptibility-weighted imaging (SWI) is a high-resolution (3D) gradient-echo sequence. It is extremely sensitive to the inhomogeneity of the local magnetic field and highly useful in identifying the small amount of hemorrhage, which may be inapparent on other MR pulse sequences. In this review, we present different pattern of an intra-parenchymal brain hemorrhage on SWI with emphasis on differential diagnosis.

Sauter-Schwinger Effect for Colliding Laser Pulses.

Via a combination of analytical and numerical methods, we study electron-positron pair creation by the electromagnetic field A(t,r)=[f(ct-x)+f(ct+x)]e_{y} of two colliding laser pulses. Employing a generalized Wentzel-Kramers-Brillouin approach, we find that the pair creation rate along the symmetry plane x=0 (where one would expect the maximum contribution) displays the same exponential dependence as for a purely time-dependent electric field A(t)=2f(ct)e_{y}. The prefactor in front of this exponential does also contain corrections due to focusing or defocusing effects induced by the spatially inhomogeneous magnetic field. We compare our analytical results to numerical simulations using the Dirac-Heisenberg-Wigner method and find good agreement.

Catapulting towards massive and large spatial quantum superposition

Large spatial quantum superposition of size ${\cal O}(1-10)~{\rm \mu \text{m}}$ for mass $m \sim 10^{-17}-10^{-14}~\text{kg}$ is required to probe the foundations of quantum mechanics and testing classical and quantum nature of gravity via entanglement in a laboratory. In this paper, we will show that it is possible to accelerate the two spin states of a macroscopic nano-crystal sourced by the inhomogeneous nonlinear magnetic field in the Stern-Gerlach type setup. We will assume that the electronic spin can be embedded at the centre of the nano-crystal, such as the nitrogen-vacancy (NV) centre of diamond. Our analysis will be generic to any dopant or any material. We will show that we can create a desired superposition size within $1-2$ seconds by catapulting the trajectories of the two spin states with a modest magnetic field gradient and then recombine the trajectories for a coherent interference. We will show the demanding nature of the precision required in the magnetic field to recover $99\%$ spin coherence confidence level at the moment of interference.

Loading a quantum gas from a hybrid dimple trap to a shell trap

Starting from a degenerate Bose gas in a hybrid trap combining a magnetic quadrupole trap and an attractive optical trap resulting from a focused laser beam, we demonstrate the efficient loading of this quantum gas into a shell-shaped trap. The shell trap is purely magnetic and relies on adiabatic potentials for atoms in an inhomogeneous magnetic field dressed by a radiofrequency (rf) field. We show that direct rf evaporation in the hybrid trap enables an efficient and simple preparation of the cold sample, well adapted to the subsequent loading procedure. The transfer into the shell trap is adiabatic and limits the final excitation of the center-of-mass motion to below 2 μm.

Research on the Gradient-Field Superconducting Magnet for Magnetoplasmadynamic Thruster Performance Improvement

Magnetoplasmadynamic thrusters (MPDTs) are characterized by their high specific impulse, high thrust, and long operational life. Hence, they are promising candidates as space electric thrusters for deep space exploration, such as for future interstellar voyages. To improve the performance of MPDTs, this article investigates the potential to enhance their plume performance using a high-gradient magnetic field based on superconductor technology. We introduce a high-intensity superconducting magnetic field constraint method based on a 100-kW MPDT, which involves making the solenoid and gradient-field superconducting magnets more compact. The central magnetic field can reach 1 T and the magnetic field homogeneity is optimized from 1.69% to 0.83%. In addition, the MHD method is used to simulate and verify the concentration-constrained acceleration of plasma by the gradient-field superconducting magnets. Experimental results show that with increases in magnetic field strength, the axial plasma plume becomes more concentrated. The plume distribution of the 100-kW gradient-superconducting MPDT is more uniform than that of a solenoid plume at the same discharge current due to better magnetic field homogeneity.

Flux penetration of an HTS coated-conductor tape by an approaching permanent magnet

Interactions between inhomogeneous magnetic fields and a superconductor lie at the heart of several high-temperature superconducting (HTS) devices. An example is the situation where the magnetic field from a small permanent magnet (PM) is applied to a coated-conductor HTS tape, such that flux penetrates the superconductor. The PM field is highly spatially inhomogeneous, such that there is significant variation in the applied field magnitude and direction across the tape. This leads to a varying local critical current density, Jc(B , θ ), across the tape, and consequently different ‘shielding’ (magnetic flux penetration) behaviour than is the case for a uniform applied field. Here, we report results from measurements and numerical simulations of the penetrating magnetic field within an HTS coated-conductor tape which occur as a PM dipole approaches from a distance. Electromagnetic simulations were performed using a finite-element model based on the H -formulation, and which incorporated experimentally-measured anisotropic Jc(B , θ ) properties from a coated-conductor tape. This showed good agreement with experimental measurements. The effects of varying key modelling assumptions and parameters were then studied, including changing the widths of the PM and HTS tape and the magnitude of Jc(B , θ ). The inhomogeneity of the PM field leads to a characteristic gull-wing distribution of magnetic flux across the superconductor at elevated applied fields. Close approach of the PM to the tape suppresses Jc in the centre of the tape, and this results in the observation of a characteristic maxima for the total shielding currents circulating in the tape. A figure of merit is introduced which uses this effect to provide a threshold definition for ‘full flux penetration’ of an HTS tape by an inhomogeneous dipole field.

A Wideband Balun-Based Microwave Device for Quantum Information Processing With Nitrogen–Vacancy Centers in Diamond

We present a wideband balun-based microwave device for quantum manipulation of nitrogen-vacancy centers in diamond. The device employs a balun with multi-section Chebyshev impedance transformer to realize the conversion from the unbalanced feed line to the balanced feed line and ensures impedance matching. A bandwidth of 2.2 GHz with the maximum resonance frequency up to 4.4 GHz has been realized, which is important for multi-qubit quantum control of the nitrogen-vacancy centers. We also carry out optically detected magnetic resonance to observe Rabi oscillations and calibrate the strength of the microwave magnetic field generated by the microwave device. With an input power of 0.5 W, the Rabi frequencies can reach 6.17 MHz and 6.06 MHz for resonant microwave at 1.40 GHz and 4.34 GHz, respectively. Combining with large bandwidth and excellent magnetic field homogeneity, the device can be applied in solid-state quantum information processing.

Rod mangle rotation patterns for adjustable magnetic field generation

The use of adjustable magnetic field sources is becoming an indispensable necessity in numerous applications. In particular, permanent magnet assemblies, such as the rod mangle design, avoid the large power consumption and Joule heating problems present in electromagnets. In this work, we present a systematic 2D study of rod mangle configurations with different number of rods. We observe that the magnetic field homogeneity increases with the number of used rods and, for a sufficiently large number, the behavior of the assembly is similar to that of a perfect Halbach. Generally, the rotation of only one rod changes the direction and decreases the homogeneity and magnitude of the magnetic field. Nevertheless, the impact of such rotation reduces with the increase of the number of rods in the system. We also study how different rotation patterns influence the generated magnetic field and demonstrate how one can: (i) reduce the magnetic field magnitude rotating its direction; (ii) reduce the magnetic field magnitude keeping its direction constant; (iii) invert the magnetic field direction, after being reduced; or (iv) rotate the magnetic field direction keeping its magnitude constant. These results are similar to those obtained by a nested Halbach. In addition to the low cost, the results show that the mangle system is highly versatile, can be easily manipulated and, consequently, has a large application potential. The magnetic model was successfully validated with experimental systems of three and four rods.

Concept for gradient-free MRI on twin natural slices.

The design of an MRI for use in space requires that the hardware be kept to an absolute minimum in terms of mass, complexity, and power. In addition, NASA requirements are that the external stray field needs to be less than 3.2 Gauss, 7 cm from the MRI enclosure. RF encoding designs with Halbach magnets offer the best chance of meeting those requirements. Spatially non-uniform magnetic fields with foliations of isomagnetic surfaces, or natural slices, may be used to provide slice selection, and to reduce further the hardware complexity, for TRansmit Array Spatial Encoding (TRASE) Magnetic Resonance Imaging (MRI) or potentially for other radio frequency (RF) encoding methods. The design of such non-uniform magnetic fields in a Halbach configuration with built-in axial gradients leads to pairs of isomagnetic surfaces centered on either side of a central maximum field strength slice. If TRASE images from slices other than the central isomagnetic surface are desired, then the Nuclear Magnetic Resonance (NMR) signals originating from the twin natural slices must be separated during image reconstruction. Here, a design for simultaneously imaging on twin slices in such an inhomogeneous magnetic field using multiple receiver coils with spatially varying RF profiles is described mathematically and numerical simulation examples are given. To achieve RF encoding on the natural slices, at least three TRASE transmit coils are required. Here a solution with twisted solenoid coils is given. To achieve the twin slice separation at least two receive coils are required. Here a solution with two solenoids is given. The MRI design presented here uses a combination of RF encoding (TRASE), a spatial encoding magnetic field (SEM, pairs of natural slices) and receive coil spatial profiles to encode enough information into the NMR signal for image slice reconstruction. The design presented here enables using Halbach magnets with a built-in axial gradient to be used for MRI. The result is a new gradient-free TRASE MRI design capable of imaging pairs of electronically selectable axial slices.

Gyro-averaging operators with magnetic field inhomogeneity

We derive expressions for the gyro-averaging operator that is applicable to electrostatic fluctuations in a spatially inhomogeneous magnetic field. Both low and high wavenumber limits are considered. The gyro-averaging operator for the former case is represented by sums of Bessel functions with different orders. A simplified expression is provided as a Padé approximant in the low wavenumber limit. This form could be used in practical computations based on the gyrofluid formulation. In the high wavenumber limit, we find that the operator naturally involves fractional derivatives whose physical interpretations are yet to be explored. Discussions are made of a potential impact of this asymptotic expression in the high wavenumber limit.

Hierarchical Multi-Resolution Graph-Cuts for Water-Fat-Silicone Separation in Breast MRI.

Water-fat separation is a non-linear non-convex parameter estimation problem in magnetic resonance imaging typically solved using spatial constraints. However, there is still limited knowledge on how to separate in vivo three chemical species in the presence of magnetic field inhomogeneities. The proposed method uses multiple graph-cuts in a hierarchical multi-resolution framework to perform robust chemical species separation in the breast for subjects with and without silicone implants. Experimental results show that the proposed method can decrease the computational time for water-fat separation and perform accurate water-fat-silicone separation with only a limited number of acquired echo images at 3 T. The silicone-separated images have an improved spatial resolution and image contrast compared to conventional scans used for regular monitoring of the silicone implant's integrity.