Helmholtz Coils Research Articles

Electromagnetic fields effects on microbial growth in cocoa fermentation: A controlled experimental approach using established growth models

Understanding the effects of electromagnetic fields is crucial in the fermentation of cocoa beans, since through precise control of fermentation conditions the sensory and nutritional properties of cocoa beans could be improved. This study aimed to evaluate the effect of oscillating magnetic fields (OMF) on the kinetic growth of the core microbial communities of the Collections Castro Naranjal (CCN 51) cocoa bean. The data was obtained by three different models: Gompertz, Baranyi, and Logistic. The cocoa beans were subjected to different OMF strengths ranging from 0 mT to 80 mT for 1 h using the Helmholtz coil electromagnetic device. The viable microbial populations of lactic acid bacteria (LAB), acetic acid bacteria (AAB), and yeast (Y) were quantified using the colony-forming unit (CFU) counting method. The logistic model appropriately described the growth of LAB and Y under magnetic field exposure. Whereas the Baranyi model was suitable for describing AAB growth. The microbial populations in cocoa beans exposed to magnetic fields showed lower (maximum specific growth rate (μmax), values than untreated controls, with AAB exhibiting the highest average growth rate value at 5 mT and Y having the lowest average maximum growth rate value at 80 mT. The lower maximum specific growth rates and longer lag phases when exposed to magnetic fields compared to controls demonstrate the influence of magnetic fields on microbial growth kinetics.

Hall sensors for magnetization loop determination in thin films

In this paper, a new method and technology to determine magnetization loops in thin films is demonstrated. The method is based on Hall sensor measurements set parallel to the thin film, which is magnetized using a Helmholtz pair coil set-up. The magnetic lines out of the thin films are measured by a Hall sensor, set parallel to the thin film. Provided that the shape of the thin film under test, preferably in the form of rectangle or circle, is well known with a length or diameter at least 100 times larger than its thickness, the precise determination of the magnetization loop of the film is possible with acceptable uncertainty, by means of calibrating the Hall sensor response with respect to the comparison with a standard method.

The Disintegration of Magnetic Fluid Layer on Liquid and Solid Substrates in Vertical Magnetic Field

The purpose of this work is to experimentally study the process of decomposition of a horizontal layer of a magnetic fluid lying on a liquid non-wettable substrate into an ordered system of drops under the influence of a vertical magnetic field.Methods. In the experiment, the studied liquids in the form of a two-layer system filling a cylindrical glass cuvette were placed on a horizontal platform in the center of the Helmholtz coils connected to a direct current source. The development of magnetic fluid layer free surface instability was monitored using a high-speed digital video camera installed at the top of the coil system.Results. The dependence of the critical strength of the magnetic field on the thickness of the torn layer and the magnetic susceptibility of the magnetic fluid is determined. The obtained experimental data are compared with the results of existing theoretical studies of the instability of the magnetic fluid layer. Compared to the case of a solid substrate, the rupture of the magnetic fluid layer on a liquid substrate occurred at lower values of the critical field strength. In this case, the development of instability of the free surface magnetic fluid layer on a solid substrate occurs at field strengths twice as high as the instability strength of the interface of the layer on a liquid substrate.Conclusion. The use of a liquid substrate makes it possible to break such layers of the magnetic fluid, the deformation of which on a solid substrate is limited only by a periodic perturbation of the surface. The magnitude of the critical strength leading to the disintegration of a continuous magnetic fluid layer on a liquid substrate increases with increasing thickness of this layer. An increase in the magnetic susceptibility of the magnetic fluid leads to a decrease in the critical field strength.

Design and Optimization of a Novel Double-Layer Helmholtz Coil for Wirelessly Powering a Capsule Robot

Design and Optimization of a Novel Double-Layer Helmholtz Coil for Wirelessly Powering a Capsule Robot

Magnetic field dependent thermal conductivity investigation of water based Fe3O4/CNT and Fe3O4/graphene magnetic hybrid nanofluids using a Helmholtz coil system setup

Magnetic hybrid nanofluids are making a name of themselves in mainstream application areas such as heat transfer, solar systems, acoustic applications, etc. These nanofluids are highly favorable as their ability to advance the properties of their constituent particles such as their thermophysical properties. This study aims to investigate the magnetic field dependent thermal conductivity of Fe3O4/CNT – water and Fe3O4/Graphene – water magnetic hybrid nanofluids. The thermal conductivity investigations are carried out with the 3ω method under a uniform magnetic field generated by a 3D Helmholtz coil system. Fe3O4/CNT – water and Fe3O4/Graphene – water magnetic hybrid nanofluids were purchased commercially as 20 wt% colloids. Then, the samples with 1, 2, 3, 4, and 5 wt% were prepared by diluting them with DI water. Thermal conductivity measurements were carried out for the samples under the external uniform magnetic field in the range of 0–250 G in both parallel and perpendicular directions to the temperature gradient generated by the thermal conductivity measurement probe. The results pointed out that the thermal conductivity of the samples increases as the magnetic field and particle concentration increase for both magnetic hybrid nanofluids. Additionally, it is obtained that the thermal conductivity enhancement of Fe3O4/Graphene – water is up to 3 times higher than Fe3O4/CNT – water samples. Moreover, the maximum thermal conductivity enhancement was observed as ∼12 % and ∼9 % for Fe3O4/CNT – water, and ∼51 % and ∼21 % Fe3O4/Graphene – water under external magnetic field application with parallel and perpendicular direction, respectively.

Complete control and strengthening over iron oxide porous structures freeze cast under oscillating magnetic fields

AbstractPorous scaffolds can be utilized in a variety of biomedical as well as mechanical applications. The process of freeze casting is a successful method to fabricate these porous structures but with ideal characteristics in only one direction (the ice‐growth direction). The application of magnetic fields led to an increase in both the microstructural control and mechanical strength in an additional orthogonal direction. The application of these weak, uniform fields (≤20 mT), in particular oscillating fields from a Helmholtz coils setup, has led to increases in mechanical strength through microstructural alignment in multiple material types. However, structures fabricated from these uniform fields have primarily been compared to each other, with little research comparing them to structures fabricated under strong, non‐uniform fields from permanent magnet setups. Therefore, iron‐oxide scaffolds were fabricated under weak, uniform fields (≤20 mT) as well as strong, non‐uniform fields (≥20 mT), and their mechanical and microstructural properties were compared to one another. The application of weak, uniform fields led to superior mechanical properties compared to those produced from the application of strong, non‐uniform fields, no distortions in the physical structure of the freeze‐cast scaffold, and the best microstructural alignment ever seen in freeze‐cast structures.

Characterization of magnetic field for loading, trapping and transferring cold atom close to the atom chip’s surface

Atom chips provide flexible technologies for implementing modern concepts in quantum optics, quantum measurement, and quantum information processing. Atom chips are miniature devices that confine, control and manipulate cold atoms using electric, magnetic, and light fields. Due to the shrinkage of scale, arbitrary magnetic traps can be generated from current sources outside the chip’s vacuum compartment, in contrast to a traditional setup. This makes it easy to change trap configurations in an experiment that involves rapid prototyping of quantum states and quantum trajectory designs in free space. In this work, we show relevant parameters needed for transferring a cold atom cloud at the recoil limit from a magneto-optical trap (MOT) to an area close to the atom chip. To create a movable magnetic potential for this transfer, we used the MOT coils and an additional pair of coils in an anti-Helmholtz configuration. The properties of the movable potential were obtained by performing the Computer Simulation Tool (CST EM Studio suite®). Furthermore, an appropriate magnetic trap on a chip is developed, based on the simulation from COMSOL Multiphysics. We used a magnetic field gradient of around 20 G/cm to transport the cold atom with a distance over 20 mm with a temperature gain below 100 micro-Kelvin. The simulation results are based on an atom chip with a size of 2×2 cm2 and a copper wire thickness of 2 mm. The atom chip consists of Z, U and I-shaped wires that generate a quadrupole magnetic field. The resulting field minimum can be made at least 7 mm away from the chip surface.

Radiation damping at clinical field strength: Characterization and compensation in quantitative measurements.

In any MR experiment, the bulk magnetization acts on itself, caused by the induced current in the RF receiver circuit that generates an oscillating damping field. This effect, known as "radiation damping" (RD), is usually weak and, therefore, unconsidered in MRI, but can affect quantitative studies performed with dedicated coils that provide a high SNR. The current work examined RD in a setup for investigations of small tissue specimens including a quantitative characterization of the spin-coil system. A custom-made Helmholtz coil (radius and spacing 16 mm) was interfaced to a transmit-receive (Tx/Rx) switch with integrated passive feedback for modulation or suppression of RD similar to preamplifier decoupling. Pulse sequences included pulse-width arrays to demonstrate the absence/ presence of RD and difference techniques employing gradient pulses or composite RF pulses to quantify RD effects during free precession and transmission, respectively. Experiments were performed at 3T in small samples of MnCl2 solution. Significant RD effects may impact RF pulse application and evolution periods. Effective damping time constants were comparable to typical T2 * times or echo spacings in multi-echo sequences. Measurements of the phase relation showed that deviations from the commonly assumed 90° angle between the damping field and the transverse magnetization may occur. Radiation damping may affect the accuracy of quantitative MR measurements performed with dedicated RF coils. Efficient mitigation can be achieved hardware-based or by appropriate consideration in the pulse sequence.

Kinetic and Parametric Analysis of the Separation of Ultra-Small, Aqueous Superparamagnetic Iron Oxide Nanoparticle Suspensions under Quadrupole Magnetic Fields.

Superparamagnetic iron oxide nanoparticles (SPIONs) have gathered tremendous scientific interest, especially in the biomedical field, for multiple applications, including bioseparation, drug delivery, etc. Nevertheless, their manipulation and separation with magnetic fields are challenging due to their small size. We recently reported the coupling of cooperative magnetophoresis and sedimentation using quadrupole magnets as a promising strategy to successfully promote SPION recovery from media. However, previous studies involved SPIONs dispersed in organic solvents (non-biocompatible) at high concentrations, which is detrimental to the process economy. In this work, we investigate, for the first time, the magnetic separation of 20 nm and 30 nm SPIONs dispersed in an aqueous medium at relatively low concentrations (as low as 0.5 g·L-1) using our custom, permanent magnet-based quadrupole magnetic sorter (QMS). By monitoring the SPION concentrations along the vessel within the QMS, we estimated the influence of several variables in the separation and analyzed the kinetics of the process. The results obtained can be used to shed light on the dynamics and interplay of variables that govern the fast separation of SPIONs using inexpensive permanent magnets.

Slit extraction and emittance results of a permanent magnet minimum-B quadrupole electron cyclotron resonance ion source

We present an experimental and simulation study of high charge state ion beams produced with a permanent magnet electron cyclotron resonance ion source (ECRIS) with minimum-B quadrupole magnetic field topology and slit extraction system. The unconventional topology generates fan-shaped plasma flux, favouring a rectangular aperture for beam extraction. We demonstrate successful low-energy transport of the slit beam, achieving up to 25 times higher beam currents compared to a round extraction aperture. Maximum beam intensities are 1.5 and 15 μA for argon 11＋ and 9＋, respectively. The results indicate microwave power-limited beam production. Emittance values are comparable to traditional ECRIS. Unlike in conventional ECRIS, the emittance increases monotonically with increasing charge state due to slit extraction, magnetic topology, and beam transport characteristics. The experimental transport efficiency for helium ranges from 32% to 50%, depending on the total beam current. The losses primarily occur due to the dipole magnet’s insufficient vertical aperture.

Fluctuation of Magnetic Field in Magnetic Nano-Particles Thermometer

In this study, some important factors have been identified for the fluctuation of the magnetic field over time, which provides a basis for improving the temperature measurement accuracy and long-term stability of the magnetic nanoparticles thermometer (MNPT). We improved the mathematical model between the relative error of the magnetic induction intensity and the temperature measurement error of MNPT. The simulation founded that when the relative error of the magnetic induction intensity is kept within the range of <0.01%, the temperature measurement error is less than 0.1 K. Then, the multi-physics (magnetic field, temperature field, and thermal stress field) coupling relationship in the Helmholtz coil is established. During the coupling process, the temperature field of the Helmholtz coil affects the resistance of coil wire, and its thermal stress field is affected by the temperature field, which also changes the coil radius. After the temperature field and thermal stress field change, the magnetic field fluctuates immediately. Further simulation found that the problem of coil temperature rise becomes more serious as the size of the Helmholtz coil increases. The method of forced air cooling can make the fluctuation of the large Helmholtz coil magnetic field < 0.01%, and improve the measurement accuracy of MNPT.

In Situ Study of the Magnetic Field Gradient Produced by a Miniature Bi-Planar Coil for Chip-Scale Atomic Devices.

The miniaturization of quantum sensors is a popular trend for the development of quantum technology. One of the key components of these sensors is a coil which is used for spin modulation and manipulation. The bi-planar coils have the advantage of producing three-dimensional magnetic fields with only two planes of current confinement, whereas the traditional Helmholtz coils require three-dimensional current distribution. Thus, the bi-planar coils are compatible with the current micro-fabrication process and are quite suitable for the compact design of the chip-scale atomic devices that require stable or modulated magnetic fields. This paper presents a design of a miniature bi-planar coil. Both the magnetic fields produced by the coils and their inhomogeneities were designed theoretically. The magnetic field gradient is a crucial parameter for the coils, especially for generating magnetic fields in very small areas. We used a NMR (Nuclear Magnetic Resonance) method based on the relaxation of 131Xe nuclear spins to measure the magnetic field gradient in situ. This is the first time that the field inhomogeneities of the field of such small bi-planar coils have been measured. Our results indicate that the designed gradient caused error is 0.08 for the By and the Bx coils, and the measured gradient caused error using the nuclear spin relaxation method is 0.09±0.02, suggesting that our method is suitable for measuring gradients. Due to the poor sensitivity of our magnetometer under a large Bz bias field, we could not measure the Bz magnetic field gradient. Our method also helps to improve the gradients of the miniature bi-planar coil design, which is critical for chip-scale atomic devices.

Ultracold plasma expansion in quadrupole magnetic field.

We present simulation results of ultracold Sr plasma expansion in a quadrupole magnetic field by means of molecular dynamics. An analysis of plasma evolution influenced by a magnetic field is given. Plasma confinement time behavior under variation of magnetic field strength is estimated. Similarity of the time dependence of the concentration and distribution of ion velocities against the parameters of the plasma and magnetic field is established. Simulation results are in agreement with the experimental ones.

Grating magneto-optical traps with complicated level structures

We study the forces and optical pumping within grating magneto-optical traps (MOTs) operating on transitions with non-trivial level structure. In contrast to the standard six-beam MOT configuration, rate equation modeling predicts that the asymmetric laser geometry of a grating MOT will produce spin-polarized atomic samples. Furthermore, the Landé g-factors and total angular momenta of the trapping transition strongly influence both the confinement and equilibrium position of the trap. Using the intuition gained from the rate equation model, we realize a grating MOT of fermionic 87Sr and observe that it forms closer to the center of the trap’s quadrupole magnetic field than its bosonic counterpart. We also explore the application of grating MOTs to molecule laser cooling, where the rate equations suggest that dual-frequency operation is necessary, but not sufficient, for stable confinement for type-II level structures. To test our molecule laser cooling models, we create grating MOTs using the D 1 line of 7Li and see that only two of the four possible six-beam polarization configurations operate in the grating geometry. Our results will aid the development of portable atom and molecule traps for time keeping, inertial navigation, and precision measurement.

A Fundamental Mechanism of Solar Eruption Initiation in a Multipolar Magnetic Field

Recently, we established a fundamental mechanism of solar eruption initiation in which an eruption can be initiated from a bipolar field through magnetic reconnection in the current sheet (CS) that is formed slowly in the core field as driven by photospheric shearing motion. Here, using a series of fully 3D MHD simulations with a range of different photospheric magnetic flux distributions, we extended this fundamental mechanism to the quadrupolar magnetic field containing a null point above the core field, which is the basic configuration of the classical breakout model. As is commonly believed, in such a multipolar configuration, the reconnection triggered in the CS originated at the null point (namely, the breakout reconnection) plays the key role in eruption initiation by establishing a positive feedback loop between the breakout reconnection and the expansion of the core field. However, our simulation showed that the key to eruption initiation in such a multipolar configuration remains the slow formation of the CS in the sheared core, rather than the onset of fast breakout reconnection. The breakout reconnection only helps the formation of the core CS by letting the core field expand faster, but the eruption cannot occur when the bottom surface driving is stopped well before the core CS is formed, even though the fast reconnection has already been triggered in the breakout CS. This study clarified the role of breakout reconnection and confirmed formation of the core CS as the key to the eruption initiation in a multipolar magnetic field.

Optimization of composite Helmholtz coils towards high magnetic uniformity

Helmholtz coils are widely used in biomedicine, aerospace, and other fields due to their simple structure and remarkable magnetic properties. However, the uniform areas they generate are not suitable for conducting scientific experiments with large equipment. In order to improve the uniformity and scale of the uniform magnetic field, a novel scheme is proposed in this paper. Two auxiliary coils are added on both sides of the Helmholtz coils to optimize the magnetic field. The non-dominated Sorting Genetic Algorithm (NSGA-II) was used to optimize the structural parameters of the improved model, while orthogonal experiments were conducted to obtain initial values for better adaptability of influencing factors. A comparison was made between the magnetic field deviation rates of the coil optimized by traditional derivative algorithms within a range of 0.2 m ∼ 0.8 m from the central axis of the magnetic field. Finally, finite element simulation was employed to verify the calculated results. The results show that the magnetic field deviation rate of the coil is less than 0.5% when the intelligent algorithm is used for optimization in the range of 0.6 m from the central axis. In addition, the structure can be extended to a three-dimensional structure, which can achieve a constant magnetic field in any direction by controlling the size and direction of the current, making it a potential application prospect in magnetic navigation technology.

AC magnetic trap for cold paramagnetic molecules

Strong-field-seeking states are the lowest-energy configurations for paramagnetic molecules in the magnetic field. Molecules in strong-field-seeking states cannot be trapped in a magnetostatic field because a magnetostatic maximum in free space is not allowed. In this paper, we propose an AC magnetic trap composed of two pairs of Helmholtz coils. The spatial magnetic field distribution is numerically calculated and the time-sequential control is depicted. We investigate the influence of the switching frequency and the electric current in the coils on the performance of our trap. Variations of the location and phase-space distribution during a whole switching cycle are simulated. Finally, we study the impact of time during which the field is switched off on the number of captured molecules in a switching cycle.

Improved strength and control over surface-magnetized titania porous structures freeze cast under oscillating magnetic fields

Fabrication processes that produce porous structures with a high strength-to-weight ratio are sought after in many industries. The process of freeze casting is a way to achieve these porous structures with high strength-to-weight ratios, but only in a single direction (the direction of the templating-ice growth). Application of a magnetic field to these structures allows for an increase of mechanical strength in an additional orthogonal direction, therefore allowing them to be applied in more complex loading scenarios. Using a Helmholtz coils setup, it is possible to apply weak, uniform fields (<10 mT) in a variety of directions, magnitudes, or frequencies. Previous research has shown that the application of these weak uniform fields, in particular, oscillating fields from a Helmholtz coils setup, has led to increased mechanical strength through microstructural alignment, but only when using iron oxide structures. To mitigate this, a surface magnetization process was used to increase the magnetic response from non-ferrimagnetic materials, specifically titania, along with a higher magnetic strength (20 mT) Helmholtz coils setup. These surface-magnetized materials and oscillating fields led to an increase of strength of 10x when compared to non-surface-magnetized materials and 2x when compared to surface-magnetized materials under no field due to decreased porosity and increased alignment of mineral bridges. This demonstrates that increased material response for non-ferrimagnetic titania can be induced through the application of an oscillating field in conjunction with the surface-magnetization process.

Creating chiral superstructures in magnetic quadrupole fields.

Creating chiral superstructures in magnetic quadrupole fields.

Bloch point nanospheres for the design of magnetic traps

Through micromagnetic simulations, this work analyzes the possibility of using an array of magnetic nanospheres hosting Bloch points (BPs) to compose a system with the features of a magnetic trap (MT). We show that a BP can be nucleated as a metastable configuration in a wide range of the nanosphere radius compared to a quasi-uniform and vortex state. We also show that the stabilized BP generates a quadrupolar magnetic field outside the nanosphere. Based on this fact, we analyze the field profile of different arrays of these nanospheres, showing that the obtained magnetic field profiles share the needed features to compose MTs. Some highlights of using an array of Bloch point nanospheres in MTs rely on the magnetic field gradients achieved, which are orders of magnitude higher than similar standard systems, and allow three-dimensional trapping. Our results could be useful in trapping particles through the intrinsic magnetization of ferromagnetic nanoparticles while avoiding the commonly used mechanisms associated with Joule heating.