Magnetic Length Research Articles

Dimensioning Optimization of the Permanent Magnet Synchronous Generator for Direct Drive Wind Turbines

In the present work, a methodology that allows optimizing the permanent magnet synchronous generator (PMSG) design by establishing limit values of magnet radius and length that maximize efficiency for the nominal parameters of the wind turbine is developed. The methodology consists of two fundamental models. One model calculates the generator parameters from the radius of the magnet base, and the other optimization model determines two optimum generators according to the optimization criteria of maximum efficiency and maximum efficiency with minimum weight starting from the axial length and the radius of the magnet base. For the optimization, the numerical method of the golden section was used. The model was validated from a 10 kW PMSG and the results of two optimum generators are presented according to the optimization criteria. In addition, when the obtained results are compared with the reference electric generator, an increase in efficiency of 1.15% and 0.81% and a reduction in weight of 30.79% and 39.15% of the optimized generators are obtained for maximum efficiency and minimum weight, respectively. Intermediate options between the maximum efficiency generator and the minimum weight generator allows for the selection of the optimum dimensioning for the electric generator as a function of the parameters from the wind turbine design.

Spatial and magnetic confinement of massless Dirac fermions

The massless Dirac fermions and the ease to introduce spatial and magnetic confinement in graphene provide us unprecedented opportunity to explore confined relativistic matter in this condensed-matter system. Here we report the interplay between the confinement induced by external electric fields and magnetic fields of the massless Dirac fermions in graphene. When the magnetic length lB is larger than the characteristic length of the confined electric potential lV, the spatial confinement dominates and a relatively small critical magnetic field splits the spatial-confinement-induced atomic-like shell states by switching on a pi Berry phase of the quasiparticles. When the lB becomes smaller than the lV, the transition from spatial confinement to magnetic confinement occurs and the atomic-like shell states condense into Landau levels (LLs) of the Fock-Darwin states in graphene. Our experiment demonstrates that the spatial confinement dramatically changes the energy spacing between the LLs and generates large electron-hole asymmetry of the energy spacing between the LLs. These results shed light on puzzling observations in previous experiments, which hitherto remained unaddressed.

One dimensional analytical model considering end effects for analysis of electromagnetic pressure characteristics of annular linear induction electromagnetic pump

A one-dimensional analytical model with the finite magnetic core length and conductive duct wall thickness of annular linear induction electromagnetic pump (ALIP) is established in this paper. Based on this model considering both the end effects and the conductive duct wall effect, the explicit analytical expressions of the electromagnetic and force fields are deduced. At the same time, the influence of end effects and the conductive duct wall effect on electromagnetic field and force field is analyzed and the components of magnetic field, induced eddy current and force field which are produced because of the existence of end effects are separated. Then, the explicit analytical expression of the developed pressure is obtained with the introduced fluid loss. Finally, to verify the feasibility and accuracy of the analytical model, the performance of a small three-phase ALIP under different frequency and current conditions are calculated analytically and the calculated results are verified by experiments data from the experimental test loop.

Micromagnetic Monte Carlo method with variable magnetization length based on the Landau–Lifshitz–Bloch equation for computation of large-scale thermodynamic equilibrium states

An efficient method for computing thermodynamic equilibrium states at the micromagnetic length scale is introduced using the Markov chain Monte Carlo method. Trial moves include not only rotations of vectors, but also a change in their magnetization length. The method is parameterized using the longitudinal susceptibility, reproduces the same Maxwell–Boltzmann distribution as the stochastic Landau–Lifshitz–Bloch equation, and is applicable both below and above the Curie temperature. The algorithm is fully parallel, can be executed on graphical processing units, and efficiently includes the long-range dipolar interaction. This method is generally useful for computing finite-temperature relaxation states for both uniform and non-uniform temperature profiles and can be considered as complementary to zero-temperature micromagnetic energy minimization solvers, with comparable computation time. Compared to the dynamic approach, it is shown that the micromagnetic Monte Carlo method is up to almost 20 times faster. Moreover, unlike quasizero temperature approaches that do not take into account the magnetization length distribution and stochasticity, the method is better suited for structures with unbroken symmetry around the applied field axis, granular films, and at higher temperatures and fields. In particular, applications to finite-temperature hysteresis loop modelling, chiral magnetic thin films, granular magnetic media, and artificial spin ices are discussed.

Modeling fluid polarization during flow in a non-uniform polarization field

A sample will undergo T1 relaxation at continuously changing applied magnetic field strengths when flowing from the Earth magnetic field outside an NMR instrument to the much stronger applied magnetic field inside an NMR instrument. The interaction between fluid flow, T1 relaxation and magnetic field distributions in the case of negligible T1 dispersion can be described by considering two length scales involved: the characteristic polarization length vT1, and the effective polarization magnet length Lm⁎. By comparing water flow experiments on a 0.9 m long Halbach magnet array with simulations using the Bloch-Torrey equation, we determined that Lm⁎ is given by the integral along flow direction x of the normalized axial polarization field distribution, p(x) = B0(x)/B0(xROI), in which xROI is the center of the polarization magnet. The fluid magnetization level Mz in a region-of-interest located at xROI is a function of the ratio s = vT1/Lm⁎ and can be generally expressed as Mz,ROI = M0⁎(s)[1–exp(-1/2 s)]. The effective equilibrium magnetization function M0⁎(s) was found to have both a sample independent contribution from the axial polarization field p(x) and a sample dependent contribution from the flow velocity profile at a given flow rate. In our experiments, the axial Halbach field was found to contribute with a weight of approximately 1/3 to M0⁎(s) and the remaining 2/3 wt is contributed by the flow velocity distribution. Based on this analysis, a set of general principles for magnetization build up modeling and online NMR instrument design has been derived.

Magnetic effect on dynamic and decoherence of exciton polaron in transition metal dichalcogenides

We investigate the dynamics and decoherence of the exciton polaron in a 2D transition metal dichalcogenides modulated by a magnetic field barrier. Using the Huybrechts method and an approximate diagonalization of exciton-phonon operators is performed to derive the fundamental energy, the first excited state energy, the effective mass and the mobility of the exciton polaron. It is found that the system presents robust state energies and the dynamic properties like effective mass is reduced. We show that the motion of exciton-polaron is accelerated by increasing the magnetic barrier length. The transition of the exciton polaron from the valence band to the conduction band is also enhanced by the external field. It is found that the decoherence of exciton polaron can be adjusted by the magnetic field barrier. Our results suggest that the magnetic field barrier (i) enhances the confinement of the electron in the system, (ii) increases mobility and the rate of the decoherence of exciton polaron. The results are in agreement with experimental works.

Interference oscillations in the optical response of a hybrid-aligned nematic with a surface disclination line to a pulsed magnetic field

The propagation of light through a hybrid-aligned nematic layer with a surface disclination line is investigated in a pulsed magnetic field. The experimental dependences of light intensity I on time t accompanied by interference oscillations are presented. The shift and expansion of the interference extrema as functions of the magnetic field pulse length during the reaction are shown. The I ( t ) dependence for the relaxation process is calculated with allowance for scattering. The calculated I ( t ) dependence agrees well with the experimental dependence over the entire hybrid-aligned nematic layer, except for the surface layer.

Terahertz magneto-optical properties of graphene hydrodynamic electron liquid

The discovery of the hydrodynamic electron liquid (HEL) in graphene [D. Bandurin \emph{et al.}, Science {\bf 351}, 1055 (2016) and J. Crossno \emph{et al.}, Science {\bf 351}, 1058 (2016)] has marked the birth of the solid-state HEL which can be probed near room temperature in a table-top setup. Here we examine the terahertz (THz) magneto-optical (MO) properties of a graphene HEL. Considering the case where the magnetic length $l_B=\sqrt{\hbar/eB}$ is comparable to the mean-free path $l_{ee}$ for electron-electron interaction in graphene, the MO conductivities are obtained by taking a momentum balance equation approach on the basis of the Boltzmann equation. We find that when $l_B\sim l_{ee}$, the viscous effect in a HEL can weaken significantly the THz MO effects such as cyclotron resonance and Faraday rotation. The upper hybrid and cyclotron resonance magnetoplasmon modes $\omega_\pm$ are also obtained through the RPA dielectric function. The magnetoplasmons of graphene HEL at large wave-vector regime are affected by the viscous effect, and results in red-shifts of the magnetoplasmon frequencies. We predict that the viscosity in graphene HEL can affect strongly the magneto-optical and magnetoplasmonic properties, which can be verified experimentally.

Magnetic Penetration Depth and Coherence Length in a Single‐Crystal YBa2Cu3O7−δ

Herein, DC magnetization measurements of high‐quality single crystals of optimally doped YBa2Cu3O7−δ(YBCO) superconductor are reported on. The temperature and orientation dependence of the magnetic penetration depth for single‐crystal YBCO is estimated from the slope of the linear M(lnH) dependence in the London regime. The penetration depth is strongly anisotropic. Also, the electrical resistivity as a function of temperature in YBCO thin film deposited by laser ablation on the SrTiO3 substrate face is investigated around the superconducting transition region under various values of the magnetic field up to 14 T. In this work, based on the resistivity measurements, the upper critical magnetic field for both directions B∥c‐axis and B⊥c‐axis is determined with a DC weak transport current density by using 90% criteria of the normal‐state resistivity value. According to the relations and , the coherence length (0) and (0) is determined, where is the magnetic flux quantum. The anisotropy parameter is calculated and analyzed within the frame of Ginzburg–Landau theory. This result indicates that anisotropy is more evident in YBCO.

Design optimization of coreless stator axial flux-switching motor

This study aimed to provide the analytical design, optimization, and three-dimensional (3D) simulation through finite element method of a coreless stator axial field flux-switching motor (AFFSM). The motor consists of two indented rotors with a coreless stator between them consisting of a magnet and winding. First of the motor electrical and magnetic design was performed and its basic parameters were calculated. Then, the optimization of the machine was evaluated implementing the Taguchi algorithm in order to minimize the motor cogging torque. Some of the basic motor dimensions, such as the magnet length and width, the rotor tooth width and height, and the back iron thickness, were selected as optimization variables, and the best combination of these variables was obtained by changing them in a certain range to achieve the desired objective. Then, the accuracy of analytical design and optimization was evaluated through forming a 3D finite element method (FEM) of the motor and investigating its performance. Comparison of the optimized and primary motor revealed that the optimal design had a better performance than the initial. Finally, a prototype of the proposed motor was fabricated and tested, which indicated that the experimental results were largely similar to the analytical results.

Buckling and Free Vibrations of a Magneto-Electro-Elastic Sandwich Panel with Flexible Core

This paper presents the buckling and out-of-plane free vibration response of a sandwich panel with flexible core for the different boundary condition. In the desired configuration of the sandwich panel, the top and bottom plates are made of magneto-electro-elastic (MEE) plates. Moreover, the in-plane electric and magnetic potential fields are neglected for the derivation of the required relations. The sandwich structure is subjected to axial force in both longitudinal and transverse directions; in addition, and the top and bottom plates are exposed to electric and magnetic fields. The governing equations of motion for MEE sandwich panel with a flexible core are derived based on the first-order shear deformation theory by neglecting the displacement of the mid-plate and using the Hamilton’s principle. Furthermore, the derived partial differential equations (PDEs) are solved. According to the obtained numerical results, the core thickness, variation of electric field, variation of magnetic field and plate length are introduced as the most influential parameters on the free vibration response of the panel as well as the critical force of buckling. As one of the results, the electric potential is inversely related to the natural frequency and buckling load, so that with increasing the electric potential, the natural frequency and critical load of the structure is also increased.Moreover, the magnetic potential is directly related to the natural frequency and buckling load of the system, and increasing trends of natural frequency and critical load are observed by increasing the magnetic potential.

Modelling a multi-spacecraft coronal mass ejection encounter with EUHFORIA

Context. Coronal mass ejections (CMEs) are a manifestation of the Sun’s eruptive nature. They can have a great impact on Earth, but also on human activity in space and on the ground. Therefore, modelling their evolution as they propagate through interplanetary space is essential. Aims. EUropean Heliospheric FORecasting Information Asset (EUHFORIA) is a data-driven, physics-based model, tracing the evolution of CMEs through background solar wind conditions. It employs a spheromak flux rope, which provides it with the advantage of reconstructing the internal magnetic field configuration of CMEs. This is something that is not included in the simpler cone CME model used so far for space weather forecasting. This work aims at assessing the spheromak CME model included in EUHFORIA. Methods. We employed the spheromak CME model to reconstruct a well observed CME and compare model output to in situ observations. We focus on an eruption from 6 January 2013 that was encountered by two radially aligned spacecraft, Venus Express and STEREO-A. We first analysed the observed properties of the source of this CME eruption and we extracted the CME properties as it lifted off from the Sun. Using this information, we set up EUHFORIA runs to model the event. Results. The model predicts arrival times from half to a full day ahead of the in situ observed ones, but within errors established from similar studies. In the modelling domain, the CME appears to be propagating primarily southward, which is in accordance with white-light images of the CME eruption close to the Sun. Conclusions. In order to get the observed magnetic field topology, we aimed at selecting a spheromak rotation angle for which the axis of symmetry of the spheromak is perpendicular to the direction of the polarity inversion line (PIL). The modelled magnetic field profiles, their amplitude, arrival times, and sheath region length are all affected by the choice of radius of the modelled spheromak.

Superconducting quadrupole magnet R&D in the interaction region of CEPC

To obtain high luminosity, compact high gradient quadrupole magnets QD0 and QF1 are required on both sides of the interaction points of the proposed Circular Electron Positron Collider (CEPC). QD0 is a double aperture superconducting quadrupole closest to the interaction point with a crossing angle between two aperture centerlines of 33 mrad. Magnetic field crosstalk between two apertures of QD0 is negligible using iron yoke, and the 3D coil end is optimized by ROXIE. In the design study, both NbTi conductor and HTS conductor are taken into account. The first step of the R&D is to design and manufacture a QD0 short model magnet with a magnetic length of 0.5 m. In this paper, the R&D status of QD0 short model magnet is described, and the design study of quadrupole magnet including NbTi technology and HTS Bi-2212 technology is presented.

Periodic magnetic geodesics on Heisenberg manifolds

We study the dynamics of magnetic flows on Heisenberg groups, investigating the extent to which properties of the underlying Riemannian geometry are reflected in the magnetic flow. Much of the analysis, including a calculation of the Mañé critical value, is carried out for $$(2n+1)$$ -dimensional Heisenberg groups endowed with any left invariant metric and any exact, left-invariant magnetic field. In the three-dimensional Heisenberg case, we obtain a complete analysis of left-invariant, exact magnetic flows. This is interesting in and of itself, because of the difficulty of determining geodesic information on manifolds in general. We use this analysis to establish two primary results. We first show that the vectors tangent to periodic magnetic geodesics are dense for sufficiently large energy levels and that the lower bound for these energy levels coincides with the Mañé critical value. We then show that the marked magnetic length spectrum of left-invariant magnetic systems on compact quotients of the Heisenberg group determines the Riemannian metric. Both results confirm that this class of magnetic flows carries significant information about the underlying geometry. Finally, we provide an example to show that extending this analysis of magnetic flows to the Heisenberg-type setting is considerably more difficult.

Large-scale balances and asymptotic scaling behaviour in spherical dynamos

SUMMARY The large-scale dynamics of convection-driven dynamos in a spherical shell, as relevant to the geodynamo, is analysed with numerical simulation data and asymptotic theory. An attempt is made to determine the asymptotic size (with the small parameter being the Ekman number, Ek) of the forces, and the associated velocity and magnetic fields. In agreement with previous work, the leading order mean force balance is shown to be thermal wind (Coriolis, pressure gradient and buoyancy) in the meridional plane and Coriolis–Lorentz in the zonal direction. The Lorentz force is observed to be weaker than the mean buoyancy force across a range of Ek and thermal forcing; the relative difference in these forces appears to be O(Ek1/6) within the parameter space investigated. We find that the thermal wind balance requires that the mean zonal velocity scales as O(Ek−1/3), whereas the meridional circulation is asymptotically smaller by a factor of O(Ek1/6). The mean temperature equation shows a balance between thermal diffusion and the divergence of the convective heat flux, indicating the presence of a mean temperature length scale of size O(Ek1/6). Neither the mean nor the fluctuating magnetic field show a strong dependence on the Ekman number, though the simulation data shows evidence of a mean magnetic field length scale of size O(Ek1/6). A consequence of the asymptotic ordering of the forces is that Taylor’s constraint is satisfied to accuracy O(Ek1/6), despite the absence of a leading-order magnetostrophic balance. Further consequences of the force balance are discussed with respect to the large-scale flows thought to be important for the geodynamo.

Coronal properties of low-mass Population III stars and the radiative feedback in the early universe

ABSTRACT We systematically investigated the heating of coronal loops on metal-free stars with various stellar masses and magnetic fields by magnetohydrodynamic simulations. It is found that the coronal property is dependent on the coronal magnetic field strength Bc because it affects the difference of the non-linearity of the Alfvénic waves. Weaker Bc leads to cooler and less dense coronae because most of the input waves dissipate in the lower atmosphere on account of the larger non-linearity. Accordingly EUV and X-ray luminosities also correlate with Bc, while they are emitted in a wide range of the field strength. Finally, we extend our results to evaluating the contribution from low-mass Population III coronae to the cosmic reionization. Within the limited range of our parameters on magnetic fields and loop lengths, the EUV and X-ray radiations give a weak impact on the ionization and heating of the gas at high redshifts. However, there still remains a possibility of the contribution to the reionization from energetic flares involving long magnetic loops.

Design Method of Bearingless Permanent Magnet Slice Motor for Maglev Centrifugal Pump Based on Performance Metric Cluster

Different from ordinary AC machines, the design of a bearingless permanent magnet slice motor (BPMSM) considers not only the torque performance, but also the passive and active suspension properties. In addition, BPMSM for a maglev centrifugal pump has unique design characteristics due to the integration of the pump head and sensors. This paper investigates evaluation and design techniques based on a cluster of performance metrics targeting on developing BPMSM for a maglev centrifugal pump. The cluster of performance metrics for BPMSM, including passive stiffness (kz, kz/kx, kz/ky, kα, and kβ) and active factors (ki and cm), is first proposed and an evaluation function fiSi,Li is constructed. Then, practical configurations of BPMSM for a maglev centrifugal pump are summarized. Based on the cluster of performance metrics, the finite-element method (FEM) is used to explore the impact of the rotor magnetization (sinusoidal, diametric, and radial method) on motor properties. Subsequently, the complete design process of BPMSM for a maglev centrifugal pump is introduced and key differences (including three crucial geometric parameters: ratio of rotor height to diameter λ, magnetic gap length δ, and stator tooth width αst) in the design considerations between BPMSM and general bearingless motors are analyzed. Finally, the upgraded performance (kz, kα, kβ, ki, cm, and fiSi,Li increased by about 29%, 38%, 33%, 31%, 21%, and 15%, respectively) of the designed candidate is obtained, which verifies the effectiveness of the proposed design methods.

Improving the performance of a water ion thruster using biased electrodes

This paper investigates whether biased electrodes can improve the performance of a microwave-discharge water ion thruster, which has been proposed as a propulsion system for CubeSats. Previously, for several microwave ion sources, the beam currents have been increased by mounting a biased electrode in the discharge chamber. The effect of a biased electrode on a water ion thruster was unclear because its magnetic field profile and characteristic length compared with the microwave wavelength are different from those used in previous studies. To investigate the effect, a thruster was divided into three parts: magnets, a microwave antenna, and a discharge chamber, which is comprised of an electrically conductive sidewall and a screen grid. The parts were isolated electrically. Bias voltages from −44 to ＋ 44V were applied to the microwave antenna or magnets by an additional power supply. These bias voltages were applied with respect to the screen grid and sidewall voltage. A maximum screen current of 12.7 mA was obtained for an antenna bias voltage of 38.2V. This is 2.3 times larger than the current without a bias voltage. This increase could be caused by a combination of an increase of plasma density and ion extraction efficiency. The plasma density could increase with the gain of the total absorbed power. The ion extraction efficiency could increase due to the change in the plasma potential distribution. Overall, the mass utilization efficiency of the water ion thruster increased from 3.3% to 7.9% and the discharge loss decreased from 280 to 150W/A at the antenna bias of 38V. The performance with water electron source was evaluated. The thrust force, specific impulse, thrust-to-power ratio, thrust efficiency were 190μN, 350s, 5.9μN/W, and 1.0%, respectively at the antenna bias of 25V.

SYNTHESIS OF THE LOW-FREQUENCY VIBRATION EXCITER

The article is devoted to synthesis of the low-frequency vibration exciter for checking of sensors of acceleration. In many areas of the modern equipment sensors of acceleration working in very low range of frequencies are widely used. For checking and graduation of such sensors the vibration exciter capable to provide rectilinear horizontal harmonic oscillations of the calibrated accelerometer in so low range of frequencies are required. Low frequencies of fluctuations cause the necessity of creation of big amplitudes of movements for ensuring the acceptable values of amplitudes of accelerations. The low-frequency electrodynamic vibration exciter with a magnetic suspension of mobile part which is a component of the National Standard of the vibration movement of the Russian Federation is so far created. However, development of the modern equipment demands expansion of frequency ranges to the area of ultralow frequencies. One of requirements shown to the vibration exciter working in the ultralow range of frequencies is increase in amplitude of horizontal movements of a mobile part as with small amplitudes the speed and acceleration of the harmonious law of the movements proportional according to the frequency of fluctuations and a square of this frequency, will have small amplitude values. One of problems of realization of a control system of the electric drive of the vibration exciter consists in that a mobile part possesses indifferent position of balance. The centre of fluctuations of a mobile part is not defined and can be in any point on magnetic conductor length. That fluctuations had the steady centre in an average point of a magnetic conductor without use of a mechanical spring, the drive is supplied with an additional control system of fluctuations, or a so-called electromagnetic spring.

Magnetic field line random walk and solar energetic particle path lengths

Context. In 2020 May-June, six solar energetic ion events were observed by the Parker Solar Probe/IS⊙IS instrument suite at ≈0.35 AU from the Sun. From standard velocity-dispersion analysis, the apparent ion path length is ≈0.625 AU at the onset of each event. Aims. We develop a formalism for estimating the path length of random-walking magnetic field lines to explain why the apparent ion path length at an event onset greatly exceeds the radial distance from the Sun for these events. Methods. We developed analytical estimates of the average increase in path length of random-walking magnetic field lines, relative to the unperturbed mean field. Monte Carlo simulations of field line and particle trajectories in a model of solar wind turbulence were used to validate the formalism and study the path lengths of particle guiding-center and full-orbital trajectories. The formalism was implemented in a global solar wind model, and the results are compared with ion path lengths inferred from IS⊙IS observations. Results. Both a simple estimate and a rigorous theoretical formulation are obtained for field-lines’ path length increase as a function of path length along the large-scale field. From simulated field line and particle trajectories, we find that particle guiding centers can have path lengths somewhat shorter than the average field line path length, while particle orbits can have substantially longer path lengths due to their gyromotion with a nonzero effective pitch angle. Conclusions. The long apparent path length during these solar energetic ion events can be explained by (1) a magnetic field line path length increase due to the field line random walk and (2) particle transport about the guiding center with a nonzero effective pitch angle due to pitch angle scattering. Our formalism for computing the magnetic field line path length, accounting for turbulent fluctuations, may be useful for application to solar particle transport in general.