Static Magnetic Field Distribution Research Articles

Simultaneous imaging of static and microwave magnetic field distributions by magneto optical indicator microscopy

Simultaneous imaging of static and microwave magnetic field distributions by magneto optical indicator microscopy

Analytical calculation method of a liquid-cooling eddy current brake considering magnetic saturation and skin effect

In this article, a clear and concise analytical method for predicting the performance of a Liquid-cooling eddy current brake (LC-ECB) is proposed. The LC-ECB has a coolant channel in the rotor to allow direct cooling of the inner surface of the stator. The static air-gap magnetic field distribution is obtained by the dynamic magnetic equivalent circuit (MEC) method, and the magnetic flux leakage and global magnetic saturation effects are fully considered. The magnetic field intensity distribution function of the eddy current reaction magnetic field is derived for the first time based on Ampere circuital theorem. Considering the local magnetic saturation and skin effect, a novel double-iteration algorithm based on the conservation principle of magnetic pressure drop is applied to obtain the transient air-gap flux density distribution, and then the brake torque expression is obtained. The finite element method (FEM) and experimental results show that the proposed method is feasible and effective. The new model is easy to program and can be easily used in the initial design and optimization of LC-ECB.

Torsional mode magnetostrictive patch transducer with TPSMs for liquid-filled pipes inspection

T (0,1) mode guided waves have no axial or radial vibration displacement. Therefore, the energy leakage is small in liquid-filled pipes, which is suitable for detecting the axial defects of the liquid-filled pipes. Fe–Co materials with high magnetostriction coefficient is fabricated. Its magnetic properties are tested. The size and distribution of static magnetic fields generated by different permanent magnets are verified by simulation. A new torsional mode guided wave magnetostrictive patch transducer with tile-shaped permanent magnets for defect detection of the liquid-filled pipes is proposed. The detection effect of the transducer on the liquid-filled pipes under different pressures is tested through experiments. The proposed transducer consists of four tile-shaped permanent magnets, eight triangular yokes, a coil, and two magnetostrictive patches. Four groups of tile permanent magnets are symmetrically distributed around the patch according to the polarity. Magnetic yokes are installed at both ends of each group of tile permanent magnets. The experiment verified that the magnetostrictive patch transducer realize the excitation and reception of guided wave signals in the magnetostrictive patches. The amplitude and signal-to-noise ratio of echo signals is improved. The position of pipe defects is determined according to the propagation time of guided wave. The influence of different pressures in the liquid-filled pipes on the amplitude of guided wave is verified.

Concept of scanning magnet with distributed winding coils for particle beam therapy

To reduce the size of a rotating gantry for a particle therapy system, it is desirable to reduce the size of the scanning system by substituting separated scanning magnets with a combined-function scanning magnet. A two-dimensionally designed scanning magnet with distributed winding coils (DW-SCM) is proposed. The DW-SCM can generate a rotating dipole magnetic field using a single power supply. The two-dimensional static magnetic field distributions of the DW-SCM were calculated using poisson superfish code and compared with those of an octupole scanning magnet (CW-SCM) and cos-theta-type scanning magnet ($\mathrm{Cos}\ensuremath{\theta}$-SCM) with the same bore diameter of 98 mm. The calculated two-dimensional magnetic field showed that the DW-SCM had the highest magnetic field strength of the three types of scanning magnets, and compared with the CW-SCM, the DW-SCM was 129% higher and the $\mathrm{cos}\text{ }\ensuremath{\theta}$-SCM was 72% higher. The good field region (radius) of field homogeneity within $\ifmmode\pm\else\textpm\fi{}0.5%$ of the DW-SCM had a symmetrical shape centered on a current phase of 30\ifmmode^\circ\else\textdegree\fi{}, and the minimum value was 24 mm. Also compared with the CW-SCM, the minimum good field region was 11% higher for the DW-SCM and 65% higher for the $\mathrm{cos}\text{ }\ensuremath{\theta}$-SCM. The results indicate that the proposed scanning magnet could generate a high-strength rotating dipole magnetic field, and it might be possible to reduce the size of a scanning system by substituting separated scanning magnets with the DW-SCM.

A three-dimensional magnetoelastic valley Hall insulator with tunable elastic wave route and frequency

Topological insulators (TIs) are a new type of quantum state materials. Due to their novel physical properties, such as topological protection defect immunity to edge states, TIs have become the focus of attention in condensed matter and material physics. At present, the research on TIs has been gradually extended to classical wave fields such as electromagnetic waves, acoustic waves, and elastic waves, and has aroused extensive research interest. However, for elastic wave systems, most TIs cannot actively control topological interface states due to the limitation of fixed structure, which hinders their application in practical situations. Here, we propose a kind of tunable three-dimensional (3D) valley Hall insulator composed of magnetoelastic materials. First, the topological phase transition can be induced by the asymmetric geometry. Then, the working frequency of topological interface states can be changed by using static magnetic fields. Second, topological phase transformation can also be induced by independently tuning the distribution of static magnetic fields or pre-stress in each unit. Based on this, reconfigurable propagation routes of interface states with arbitrary shapes can be realized by tuning the distribution of static magnetic fields or pre-stress in each unit. Finally, considering the sandwich structure composed of different magnetic fields or pre-stress distribution modes, the waveguide with tunable width and route is designed by coupling edge and bulk states, which is convenient for application and better energy transfer. This study provides a reference for the design of a tunable intelligent elastic waveguide.

Numerical investigation of plasma characteristics for an ion thruster discharge chamber

A 2D fully kinetic particle-in-cell plus Monte Carlo collision model is developed to simulate the plasma characteristics of the ion thruster discharge chamber. The interaction between charged particles and the dynamic electric field, the confinement of the static magnetic field, and the particle collisions are simulated. The self-similar scaling method, the multigrid method, sub-cycle time, and neutral particle pre-distribution techniques work together to perform the full-particle and full-kinetic simulation. The steady-state results such as particle number, current flows, static magnetic field, electric potential, particle density, and energy distributions are obtained and discussed. Moreover, the plasma generation and transport, the dynamic behavior of neutral particles, and the influence of the gas flow rate on the plasma are analyzed. The density distribution of charged particles indicates that the electrons are strongly constrained by the magnetic field, and the magnetization characteristics are distinguished from that of ions. Neutral particles are heavily ionized and are depleted along the central line, which is consistent with the area with a higher ionization rate. At last, it is found that the energy and density of charged particles are sensitive to the change of the gas flow rate.

Study on Uniform Plasma Generation Mechanism of Electron Cyclotron Resonance Etching Reactor

Electron cyclotron resonance (ECR) plasma sources have been used widely in ultra large-scale integration (ULSI) manufacturing processes for many years. ECR plasma sources can generate plasma stably, even at very low pressures below 0.1 Pa, and the plasma’s spatial distribution can be controlled easily using the static magnetic field distribution. Furthermore, the plasma generation region can be located away from the chamber walls, which leads to reduced plasma contamination from the walls. In addition, uniform plasmas can be generated using these sources to achieve uniform plasma processing, which is crucial for large-diameter wafers such as 300 or 450 mm wafers. However, the mechanism for generation of uniform plasma using an ECR plasma source for semiconductor manufacturing is not well understood because of the complexity of the microwave propagation in the plasma. We use a plasma simulation technique to understand the uniform plasma generation mechanism in this work. We have conducted an integrated ECR plasma simulation of an etcher used for semiconductor manufacturing that combines 1) a microwave analysis in magnetized plasma, 2) a plasma generation analysis, and 3) a plasma diffusion analysis. We obtained consistent solutions for each analysis in practical calculation times of a few minutes. Additionally, we verified the simulation results by comparison with experimental plasma density measurements. We then discussed the uniform plasma generation mechanism in the ECR plasma source using simulated results for the microwave distribution in the etching reactor.

Design and research of deep slot universal motor for electric power tools

In this paper, a deep slot universal motor is designed for electric power tools. The deep slot structure can reduce the cost of universal motor materials and improve production efficiency. A mathematical model of a universal motor is established and a simulation model is established by MagNet finite element software. Then, the static magnetic field distribution and load characteristics of the deep slot universal motor are analyzed. Different methods are adopted to improve the commutation spark of a deep slot universal motor. Finally, three 550-W and 9500-rpm prototype motors are tested. Simulation and experimental results indicate that an uneven air gap, brush offset by a certain angle, as well as different turns of the lead coil and the lag coil can effectively improve the commutation spark of a deep slot universal motor.

Improvement of SiC Crystal Growth Rate and Uniformity via Top-Seeded Solution Growth under External Static Magnetic Field: A Numerical Investigation.

Silicon carbide (SiC) is an ideal material for high-power and high-performance electronic applications. Top-seeded solution growth (TSSG) is considered as a potential method for bulk growth of high-quality SiC single crystals from the liquid phase source material. The crystal growth performance, such as growth rate and uniformity, is driven by the fluid flow and constitutional flux in the solution. In this study, we numerically investigate the contribution of the external static magnetic field generated by Helmholtz coils to the fluid flow in the silicon melt. Depending on the setup of the Helmholtz coils, four static magnetic field distributions are available, namely, uniform vertical upward/downward and vertical/horizontal cusp. Based on the calculated carbon flux coming to the crystal surface, the vertical downward magnetic field proved its ability to enhance the growth rate as well as the uniformity of the grown crystal.

Inverse Problem Analysis of Distribution of Power Generation Current inside PEFC Using Magnetic Field around Fuel Cell

A PEFC (polymer electrolyte fuel cell) is expected to be used as a power supply of the homes and electric vehicles, since it can generate electricity at low temperatures and the startup speed is fast. Since the power generation efficiency of PEFC is influenced by the mass transfer of hydrogen, oxygen and steam in it, it is important to clarify the current distribution inside MEA that is closely related to the mass transfer. In this paper, the evaluation method of the distribution of the power generation current inside MEA using the distribution of static magnetic field around PEFC is proposed. The generated current distribution in the MEA is calculated by inverse problem analysis using Ampere's law from the distribution of the magnetic field around the fuel cell. The evolutionary strategy method is used by this inverse problem analysis. In addition, the current distribution estimation accuracy inside the MEA has been improved by using the weighting coefficient according to the distance between the magnetic sensor and each position in the MEA.

Optimal Design for a Portable NMR- and MRI-Based Multiphase Flow Meter

In this paper, an optimal design for nuclear magnetic resonance and magnetic resonance imaging based multiphase flow meter is presented. The overall apparatus consists of an electromagnetic coil surrounded by 12 rings of Halbach arrays of 12 magnets each. Both the coil and the magnets arrays were simultaneously and optimally designed using a three-dimensional (3-D) finite-element method (FEM) package and particle swarm optimization algorithm. The aim of the design is to uniformly prepolarize the hydrogen spins of the atoms composing the flowing fluid, with a predefined static magnetic field, before they enter into the measurement area within which they are homogenously polarized with a perpendicular ac magnetic field. Cheap and lightweight hardware are the other two design parameters of the design. Results of extensive 3-D simulations of the design indicate that an optimized and homogenous static magnetic field distribution could be achieved within an area of 40 mm diameter and 606 mm length. In addition, using a multiturn coil of 6 cm diameter and 2000 turns, an ac magnetic field of 13 mT amplitude and 112 ppm homogeneity could be achieved, which is enough to handle fluids flowing at speed of up to 2 m/s. Experimental validation, which was done using newly constructed two Halbach arrays of cuboid and trapezoid magnet elements, respectively, indicates a good match with FEM simulations.

Alternating magnetic force microscopy: simultaneous observation of static and dynamic magnetic field in three-dimensional space

In the scanning magnetic domain by using the conventional magnetic force microscopy (MFM), a laser beam reflection is used to detect the static magnetic force between probe and sample. Therefore, for the MFM, it is a challenge to directly detect the dynamic magnetic force between probe and sample under an external alternating-current (AC) magnetic field. In this study, it is proved that in an alternating magnetic force microscopy (A-MFM) a sensitive Co-GdO<sub><i>x</i></sub> superparamagnetic probe can be usedto detect the dynamic magnetic force under an external AC magnetic field (frequency <i>ω</i><sub>m</sub>). In the present method, the magnetization of Co-GdO<sub><i>x</i></sub> probe is modulated by an external AC magnetic field. Collecting <i>ω</i><sub>m</sub> and 2<i>ω</i><sub>m</sub> signals by using the combination of phase-locked loop (PLL) and lock in amplifiers can accurately represent the static (DC, which stands for direct current) magnetic field areas (the external AC magnetic field has no effect on the magnetized status of the sample) and dynamic (AC) magnetic field areas (the external AC magnetic field changes the magnetized status of the sample) of an anisotropic Sr ferrite sintered magnet at the same time, respectively. The Sr ferrite sample is a single-domain-type magnet where magnetization mainly changes via magnetic rotation. The A-MFM method can measure the strength and identify the polarities of the static magnetic field of sample with a DC demagnetized state. By modifying the traditional tapping-lift mode into a tapping-multiply lift mode, the A-MFM by using superparamagnetic tips can measure the static and dynamic magnetic field distribution in three-dimensional (3D) space. It is proved that the static and dynamic magnetic field as a function of the distance <i>z</i> between probe and sample are both expressed as <i>H<sub>z</sub></i>(<i>z</i>) = <i>H<sub>z</sub></i>(0)·exp(–<i>kz</i>). The experimental data are consistent with the previous theoretical calculations. The A-MFM can be used to study the dynamic magnetization process and to evaluate the magnetic homogeneity (microstructural homogeneity) of magnetic materials.

Controllability of magnetorheological shock absorber: I. Insights, modeling and simulation**Part of work was done while Xian-Xu ‘Frank’ Bai was a joint PhD student of Chongqing University and University of Maryland, College Park.

The smart fluids—magnetorheological (MR) fluids show the potentials for high-speed shock mitigation applications because of their fast response and large controllable damping range. In order to match the high-speed applications of MR fluids, the structural design and controllability evaluation of MR actuators are of great significance. Controllability of MR actuators are defined to be evaluated by the mechanical behaviors (i.e. damping force controllability), including the controllable damping force range, dynamic range and constant stroking load velocity range, and response time performance (i.e. real-time controllability). We proposed an internal bypass MR shock absorber (MRSA) (Bai et al 2013a IEEE Trans. Mag. 49 3422–5), which presents the possibility in vibration control and shock mitigation applications. In the internal bypass MRSA, the gap formed by the coaxially assembled inner and outer cylinders is the flow channel for MR fluids. The electromagnetic coil windings evenly wound on the outer wall of the inner cylinder and the active rings contribute to the MR effect, and continuous adjustable damping is realized through applying current into the electromagnetic coil windings. In Part I of this paper, the principle and the mechanical behaviors of the internal bypass MRSA is further in-deeply investigated via theoretical modeling and simulations. Based on the fluid dynamics—parallel plate models and the finite element analysis software ANSYS/Fluent, the mathematical model and the fluidic entities of the internal bypass MRSA are established, respectively. Response time model of the internal bypass MRSA under a voltage driver is established. The static magnetic field distributions of the internal bypass MRSA under different current excitations are obtained via ANSYS/Workbench. The response times of the internal bypass MRSA under transient impulse current excitation and transient impulse voltage excitation are obtained by transient electromagnetic simulation. The damping force performance of the conventional unicoil MRSA with an electromagnetic coil winding in positon, multicoil MRSA with five electromagnetic coil windings in positon and the internal bypass MRSA are compared and analyzed. As compared with the conventional unicoil and multicoil MRSAs with coupled structures of the piston and the electromagnetic coil windings, the internal bypass MRSA with decoupled arrangement shows much better damping force controllability. Experimental test and controllability evaluation of the internal bypass MRSA will be conducted and presented in Part II of this paper.

Magnetic imaging of a single ferromagnetic nanowire using diamond atomic sensors

Recent advances in nanorobotic manipulation of ferromagnetic nanowires bring new avenues for applications in the biomedical area, such as targeted drug delivery, diagnostics or localized surgery. However, probing a single nanowire and monitoring its dynamics remains a challenge since it demands high precision sensing, high-resolution imaging, and stable operations in fluidic environments. Here, we report on a novel method of imaging and sensing magnetic fields from a single ferromagnetic nanowire with an atomic-scale sensor in diamond, i.e. diamond nitrogen-vacancy (NV) defect center. The distribution of static magnetic fields around a single Co nanowire is mapped out by spatially distributed NV centers and the obtained image is further compared with numerical simulation for quantitative analysis. DC field measurements such as continuous-wave ODMR and Ramsey sequence are used in the paper and sub Gauss level of field sensing is demonstrated. By imaging magnetic fields at a single nanowire level, this work represents an important step toward tracking and probing of ferromagnetic nanowires in biomedical applications.

Comparative Study of Continuous Wave and Pulse Wave Electromagnetic Acoustic Resonance for the Measurement of Pipe Wall Thickness

This paper describes the measurement of wall thinning by continuous wave electromagnetic acoustic resonance (CW-EMAR) and pulse wave electromagnetic acoustic resonance (PW-EMAR). Simulated pipe wall thinning specimens are measured. The two methods show accurate results in the areas with a low variation of thickness, and the accuracy decreases when the thickness changes drastically. The two methods are analyzed by using the finite element method. The results show that with the increase of the bottom inclination, the accuracy of the two methods is reduced. For thin specimens, the reason for the larger error of CW-EMAR method is due to that CW-EMAR method is more easily affected by ultrasonic scattering and nonuniform distribution of static magnetic field.

Current density analysis of electron transport through molecular wires in open quantum systems.

The current density in molecular wires connected to contacts is investigated within the nonequilibrium Green's function formalism combined with the Landauer approach. Energy-dependent and total current density through a series of molecular junctions are calculated in real space representation. A rich variety of current patterns including pronounced ring currents ("vortices") are found even in the defect-free minimal building blocks of molecular devices. The influences of contact positions, functional groups as well as atomic defects on the transport properties are examined systematically for prototypical ortho-, meta-, and para-substituted benzenes as well as heteroaromatic systems. It is found that substitutional functional groups mainly shift the molecular levels and retain characteristic transport channels, while a significant change of electronic pathways and conductance is induced by hetero-aromaticity. The current distribution is used to calculate the static magnetic field distribution in the carbon-based conductors. © 2017 Wiley Periodicals, Inc.

Magnetic field deformation due to electron drift in a Hall thruster

The strength and shape of the magnetic field are the core factors in the design of the Hall thruster. However, Hall current can affect the distribution of static magnetic field. In this paper, the Particle-In-Cell (PIC) method is used to obtain the distribution of Hall current in the discharge channel. The Hall current is separated into a direct and an alternating part to calculate the induced magnetic field using Finite Element Method Magnetics (FEMM). The results show that the direct Hall current decreases the magnetic field strength in the acceleration region and also changes the shape of the magnetic field. The maximum reduction in radial magnetic field strength in the exit plane is 10.8 G for an anode flow rate of 15 mg/s and the maximum angle change of the magnetic field line is close to 3° in the acceleration region. The alternating Hall current induces an oscillating magnetic field in the whole discharge channel. The actual magnetic deformation is shown to contain these two parts.

Targeted inductive heating of nanomagnets by a combination of alternating current (AC) and static magnetic fields

The conversion of electromagnetic energy into heat by nanomagnets has the potential to be a powerful, non-invasive technique for cancer therapy by hyperthermia and hyperthermia-based drug release, while temperature controllability and targeted heating are challenges to developing applications of such magnetic inductive hyperthermia. This study was designed to control the hyperthermia position and area using a combination of alternating current (AC) and a static magnetic field. MnZn ferrite (MZF) nanoparticles which exhibited excellent hyperthermia properties were first prepared and characterized as an inductive heating mediator. We built model static magnetic fields simply using a pair of permanent magnets and studied the static magnetic field distributions by measurements and numerical simulations. The influence of the transverse static magnetic fields on hyperthermia properties was then investigated on MZF magnetic fluid, gel phantoms and SMMC-7721 cells in vitro. The results showed a static magnetic field can inhibit the temperature rise of MZF nanoparticles in an AC magnetic field. But in the uneven static magnetic field formed by a magnet pair with repelling poles face-to-face, the heating area can be restricted in a central low static field; meanwhile the side effects of hyperthermia can be reduced by a surrounding high static field. As a result we can position the hyperthermia area, protect the non-therapeutic area, and reduce the side effects just by using a well-designed combination of AC and static field.

Static magnetic solution in magnetic composites with arbitrary susceptibility inhomogeneity and anisotropy

The static magnetic solutions in magnetic composites with arbitrary susceptibility inhomogeneity and anisotropy are accurately computed using an efficient numerical algorithm based on a proposed Fourier spectral iterative perturbation method for 3-dimensional systems. An advantage of this method is that the interphase boundary conditions are automatically considered without explicitly tracking interphase interfaces in the composites. This method can be conveniently implemented in phase-field modeling of microstructure evolution in systems with inhomogeneous susceptibility as well as inhomogeneous spontaneous magnetization distributions. Based on the proposed method, the effects of microstructures including the susceptibility mismatch between the inclusions and matrix, inclusions volume fraction, and inclusions arrangement on the effective susceptibility and local static magnetic field distribution of the composite are investigated. It is found that the interactions among the inclusions embedded in the matrix play critical roles in determining the composite properties.

Experimental optimization in ion source configuration of a miniature electron cyclotron resonance ion thruster

A miniature ion thruster has been proposed in recent years for a small propulsion system applied in space missions such as deep space exploration, precise high-stability attitude and position control. An electron cyclotron resonance (ECR) ion thruster is free from contamination and degradation of electron emission capacity and will offer a potentially longer thruster lifetime than that in the electron bombardment type. The microwave ECR ion source with a 20-mm diameter designed here consists of two annular permanent magnets (SmCo), ring coupling antenna and a grid system including screen and acceleration. For the ion source performance optimization, with a fixed magnetic structure, the antenna position and cavity length in the discharge chamber can be adjusted to strengthen electron ECR heating and increase ion beam extraction. According to the distribution of static magnetic field and the ECR layer measured by Gauss meter, three possible sizes of antenna position (L1) are set; depending on the cut-off characteristics of the discharge chamber and the distribution of microwave electric field calculated by finite element method, six candidate sizes of cavity length (L2) are set. By comparing the difference in plasma discharge and ion beam extraction, the optimal structure of ion source can be obtained. Experimental results show that for a given antenna position, there is a cavity length not too long or too short to extract the maximum ion beam. And the launch of microwave from strong magnetic field near ECR layer is conductive to lossless wave propagation in plasma and highly efficient electron ECR heating. To maintain a plasma in very low power and flow, the size combination of 0.6-mm in L1 and 5-mm in L2 is selected as the preferred structure. The performances of miniature ECR ion source, that is, ion beam current, discharge loss, propellant utilization efficiency, thrust and specific impulse are 5.4 mA, 389 W/A, 15%, 163 μup N and 1051 s, respectively, at an incident power of 2.1 W and argon flow of 14.9 μg/s.