Shape Of Magnetic Field Research Articles

Full-physics 3-D heterogeneous simulations of electromagnetic induction fields on level and deformed sea ice

AbstractIn this paper we explore simulated responses of electromagnetic (EM) signals relative to in situ field surveys and quantify the effects that different values of conductivity in sea ice have on the EM fields. We compute EM responses of ice types with a three-dimensional (3-D) finite-volume discretization of Maxwell’s equations and present 2-D sliced visualizations of their associated EM fields at discrete frequencies. Several interesting observations result: First, since the simulator computes the fields everywhere, each gridcell acts as a receiver within the model volume, and captures the complete, coupled interactions between air, snow, sea ice and sea water as a function of their conductivity; second, visualizations demonstrate how 1-D approximations near deformed ice features are violated. But the most important new finding is that changes in conductivity affect EM field response by modifying the magnitude and spatial patterns (i.e. footprint size and shape) of current density and magnetic fields. These effects are demonstrated through a visual feature we define as ‘null lines’. Null line shape is affected by changes in conductivity near material boundaries as well as transmitter location. Our results encourage the use of null lines as a planning tool for better ground-truth field measurements near deformed ice types.

Local atomic arrangement in mechanosynthesized Co x Fe1−x−y Ni y alloys studied by Mössbauer spectroscopy

Mechanosynthesized Co x Fe1−x−y Ni y alloys were examined using X-ray diffraction (XRD) and Mössbauer spectroscopy. In order to explain the shape of hyperfine magnetic field (HMF) distributions for the alloys, a local environment model based on a multinomial distribution was proposed. The model was in agreement with the XRD data and confirmed that the studied alloys were disordered solid solutions. It was successfully applied to describe the samples with bcc and fcc crystalline lattice type within the relatively broad range of components concentration. The results showed that the change of the crystalline lattice type does not cause an abrupt change of the HMF value. Moreover, a mean number of unpaired spins for the first coordination sphere may be used as a parameter to describe the HMF value experienced by 57Fe nucleus. Finally, a set of the most probable atomic configurations and their corresponding contributions to the HMF distribution were obtained.

Ground-state magnetic phase diagram of bow-tie graphene nanoflakes in external magnetic field

The magnetic phase diagram of a ground state is studied theoretically for graphene nanoflakes of bow-tie shape and various sizes in external in-plane magnetic field. The tight-binding Hamiltonian supplemented with Hubbard term is used to model the electronic structure of the systems in question. The existence of the antiferromagnetic phase with magnetic moments localized at the sides of the bow-tie is found for low field and a field-induced spin-flip transition to ferromagnetic state is predicted to occur in charge-undoped structures. For small nanoflake doped with a single charge carrier, the low-field phase is ferrimagnetic and a metamagnetic transition to ferromagnetic ordering can be forced by the field. The critical field is found to decrease with increasing size of the nanoflake. The influence of diagonal and off-diagonal disorder on the mentioned magnetic properties is studied. The effect of off-diagonal disorder is found to be more important than that of diagonal disorder, leading to significantly widened distribution of critical fields for disordered population of nanoflakes.

Multi-coil magnetic field modeling

The performance of multi-coil (MC) magnetic field modeling is compared to dedicated wire patterns for the generation of spherical harmonic (SH) shapes as these are the workhorse for spatial encoding and magnetic field homogenization in MR imaging and spectroscopy. To this end, an example 48 channel MC setup is analyzed and shown to be capable of generating all first through fourth order SH shapes over small and large regions-of-interest relevant for MR investigations. The MC efficiency for the generation of linear gradient fields shares the same order of magnitude with classic and state-of-the-art SH gradient coils. MC field modeling becomes progressively more efficient with the synthesis of more complex field shapes that require the combination of multiple SH terms. The possibility of a region-specific optimization of both magnetic field shapes and generation performance with the MC approach are discussed with emphasis on the possible trade-off between the field accuracy and generation efficiency.MC shimming has been shown previously to outperform current SH shimming. Along with the efficiency gains of MC shimming shown here, the MC concept has the potential to (1) replace conventional shim systems that are based on sets of dedicated SH coils and (2) allow optimal object-specific shim solutions similar to object-specific RF coils.

Characterization of the pedestal in Alcator C-Mod ELMing H-modes and comparison with the EPED model

A dedicated series of ELMing H-mode discharges on Alcator C-Mod spanning a broad range of plasma parameters, including plasma current (400–1000 kA), magnetic field (3.5–8 T), and plasma shaping, are presented with experimental scalings of the plasma pedestal with bulk plasma and engineering parameters. The H-modes presented achieve pedestals with densities spanning 5 × 1019–2.5 × 1020 m−3 and temperatures of 150–1000 eV (corresponding to 5–40 kPa in the pressure pedestal), over a width of 3–5% of poloidal flux. The observed pedestal structure is compared with the most recent iteration of the EPED class of models, which uniquely predict the pedestal width and height for a set of scalar input parameters via a combination of stability calculations for peeling–ballooning MHD modes and kinetic-ballooning modes.

The first DC performance test and analysis of CC conductor short sample at ASIPP conductor test facility

The first DC performance experiments of ITER correction coil (CC) conductor short sample have been carried out in the conductor test facility of Institute of Plasma Physics, CAS (ASIPP) in January this year. Those experiments aim to investigate the DC performance of ITER CC conductor. The tested conductor short sample is bended as a half circle with the diameter of 270mm to meet the background magnetic field shape. The half circle part of sample is longer than the final twist pitch. The current sharing temperature (Tcs) in the 3.86T external magnetic field (Bex), ≤12kA could be measured including the critical current (Ic) run. There is no obvious impact of 1000 cycles on DC performance. Those measured Tcs results are in agreement with the expected results from strand scaling.

Boost of plasma current with active magnetic field shaping coils in rotamak discharges

A set of magnetic shaping coils is installed on the Prairie View (PV) rotamak for the study of active plasma shape control in the regimes with and without toroidal field (TF). In the spherical tokamak regime (with TF), plasma current Ip can be boosted by 200% when all five shaping coils (connected in series) are energized. The enhancement of current drive efficiency is mainly attributed to the radial compression and the substantially axial extension of the plasma column; this in turn improves the impedance matching and thus increases antenna input power. In the field-reversed configuration (without TF), plasma current can be boosted by 100% when one middle coil is used; the appearance of radial shift mode limits the achievable value of Ip. The experiments clearly demonstrate that the plasma shape control plays a role in effectively driving plasma current in rotamaks.

ENERGY INJECTION VIA FLUX EMERGENCE ON THE SUN DEPENDING ON THE GEOMETRIC SHAPE OF MAGNETIC FIELD

Flux emergence is a complicated process involving flow and magnetic field, which provides a way of injecting magnetic energy into the solar atmosphere. We show that energy injection via this complicated process is characterized by a physical quantity called the emergence velocity, which is determined by the spatial relationship between the flow velocity and magnetic field vectors. By using this quantity, we demonstrate that the geometric shape of magnetic field might play an important role in the energy injection via flux emergence.

Global configuration of Saturn's magnetic field derived from observations

Magnetic field observations from the Cassini spacecraft are averaged into 1 × 1 RS (1 RS = 60238 km) meridional bins at Saturn. A Runge‐Kutta procedure uses the bin‐averaged field components to estimate the global configuration of Saturn's magnetic field and map field lines from the equator to Saturn's ionosphere. Within ∼18 RS of Saturn, the mapping gives a good representation of the meridional shape of Saturn's magnetic field and confirms that beyond L ∼ 6, the field departs from a dipole and becomes a smoothly‐warped magnetodisk. The disk warps upwards by as much as ∼1 RS at radial distances of ∼15 RS, with the warping increasing with radial distance. When traced back to Saturn, the bin average field lines intersect the ionosphere at latitudes similar to those expected on the basis of recent magnetic field models; the ionospheric pierce latitudes can differ by as much as 6° from those expected from a simple offset dipole.

Design and development of nanofinishing process for 3D surfaces using ball end MR finishing tool

A new precision finishing process for nanofinishing of 3D surfaces using ball end MR finishing tool is developed. The newly developed finishing process is used to finish ferromagnetic as well as nonmagnetic materials of 3D shapes using specially prepared magnetorheological polishing (MRP) fluid. The existing MR finishing devices and methods are likely to incapable of finish 3D intricate surfaces such as grooves in workpiece or complex in-depth profiles in the mold due to restriction on relative movement of finishing medium and workpiece. In this newly developed finishing device, the ball end MR finishing tool is used for finishing different kinds of 3D surfaces, as there is no limitation on relative movement of finishing medium and workpiece. It can finish the work surfaces similarly as the machining of 3D surfaces by CNC ball end milling cutter and open a new era of its applications in future. The developed process may have its potential applications in aerospace, automotive and molds manufacturing industries. A computer controlled experimental setup is designed and manufactured to study the process characteristics and performance. The magnetostatic simulations were done on ferromagnetic as well as nonferromagnetic materials of 3D surfaces to observe the ball end shape of magnetic field at the tip of the MR finishing tool. The experiments were performed on flat EN31 and groove surface of copper workpieces in the developed MR finishing setup to study the effect of finishing time on final surface roughness.

Magnetic field modeling with a set of individual localized coils

A set of generic, circular individual coils is shown to be capable of generating highly complex magnetic field distributions in a flexible fashion. Arbitrarily oriented linear field gradients can be generated in three-dimensional as well as sliced volumes at amplitudes that allow imaging applications. The multi-coil approach permits the simultaneous generation of linear MRI encoding fields and complex shim fields by the same setup, thereby reducing system complexity. The choice of the sensitive volume over which the magnetic fields are optimized remains temporally and spatially variable at all times. The restriction of the field synthesis to experimentally relevant, smaller volumes such as single slices directly translates into improved efficiency, i.e. higher magnetic field amplitudes and/or reduced coil currents. For applications like arterial spin labeling, signal spoiling and diffusion weighting, perfect linearity of the gradient fields is not required and reduced demands on accuracy can also be readily translated into improved efficiency. The first experimental realization was achieved for mouse head MRI with 24 coils that were mounted on the surface of a cylindrical former. Oblique linear field gradients of 20 kHz/cm (47 mT/m) were generated with a maximum current of 1.4 A which allowed radial imaging of a mouse head. The potential of the new approach for generating arbitrary magnetic field shapes is demonstrated by synthesizing the more complex, higher order spherical harmonic magnetic field distributions X2– Y2, Z2 and Z2 X. The new multi-coil approach provides the framework for the integration of conventional imaging and shim coils into a single multi-coil system in which shape, strength, accuracy and spatial coverage of the magnetic field can be specifically optimized for the application at hand.

Study of the Effect of a Vacuum Arc Current Interruption in a Nonuniform Magnetic Field

The effect of the magnetic field having axial and radial components on the vacuum arc stability and arc extinction was studied experimentally. It has been demonstrated that the magnetic field configuration affects the arc discharge channel shape. It has been shown also that there is a critical current value in the external magnetic field. With the current values below the critical value, high-frequency voltage oscillations appeared on the arc indicating the arc instability. The critical current value depends on the value of each of the magnetic field components. The effect of the magnetic field component values on the conditions of the vacuum arc current breaking was investigated. A physical model has been suggested, and a mathematical 3-D-model has been developed that describes the current channel shape in the external nonuniform magnetic field. The trajectories of ion motion in the vacuum arc current channel have been calculated. It has been demonstrated that when some part of ion trajectories go beyond the bounds of the anode, it can result in the arc burning instability.

On the magnetic field architecture required to capture superparamagnetic nanoparticles in a microcapillary flow

The magnetic field required for the capture of superparamagnetic nanoparticles flowing in a microcapillary array under a constant pressure regime was investigated in vitro. It was found that the nanoparticles were captured in locations that varied significantly in magnetic field strength, and that the location of capture was strongly related to the net volumetric flow through the microcapillary array. A hypothesis is presented to explain these data in which the magnetic field of the trapping Neodymium Iron Boron (NdFeB) block magnet is divided into two zones that act differently on the nanoparticles. In the first ‘steering zone’, nanoparticles are moved closer to the capillary wall where they can be captured downstream in the second ‘capture zone’. This hypothesis can be used in the future to design magnetic field shapes for more effective particle capture at higher flow rates with lower field strengths.

The effect of 50 hz magnetic field of different shape on oxygen metabolism in blood platelets: in vitro studies

The aim of the study was to assess the influence that the shape of low frequency magnetic field may have on catalase and superoxide dismutase activity, malondialdehyde concentration and free radicals generation in human blood platelets. The suspension of human blood platelets was exposed for 15 min to 50 Hz magnetic field of different shape, and flux density of 10 mT. The determinations of free radicals, malondialdehyde and catalase showed increased values compared with the initial level, regardless of the shape of the magnetic field applied. In contrast, superoxide dismutase activity was lower than at the onset of the experiment. The findings indicate that the oxidative stress resulting from exposure to 50 Hz magnetic field of 10 mT induction may produce a number of adverse effects within the cell and thus may lead to systemic disturbances in the human body.

ANALYTICAL EXPRESSION OF THE MAGNETIC FIELD CREATED BY TILE PERMANENT MAGNETS TANGENTIALLY MAGNETIZED AND RADIALS CURRENT IN MASSIVE DISKS

In this paper, we present new expressions for calculating the magnetic field produced by either tile permanent magnets tangentially magnetized or by radial currents in massive disks. These expressions are fully analytical, that is, we do not use any special functions for calculating them. In addition, they are three-dimensional and can be used for calculating the magnetic field for all regular points in space. The expressions commonly used for calculating the magnetic field produced by radial currents in massive disks are often based on elliptic integrals or semi-analytical forms. We propose in this paper an alternative analytical method that can also be used for tile permanent magnets. Indeed, by using the analogy between the coulombian model and the amperian current model, radial currents in massive disks can be represented by using the fictitious magnetic pole densities that are located on two faces of a tile permanent magnet tangentially magnetized. The two representations are equivalent and thus, the shape of magnetic field produced is the same for all points in space, with a smaller value in the case of it is produced by radial currents in massive disks. Such expressions can be used for realizing easily parametric studies.

Evaluation of magnetic field distribution of superconducting bulk magnets with the same pole arrangement

We study on the construction of superconducting permanent magnets by RE123 bulk materials and the investigation of these industrial applications such as a magnetic separation. The bulk magnets can generate strong magnetic field in comparison with common permanent magnets and iron-cored electromagnets. In order to propagate an industrial application of bulk magnet in the feature, it is necessary to form various shape of magnetic field, as well as its strength is enlarged. In this paper, the strong magnetic field in the radial direction has been constructed by a pair of bulk magnets. Two Gd123 bulk materials are magnetized by the IMRA method and these are arranged face-to-face with the same pole facing each other. When the radial component of magnetic fields, B x , is measured in the open space between the magnetic poles with a gap of 50 mm, the area of | B x | > 0.5 T increases to 148% compared with the single pole.

Quantum rings of arbitrary shape and non-uniform width in a threading magnetic field

The electronic states of quantum rings with centerlines of arbitrary shape and non-uniform width in a threading magnetic field are calculated. The solutions of the Schr\"odinger equation with Dirichlet boundary conditions are obtained by a variational separation of variables in curvilinear coordinates. We obtain a width profile that compensates for the main effects of the curvature variations in the centerline. Numerical results are shown for circular, elliptical, and lima\ifmmode \mbox{\c{c}}\else \c{c}\fi{}on-shaped quantum rings. We also show that smooth and tiny variations in the width may strongly affect the Aharonov--Bohm oscillations.

Thermodynamic, kinetic, and magnetic properties of aNi54Fe19Ga27magnetic shape-memory single crystal

Ferromagnetic Heusler alloys exhibiting martensitic transformations are known to change their shape in an external magnetic field. Magnetization, electric resistance, and specific heat as a function of temperature are examined in ${\mathrm{Ni}}_{54}{\mathrm{Fe}}_{19}{\mathrm{Ga}}_{27}$ single crystal. Structural transition appears as sharp anomaly in these dependencies. This points to an avalanchelike character of martensitic transformation. The jump in resistivity at the structural phase transition and the lower density of states at the Fermi level in the martensite phase supports the hypothesis of the Jahn-Teller origin of the martensitic transformation. Magnetic measurements show that transformation to the martensitic phase is accompanied by the increase of spontaneous magnetization and an increase of magnetocrystalline anisotropy. Magnetization increase is due to different Curie temperatures of austenite and martensite. These were determined from critical behavior using Arrott plot. Additional analysis of magnetic behavior indicates ferrimagnetic ordering in this nonstoichiometric compound. Intrinsic properties of the compound are analyzed with respect to both of the actuation modes possible in magnetic shape-memory alloys. However, neither a magnetically induced martensite transformation nor a magnetically induced reorientation of variants has been observed.

Coronal Loop Model Including Ion Kinetics

We present a kinetic coronal loop model including collisions and wave-particle interactions to study the mechanisms of loop heating. The model is based on a quasi-linear treatment of the Vlasov equation for the reduced velocity distribution functions of the protons, which are the only ions considered in the loop plasma. For the energy input into the loop, we assume that linear Alfven waves penetrate the loop from its footpoints and heat the protons via wave-particle interactions and wave absorption. Through Coulomb collisions between protons and electrons some thermal energy can be transferred to the electrons. It turns out that in such a model protons are hotter than electrons, and the scale length for proton heating along the loop is determined by the dissipation scale of the ion-cyclotron waves. Through the gyrofrequency this scale is connected to the cross section area of the loop and, thus, to the spatial variation of the magnetic field shaping the coronal loop. Furthermore, it is shown that in the case of a nearly homogeneous flux tube cross section, an almost flat temperature profile occurs along the major part of the loop with an enhanced plasma density. These plasma parameter profiles are consistent with those of loops having temperatures between 1 and 1.5 MK as observed in extreme-ultraviolet emission. However, if the magnetic field lines are more strongly diverging from the footpoints to the loop apex, the proton heating is found to be more uniform, resulting in a higher temperature and lower density along the loop. These profiles are similar to those observed in X-ray loops.

Effects of magnetic field configuration on thrust performance in a miniature microwave discharge ion thruster

The effects of magnetic field configuration on thrust performance in a miniature microwave discharge ion thruster were investigated in order to improve thrust performance. First, the extracted ion beam current was measured for various levels of strength of the magnetic field. It was found that there is an optimum magnitude of the magnetic field. That this is due to the tradeoff between magnetic mirror confinement and microwave-plasma coupling was confirmed by measurement of the ion saturation current into the antenna of the ion thruster. The ion saturation current was found to decrease with an increase in magnetic field strength, due to the improvement in magnetic mirror confinement. The estimated electron temperature also decreases with an increase in magnetic field strength. This result shows that the increase in magnetic field strength leads to a decrease in microwave-plasma coupling. Next, the ion beam current for three magnetic field shapes was measured by changing the length of the central yoke. The results show that the optimum magnetic field shape depends on the mass flow rate because of the tradeoff between magnetic confinement and ionization probability. For the configurations tested, the 3 mm length central yoke is optimal for low mass flow, whereas 7 mm is the best for high mass flow. Overall, the extracted ion beam current is 21.4 mA, at a xenon mass flow rate of 0.036 mg/s, beam voltage of 1500 V, and incident microwave power of 16 W.