Magnetic Field Excitation Research Articles

Nonlinear Landau damping of plasma waves with orbital angular momentum.

We present, using three-dimensional particle-in-cell simulations, an observation that orbital angular momentum (OAM) is transferred to resonant electrons proportionally to longitudinal momentum when Laguerre-Gaussian plasma waves are subjected to Landau damping. A higher azimuthal mode number leads to a larger net orbital angular momentum transfer to particles traveling close to the phase velocity of the plasma wave, implying a population of electrons that are orbiting the same center of rotation as the plasma wave. This observation has implications on magnetic field excitation as a result of the formation and damping of OAM plasma waves. The energy distributions of electrons in damping Laguerre-Gaussian plasma waves are significantly changed as a function of azimuthal mode number. This leads to larger numbers of lower energy particles tending towards a significant narrowing of the energy distribution of accelerated particles.

Investigation of dynamic properties of isotropic and anisotropic magnetorheological elastomers with a hybrid magnet shear test rig

Magnetorheological elastomers (MREs) exhibit instantaneous and reversible adaptability of stiffness and damping properties under the influence of magnetic field, which can be implemented in the development of controllable devices. The main MRE components are normally elastomeric matrix and magnetisable particles. Depending on the distribution of the particles in the matrix, MREs can be classified into isotropic and anisotropic. This work experimentally explored, compared, and modelled the dynamic characteristics of both isotropic and anisotropic MREs with different iron particle weight fractions (17%, 22%, and 32%). A novel shear test rig was designed with hybrid magnets system, i.e. permanent magnet and electromagnets, to fulfil the characterisation tasks. The involvement of the hybrid magnets effectively cuts down the maximum electric current and energy consumption of the rig. The tests were conducted under sinusoidal shear motions with excitation frequency ranging from 0.1 Hz to 2 Hz and shear strain varying from 20% to 60% to record the force-displacement hysteresis of MRE samples. Four different levels of magnetic field (0.02, 0.54, 0.77, 1.01 T) were supplied by the hybrid magnetic system and were considered in the tests to evaluate the influence of the magnetic fields. Furthermore, characterised hysteretic behaviours for both isotropic and anisotropic MRE were modelled by a strain stiffening phenomenological model with ideal accuracy under the shear excitation inputs and magnetic fields considered.

Development of Brushless Claw Pole Electrical Excitation and Combined Permanent Magnet Hybrid Excitation Generator for Vehicles

Aiming at the problems of large excitation loss and low power generation efficiency of silicon rectifier generators and the unstable output voltage of permanent magnet (PM) generators, a hybrid excitation generator (HEG) with suspended brushless claw pole electrical excitation rotor (EER) and combined magnetic pole PM rotor is proposed in the present work. With only one fractional slot winding stator, the generator adopts PM field as the main magnetic field and electrical excitation field as the auxiliary magnetic field, which not only retains the advantages of high efficiency of PM generators but also effectively reduces excitation consumption. The main structure parameters and the design method were analyzed, and a simulation analysis of no-load magnetic field distribution and flux regulation ability was carried out using finite element software to verify the rationality of the hybrid excitation parallel magnetic circuit design. Moreover, the no-load, load, regulation, and voltage regulation characteristics of the designed generator were tested, and the results show that the designed generator has a wide range of voltage regulation, which can ensure stable output voltage under variable speed and load conditions.

Research on inversion method with multi-transceiver distance in underground space based on apparent conductivity

The layered ground under the excitation of a harmonic magnetic field will generate a secondary magnetic field. On the condition of low induction number, the apparent conductivity can be obtained by using the normalized secondary field measured by the instrument directly. By discretizing the integral of relative response function density, the expression of contribution value of conductivity of each layer to the apparent conductivity can be obtained. By using multiple sets of transceiver distance, we can obtain different apparent conductivity observation equations, then the conductivity and thickness of each layer can be solved by Gauss-Newton method. A three-layer medium model containing a V-shaped tunnel is designed for inversion simulation calculations. The results show that the higher the SNR of observed signal, the more stable the inversion result. The maximum relative error of thickness is about 10%.

Application of Linear Gradient Magnetic Field in Arterial Profile Scanning Imaging.

Background and Objectives: Cardiovascular and cerebrovascular diseases caused by arterial stenosis and sclerosis are the main causes of human death. Although there are mature diagnostic techniques in clinical practice, they are not suitable for early disease prediction and monitoring due to their high cost and complex operation. The purpose of this paper is to study the coupling effect of arterial blood flow and linear gradient magnetic field, and to propose a method for the reconstruction of the arterial profile, which will lay a theoretical foundation for new electromagnetic artery scanning imaging technology. Methods and Models: A combination coil composed of gradient coils and drive coils is applied as a magnetic field excitation source. By controlling the excitation current, a linearly gradient magnetic field with a line-shaped zero magnetic field is generated, and the zero magnetic field is driven to scan in a specific direction. According to the magnetoelectric effect of blood flow, under the action of the external magnetic field, the voltage signals on the body surface can be detected by measuring electrodes. The location of the artery center line can be determined by the time–space relationship between voltage signals and zero magnetic field scanning. In addition, based on the reciprocity theorem integral equation, a numerical model between the amplitude of the voltage signal and the arterial radius is derived to reconstruct the arterial radius. The above physical process was simulated in the finite element analysis software COMSOL, and the voltage signals obtained from the simulation verified the arterial profile reconstruction. Results: Through finite element simulation verification, the imaging method based on a linear gradient magnetic field has a numerical accuracy of 90% and a spatial resolution of 1 mm. Moreover, under 100 Hz low-frequency alternating current excitation, the single scanning time is 0.005 s, which is far shorter than the arterial blood flow change cycle, meeting the requirements of real-time imaging. The results demonstrate the effectiveness and high theoretical feasibility of the proposed method in real-time arterial imaging. Conclusions: This study indicates the potential application of linear gradient magnetic fields in arterial profile imaging. Compared with traditional electromagnetic imaging methods, the proposed method has the advantages of fast imaging speed and high resolution, showing the certain application value in early real-time imaging of arterial disease. However, further studies are necessary to confirm its effectiveness in clinical practice by more medical data and real cases.

A formalism to investigate the optimal transmit efficiency in radiofrequency shimming.

Transmit efficiency specifies the amplitude of the magnetic resonance excitation field produced over a region of interest with respect to the radiofrequency (RF) power deposited in the sample. This metric is highly important at ultra-high field magnetic resonance imaging (≥7 T), where excitation inhomogeneities and electric field interference effects could prevent achieving the desired flip angle distribution while satisfying the power safety limits. The aim of this work was to introduce an approach to calculate a theoretical upper bound on the transmit efficiency (OPTXE) for RF shimming, independent from any particular coil design. We computed the OPTXE for head-mimicking uniform spherical samples and a realistic heterogeneous head model by maximizing the square of the net transmit field per unit power deposition. The corresponding RF shimming weights were used to combine the analytical surface current modes into ideal current patterns. OPTXE grew monotonically as the target excitation voxel approached the surface of the object, and overall decreased at higher field strengths, presenting similar trends in both the uniform sphere and heterogeneous head model. Arrays with an increasing number of loops could closely approach OPTXE in the central region of the object, but performance decreased closer to the surface and at higher magnetic field strengths. The performance of 32 loops for a two-dimensional excitation region at 7 T increased from 34% to 93% when they were arranged based on the shape of the ideal current patterns. OPTXE provides an absolute reference to evaluate coil designs and RF shimming algorithms, whereas ideal current patterns could serve as guidelines for novel coil designs at ultra-high field. The uniform sphere model enables rapid analytic simulations and provides a good approximation of the OPTXE distribution in a realistic heterogeneous head model with comparable dimensions.

Magnetic Field Generation System of the Magnetic Probe With Diamond Quantum Sensor and Ferromagnetic Materials for the Detection of Sentinel Lymph Nodes With Magnetic Nanoparticles

We have developed the magnetic probe with a diamond quantum sensor and electromagnetic coils to detect sentinel lymph nodes (SLNs), as the medical application of diamond quantum sensing. The probe magnetizes magnetic nanoparticles (MNPs) accumulated in SLNs and detect the magnetic fields of magnetized MNPs. In this study, we designed a ferromagnetic core that has a unique shape and optimized the magnetic field generation system for improving the detection performance such as the magnetic sensitivity and detectable distance. The proposed magnetic core enhances an excitation magnetic field strength at a longer distance and suppresses a bias field strength at a location of the diamond quantum sensor. The increment of the excitation field is approximately 370% and the decrement of the bias field is approximately 45%. In addition to the proposed magnetic core, even applying a cancellation coil, the excitation field strength is tenfold compared with the coil only structure, suggesting the larger magnetization of MNPs generates the large magnetic field for longer detectable distance.

Compact Induction Accelerator of Laser Plasma for Ion Energy up to 1 MeV

The collective acceleration of laser plasma ions in a rapidly increasing magnetic field (108 T/s) excited by a powerful current pulse in a low-inductive conical spiral expanding in the direction of plasma acceleration has been studied. A mathematical model and an algorithm for calculating the radial Br and axial Bz components of the magnetic field in the approximation of a conical spiral by a system of rings of variable radius are proposed to analyze the factors affecting the efficiency of such acceleration. Based on computer modeling and an experimental variation of magnetic-field excitation parameters, the regime of effective ion acceleration is obtained. With the help of time-of-flight collector measurements, the velocities of ions whose atomic mass differs by two orders of magnitude are determined. The maximum velocity of both light ions (lithium) and heavy ions (lead) exceeds 106 m/s, and the corresponding energy for lead ions is ~1 MeV. The efficiency of collective acceleration with the direct acceleration of laser plasma ions in a high-current high-voltage diode with magnetic insulation is compared.

A Ka‐band power divider/combiner using two side‐inserted magnetic coupling semicircular ring probes

A Ka-band power divider/combiner with dual magnetic coupling semicircular ring probes is proposed in this paper. Firstly, a broadband microstrip-to-waveguide transition with semicircular ring probe is designed based on the side-inserted structure of magnetic field excitation in a rectangular waveguide. The insertion loss of the proposed transition is less than 0.7 dB in Ka-band assisted by the dual symmetrical broadband probes and rectangular waveguide. Then, the divider/combiner is proposed using the new transition with magnetic coupling from narrow wall into the rectangular waveguide. The bandwidth of the divider/combiner is more than 9 GHz (from 27 to 36.7 GHz), and the insertion loss of the single divider/combiner is less than 3.3 dB. Finally, the performance of the proposed divider/combiner is validated through simulations and measurements. The proposed design has potential applications in microstrip circuits.

Excitation and Magnetic Field Performances of a Prototype REBCO Sextupole Coil at 4.2 K

A prototype REBa2Cu3Oy (REBCO) sextupole coil was designed and fabricated for the chromaticity correction of an asymmetric–energy collider, named SuperKEKB. The REBCO sextupole coil consists of the six two-layer-rectangular coils, which are wound with a 4-mm-wide coated conductor and impregnated with the epoxy resin. The inner bore radius and coil length are 75 mm and 200 mm, respectively. The designed sextupole field gradient is 200 T/m2 at an operating current of 250 A. We conducted a series of the experiments on the REBCO sextupole coil after the fabrication. In this paper, we present the experimental results of the excitation test and magnetic field measurement at 4.2 K.

Mechanical Responses of a Combined Support Structure for a Nb3Sn Sextupole Magnet During Assembly and Thermal Cycle

The Institute of Modern Physics (IMP) in Lanzhou, China, is currently developing a complicated superconducting magnet system composed of four solenoids and six sextupole coils based on a sextupole-in-solenoid configuration. The magnet is made of a single Nb3Sn wire and designed for a fourth-generation ECR ion source operating at frequency of 45 GHz. To effectively prevent the wires motion from performance degradations and a quench, a pre-stress is essential for the superconducting magnet winding and strengthening. Focusing on the sextupole magnet's assembly and the pre-load procedure, a combined support structure with temporary bladders, load pads, and load keys, as well an aluminum shell was designed and fabricated in this work. A whole assembly process was fulfilled with a dummy structure at room temperature and a cooling-down and warming-up cycle to show its validity. Some low-temperature resistance strain gauges, together with a half-bridge compensation, were used to monitor strain of the aluminum shell during the pre-load control performance and thermal cycle process. A parametric study on the equivalent friction of the combined structure was carried out for the mechanical characteristics based on FEM simulation to show a reasonable agreement with the experiment measurements as a proper friction coefficient chosen. Finally, a real sextupole magnet was assembled with the combined support structure and tested for excitation. It was illustrated that a proper pre-load state realized by the support structure and a high excitation current about 800 A was reached so that the excited magnetic field gained 7 T as the designed value.

An arbitrary waveform magnetic nanoparticle relaxometer with an asymmetrical three-section gradiometric receive coil

Magnetic nanoparticles (MNPs) have a wide range of clinical applications for imaging, therapy, and biosensing. Superparamagnetic MNPs can be directly visualized with high spatiotemporal resolution using the magnetic particle imaging (MPI) modality. The image resolution of MPI depends on the relaxation properties of the MNPs. Therefore, characterization of MNP response under alternating magnetic field excitation is necessary to predict MPI imaging performance and develop optimized MNPs. Biosensing applications also make use of the change in the relaxation response of MNPs after binding to a target agent. As MNP relaxation properties change with temperature and viscosity, noninvasive probing of these microenvironmental properties is possible. In this work, we present an untuned relaxometer to measure the relaxation properties of the MNPs in a wide frequency and amplitude range. The developed relaxometer can produce above 80 mTpp magnetic field at up to 60 kHz frequency, and above 14 mTpp at up to 150 kHz frequency. An asymmetrical three-section gradiometer receive coil is used to cancel the direct coupled signal from the transmit coil. The position of one of the receive coil sections is manually tuned using a rotating knob for improved decoupling. The tuning coil section has a lower number of turns compared to the other sections to decrease the sensitivity to mechanical movement. By tuning the knob, the transmit-receive coupling can be decreased below ?80 dB. We analyzed the x-space image resolution, harmonic levels, and effect of the number of used harmonics on the resolution for two different commercially available superparamagnetic iron oxide MNPs (Perimag and Synomag-D) in a multifrequency/multiamplitude measurement scheme. The magnetization properties of MNPs for arbitrary waveforms can be measured efficiently using the developed relaxometer.

An analysis of generator trip caused by mistransmitting high temperature signal from excitation transformer

The excitation transformer is a device that specifically provides three-phase AC excitation power for the generator excitation system. The excitation system converts the three-phase power to the generator rotor DC power through the thyristor to form the generator excitation magnetic field, which is adjusted by the excitation system thyristor trigger angle, to achieve the purpose of adjusting the motor terminal voltage and reactive power. This article analyzes an accident in which the excitation transformer high temperature signal is erroneously caused to cause the unit to trip. The unplanned shutdown accident is specifically analyzed from the aspects of on-site situation investigation, cause analysis, main problems exposed, treatment and preventive measures.

Amplitude and phase of higher harmonic of magnetic nanoparticles’ magnetization under low frequency magnetic field

The rotational relaxations exist in magnetic nanoparticles (MNPs) exposed to AC excitation magnetic field, including Brownian rotational relaxation and Neel rotational relaxation. The magnetization of MNPs exposed in AC magnetic field may be affected by rotational relaxation (Brownian relaxation play a dominated role when frequency is less than 1 kHz), and phase lag of magnetization harmonics are existed. In this paper, we employ a Fokker-Planck Equation, which describe accurately AC magnetization dynamic (dominated by Brownian rotational relaxation), analyze the difference of AC magnetization based on Fokker-Planck Equation and Langevin Function, study magnetization harmonic and phase lag dependence on Brownian rotational relaxation, impose a simple, empirical harmonic model. The simulation and experiment results show that the harmonic model can describe accurately AC magnetization when the Brownian rotational relaxation is dominated in MNPs. It is expected to be useful for improving MPI and MNPT.

Effect of excitation mode on the magnetic field detection limit of magnetoelectric composite cantilevers

Magnetic field excitation of strain-coupled magnetoelectric composite cantilevers in different bending modes is investigated for magnetic field sensing, yielding the sensitivity, noise, and magnetic field detection limit. An analytic theory covering the resonant magnetoelectric response and thermal vibration noise of arbitrary bending modes and the Johnson–Nyquist noise from the composite and electronics is presented, and detection limit results of thin film FeCoBSi–Si–AlN composite cantilevers are calculated for the first three bound–free and free–free bending modes over a wide range of dimensions. We use size-scaling to yield the same 1 kHz resonance frequency for all modes and dimensions, constant quality factors Qf = 1000, and thickness-independent experimental material parameters. Magnetic field detection limits in the 1 pT/Hz1/2 to 100 fT/Hz1/2 range are predicted for practical cantilever dimensions, whereby higher modes are found to yield lower detection limits at similar functional layer thicknesses but a greater cantilever size. All detection limits are found to be thermal vibration noise limited and for different modes to display the same 1/size2 scaling behavior but require different FeCoBSi–Si–AlN layer thickness ratios.

Extending Absorption Edge through the Hybrid Resonator-Based Absorber with Wideband and Near-Perfect Absorption in Visible Region.

Metamaterial absorber with the unexpected capability for harvesting electromagnetic energy has been regarded as a potential route for various applications, including chemical/biological sensing, cloaking and photovoltaic applications. In this study, we presented the simple absorber design made with Al/SiO2/Al sandwich structures through the involvement of hybrid dual-resonators that could allow the wideband light absorption covered from 450 nm to 600 nm with average absorptivity above 95%. Examinations of excited electric field, magnetic field and total magnitude of electric field in three-dimensional space at resonances were performed to clarify the origin of resonant behaviors. In addition, an equivalent inductance–capacitance circuit model was proposed that could qualitatively explore the geometry-dependent absorption characteristics by modulating the constitutive parameters of hybrid resonators. As a result, the designed light absorber might enable to be practically applied for various optical-management and photovoltaic applications, and even offered the tunability for other desired frequency regions.

Electric fields and local magnetic field enhance Ag-BiVO4-MnOx photoelectrochemical and photocatalytic performance

The Ag-BiVO4-MnOx multi-stage interface heterojunction photocatalyst is prepared via photo-deposition method. The oxidation co-catalyst MnOx and reduction co-catalyst Ag are selectively deposited onto (0 1 0) and (1 1 0) facets of BiVO4. Under ultraviolet (UV) light, MnOx is deposited onto the BiVO4 (1 1 0) facet, causing its energy band to move upward and bend at the interface to produce a p-n junction and a built-in electric field that points from BiVO4 towards MnOx. Simultaneously, the difference in energy levels allows the BiVO4 to form surface heterojunction and produce a built-in electric field that points from (0 1 0) facet towards the (1 1 0) facet. A Schottky junction is formed when Ag is deposited onto the BiVO4 (0 1 0) facet to create space charge region without free carriers. A built-in electric field is formed that points from Ag to BiVO4. The electric fields generated via light excitation and a local magnetic field formed by plasmonic Ag improve the photocatalytic activity of the Ag-BiVO4-MnOx photocatalyst, the degradation rate of Ag-BiVO4-MnOx is 5.9 times higher than that of the BiVO4. This work not only promotes the separation efficiency of photogenerated charge carriers, but also pays attention to the synergistic effect of electric fields generated by light excitation and local magnetic field formed by plasmonic Ag, which provides a new idea for photocatalysis research.

Comparison of Anisotropic Energy-Based and Jiles–Atherton Models of Ferromagnetic Hysteresis

In this article, we apply an anisotropic extension for the energy-based and the Jiles–Atherton (JA) hysteresis models to simulate both unidirectional alternating and rotational magnetic field excitations. The results show a good agreement with measurements for unidirectional alternating fields. However, the results for rotational fields, especially at high magnitudes, show a significant discrepancy with the measurement data. We demonstrate that the JA model with appropriate parameters can estimate the losses in alternating cases up to 1.5 T, whereas both models give reasonable loss estimation up to 1 T in rotational cases.

Research on the Orthogonal Fundamental Mode Fluxgate Sensor Circuit

Currently, magnetic fluxgate circuits used for magnetic field measurements mostly adopt parallel excitation second harmonic generation. The magnetic fluxgate developed by this method cannot possess both low noise and high bandwidth at the same time. The orthogonal fundamental mode fluxgate is used in this paper: the excitation magnetic field is orthogonal to the magnetic field to be measured, and the external magnetic field is detected by measuring the voltage signal in the pick-up coil. The excitation magnetic field changes with the parallel excitation second harmonic method while the direction of the excitation magnetic field does not change in the orthogonal fundamental mode scheme, which can effectively reduce the influence of Barkhausen noise. The magnetic fluxgate circuit is designed based on the orthogonal fundamental mode scheme and its performance indexes are tested. The range of the magnetic fluxgate is ±100000 nT, the sensitivity is $100~\mu \text{V}$ /nT, the output noise is 8.9 pT/rt(Hz)@1 Hz, and the bandwidth is DC-15 kHz. The orthogonal fundamental mode fluxgate improves the bandwidth performance while enabling low-noise magnetic field measurements in practical applications.

Multiple-Transmit Focusing for the Nondestructive Testing of Downhole Casings Based on Borehole Transient Electromagnetic Systems

Borehole transient electromagnetic (TEM) array has proven to be efficient for the downhole nondestructive testing (NDT) of metal casings through the eddy-current property. However, restricted by bad downhole conditions, the simple increase of the receiving array is not sufficient enough for improving the downhole NDT performance. In this paper, we present a multiple-transmit focusing (MTF)-based borehole TEM system for the NDT of downhole casings. On the basis of the borehole TEM signal model, the response of the multiple-transmit array is derived by employing the matrix form of the borehole TEM response. It was shown that the excited magnetic field by the multiple-transmit array can be focused by weighting the current of each transmitter. Using this property, a modified linear constrained minimum variance-based multiple-transmitting array weighting method was proposed to realize the MTF. Moreover, a subarray partition approach was proposed to simplify the MTF realization, where the subarray weighting and mean square error were also analyzed. The MTF method performance was verified by applying it to a borehole TEM system for the NDTs of downhole casings. Finally, simulations and experiments were conducted, and the results demonstrated the effectiveness of the proposed method.