Homogeneous Magnetic Field Research Articles

Hall current impacts on unsteady MHD free convective flow past an infinite vertical porous surface

AbstractIn this paper, the magnetohydrodynamic free convective flow of an incompressible electrically conducting fluid over a vertical plate embedded in a porous medium is considered. A homogeneous transverse magnetic field is applied in the presence of a heat source and chemical reaction in a rotating frame, taking Hall current effects into account. The momentum equations for the fluid flow in a porous medium were determined by Brinkman modeling. At the undisturbed state, both the plate and fluid have an rigid body rotation due to the constant angular velocity, perpendicular to the infinite vertical plane surface. The vertical surface is subject to the homogeneous constant suction and the heat on the surface vary by time about a nonzero constant rate whereas the temperature of free stream is engaged to be constant. The accurate solutions for the velocity, temperature, and concentration distributions were acquired systematically using the perturbation method. The consequences of an assortment of governing flow parameters on the velocity, temperature, and concentration were analyzed through graphical profiles. The computational results for the skin friction, Nusselt number, and Sherwood number in a tabular format were also examined.

On the ridge of instability in ferrofluidic Couette flow via alternating magnetic field

There is a huge number of natural and industrial flows, which are subjected to time-dependent boundary conditions. The flow of a magnetic fluid under the influence of temporal modulations is such an example. Here, we perform numerical simulations of ferrofluidic Couette flow subject to time-periodic modulation (with frequency Omega _H) in a spatially homogeneous magnetic field and report how such a modulation can lead to a significant Reynolds number Re enhancement. Consider a modified Niklas approximation we explain the relation between modulation amplitude, driving frequency and stabilization effect. From this, we describe the system response around the primary instability to be sensitive/critical by an alternating field. We detected that such an alternating field provides an easy and in particular accurate controllable key parameter to trigger the system to change from subcritical to supercritical and vice versa. Our findings provide a framework to study other types of magnetic flows driven by time-dependent forcing.

ION-CYCLOTRON ABSORPTION OF FAST WAVES IN A CYLINDRICAL CURRENT-CARRYING PLASMA

Influence of a longitudinal stationary current on the absorption and the radial structure of fast waves in a cylindrical current-carrying plasma is discussed. To evaluate the dispersion equation for fast waves, there was used the dielectric tensor taking into account the radial current structure and geometry of the confining helical magnetic field by the plasma safety factor. It is shown that the damping rate of fast waves in a non-equilibrium current-carrying plasma differ from those for an equilibrium plasma column in a homogeneous magnetic field nearby the cutoffs and resonances due to the rotational transformation (including shear-effects) of the helical magnetic field lines.

A High-Order Integrator for the Schrödinger Equation with Time-Dependent, Homogeneous Magnetic Field

A High-Order Integrator for the Schrödinger Equation with Time-Dependent, Homogeneous Magnetic Field

Improved 7\xa0Tesla transmit field homogeneity with reduced electromagnetic power deposition using coupled Tic Tac Toe antennas

Recently cleared by the FDA, 7 Tesla (7 T) MRI is a rapidly growing technology that can provide higher resolution and enhanced contrast in human MRI images. However, the increased operational frequency (~ 297 MHz) hinders its full potential since it causes inhomogeneities in the images and increases the power deposition in the tissues. This work describes the optimization of an innovative radiofrequency (RF) head coil coupled design, named Tic Tac Toe, currently used in large scale human MRI scanning at 7 T; to date, this device was used in more than 1,300 neuro 7 T MRI scans. Electromagnetic simulations of the coil were performed using the finite-difference time-domain method. Numerical optimizations were used to combine the calculated electromagnetic fields produced by these antennas, based on the superposition principle, resulting in homogeneous magnetic field distributions at low levels of power deposition in the tissues. The simulations were validated in-vivo using the Tic Tac Toe RF head coil system on a 7 T MRI scanner.

Imaging Damage in Steel Using a Diamond Magnetometer

We demonstrate a simple, robust, and contactless method for nondestructive testing of magnetic materials such as steel. This uses a fiber-coupled magnetic sensor based on nitrogen-vacancy centers (NVCs) in diamond without magnetic shielding. Previous NVC magnetometry has sought a homogeneous bias magnetic field on the diamond to improve the sensitivity. In contrast, here we show that the spatial resolution for imaging is improved by applying an inhomogeneous magnetic field to the steel even though this leads to an inhomogeneous magnetic field on the diamond. Structural damage in the steel distorts the inhomogeneous magnetic field and by detecting this distortion we reconstruct the damage profile through quantifying the shifts in the NVC Zeeman splitting. With a 1-mm magnet as the source of our inhomogeneous magnetic field, we achieve a high spatial resolution of 1 mm in the plane parallel and 0.1 mm in the direction perpendicular to the surface of the steel. This works even when the steel is covered by a nonmagnetic material. The lift-off distance of our sensor head from the surface of 316 stainless steel is up to 3 mm.

Near-Field Fractal RFID Reader Antenna for Item Level Tagging

This paper proposes a novel methodology for designing near-field fractal antenna operating at European Ultra-High Frequency (UHF) for Radio Frequency Identification (RFID) systems. The principle of Oppositely Directed Current (ODC) technique is implemented to generate homogeneous magnetic field above the surface area meant for near-field RFID application. The conceptual theoretical framework of the design based on the fractal structure can be applied. The fabricated RFID reader antenna operates at 867 MHz with reflection coefficient S11 -30 dB, thus the reader antenna covering UHF RFID band range. The reading capabilities of the antenna are determined by the interrogation area of, 190mm x 243mm. It is adequate for object tracking and detection systems in pharmaceutical management and chemical industries.

Entangled quantum refrigerator based on two anisotropic spin-1/2 Heisenberg XYZ chain with Dzyaloshinskii–Moriya interaction

A quantum refrigerator based on anisotropic two spin-1/2 Heisenberg XYZ model in external magnetic fields with Dzyaloshinski–Moriya (DM) interaction is suggested. It consists of establishing different expressions of efficiency and thermodynamic quantities as a function of the homogeneous magnetic fields $$B_z$$ and the DM interaction ( $$D_z$$ ). A multipartite concurrence is employed as a measure of thermal entanglement in order to investigate the relationship between the entanglement and thermodynamic quantities. Many particular results related to variations in the coupling constants of the system are shown. However, the results ensure that a variation of the coupling constants has a significant impact on the results of the suggested system. All these investigations are established with respect to the conservation of the second law of thermodynamics.

In situ electromagnet with active cooling for real-time magneto-optic Kerr effect spectroscopy.

We present a compact in situ electromagnet with an active cooling system for use in ultrahigh vacuum environments. The active cooling enhances the thermal stability and increases the electric current that can be applied through the coil, promoting the generation of homogeneous magnetic fields, required for applications in real-time deposition experiments. The electromagnet has been integrated into a reflectance difference magneto-optic Kerr effect (RD-MOKE) spectroscopy system that allows the synchronous measurement of the optical anisotropy and the magneto-optic response in polar MOKE geometry. Proof of principle studies have been performed in real time during the deposition of ultra-thin Ni films on Cu(110)-(2 × 1)O surfaces, corroborating the extremely sharp spin reorientation transition above a critical coverage of 9 monolayers and demonstrating the potential of the applied setup for real-time and in situ investigations of magnetic thin films and interfaces.

FT-ICR mass spectrometry: Superconducting magnet, external ion source, ion-molecule reactions, and ion-ion traps.

The world of Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry has witnessed, especially in the last 30 years significant advances in many fields of science, such as electronics, magnets, new ICR cell designs, developed ICR event sequences, modern external ionization sources, and linear ion beam guides, as well as modern vacuum technology. In this review, a brief account is given focusing especially on the studies performed in Wanczek's group and ICR research laboratory at the University of Bremen. An FT-ICR mass spectrometer has been developed with a high magnetic field superconducting magnet, operating at 4.7 T. At this magnetic field, a trapping time of 13.5 h was obtained with 30% efficiency. For the tetrachloromethane molecular ion, m/z 166, a mass-resolving power m/Δm = 1.5 × 106 was measured at a pressure of 2 × 10-8 Torr. The transition from magnet sweep to frequency sweep and the application of Fourier-transform has greatly enhanced the ICR technology. External ion sources were invented and differential pumping schemes were developed for enabling ultrahigh vacuum condition for ICR detection, while guiding ions at relatively higher pressures, during their flight to the ICR cell. With the external ion source, a time-of-flight ICR tandem instrument is built. A method to measure the ion flight time and to trap the ions in the ICR cell is described. Many ICR cell characteristics such as z-axis ion ejection and coupling of radial and axial ion motions in a superposed homogeneous magnetic and inhomogeneous trapping electric field were extensively studied. Gas-phase ion-molecule reactions of several reactive inorganic compounds with a focus on phosphorous and sulfur as well as silicon chemistry were also studied in great detail. The gas-phase ion chemistry of several trifluoromethyl-reagents such as trifluoromethyltrimethylsilane and tris(trifluoromethyl)phosphine were also investigated in ICR. Dual polarities multisegmented ICR cells were invented and deeply characterized. Sophisticated ICR pulse event programs were developed to enable long-range ion-ion interactions between simultaneously trapped positive and negative ions.

Dynamics of self-accelerating electron beams in a homogeneous magnetic field

We examine the dynamics of electron beams that, in free space, are self-accelerating, in the presence of an additional magnetic field. We focus our attention in the case of Airy beams that follow parabolic trajectories and in generalized classes of beams associated with power-law trajectories. We study the interplay between beam self-acceleration and the circular motion caused by the magnetic field. In the case of Airy beams, using an integral representation, we find closed-form solutions for the electron wavefunction. We also derive asymptotic formulas for the beam trajectories both for Airy beams and for self-accelerating power-law beams. A ray optics description is rather useful for the interpretation of the beam dynamics. Our results are in excellent comparison with direct numerical simulations.

Weak Magnetic Fields Enhance the Efficacy of Radiation Therapy

PurposeThe clinical efficacy of radiation therapy is mechanistically linked to ionization-induced free radicals that cause cell and tissue injury through direct and indirect mechanisms. Free radical reaction dynamics are influenced by many factors and can be manipulated by static weak magnetic fields (WMF) that perturb singlet-triplet state interconversion. Our study exploits this phenomenon to directly increase ionizing radiation (IR) dose absorption in tumors by combining WMF with radiation therapy as a new and effective method to improve treatment. Methods and MaterialsCoils were custom made to produce both homogeneous and gradient magnetic fields. The gradient coil enabled simultaneous in vitro assessment of free radical/reactive oxygen species reactivity across multiple field strengths from 6 to 66 G. First, increases in IR-induced free radical concentrations using oxidant-sensitive fluorescent dyes in a cell-free system were measured and verified. Next, human and murine cancer cell lines were evaluated in in vitro and in vivo models after exposure to clinically relevant doses of IR in combination with WMF. ResultsCellular responses to IR and WMF were field strength and cell line dependent. WMF was able to enhance IR effects on reactive oxygen species formation, DNA double-strand break formation, cell death, and tumor growth. ConclusionsWe demonstrate that the external presence of a magnetic field enhances radiation-induced cancer cell injury and death in vitro and in vivo. The effect extends beyond the timeframe when free radicals are induced in the presence of radiation into the window when endogenous free radicals are produced and therefore extends the applicability of this novel adjunct to cancer therapy in the context of radiation treatment.

Modulation instability in the nonlinear Schrödinger equation with a synthetic magnetic field: Gauge matters

We theoretically investigate the phenomenon of modulation instability for systems obeying nonlinear Schr\"odinger equation, which are under the influence of an external homogeneous synthetic magnetic field. For an initial condition, the instability is detected numerically by comparing dynamics with and without a small initial perturbation; the perturbations are characterized in a standard fashion by wavevectors in momentum space. We demonstrate that the region of (in)stability in momentum space, as well as time-evolution in real space, for identical initial conditions, depend on the choice of the gauge (i.e., vector potential) used to describe the homogeneous synthetic magnetic field. This superficially appears as if the gauge invariance is broken, but this is not true. When the system is evolved from an identical initial condition in two different gauges, it is equivalent to suddenly turning on the synthetic magnetic field at $t=0$. This gives rise, via Faraday's law, to an initial instantaneous kick of a synthetic electric field to the wavepacket, which can differ for gauges yielding an identical uniform magnetic field at $t>0$.

Convenient Peierls phase choice for periodic atomistic systems under magnetic field

Peierls phase factors multiplying the hopping elements of tight-binding-like Hamiltonians conveniently describe the effect of slowly varying magnetic fields on electrons. This paper provides practical recipes to determine Peierls phase factors such that they preserve the translation symmetry of periodic two-dimensional systems and of quasi-one-dimensional systems that are periodic or with periodic subcomponents, such as Hall bars, in the presence of a homogeneous magnetic field. This allows the applicability of convenient numerical techniques based on the spatial periodicity of the Hamiltonian as demonstrated here on some examples.

The Influence of a Static, Homogeneous Magnetic Field on the Sorption Properties of Soybean Meal During Maritime Transport

The Influence of a Static, Homogeneous Magnetic Field on the Sorption Properties of Soybean Meal During Maritime Transport

ПОРІВНЯННЯ РІЗНИХ МЕТОДІВ ВИМІРЮВАННЯ МАГНІТНИХ ПОЛІВ У СОНЯЧНИХ СПАЛАХАХ

We present a comparison of possibilities of three methods of magnetic field measurements using the Zeeman effect, namely, method of ”center of gravity”, splitting of Stokes V peaks and analysis of bisectors of I ± V profiles. We note that first method gives very averaged data in form of effective magnetic field Beff which presents the lower limit of local magnetic field in spatially unresolved structures. Splitting of Stokes V peaks DlV can present local magnetic field B, but only in a case when this splitting exceeds considerably the splitting of peaks of the Stokes I gradient dI/dl. Analysis of bisectors of I ± V profiles presents simple method for rapid diagnostics of magnetic field inhomogeneity. In particular, in case of really weak and homogeneous magnetic field, bisectors of I + V and I – V profiles should be parallel to each other. If these bisectors are non-parallel having some extrema, this could be an evidence of hidden presence of strong spatially unresolved magnetic field. Last method is very subtle but needs careful accounting of instrumental effects in line profiles. In this paper, we present also new data related to magnetic fields in limb solar flare of 14 July 2005. Spectral observations of this flare were carried out with the Echelle spectrograph of the Horizontal Solar Telescope of the Astronomical Observatory of Taras Shevchenko National University of Kyiv. In order to measure the magnetic fields in this flare, I ± V profiles of Нa line were studied. It was found that effective magnetic field Вeff in the flare reached 850 ± 100 G on height 16 Mm. However, the spectral evidences to yet stronger local fields of 104 – 105 G range were found. In particular, for several places of the flare, a weak circular polarization of opposite signs was found in line wings on distances of 1.8-2.2 Å from line center. If this polarization to interpret as manifestations of the Zeeman effect, the corresponding magnetic field is » 85–100 kG.

High homogeneity permanent magnet for diamond magnetometry

Halbach magnets are a source of homogeneous magnetic field in an enclosed volume while keeping stray fields at a minimum. Here, we present the design, construction, and characterization for a stack of two Halbach rings with 10 cm inner diameter providing a homogeneous (<100 ppm over 1.0×1.0×0.5cm3) magnetic field of around 105 mT, which will be used for a diamond based microwave-free widefield imaging setup. The final characterization is performed with a novel fiberized diamond-based sensor on a 3D translation stage documenting the high homogeneity of the constructed Halbach array and its suitability for the proposed use.

Omnidirectional Free-Degree Wireless Power Transfer System Based on Magnetic Dipole Coils for Multiple Receivers

Wireless power transfer (WPT) technology offers a potential solution for the energy-supply problem. In this paper, we propose a novel magnetic coupling mechanism for omnidirectional and multiple-pickup energy transfer based on the magnetic dipole coils. Firstly, a cubic transmitter structure capable of generating three-dimension (3D) homogeneous magnetic field is fabricated to weaken directional sensitivity of receiving coil. Furthermore, the direction of current flowing through each dipole coil is also studied in detail. Numerical and simulated analysis is implemented to verify the omnidirectionality of the transmitter. Secondly, the equivalent circuit model for WPT systems with ferrite is analyzed. In addition, optimal loads, power distribution and transfer efficiency for multiple receivers are discussed and used to achieve the proper system design. Finally, experimental prototype is set up to validate the transmission performance of the proposed WPT system. The results have showed that genuine 3D high degree of freedom (DoF) can be achieved. Meantime, above 60% efficiency at least 30 W of total output power can be obtained for WPT systems with eight pick-ups. We believe the proposed system will give a new guideline for future low power electronic applications, such as monitoring sensors, miniature robots and household devices.

Магнитный аналог вращения Квинке

Opportunity to observe magnetic analog of the Quincke rotation effect, namely, spontaneous rotation of a spherical particle in homogeneous DC magnetic field (B), is analyzed. Two experimental versions: nonmagnetic particle suspensioned in ferromagnetic liquid and, second, - diamagnetic sphere levitated in vacuum, are considered. In the first case spontaneous rotation is possible in the fields B &lt; 0.1 T for relaxation times of the particle magnetization ~ 10-3 s. Indispensible condition of accelerated spinning of the diamagnetic sphere is confinement of its rotation degree of freedom by only one rotation axis orthogonal to the vector B.

Large eddy simulation of Taylor-Couette turbulent flow under transverse magnetic field

By the large eddy simulation method, the turbulent Taylor-Couette flow of conducting fluid under a homogenous transverse magnetic field is investigated through using the computational fluid dynamic software ANSYS Fluent 17.0. The flow is confined between two infinitely long cylinders, thus a periodic boundary condition is imposed in the axial direction. The inner cylinder rotates while the outer one is at rest, and their radius ratio is 1/2. Two Reynolds numbers of 3000 and 5000 are considered in the simulations, and the Hartmann number is varied from 0 to 50. In the present study, we assume a lower magnetic Reynolds number &lt;inline-formula&gt;&lt;tex-math id="Z-20210904222045"&gt;\begin{document}$Re_{\rm m} \ll 1$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210389_Z-20210904222045.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210389_Z-20210904222045.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, i.e., the influence of the induced magnetic field on the flow is negligible in comparison with the imposed magnetic field. The evolution of Taylor vortices, velocity profile of mean flow, and turbulent kinetic energy distribution under the transverse magnetic field are analyzed and compared with the results of the axial magnetic field counterpart. It shows that the imposed magnetic field has a significant damping effect on the Taylor-Couette flow. The twisted Taylor vortices break into small-scale vortex structures under the transverse magnetic field and they arrange themselves along the magnetic field. The fluctuations which are perpendicular to the magnetic field are suppressed effectively, while the one which is parallel to the magnetic field is nearly uninfluenced, resulting in quasi-two-dimensional elongated structure in the flow field. As anticipated, in a sufficiently strong magnetic field, the turbulent Taylor-Couette flow may eventually decay to a Couette laminar flow. In the outer cylinder and the area perpendicular to the direction of the magnetic field, the suppression effect is even stronger than those in any other places and fewer vortices are observed in the simulations. The turbulent kinetic energy is transferred firstly from large eddies to intermediate eddies, then to small eddies, and finally dissipated due to the viscous and Joule effect. As the Reynolds number increases, the suppression effect of the magnetic field weakens, and the flow behaves divergently in different areas of the apparatus. Compared with the axial magnetic field, the transverse magnetic field has a weak suppression effect on the flow field, and the profiles of related variables are obviously anisotropic.