Transient Magnetic Field Research Articles

Magneto‐Structural analysis of rotating electrical machines

AbstractThe vibrations of the stator core of a rotating electrical machine induce acoustic noise. These oscillations of the stator yoke are excited of the force density due to the magnetic field in the air gap. This requires a transient magnetic field analysis coupled with a dynamic mechanical analysis. Coupling these two different physical fields results in a high numerical effort and usually one direction of the interaction is disregarded. This paper presents a method to calculate the vibrations of a stator core under design operating conditions. For this purpose, harmonic electromagnetic excitation forces have been calculated in a linear magnetic field analyses using the finite element method. The resulting forces have been applied to a linear structural dynamic FE model in the frequency domain. The results of the calculations are harmonic velocities specified by amplitude and phase from the structural surface of the stator core. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamic Analysis of Transformers With Second-Generation High-Temperature Superconductors

The high-temperature superconducting (HTS) transformer is one of the potential applications of high-temperature superconductors. When a HTS transformer was put into the power grid, the fault current at the secondary winding loads or the fault passing through current of the power grid may occur. During this transient process, the voltage, current, magnetic field, and temperature distribution would take great changes. Not only it affects the power grid but also the HTS transformer. This paper investigated the dynamic analysis of transformers with secondary-generation high-temperature superconductors based on COMSOL Multiphysics simulation software. Combining the electric circuit, magnetic field, and heat transfer, the factors that influenced the magnitude of fault current, temperature distribution were concluded.

Control of transmission of right circularly polarized laser light in overdense plasma by applied magnetic field pulses.

The effect of a transient magnetic field on right-hand circularly polarized (RHCP) laser light propagation in overcritical-density plasma is investigated. When the electron gyrofrequency is larger than the wave frequency, RHCP light can propagate along the external magnetic field in an overcritical density plasma without resonance or cutoff. However, when the magnetic field falls to below the cyclotron resonance point, the propagating laser pulse will be truncated and the local plasma electrons resonantly heated. Particle-in-cell simulation shows that when applied to a thin slab, the process can produce intense two-cycle light pulses as well as long-lasting self-magnetic fields.

Optical properties of a grating-nanorod assembly structure for solar cells

This paper proposes a grating-nanorod assembly structure that can be applied to silicon solar cells. The optical properties of the assembly structure are examined by applying the finite difference time domain method in the 300–1100nm wavelength region, where the average spectral absorptance of the structure can reach 0.955. This high absorptance is attributed to guided mode resonance and microcavity effect. The transient and steady-state magnetic field distribution of the structure reveals the underlying mechanisms of such extraordinary phenomena. Absorptance is further investigated at different diameters and lengths of the nanorod component. The effects of incident angle on absorptance are also discussed. The solar cells of the structure can yield an optimum conversion efficiency of 25.91%. Thus, the proposed structure can be applied to silicon solar cells.

Investigations of ultrafast charge dynamics in laser-irradiated targets by a self probing technique employing laser driven protons

The divergent and broadband proton beams produced by the target normal sheath acceleration mechanism provide the unique opportunity to probe, in a point-projection imaging scheme, the dynamics of the transient electric and magnetic fields produced during laser-plasma interactions. Commonly such experimental setup entails two intense laser beams, where the interaction produced by one beam is probed with the protons produced by the second. We present here experimental studies of the ultra-fast charge dynamics along a wire connected to laser irradiated target carried out by employing a ‘self’ proton probing arrangement – i.e. by connecting the wire to the target generating the probe protons. The experimental data shows that an electromagnetic pulse carrying a significant amount of charge is launched along the wire, which travels as a unified pulse of 10s of ps duration with a velocity close to speed of light. The experimental capabilities and the analysis procedure of this specific type of proton probing technique are discussed.

Reduction of Transient Magnetic Field Due to Lightning by a Shielded Room in a Building

The protection of a control center from transient currents and voltages due to lightning is an important problem. This paper investigates the effect of a shielded room on transient currents and induced magnetic fields in the room. The experimental and FDTD simulation results in a scaled-down shielded room show that steel poles around the room can reduce the transient magnetic fields noticeably, and the magnetic fields inside the shielded room are reduced to about one-third. An on-site test in a skyscraper under construction is carried out, and the transient magnetic field in a server room of a control/data center is measured to show that it is reduced to nearly two-thirds by shielded meshes when lightning strikes the building. The FDTD simulations show a reasonable accuracy in comparison with the experimental and on-site test results, and can be applied to design a shielded room.

A magnetoplasmonic electrical-to-optical clock multiplier

We propose and investigate an electrical-to-optical clock multiplier, based on a bismuth-substituted yttrium iron garnet (Bi:YIG) magnetoplasmonic Mach-Zehnder interferometer (MZI). Transient magnetic fields induce a precession of the magnetization vector of the Bi:YIG, which in turn modulates the nonreciprocal phase shift in the MZI arms, and hence the intensity at the output port. We show that the device is capable of modulation depth of 16.26 dB and has a tunable output frequency between 279.9 MHz and 5.6 GHz. Correspondingly, the input electrical modulation frequency can be multiplied by factors of up to 2.1×103 in the optical signal. Such a device is envisioned as a critical component in the development of hybrid electrical-optical circuitry.

Deep Transcranial Magnetic Stimulation Using Figure-of-Eight and Halo Coils

Transcranial magnetic stimulation (TMS) is a technique for noninvasive stimulation of the human brain. Stimulation is produced by generating a brief, transient high intensity magnetic field by passing a brief, transient electric current through a magnetic coil placed upon the scalp. The induced electrical currents in the underlying cortical tissue produce a localized axonal depolarization [1-3]. TMS has become a major tool in brain research and, potentially, a promising treatment for various neurobehavioral disorders. Recently, interests in stimulating deeper cortical, subcortical and limbic areas have arisen and have become an active research topic in transcranial magnetic stimulation, because several studies show that activation of deeper prefrontal and limbic regions may increase the antidepressant effect [4-5]. Standard TMS is applied with an electromagnetic coil called a round, or figure-of-eight (Fo8) coil which induces stimulation in cortical regions mainly just superficially under the windings of the coil. These coils are not suitable for stimulating deep brain regions directly. As the fields induced by these coils decrease rapidly as a function of depth, only very high intensities would allow functional stimulation of deep brain regions and such intensities would lead to painful scalp stimulation. Furthermore, the risk of seizure is increased. These limitations have led to the development of novel coil designs suitable for deep transcranial magnetic stimulation (dTMS). The Halo-coil, a large circular coil being placed around the head was developed for dTMS. It was shown that the Halo-coil working with a typical round coil at the top of the head can increase the fields at depth in the brain [6]. The present study was to study the field characteristics of Fo8 coil which was placed at the left dorsolateral prefrontal cortex (DLPFC) working with a Halo-coil as shown in Fig. 1. The purpose of this study is to show the field penetration depth by Fo8 coil can be further increased under the help of the Halo coil.

Evidence for torque caused by a magnetic impulse on a nonmagnetic torsion pendulum

I have performed an experiment which is a variant of the one suggested recently by F. O. Minotti and T. E. Raptis. The aim of this experiment is to check the generation of a pulsed gravitational potential by a transient magnetic field as predicted by a scalar tensor theory of gravity. Such a scalar tensor theory allows an enhanced coupling of its fundamental long-range real scalar field as well as its external scalar field to the electromagnetic field, according to the author's previous papers. Several possible sources that could mimic the predicted effect have been ruled out. Finally, it seems likely that the predicted effect is there with the right order of magnitude, as expected.

Modeling of electrodynamic‐mechanical coupling phenomena in smart magnetoelectroelastic materials

AbstractThe coupling of electric, magnetic and mechanical phenomena may have various reasons. The famous Maxwell equations of electrodynamics describe the interaction of transient magnetic and electric fields. On the constitutive level of dielectric materials, coupling mechanisms are manyfold comprising piezoelectric, magnetostrictive or magnetoelectric effects. Electromagnetically induced specific forces acting at the boundary and within the domain of a dielectric body are, within a continuum mechanics framework, commonly denoted as Maxwell stresses. In transient electromagnetic fields, the Poynting vector gives another contribution to mechanical stresses. First, a system of transient partial differential equations is presented. Introducing scalar and vector potentials for the electromagnetic fields and representing the mechanical strain by displacement fields, seven coupled differential equations govern the boundary value problem, accounting for linear constitutive equations of magnetoelectroelasticity. To reduce the effort of numerical solution, the system of equations is partly decoupled applying generalized forms of Coulomb and Lorenz gauge transformations [1,2]. A weak formulation is given to establish a basis for a finite element solution. The influence of constitutive magnetoelectric coupling on electromagnetic wave propagation is finally demonstrated with a simple one‐dimensional example. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental evidence of electrification processes during the 2009 L'Aquila earthquake main shock

AbstractTwo types of coseismic magnetic field events are simultaneously observed: transient offset events and magnetic field signal that occurred at the destructive, Mw6.1 L'Aquila earthquake main shock. The offset event, conventionally interpreted as a signature of piezomagnetic effects, however, could not be explained as such. In the second type of coseismic event, the transient magnetic signal starts simultaneously with the offset event and reaches amplitude of 0.8 nT in the total magnetic field. The signal is a local one; its amplitude shape resembles diffusion‐like form with time scale characteristics that are indicative for a source deep in the crust. The polarity of the transient signal is in the horizontal plane and nearly parallel to the L'Aquila fault strike.

OBSERVATIONS OF AN ENERGETICALLY ISOLATED QUIET SUN TRANSIENT: EVIDENCE OF QUASI-STEADY CORONAL HEATING

Increasing evidence for coronal heating contributions from cooler solar atmospheric layers, notably quiet Sun (QS) conditions, challenges standard solar atmospheric descriptions of bright transition region (TR) emission. As such, questions to the role of dynamic QS transients in contributing to the total coronal energy budget are elevated. Using observations from the {\it Atmospheric Imaging Assembly} and {\it Heliosemic Magnetic Imager} on board the {\it Solar Dynamics Observatory}, and numerical model extrapolations of coronal magnetic fields, we investigate a dynamic QS transient energetically isolated to the TR and extruding from a common footpoint shared with two heated loop arcades. A non-casual relationship is established between episodic heating of the QS transient and wide-spread magnetic field re-organization events, while evidence is found favoring a magnetic topology typical of eruptive processes. Quasi-steady interchange reconnection events are implicated as a source of the transient's visibly bright radiative signature. We consider the QS transient's temporally stable ($\approx$\,35\,min) radiative nature occurs as a result of the large-scale magnetic field geometries of the QS and/or relatively quiet nature of the magnetic photosphere, which possibly act to inhibit energetic buildup processes required to initiate a catastrophic eruption phase. This work provides insight to the QS's thermodynamic and magnetic relation to eruptive processes quasi-steadily heating a small-scale dynamic and TR transient. This work elevates arguments of non-negligible coronal heating contributions from cool atmospheric layers in QS conditions, and increases evidence for solar wind mass feeding of dynamic transients therein.

Superconductor Analog-to-Digital Converter for High-Resolution Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) requires a sensitive radio receiver to detect the weak resonant magnetic field from nuclear spins, usually hydrogen in biological tissues. In a typical static magnetic field of 1.5 T, a transient RF magnetic field at 63 MHz must be measured to fT amplitudes. This must be amplified and then digitized with an analog-to-digital converter (ADC) with a high dynamic range (typically 16 bits), in order to obtain high image resolution. As the magnetic field is increased in order to obtain even finer resolution, the required dynamic range is expected to increase further. A superconductor ADC (oversampling at 20 GHz) has previously been demonstrated to exhibit very high dynamic range (up to 24 bits) as part of a receiver for broadband digital-RF communication. Here, we present preliminary results on a similar ADC applied to a 4 T commercial small-animal MRI system, substituted for the built-in 16-bit ADC. The superconductor ADC was mounted on a 4 K cryocooler in a magnetically shielded container outside the MRI room and functioned well despite the presence nearby of a large static field and a high-power RF transmitter. Images were obtained either using direct digitization of the 170-MHz signals or, alternatively, using a low-noise analog mixer and digitization of a 1-MHz intermediate frequency signal. While the results demonstrated the superior dynamic range of the superconductor ADC, further improvement was limited by the thermal noise from the room-temperature pickup coil. Current work is investigating the use of a cryocooled coil (which may be superconducting) and a cold low-noise preamplifier in order to take full advantage of the large dynamic range of the superconductor ADC. This should permit improved spatial resolution, allowing one to “zoom in” to a region of interest.

Exploring human brain neuronal currents with phase MRI

ABSTRACTPurpose: Brain activity‐associated neuronal currents produce weak transient magnetic fields that would affect both magnitude and phase of the local MRI signal, but these very small signal changes are not reliably detectable with conventional fMRI methodologies. A recent simulation study, using a realistic model specifically for human cerebral cortex, indicates that the phase signal change induced by spontaneous activity may reach a detectable level (up to 0.2°) in favorable conditions. This study aimed to investigate neuronal current‐induced signal changes in human visual cortex with phase MRI. Materials and Methods: Six healthy subjects participated in a phase fMRI study using a temporally well‐controlled visual stimulation paradigm with a known neuronal firing pattern in visual cortex. The precise timing of the paradigm provides a means of detecting and testing the neuronal current‐induced phase signal changes, and placing a series of acquisition windows to fully cover the entire response duration enables a thorough detection of any detectable phase signal changes induced by the stimulus‐evoked neuronal currents. Results: The presented phase MRI method demonstrated to be reliable, and the improved phase measure has achieved a sensitivity level of 0.2° for detecting any significant phase signal changes under a practical length of fMRI session. The test found no sign of any significant neuronal current‐induced phase signal changes in any subject and study. Conclusions: Under the experimental condition, the upper limit of the neuronal current‐induced phase signal changes was found to be less than 0.2° in the human visual cortex, consistent with the model prediction.

Cometary impact effects at the Moon: Implications for lunar swirl formation

Relatively recent cometary impacts at the Moon could leave unique traces of their origins: high impact velocities and volatile abundances, combined with the presence of a dust- and ice-laden coma, may thermally and mechanically process the lunar surface in ways distinct from the impact of an asteroid. Here we analytically and numerically assess the consequences of a cometary impact at the Moon by considering the combined effects of a collision by the nucleus and inner coma. Our results show that cometary impacts entrain the finest fraction of lunar soil grains (<10μm) over regional scales (∼100–1000km), produce large masses of vaporized material, and likely generate transient magnetic fields that could exceed the Earth’s surface field strength by a factor of 104. This combination of processes is consistent with a mechanism to generate lunar swirls: the diffuse, meandering disturbances in brightness and regolith texture that curl across much of the lunar far-side and are also commonly (but not exclusively) associated with magnetic anomalies. Previous observations of swirl features indicate that bright regions also possess a peculiar, altered regolith structure, which can be produced by the removal of fine soil grains. Regional scouring by an impacting comet explains both the structure and albedo variations: large dynamic pressures entrain the smallest grains within a near-surface flow of dusty plasma, disrupting the backscattering, “fairy-castle” structure of lunar soils in equilibrium with the airless environment. The resulting surface is brightened by compaction of the previously open, porous macrostructure. Darker lanes observed within swirl regions are interpreted as possible melt and/or vapor deposits. Finally, the intense magnetic fields generated during high-speed cometary impacts provide an explanation for correlations between swirl locations and magnetic anomalies.

Electromagnetic Field Effect and Analysis of Composite Structure

The electromagnetic and thermal response of composites subjected to magnetic fields is simulated by solving Maxwell and heat transfer equations simultaneously. The developed analysis accounts for the anisotropic nature of the electrical and thermal properties in three dimensions. A finite-element code is developed to predict the response of composite structures subjected to transient magnetic fields. The analysis has been validated against a closed-form solution and applied to simulate the induction heating process of composite cylinders. The developed analysis can be applied to the design of modern electrical weapons and used to simulate composite manufacturing processes such as induction cure.

Key enabling design features of the ITER HNB Duct Liner

The Duct Liner (DL) for the ITER Heating Neutral Beam (HNB) is a key component in the beam transport system. Duct Liners installed into equatorial ports 4 and 5 of the Vacuum Vessel (VV) will protect the port extension from power deposition due to re-ionisation and direct interception of the HNB. Furthermore, the DL contributes towards the shielding of the VV and superconducting coils from plasma photons and neutrons.The DL incorporates a 316L(N)-IG, deep-drilled and water cooled Neutron Shield (NS) whose internal walls are lined with actively cooled CuCrZr Duct Liner Modules (DLMs). These Remote Handling Class 2 and 3 panels provide protection from neutral beam power. This paper provides an overview of the preliminary design for the ITER HNB DL and focusses on critical features that ensure compatibility with: high heat flux requirements, remote maintenance procedures, and transient magnetic fields arising from major plasma disruptions.The power deposited on a single DLM can reach 300kW with a peak power density of 2.4MW/m2. Feeding coolant to the DLMs is accomplished via welded connections to the internal coolant network of the NS. These are placed coaxially to allow for thermal expansion of the DLMs without the use of deformable connections. Critically, the remote maintenance of individual DLMs necessitates access to water connections and bolts from the beam facing surface, thus subjecting them to high heat flux loads. This design challenge will become more prevalent as fusion devices become more powerful and remote handling becomes a necessity. The novel solutions implemented to overcome this are detailed and their performance scrutinised.The designs presented include tungsten caps that protect bolted and remotely welded connections by radiating heat away, and explosion bonded CuCrZr/Stainless Steel panels designed to reduce by a factor of 10 the eddy current torques caused by plasma disruptions.

MEJORA DE LA EFICIENCIA DEL PROCESO DE CONFORMADO ELECTROMAGNÉTICO

The present study addresses a novel solution to the problems posed by low electrical conductivity metallic materials that want to be deformed by Electro Magnetic Forming (EMF). The EMF is a forming process that is part of the so-called High Speed Deformation. By electromagnetic forming surface transient high intensity magnetic fields are generated over the workpiece to deform that cause the appearance of Lorentz forces. These repulsive forces act instantaneously and for a short period of time between the workpiece and the object that is opposed to it and that generates it, the EMF coil. One of the drawbacks of the technology is in the fact that the efficiency of the induction phenomenon is proportional to the electrical conductivity of the object to deform. Steels, due to their low conductivity when compared against materials such as copper or aluminum, are little susceptible to be shaped with guarantees. In this study we have worked with H360LA microalloyed steel with low electrical conductivity. In order to improve the surface conductivity several tubes of the material were electrolytically coated with copper and expanded using the EMF. The tubes with copper coating had an increase of the circumferential deformation of 16% relative to that experienced by the uncoated tubes.

Transient Magnetic Field Analysis of PF Converter Unit

This paper presents the transient magnetic field analysis for PF converter unit. Different short circuit conditions and different magnetic field sources are considered in the analysis. An analysis model is built for each magnetic field source first. And then a magnetic field analysis is performed for the worst short circuit condition. The analysis results presented in this paper will be a very important reference for the confirmation of EMC test level and the EMC design of instrument and control systems.

Fast Algorithm to Obtain the Torque Characteristics With Respect to Load Angle of Synchronous Machines Using Finite Element Method

Synchronous machines (SMs) have wide applications in many areas including industrial, agricultural, and domestic. The torque-load angle relationship is an essential parameter governing the characteristics of SMs. Conventionally, for each load angle, a transient magnetic field model using the time-domain finite element method (FEM) needs to be solved, and hence it is very time-consuming to obtain the complete characteristics of SMs. In this paper, a fast variable phase method, which computes the transient process of the time-domain FEM once only and not repeatedly, is proposed to find the entire torque-load angle curve of SMs. By adjusting the time-step size and the nonlinear iteration error tolerances, the proposed method can significantly reduce the computing time when compared with those of conventional methods.