Rotating Magnetic Field Research Articles

Escape of cosmic rays from perpendicular shocks in the circumstellar magnetic field

We investigate the escape process of cosmic rays (CRs) from perpendicular shock regions of a spherical shock propagating to a circumstellar medium with the Parker-spiral magnetic field. The diffusive shock acceleration in perpendicular shocks of supernova remnants (SNRs) is expected to accelerate CRs up to PeV without upstream magnetic field amplification. Red supergiants (RSGs) and Wolf-Rayet (WR) stars are considered as progenitors in this work. We perform test particle simulations to investigate the escape process and escape-limited maximum energy without magnetic field amplification in the upstream region, where the magnetic field strength and rotation period expected from observations of RSGs and WR stars are used. We show that particles escape to the far upstream region while moving along the equator or poles and the maximum energy is about $10-100~{\rm TeV}$ when SNRs propagate to free wind regions of RSGs and WR stars. In most cases, the escape-limited maximum energy is given by the potential difference between the equator and pole. If progenitors are oblique rotators and SNRs are in the early phase just after the supernova explosion, the escape-limited maximum energy is limited by the half wavelength of the wavy current sheet. In addition, for RSGs, we show that the luminosity of CRs accelerated in the wind region is sufficient to supply the observed CR flux above $10~{\rm TeV}$ if a strong magnetic field strength is sustained in most RSGs. In terms of the CR luminosity, SNRs propagating to the free wind of WR stars can contribute to PeV CRs. As long as no magnetic field amplification works around SNR shocks, the maximum energy is decided by the magnetic field strength in the wind region, which depends on the rotation period, stellar wind, and surface magnetic field of RSGs and WR stars. Therefore, we need to observe these quantities to understand the origin of CRs.

Research on Hydrogen Production by Water Electrolysis Using a Rotating Magnetic Field

In this paper, the effect of rotating magnetic fields on hydrogen generation from water electrolysis is analyzed, aiming to provide a research reference for hydrogen production and improving hydrogen production efficiency. The electrolytic environment is formed by alkaline solutions and special electrolytic cells. The two electrolytic cells are connected to each other in the form of several pipes. The ring magnets are used to surround the pipes and rotate the magnets so that the pipes move relative to the magnets within the ring magnetic field area. Experimentally, the electrolysis reaction of an alkaline solution was studied by using a rotating magnetic field, and the effect of magnetic field rotation speed on the electrolysis reaction was analyzed using detected voltage data. The experimental phenomenon showed that the faster the rotation speed of the rotating magnetic field, the faster the production speed of hydrogen gas.

Investigation of a temperature-sensitive ferrofluid to predict heat transfer and irreversibilities in LS-3 solar collector under line dipole magnetic field and a rotary twisted tape

This article numerically studies the performance of the LS-3 parabolic trough collector. The parabolic trough solar system is equipped with an innovative rotary twisted tape. The absorber tube and twisted tape have the same length. Moreover, line dipole magnetic field is applied to improve the performance of solar collector. In addition, the characteristics of the irreversibilities are investigated. Commercial temperature-sensitive ferrofluid is selected for working fluid, and the nanoparticles along with the base fluid assumed to be uniform. The effect of rotational speed, twisted ratio, and line dipole magnetic field are compared with plain tube. The heat flux profile on absorber tube is ascertained by the Monte Carlo Ray Tracing method. Curve fitting method is used to obtain the mathematical function of the heat flux profile. To solve the equations including continuity, momentum, energy, and SST κ−ω model, the finite volume procedure is selected. The more convective heat transfer coefficient occurs by dipole magnetic field, the maximum enhancement reaches over 97% at B = 100 G. In addition, the twisted tape can increase heat transfer by 305% at twisted ratio = 2.5 and rotational speed = 200 rpm. Furthermore, if the twisted tape has rotational speed, the thermal entropy generation decreases.

Solar wind magnetic holes can cross the bow shock and enter the magnetosheath

Abstract. Solar wind magnetic holes are localized depressions of the magnetic field strength, on timescales of seconds to minutes. We use Cluster multipoint measurements to identify 26 magnetic holes which are observed just upstream of the bow shock and, a short time later, downstream in the magnetosheath, thus showing that they can penetrate the bow shock and enter the magnetosheath. For two magnetic holes, we show that the relation between upstream and downstream properties of the magnetic holes are well described by the MHD (magnetohydrodynamic) Rankine–Hugoniot (RH) jump conditions. We also present a small statistical investigation of the correlation between upstream and downstream observations of some properties of the magnetic holes. The temporal scale size and magnetic field rotation across the magnetic holes are very similar for the upstream and downstream observations, while the depth of the magnetic holes varies more. The results are consistent with the interpretation that magnetic holes in Earth's and Mercury's magnetosheath are of solar wind origin, as has previously been suggested. Since the solar wind magnetic holes can enter the magnetosheath, they may also interact with the magnetopause, representing a new type of localized solar wind–magnetosphere interaction.

On the dynamics of curved magnetoactive soft beams

Recently, hard-magnetic microparticles were embedded into soft matrix to manufacture a new type of magnetoactive soft materials, i.e. hard-magnetic soft (HMS) materials. Due to the promising applications of HMS materials in the areas of soft robotics, flexible electronics, biomedicine, etc., there is a burgeoning trend in investigating the mechanical responses of HMS structures. Special attention has been placed on the static deformations of initially straight HMS beams. However, to fulfill the potential applications of HMS materials, it is also necessary to examine the dynamical behaviors of HMS beams. The present study aims to develop a large-deformation dynamical model of curved HMS beams actuated by a harmonically rotational magnetic field, which is varying periodically with time. This dynamical curved HMS beam model could be easily reduced to the corresponding static one, which is then solved both analytically and numerically. To tackle the dynamic problem, the derived governing equation for nonlinear vibrations of the curved HMS beam is first discretized by using the Galerkin method and then solved by a fourth-order Runge-Kutta integration algorithm. It is found that the curved HMS beam tends to align with the direction of the applied constant magnetic field as the strength of the applied field increases. The curved HMS beam would vibrate periodically if a periodically rotational magnetic field is applied. Furthermore, when the rotational frequency of the magnetic field is close to the natural frequency of the beam, the system resonates and the vibration amplitude of the curved beam becomes extremely large.

Viscometer for investigating the viscosity of magnetic fluid media

The rational application of magnetic fluids in various technical systems requires their stability analysis, hydrodynamic particle size, dissipative processes in the colloid control. The existing viscometers do not fully meet all requirements, in particular, they do not allow research in a magnetic field of given magnitude and certain topography. It is proposed to determine rheological properties of magnetic fluids using rotary viscometer, in which two measuring gaps are structurally designed and filled with studied fluid, which is important for calculating characteristics of low viscosity magnetic media. The device design implements practically homogeneous magnetic field, varying within wide range and directed perpendicular to liquid shear stress. The device allows to steadily maintain shear rate in medium under study, and to determine viscosity with a sufficiently large change. A small magnetic fluid volume is required to estimate rheological characteristics. The accuracy of measurements of the internal friction force, device heating temperature, coaxial cylinders rotation speed and magnetic field inside investigated medium meet the requirements of international standards. Laminar fluid flow process in the device gap is described mathematically. This made it possible to justify the construction geometrical dimensions in order to increase the measurements and determine rheological properties of low-viscosity magnetic materials. The formulas for calculation of Newtonian fluid viscosity coefficient, shear stress and shear rate based on experimental data were obtained.

Magnetic field and Faraday rotation from large-scale interstellar medium to plasma near the black-hole horizon

Magnetic fields appear to be ubiquitous in the astrophysical environments of our Universe, but are still poorly understood despite playing an important role in understanding different-scale celestial objects (e.g., Earth, planets, stars, normal galaxies, active galactic nuclei, clusters etc.). We briefly review the results and progress in magnetic field estimation based on rotation measures from the large-scale interstellar medium, megaparsec-scale radio jets to the plasma near the black-hole horizon. We find that there is no evident correlation between electron density and magnetic field in warm ionized plasma, which suggests that the estimation of the magnetic field from the ratio of the rotation measure and dispersion measure correctly reflects the intrinsic field. The estimation of the magnetic field from the rotation measure is also discussed near the black hole horizon in M 87. The Square Kilometre Array will greatly improve the understanding of the magnetic field in our Universe, which will provide high-quality polarization imaging and extragalactic rotation measures in the near future.

Rolling microswarms along acoustic virtual walls

Rolling is a ubiquitous transport mode utilized by living organisms and engineered systems. However, rolling at the microscale has been constrained by the requirement of a physical boundary to break the spatial homogeneity of surrounding mediums, which limits its prospects for navigation to locations with no boundaries. Here, in the absence of real boundaries, we show that microswarms can execute rolling along virtual walls in liquids, impelled by a combination of magnetic and acoustic fields. A rotational magnetic field causes individual particles to self-assemble and rotate, while the pressure nodes of an acoustic standing wave field serve as virtual walls. The acoustic radiation force pushes the microswarms towards a virtual wall and provides the reaction force needed to break their fore-aft motion symmetry and induce rolling along arbitrary trajectories. The concept of reconfigurable virtual walls overcomes the fundamental limitation of a physical boundary being required for universal rolling movements.

The behavior and heat generation effect of a magnetic rod-like particle suspension in an alternating and a rotating magnetic field

We have investigated the behavior of magnetic rod-like particles and their relationship with the heat generation effect for both alternating and rotating magnetic fields, by means of Brownian dynamics simulations. As a common feature for both type of magnetic field variations, in the case of significantly strong particle-particle interaction strengths, densely-packed clusters are formed, which do not contribute to the heat generation. For the case of the alternating magnetic field, in the intermediate frequency range, linear thick chain-like clusters are formed and the constituent rod-like particles themselves tend to rotate to follow the change in the field. In contrast, for the case of the rotating field, linear clusters rotate as a whole body to respond to the magnetic field rotation. In both type of magnetic field variations, the magnetic interactions between the constituent particles in a cluster tend to function to suppress the relaxational motion of the rod-like particles, which, as a result, leads to increase in the heat generation effect in certain situations. In a relatively large frequency region, the rotating applied magnetic field gives rise to a larger heat generation effect, whereas in contrast, in the lower frequency region the alternating magnetic field yields the larger heating effect. Highlights The behavior of magnetic rod-like magnetic particles has been elucidated in the case of an alternating and a rotating magnetic field. The magnetic interactions between the constituent particles in a cluster tend to suppress the relaxation motion of the rod-like particles. The frequency and strength of a time-dependent magnetic field have complex influences on the heating effect. In a relatively large frequency region, the rotating applied magnetic field gives rise to a larger heat generation effect. In a relatively lower frequency region, the alternating magnetic field is superior to the rotating field.

Macrosegregation Evolution in Eutectic Al-Si Alloy under the Influence of a Rotational Magnetic Field

Using magnetic stirring during solidification provides a good opportunity to control the microstructure of alloys, thus controlling their physical properties. However, magnetic stirring is often accompanied by a change in local concentrations, and new structures form which could harm the physical properties. This research paper investigated the effect of forced melt flow by a rotating magnetic field (RMF) on the macrostructure of an Al-Si eutectic alloy. To serve this purpose, Al-12.6 wt% Si alloy samples were solidified in a vertical Bridgman-type furnace equipped with a rotating magnetic inductor to induce the flow in the melt. The diameter and length of the sample are 8 mm and 120 mm, respectively. The solidification parameters are a temperature gradient (G) of 6 K/m, and the solid/liquid front velocity (v) of 0.1 mm/s. These samples were divided into parts during the solidification process, where some of these parts are solidified under the effect of RMF stirring while others are solidified without stirring. The structure obtained after solidification showed a distinct impact of stirring by RMF; new phases have been solidified which were not originally present in the structure before stirring. Besides the eutectic structure, the new phases are the primary aluminum and the primary silicon. The Si concentration and the volume fraction of each phase were measured using Energy-Dispersive Spectroscope (EDS)and new image processing techniques. The experimental results reveal that applying the RMF during the solidification has a distinct effect on the macrostructure of Al-Si eutectic alloys. Indeed, the RMF provokes macro-segregation, reduces the amount of eutectic structure, and changes the sample’s Si concentration distribution.

Two-Phase System for Generating a Higher-Frequency Rotating Magnetic Field Excited Causing Hyperthermic Effect in Magnetic Fluids

This article presents a new method of excitation for a fast-changing rotating magnetic field (RMF) of higher frequencies (HF) causing the hyperthermic effect in magnetic fluids. The method proposed here uses a magnetic field exciter (inductor) consisting of a ferrite magnetic circuit and a system of coils connected in a two-phase arrangement. The proposed system is powered by two higher-frequency rectangular signals, with a 90-degree phase shift between each other, through HF transformers with ferrite cores. This paper presents the outcomes of the operation of RMFs in the frequency range of 38 kHz to 190 kHz, with a value of amplitude of magnetic field intensity H equal to 20 kA/m and increasing temperature, in a sample of APG513 magnetic liquid. The obtained results show that, in the range of the magnetic field intensities of moderate values, at a constant frequency f, the values of the time derivative of temperature are proportional to the square of the magnetic field intensity dT/dt~H2. Moreover, the values of the temperature rate, which are measured with the constant value of the magnetic field intensity, are proportional to the square of the frequency dT/dt~f2. At higher amplitudes of the RMF, the relationship dT/dt~H2 is no longer fulfilled, and an inflexion point of this function appears. In the case of the highest values of the achieved intensity amplitudes (H = 20 kA/m), the parameter of the Langevin function achieves a value equal to ξ = 6.

Effect of crucible wall roughness on the laminar/turbulent flow transition of the Ga75In25 alloy stirred by a rotating magnetic field

The critical magnetic induction (Bcr) values of a melt flow produced by a rotating magnetic field (RMF), remaining laminar or turbulent, are essential in different solidification processes. In an earlier paper (Metall Res Technol 100: 1043–1061, 2003), we showed that Bcr depends on the crucible radius (R) and frequency of the magnetic field (f). The effect of wall roughness (WR) on Bcr was investigated in this study. Using ten different wall materials, we determined the angular frequency (ω) and Reynolds number (Re) as a function of the magnetic induction (B) and f using two different measuring methods (pressure compensation method, PCM; height measuring method, HMM). The experiments were performed at room temperature; therefore, the Ga75wt%In25wt% alloy was chosen for the experiments. Based on the measured and calculated results, a simple relationship was determined between Bcr and Re*, f, R, and WR, where the constants K1, K2, K3, and K4 depended on the physical properties of the melt and wall material:Bcrleft({Re}^{*},f,R,WRright)=frac{{Re}^{*}}{{R}^{2}}({K}_{1} {f}^{-K2}+{K}_{3} {f}^{-K4} WR)

Atomic-level-mixing contribution to the P,T -odd Faraday effect as an enhancement factor in the search for P,T -odd interactions in nature

Recently, a $\mathcal{P},\mathcal{T}$-odd Faraday effect (PTFE), optical rotation in an external electric field parallel to the light propagation direction, was discussed as a promising tool for observation of the electron electric dipole moment in the intracavity absorption spectroscopy with atoms or molecules. The main mechanism leading to this effect, the linear Stark splitting of atomic levels (PTFE, LS) is well known. This mechanism is similar to the Zeeman splitting mechanism in case of the ordinary Faraday effect, i.e., optical rotation in an external magnetic field. In this paper we analyze the other possible mechanisms of PTFE, in particular, the atomic-level-mixing mechanism (PTFE, LM). In our analysis we perform considering PTFE (as well as the ordinary FE) as a kind of electro- (magneto-) optical circular birefringence. For the ordinary FE the level-mixing mechanism (FE, LM) is known apart from the main Zeeman level splitting mechanism (FE, LS). However, (PTFE, LM) mechanism for the PTFE was never considered in literature. Usually, the (FE, LS) mechanism dominates and the same was expected for (PTFE, LS). We demonstrate, however, that with heavy diatomic molecules, in particular in PbF, the contribution of (PTFE, LM) may essentially exceed the (PTFE, LS). This promises an additional enhancement of $\mathcal{P},\mathcal{T}$-odd effects in experiments with PbF molecule and makes this molecule one of the best candidates for performing such experiments.

Liquid metal hydraulic actuation and thermal management based on rotating permanent magnets driven centrifugal pump

Liquid metals are quite prominent in the frontier fields of advanced heat dissipation because of their excellent thermal conductivity, electromagnetically drivable capability and wide range of working temperature. However, driving liquid metals that would work in tough situations with requirements of small scale, low vibration, leakage free and corrosion proof still remains a challenge. Here, we proposed a split electromagnetic centrifugal pump for liquid metal hydraulic actuation and thermal management. Owing to the separate design of the rotating magnets and the pump chamber, the metallic fluid in the pump would not contact the outside which well fulfills the above-mentioned demands. As it was experimentally demonstrated, the liquid metal in the pump is directly driven by its electromagnetic interaction with tunable rotational magnetic field. The working performance of the pump was justified through numerical simulations and a series of conceptual measurements. Moreover, the centrifugal pump was found to have achieved an output of 253.4 mL/min and 6.2 kPa. To illustrate the practical value of the centrifugal pump, a mini-channel cooling system and liquid metal enabled reconfigurable antenna system were fabricated and tested, which showed its further potential application in thermal management system and self-cooled reconfigurable antenna device.

Higher harmonics in planar Hall effect induced by cluster magnetic multipoles

Antiferromagnetic (AFM) materials are attracting tremendous attention due to their spintronic applications and associated novel topological phenomena. However, detecting and identifying the spin configurations in AFM materials are quite challenging due to the absence of net magnetization. Herein, we report the practicality of utilizing the planar Hall effect (PHE) to detect and distinguish “cluster magnetic multipoles” in AFM Nd2Ir2O7 (NIO-227) fully strained films. By imposing compressive strain on the spin structure of NIO-227, we artificially induced cluster magnetic multipoles, namely dipoles and A2- and T1-octupoles. Importantly, under magnetic field rotation, each magnetic multipole exhibits distinctive harmonics of the PHE oscillation. Moreover, the planar Hall conductivity has a nonlinear magnetic field dependence, which can be attributed to the magnetic response of the cluster magnetic octupoles. Our work provides a strategy for identifying cluster magnetic multipoles in AFM systems and would promote octupole-based AFM spintronics.

Effect of rotating magnetic field on NiAl(Si) phase evolution and mechanical properties of special nickel brass

The rotating magnetic field (RMF) has been proven as one of effective method and widely used for solidification controlling of metallics materials. In this present study, a rotating magnetic field was applied during the solidification of special nickel brass, and corresponding effect on solidification microstructure and mechanical properties were investigated and revealed. Results demonstrated that the application of RMF greatly affects the lattice parameters of the synthesized NiAl(Si) phase. It also can be determined that the applied RMF can effectively eliminate the aggregation of the NiAl(Si) phase at grain boundaries, which is favorable and beneficial for homogenous distribution of NiAl(Si) phase. Additionally, the applied RMF also refines the brass matrix and induces the morphology evolution of the NiAl(Si) phase changing from bulk to spherical morphology. Accordingly, the enhanced mechanical properties with 627 MPa in ultimate tensile strength (UTS), 518 MPa in yield strength (YS) and 3.6% in elongation (EL) can be obtained under the RMF with optimal current intensity of 30 A, corresponding to the improvement of about 6.1%, 5.0% and 33.3% over the respective properties of the special nickel brass without applied RMF. In addition, underlying mechanisms for the effect of RMF on the solidification process of special nickel brass and evolution of NiAl(Si) phase, as well as the resultant mechanical properties was discussed and clarified. It is anticipated that this present study would provide a feasible and effective approach to tailor the solidification microstructure and mechanical properties of the commercial brass for industrial application.

Energizing charged particles by an orbit instability in a slowly rotating magnetic field.

The stability of charged particle motion in a uniform magnetic field with an added spatially uniform transverse rotating magnetic field (RMF) is studied analytically. It is found that the stability diagram of a single particle's orbit depends critically on the chosen boundary conditions. We show that for many boundary conditions and wide regions in the parameter space, RMFs oscillating far below the cyclotron frequency can cause linear instabilities in the motion which break μ invariance and energize particles. Such energization may appear at odds with the adiabatic invariance of μ; however, adiabatic invariance is an asymptotic result and does not preclude such heating by magnetic fields oscillating at slow frequencies. This mechanism may contribute to heating in the edge plasma of field-reversed configurations (FRCs) in rotamak-FRC experiments. Furthermore, these RMF-driven instabilities may significantly enhance azimuthal current drive during the formation of FRCs in such devices.

Quantized charge-pumping in higher-order topological insulators

We study the quantized charge pumping of higher-order topological insulators (HOTIs) with edge-corner correspondences based on the combination of the rotation of in-plane magnetic field and the quantum spin Hall effect. A picture of a specific charge pumping process is uncovered with the help of the non-equilibrium Green's function method. Significantly, we demonstrate that the quantized charge pumping current is achieved without the participation of bulk states, and the charges move along the boundary of the sample. Furthermore, the effects of external parameters on the pumping current is also studied. We find that the magnitude and direction of the pumping current can be manipulated by adjusting the coupling strength between the leads and sample. Our work deepens the understanding of the charge pumping in HOTIs and extends the study of their transport properties.

The impact of alternating and rotating regimes on the heating characteristics of magnetic bacterial cellulose structure

The hyperthermia efficiency itself and the heating effect are determined not only by the material properties but also by the technical-application factors. For this reason, we subjected our experimental materials to experiments in an alternating magnetic field (AMF) as well as in a rotating magnetic field (RMF). According to the known numerical models as well as the results of comparative experiments, the RMF used is expected to be more efficient in heat generation than the AMF. Such tendency can be changed in the systems of higher density. This affects the particles' freedom of movement during field application, and thus there is a block of rotation. To research this, bacterial cellulose (BC) was magnetized in contact with magnetic fluid. By magnetic modification of BC, another range of potential applications is opened up including hyperthermia. Obtained specific absorption rates (SAR) at 3 different frequencies point to the possibility of effective tuning of the heating effect in conditions of alternating magnetic fields. Preliminary temperature evolution data of magnetized cellulose samples in both RMF and AMF indicate an increase in heating rate with applied field intensity proportional to H2 up to 4.3 kA∙m−1. In this AMF value, an inflection point is observed when data are plotted as d2T/dt2. In the case of RMF such tendency is not observed which permits a better temperature evolution prediction even in higher applied fields.

Effect of rotating magnetic field (RMF) on the microstructure and mechanical properties of Al–2%Fe eutectic alloy

The effect of rotating magnetic field (RMF) on the solidification of microstructures and mechanical properties of Al–2%Fe (wt.%) eutectic alloy are studied in this manuscript. SEM, EDS and TEM techniques are adopted to analyze the evolution of Al3Fe intermetallics morphology in Al–2%Fe eutectic alloy. Without RMF, the length of Al3Fe intermetallics with large plate-like and long acicular shape are about 173 μm and 189 μm, respectively. With the RMF, the Fe-rich intermetallics is refined due to collision and shear action by the Lorentz force. When the intensity of the RMF reaches to 80 V, the average length of acicular Al3Fe intermetallics is 17 μm, and the average diameter of plate-like Al3Fe is 40 μm. However, when the intensity reaches to 100 V, the Al3Fe intermetallics coarsens. With the introduction of RMF, the tensile strength and elongation of the Al–2%Fe alloy increase accordingly. When the intensity of the RMF is 80V, the maximum tensile strength and elongation are 94.24 MPa and 13.37%, respectively. The influential mechanism of RMF is also discussed in this manuscript.