Vertical Magnetic Field Gradient Research Articles

Thermal and electrical design of superconducting demonstrator for magnetic density separation

In this paper the focus is on thermal and electrical design aspects of a NbTi-based demonstrator magnet for magnetic density separation (MDS) that is being constructed at the University of Twente. MDS is a recycling technology that allows the separation of non-magnetic particles based on their mass density, using a vertical magnetic field gradient and a ferrofluid. To minimize the distance between the planar array of racetrack coils and the ferrofluid bath, the system is conduction-cooled. First the thermal design is presented, which shows that the coils can operate below 4.5 K with sufficient margin using a single cryocooler. High-purity aluminium heat drains enable a low thermal gradient across the cold mass. The current path is introduced, as well as the adopted protection scheme. The magnet’s stored energy can safely be dumped in the coils. Diodes are placed (anti-)parallel to the coils in the cold to prevent high terminal voltages. In the case of a quench in the superconducting part of the current leads or an external anomaly, a switch is opened and the current is forced through a resistor in series with the diodes, causing a deliberate transition of the coils to the normal state and thus a fast ramp-down.

Mechanical design of a superconducting demonstrator for magnetic density separation

The focus of this paper is on the mechanical design of a NbTi-based demonstrator magnet for magnetic density separation (MDS) that is being constructed at the University of Twente. MDS is a new recycling technology that allows the separation of non-magnetic particles based on their mass density, using a vertical magnetic field gradient and a ferrofluid. The unique mechanical design challenge for this type of magnet is the desired minimization of the distance between a sim1 m2 planar array of cryogenic racetrack coils and the ambient-temperature ferrofluid bath. The optimization of the magnet geometry results in a distance between the coils and ferrofluid of 50 mm. This is made possible by opting for conduction-cooling, for the inclusion of room-temperature rods that pass through the cold mass to support the cryostat, and for the geometry of the cassette that reacts to the Lorentz force.

Optimum Coil-System Layout for Magnet-Driven Superconducting Magnetic Density Separation

This article discusses the optimum layout of coils of a superconducting magnet system for magnetic density separation (MDS). MDS is a novel separation technology that combines a vertical magnetic field gradient with a ferrofluid to separate mixtures of non-magnetic particles based on their mass density. The MDS process can separate more than two types of particles in a single process step, thereby distinguishing it from other separation techniques using a ferrofluid. The authors are currently constructing a superconducting MDS demonstrator. Ideally, the gradient of the magnetic field magnitude should change only with the distance above the magnet but remain constant in a horizontal plane. In principle, such an ideal field profile can be generated with an infinite harmonic sheet current. In practice, edge effects appear due to the necessity of using a finite number of coils. These cause a horizontal component in the field gradient and also change the vertical component. We compare the vertical magnetic field gradient of various coil layouts to see which configuration performs best. To facilitate ease of production, the analysis is restricted to flat racetrack coils. The main result is that the specific shape of a racetrack coil has a larger influence on the vertical gradient than the number of coils. The feed particles need to be pushed through the separation chamber from the insertion to the collection point. One option to realize this is to use an MDS setup in which the magnet is inclined with respect to the horizontal plane. This tilting results in a horizontal magnetic force component, which drives feed particles through the fluid bed. We show that a three-coil layout provides the largest usable fluid bed depth for a wide range of tilt angles.

Fundamental Electromagnetic Configuration for Generating One-Directional Magnetic Field Gradients

In this article, electromagnet layouts are presented, which generate a magnetic field with a magnitude gradient that does not vary significantly in a horizontal plane but decreases monotonically with the vertical height above the magnet. Such a one-direction magnetic field gradient is a specific requirement for magnetic density separation (MDS), a novel recycling technology that combines a vertical magnetic field gradient with a ferrofluid to separate a mixture of non-magnetic materials based on their mass density. We are assembling the first superconducting magnet to be used for this application. In contrast to other separation technologies that use ferrofluid, multiple products can be separated in a single process step. First, the idealized current distribution is introduced that produces such a magnetic field with a magnitude that decays only in one direction. This ideal field can be approximated with practical coil configurations, which are evaluated with a Fourier analysis to derive an optimal cross-sectional layout based on flat racetrack coils. The analysis concludes with a discussion of the effect of winding pack thickness on the value of the magnetic field above the magnet system and the peak field inside the winding pack. The conclusions of this study are applicable not just for MDS but for any application that requires a magnetic field gradient that changes only in one direction.

Optically pumped Cs magnetometers enabling a high-sensitivity search for the neutron electric dipole moment

An array of sixteen laser-pumped scalar Cs magnetometers was part of the neutron electric dipole moment (nEDM) experiment taking data at the Paul Scherrer Institute in 2015 and 2016. It was deployed to measure the gradients of the experiment's magnetic field and to monitor their temporal evolution. The originality of the array lies in its compact design, in which a single near-infrared diode laser drives all magnetometers that are located in a high-vacuum chamber, with a selection of the sensors mounted on a high-voltage electrode. We describe details of the Cs sensors' construction and modes of operation, emphasizing the accuracy and sensitivity of the magnetic field readout. We present two applications of the magnetometer array directly beneficial to the nEDM experiment: (i) the implementation of a strategy to correct for the drift of the vertical magnetic field gradient and (ii) a procedure to homogenize the magnetic field. The first reduces the uncertainty of the new nEDM result. The second enables transverse neutron spin relaxation times exceeding 1500 s, improving the statistical sensitivity of the nEDM experiment by about 35% and effectively increasing the rate of nEDM data taking by a factor of 1.8.

Solar photosphere magnetization

Context.A recent review shows that observations performed with different telescopes, spectral lines, and interpretation methods all agree about a vertical magnetic field gradient in solar active regions on the order of 3 G km−1, when a horizontal magnetic field gradient of only 0.3 G km−1is found. This represents an inexplicable discrepancy with respect to the divB = 0 law.Aims.The objective of this paper is to explain these observations through the lawB = μ0(H+M) in magnetized media.Methods.Magnetization is due to plasma diamagnetism, which results from the spiral motion of free electrons or charges about the magnetic field. Their usual photospheric densities lead to very weak magnetizationM, four orders of magnitude lower thanH. It is then assumed that electrons escape from the solar interior, where their thermal velocity is much higher than the escape velocity, in spite of the effect of protons. They escape from lower layers in a quasi-static spreading, and accumulate in the photosphere. By evaluating the magnetic energy of an elementary atom embedded in the magnetized medium obeying the macroscopic lawB = μ0(H+M), it is shown that the Zeeman Hamiltonian is due to the effect ofH. Thus, what is measured isH.Results.The decrease in density with height is responsible for non-zero divergence ofM, which is compensated for by the divergence ofH, in order to ensure div B = 0. The behavior of the observed quantities is recovered.Conclusions.The problem of the divergence of the observed magnetic field in solar active regions finally reveals evidence of electron accumulation in the solar photosphere. This is not the case of the heavier protons, which remain in lower layers. An electric field would thus be present in the solar interior, but as the total charge remains negligible, no electric field or effect would result outside the star.

Interplay of cell death signaling pathways mediated by alternating magnetic field gradient

The ability to control or manipulate the pathways leading to cell death plays a pivotal role in cancer treatment. We demonstrate magneto-actuation of magnetic nanoparticles (MNPs) to induce different cell death signaling pathways, exemplifying the intricate interplay between apoptosis and necrosis. In vitro cell experiments show the cell viabilities decreases with increasing field strength and is lower in cells treated with low aspect ratio MNPs. In a strong vertical magnetic field gradient, the MNPs were able to apply sufficient force on the cell to trigger the intracellular pathway for cell apoptosis, thus significantly reducing the cell viability. The quantification of apoptotic and necrotic cell populations by fluorescence dual staining attributed the cell death mechanism to be predominantly apoptosis in a magnetic field gradient. In contrast, the MNPs in an alternating magnetic field gradient can effectively rupture the cell membrane leading to higher lactate dehydrogenase leakage and lower cell viability, proving to be an effective induction of cell death via necrosis.

Vertical magnetic field gradient in the photospheric layers of sunspots

We investigate the vertical gradient of the magnetic field of sunspots in the photospheric layer. Independent observations were obtained with the SOT/SP onboard the Hinode spacecraft and with the TIP-2 mounted at the VTT. We apply state-of-the-art inversion techniques to both data sets to retrieve the magnetic field and the corresponding vertical gradient. In the sunspot penumbrae we detected patches of negative vertical gradients of the magnetic field strength, i.e.,the magnetic field strength decreases with optical depth in the photosphere. The negative gradient patches are located in the inner and partly in the middle penumbrae in both data sets. From the SOT/SP observations, we found that the negative gradient patches are restricted mainly to the deep photospheric layers and are concentrated near the edges of the penumbral filaments. MHD simulations also show negative gradients in the inner penumbrae, also at the locations of filaments. Both in the observations and simulation negative gradients of the magnetic field vs. optical depth dominate at some radial distances in the penumbra. The negative gradient with respect to optical depth in the inner penumbrae persists even after averaging in the azimuthal direction, both in the observations and, to a lesser extent, also in MHD simulations. We interpret the observed localized presence of the negative vertical gradient of the magnetic field strength in the observations as a consequence of stronger field from spines expanding with height and closing above the weaker field inter-spines. The presence of the negative gradients with respect to optical depth after azimuthal averaging can be explained by two different mechanisms: the high corrugation of equal optical depth surfaces and the cancellation of polarized signal due to the presence of unresolved opposite polarity patches in the deeper layers of the penumbra.

Prominence and Filament Eruptions Observed by the Solar Dynamics Observatory: Statistical Properties, Kinematics, and Online Catalog

We present a statistical study of prominence and filament eruptions observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). Several properties are recorded for 904 events that were culled from the Heliophysics Event Knowledgebase (HEK) and incorporated into an online catalog for general use. These characteristics include the filament and eruption type, eruption symmetry and direction, apparent twisting and writhing motions, and the presence of vertical threads and coronal cavities. Associated flares and white-light coronal mass ejections (CME) are also recorded. Total rates are given for each property along with how they differ among filament types. We also examine the kinematics of 106 limb events to characterize the distinct slow- and fast-rise phases often exhibited by filament eruptions. The average fast-rise onset height, slow-rise duration, slow-rise velocity, maximum field-of-view (FOV) velocity, and maximum FOV acceleration are 83 Mm, 4.4 hours, 2.1 km/s, 106 km/s, and 111 m/s^2, respectively. All parameters exhibit lognormal probability distributions similar to that of CME speeds. A positive correlation between latitude and fast-rise onset height is found, which we attribute to a corresponding negative correlation in the average vertical magnetic field gradient, or decay index, estimated from potential field source surface (PFSS) extrapolations. We also find the decay index at the fast-rise onset point to be 1.1 on average, consistent with the critical instability threshold theorized for straight current channels. Finally, we explore relationships between the derived kinematics properties and apparent twisting motions. We find that events with evident twist have significantly faster CME speeds and significantly lower fast-rise onset heights, suggesting relationships between these values and flux rope helicity.

Lengths of Lifted Laminar Flames Under Vertical Magnetic Field Gradient

The effect of a vertical magnetic field gradient on stoichiometric lengths of methane-air diffusion lifted flames was numerically investigated. The study was developed for understanding the evolution of these lengths when coaxial jet air velocity was varied keeping constant fuel velocity. As the air jet velocity is increased, gradient in mixture fraction and liftoff height increase and flame length decreases. Calculated reacting flow results of liftoff and flame length are presented for two cases, with and without magnetic field. For the same initial flow conditions, the application of the negative magnetic field gradient results in an increase of the stoichiometric flame lengths and a reduction of liftoff heights. For the lifted flame under magnetic field gradient, the Froude number was modified to take into account the magnetic buoyancy effect and it is shown that the correlation proposed by Altenkirch et al. (1976) can still be applied to represent lengths of lifted flames.

Controlling Gravity Impact on Diffusion Flames by Magnetic Field

The ability to use a magnetic field as a means for controlling the role of gravity buoyancy on the combustion process is demonstrated by applying a strong vertical magnetic field gradient on a laminar gas jet diffusion flame. The confirmation is based on a comparison of flame appearance; in particular, length variation, to both elevated gravity (higher than earth’s gravity) and zero-gravity combustion experimental data. The comparison parameter is the dimensionless number G, defined as the ratio of gravity level generated by magneto-gravity buoyancy to earth’s gravity. The more important results are as follows. First, for G > 1, good agreement between magnetic and centrifuge length scaling laws reveals that the slight decrease of flame length according to L ∼ G−1/8 is the result of increasing artificial magnetically induced gravity strength. It ensues that flame thinning, bluing, lifting, and extinction are produced by similar mechanisms previously identified in centrifuge diffusion flames. Thereafter, at G ≅ 0, the flame assumes a nearly hemispheric shape and a blue color in perfect similarity to nonbuoyant flames under zero-gravity conditions generated in drop towers. Another important fact is that the magnetic field offers the ability to observe the flame behavior at low gravity levels 0 < G < 1. A primary interesting result is that flame length varies strongly, following the scaling law L ∼ G−1/2.

Comparison between electromagnetic induction and fluxgate gradiometer measurements on the buried remains of a 17th century castle

This study aimed to evaluate the different configurations of an electromagnetic induction (EMI) sensor, the EM38DD (Geonics Limited, Canada) with fluxgate gradiometer measurements on an archaeological site. The EM38DD allows measuring both the apparent magnetic susceptibility (MSa or χ a) and the apparent electrical conductivity (ECa or σ a) in two different coil orientations. A gradiometer measures the lateral variations of the vertical magnetic field gradient, caused by the induced and remanent magnetisations. An archaeological site where historical documents indicated the presence of a 17th century brick castle was selected as a test area. The results of the first survey with the EM38DD showed very strong magnetic anomalies in the central field, which were caused by the brick remains of the castle. Therefore, a smaller area was chosen within this field to compare the different configurations of the EM38DD with the gradiometer at the same measurement resolution. The most useful results with the EM38DD were obtained from the MSa measured in a vertical coplanar orientation. Its anomalies corresponded well with the gradiometer anomalies. The gradiometer anomalies were sharper defined than the EM38DD anomalies, but were complicated by the bipolar response pattern. The MSa map in horizontal coplanar orientation was very difficult to interpret, due to the less optimal spatial sensitivity. The wall remains were not visible in the ECa map in horizontal coplanar orientation, although other interesting anomalies were detected.

Numerical computation of magnetothermal convection of water in a vertical cylindrical enclosure

Numerical computations were carried out to clarify the effect of Kelvin force on the flow of water in a vertical cylindrical enclosure heated from below and cooled from above under a vertical magnetic field gradient. Since the Kelvin force that is produced by the magnetic field gradient depends on the position and the size of a circular electric coil, the coil was placed at either the hot or the cold plate and the coil diameter was set to be 2.5 or 5 times that of the enclosure. First, to understand the mechanism of the generation of the magnetothermal convection induced by the Kelvin force alone, the transition of velocity and temperature fields were visualized under a non-gravitational field. Under a gravitational field, the average Nusselt number decreased with increase of magnetic strength when the coil was placed at the lower end plate which was heated isothermally so that the generation of natural convection was suppressed by the Kelvin force. On the contrary, the average Nusselt number increased with increase of magnetic strength when the coil was placed at the upper end plate which was cooled isothermally so that the natural convection was enhanced by the Kelvin force.

On the similarity between gravity and magneto-gravity convection within a non-electroconducting fluid in a differentially heated rectangular cavity

The magneto-gravity convection induced by a vertical magnetic field gradient applied to a non-electroconducting paramagnetic liquid in a differentially heated rectangular cavity is investigated numerically and experimentally. Due to the magnetic susceptibility temperature dependence of the fluid, the magnetic body force drives a thermoconvective motion in the cavity. Depending on the sign of the magnetic gradient, the vertical magnetic force in the liquid can be parallel or antiparallel to gravity. Hence, buoyancy convection in the cavity results of the combination of the vertical magnetic force and of gravity. Expressed in terms of Nu versus Ra, the experimental and numerical results are compared to the correlation obtained by Catton without magnetic field. Agreement between the two curves clearly points out the similarity between gravity induced convection and magneto-gravity convection.

Vertical Lorentz Force and Cross‐Field Currents in the Photospheric Magnetic Fields of Solar Active Regions

We demonstrate that the vertical Lorentz force and a corresponding lower limit of the cross-field electric current density can be calculated from vector magnetograms of solar active regions obtained at a single height in the solar atmosphere, provided that the vertical gradient of the magnetic field strength is known at this height. We use a predicted vertical magnetic field gradient derived from a previous analysis. By testing various force-free solutions, we find that the numerical accuracy of our method is satisfactory. Applying the method to active region photospheric vector magnetograms, we find vertical Lorentz forces ranging from several hundredths to a few tenths of the typical photospheric gravitational force, and typical cross-field current densities up to several times 10 mA m-2. The typical vertical current density is found to be 2-3 times smaller, on the order of 10-15 mA m-2. These differences are above the associated uncertainties. The values of the cross-field currents decrease in an averaged vector magnetogram, but the ratio of the cross-field to the vertical current density increases, also above the uncertainties. We conclude that the photospheric active region magnetic fields are not force-free, contrary to the conjectures of some recent studies.

Numerical Prediction on Heat Transfer Phenomenon in Paramagnetic and Diamagnetic Fluids Under a Vertical Magnetic Field Gradient

Two-dimensional numerical computations were carried out in order to clarify the effect of magnetic field on paramagnetic and diamagnetic fluids in a vertical cylindrical container with thermal and vertical magnetic field gradients under both gravitational and nongravitational fields. In both gravitational and nongravitational fields, the magnetizing force that was produced by the gradient of magnetic field could be utilized to control the heat transfer rate for paramagnetic and diamagnetic fluids. These phenomena could be successfully explained by visualizing the field of magnetizing force.

Numerical analysis of a hydrogen-oxygen diffusion flame in vertical or horizontal gradient of magnetic field

The magnetic force is essentially in proportion to the mass density and the magnetic susceptibility of paramagnetic chemical species and the gradient of magnetic field. In this study, the effect of magnetic field on OH radical distribution in a hydrogen-oxygen diffusion flame was investigated by numerically solving the equations of reactive gasdynamics and magnetism to explore a possibility of combustion control by magnetic force. Particularly, we attempted the analysis for three kinds of cases with different spatial distributions of the magnetic field: Case 1 is without the magnetic field, Case 2 is under the vertical gradient of magnetic field, and Case 3 is under the horizontal gradient of magnetic field. According to the analysis, because the mass density and the magnetic susceptibility of O 2 are much larger than those of other chemical species in the peripheral region of the flame, O 2 was strongly influenced by the magnetic force in that region. Consequently, this force induced the flow of gas including a large amount of O 2 and indirectly changed the OH distribution in the flame. The horizontal magnetic gradient (Case 3) was about 1.2 times more effective in the OH density change than the vertical gradient (Case 2).

Magnetic control of thermal convection in electrically non-conducting or low-conducting paramagnetic fluids

An inhomogeneous magnetic field exerts a magnetization force on all materials, including electrically conducting and non-conducting fluids. A recent experiment demonstrated that this force can enhance or suppress convection in a paramagnetic aqueous solution heated from either below or above. In this paper, to clarify the mechanism of the observed phenomena, we numerically simulated the effect of a vertical magnetic field gradient on thermal convection in paramagnetic fluids. These simulated results agree with experimental observations. Our study shows that the magnetic buoyancy force has potential applications for controlling convection and enhancing the heat transfer efficiency in either electrically non-conducting or low-conducting fluids.

Suppression/reversal of natural convection by exploiting the temperature/composition dependence of magnetic susceptibility

Natural convection, driven by temperature or concentration gradients or both, is an inherent phenomenon during solidification of materials on Earth. This convection has practical consequences (e.g., effecting macrosegregation) but also renders difficult the scientific examination of diffusive/conductive phenomena during solidification. It is possible to halt, or even reverse, natural convection by exploiting the variation (with temperature, for example) of the susceptibility of a material. If the material is placed in a vertical magnetic field gradient, a buoyancy force of magnetic origin arises and, at a critical field gradient, can balance the normal buoyancy forces to halt convection. At higher field gradients the convection can be reversed. The effect has been demonstrated in experiments at Marshall Space Flight Center where flow was measured by particle image velocimetry in MnCl2 solution in a superconducting magnet. In auxiliary experiments the field in the magnet and the properties of the solution were measured. Computations of the natural convection, its halting and reversal, using the commercial software FLUENT® were in good agreement with the measurements.