Helmholtz Coils Research Articles

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.

HCS background magnetic field study HYTARO+

Context. HYTARO+ is an analytical model developed to evaluate the influence of the solar magnetic dipole in the topology of the local structure of the heliospheric current sheet (HCS). Aims. Statistical methods such as MVA, which are typically used in the study of the HCS come with two limitations. On one hand, they do not provide any information about the physical properties of the current sheet and, on the other, they do not offer the possibility of defining the local structure of the HCS from the physical conditions that prevail in the two sectors that separate the sheet. HYTARO model describes the interaction between the magnetic field that defines the structure of the current sheet and the plasma confined within it. HYTARO+ goes deeper into the study of the background magnetic field that is included in the HYTARO model with the aim of identifying its source. Methods. Multipole expansion is used to provide a general approach to any physical phenomenon of some system, thereby allowing for a potential function representation. In our case, we considered the dipolar and quadrupolar Sun magnetic field contribution in the HCS. Results. In the development of the HYTARO+ model, we estimated the components of the dipole and quadrupole magnetic fields at 1 AU. In the present study, we establish that the Z component of the dipole field and the X component of the quadrupole field are the more relevant terms of the background field present in the data that correspond to the discontinuities. A summary of the contribution of the multipolar components along the 23 solar cycle is incorporated.

The Enhancement of Microstructures and Mechanical Characteristics for Sand Casting A357 Alloys with Magnetic Fields by Helmholtz Coils

The Enhancement of Microstructures and Mechanical Characteristics for Sand Casting A357 Alloys with Magnetic Fields by Helmholtz Coils

Simulation validation based on emission coil of three-axis square Helmholtz magnetic source

In electromagnetic detection, the received signal strength is largely determined by the transmitter’s signal strength. Therefore, it is necessary to select and design the transmitting coil to meet the requirements of the electromagnetic launching system. To obtain better-transmitting coils, Maxwell is used to establishing square and toroidal coil models for calculation and compare magnetic field intensity and uniformity. To cope with electromagnetic exploration in a complex environment, the electromagnetic field model of the three-axis toroidal coil and square Helmholtz coil is established. The magnetic field simulation results verify that the three-axis square Helmholtz coil has higher emission field intensity under the same excitation condition, which is suitable for electromagnetic exploration.

Damped oscillation of a magnetically-trapped superconducting micro-particle in superfluid helium: measurement of viscosity based on a hydrodynamic analysis

We have developed a trap of a single superconducting micro-particle using a quadrupole magnetic field in superfluid helium. By displacing the particle from a stable static position and then releasing it, the particle was observed to oscillate around the stable position. Analyzing the damped oscillation, we have demonstrated the measurement of the viscosity of superfluid helium.

Multiwavelength observations of PSR J2021+4026 across a mode change reveal a phase shift in its X-ray emission

Context. We have investigated the multiwavelength emission of PSR J2021+4026, the only isolated γ-ray pulsar known to be variable, which in October 2011 underwent a simultaneous change in γ-ray flux and spin-down rate, followed by a second mode change in February 2018. Multiwavelength monitoring is crucial to understand the physics behind these events and how they may have affected the structure of the magnetosphere. Aims. The monitoring of pulse profile alignment is a powerful diagnostic tool for constraining magnetospheric reconfiguration. We aim to investigate timing or flux changes related to the variability of PSR J2021+4026 via multiwavelength observations, including γ-ray observations from Fermi-LAT, X-ray observations from XMM-Newton, and a deep optical observation with the Gran Telescopio Canarias. Methods. We performed a detailed comparison of the timing features of the pulsar in γ and X-rays and searched for any change in phase lag between the phaseogram peaks in these two energy bands. Although previous observations did not detect a counterpart in visible light, we also searched for optical emission that might have increased due to the mode change, making this pulsar detectable in the optical. Results. We have found a change in the γ-to X-ray pulse profile alignment by 0.21 ± 0.02 in phase, which indicates that the first mode change affected different regions of the pulsar magnetosphere. No optical counterpart was detected down to g′ = 26.1 and r′ = 25.3. Conclusions. We suggest that the observed phase shift could be related to a reconfiguration of the connection between the quadrupole magnetic field near the stellar surface and the dipole field that dominates at larger distances. This is consistent with the picture of X-ray emission coming from the heated polar cap and with the simultaneous flux and frequency derivative change observed during the mode changes.

A calibration method of ground-airborne frequency domain electromagnetic receving system based on magnetic source excitation

In the ground-airborne frequency domain electromagnetic detection, the receving system converts the magnetic field signal into an electrical signal representing the magnetic field signal. In order to accurately obtain the relationship between the magnetic field signal and the electrical signal, it is necessary to avoid the influence of environment and other factors on the system accuracy through on-site calibration. This requires high calibration accuracy, simple operation, portable equipment and strong adaptability to the environment. However, the existing calibration methods put forward higher requirements for instruments and environment. For example, the calibration of uniform magnetic field generated by Helmholtz coil needs to shield external magnetic field interference. In this paper, an overall calibration method of measurement system based on magnetic source excitation is proposed to meet the requirements of electromagnetic field accurate measurement in ground-airborne frequency domain. Firstly, based on the electromagnetic measurement method in ground-airborne frequency domain, the overall calibration method and principle of the detection system are introduced. Then, based on the computable standard magnetic field generated by the magnetic source transmitting coil, determine the measured voltage obtained by the receiving system, get the calibration coefficient of the receiving system. At last,the source of standard uncertainty related to calibration is analyzed, and the uncertainty is evaluated. In addition, the method is verified by comparing the calibration results with those obtained by Braunbeck coil calibration.

Determination of the critical contact separation length for DC interruption in a vacuum subject to an external transverse magnetic field

The objective of this work is to experimentally determine the critical contact separation length Lcritical for DC interruption in vacuum under an external transverse magnetic field (TMF), according the interruption behaviors and vacuum arc characteristics. L is defined as the separation length at the instant of TMF application. Lcritical is the critical and the minimal necessary separation length of the contacts for a successful DC interruption in vacuum. A pair of Helmholtz coils was used to generate the external TMF. A high-speed camera was used to record the vacuum arc appearance. The experimental results quantitatively determine Lcritical for DC vacuum interruption under a certain DC and the external TMF. When the DC ranged from 100 to 500 A under an external TMF of 30 mT, Lcritical ranged from 2.1 to 3.7 mm and did not show linear variation. Second, the extinguishing time of DC vacuum interruption te at various currents significantly increased in a similar pattern under decreasing L in the range of 0.3–1.6 ms. The decrease in L mainly increased the duration of the stable stage and had little apparent influence on the duration of the unstable stage of the DC vacuum arc. Third, the physical mechanism determining Lcritical in DC vacuum interruption under a TMF could be explained from two aspects: the expansion of the vacuum arc column and the diffusion of the metal plasma. In successful DC vacuum interruption, Lcritical should be large enough for the formation and expansion of the “C”-shaped arc column, and enough space should be available for the diffusion of the metal plasma.

Magnetically threaded thin discs in the presence of the quadrupole magnetic field

ABSTRACT Neutron stars might have multipole magnetic fields as implied by recent observations of pulsars. The presence of the quadrupole field might have an effect on the interaction between the disc and the neutron star depending on the location of the inner radius of the disc, and the strength of the quadrupole field. For a quadrudipole stellar field, we calculate the toroidal field generated within the disc, the magnetospheric radius, and the torque exerted on to the star. Also, we deduce the effect of the rotation of the star on the magnetospheric radius which is relevant even for pure dipole magnetic fields.

An efficient magnetic approach for manipulating dispersed non-magnetic particles on surfaces and microchannels

Recently, there has been significant interest in understanding the behavior of magnetic particles in aqueous media under external excitations. The dynamics and self-organization of these systems, especially in the presence of time-dependent excitations, have been extensively studied due to their relevance in both fundamental physics and technological applications. In this study, we employ three Helmholtz coils to generate elliptically polarized fields. This configuration is utilized to manipulate micrometer-sized nickel and iron particles located at the bottom of a container filled with an aqueous medium. By analyzing video-microscopy images, we investigate the time-dependent displacement of these particles. Notably, we observe the formation of chain structures known as “rollers”, whose movement on the vessel’s bottom plane can be controlled through the applied magnetic field. Furthermore, we demonstrate that the introduction of glass microspheres results in induced motion due to the displacement and drag exerted by the magnetic structures. Exploiting this effect, we successfully control the movement of glass beads along microchannels. Our findings suggest potential applications for targeted cargo delivery using this simple approach.

Magnetohydrodynamics simulation of magnetic flux rope formation in a quadrupolar magnetic field configuration

Magnetic flux ropes (MFRs) play an important role in high-energetic events like solar flares and coronal mass ejections in the solar atmosphere. Importantly, solar observations suggest an association of some flaring events with quadrupolar magnetic configurations. However, the formation and subsequent evolution of MFRs in such magnetic configurations still need to be fully understood. In this paper, we present idealized magnetohydrodynamics (MHD) simulations of MFR formation in a quadrupolar magnetic configuration. A suitable initial magnetic field having a quadrupolar configuration is constructed by modifying a three-dimensional linear force-free magnetic field. The initial magnetic field contains neutral lines, which consist of X-type null points. The simulated dynamics initially demonstrate the oppositely directed magnetic field lines located across the polarity inversion lines (PILs) moving towards each other, resulting in magnetic reconnections. Due to these reconnections, four highly twisted MFRs form over the PILs. With time, the foot points of the MFRs move towards the X-type neutral lines and reconnect, generating complex magnetic structures around the neutral lines, thus making the MFR topology more complex in the quadrupolar configuration than those formed in bipolar loop systems. Further evolution reveals the non-uniform rise of the MFRs. Importantly, the simulations indicate that the pre-existing X-type null points in magnetic configurations can be crucial to the evolution of the MFRs and may lead to the observed brightenings during the onset of some flaring events in the quadrupolar configurations.

Runaway electron mitigation with pulsed localized vertical magnetic field perturbation in ADITYA tokamak

To reduce the risk of severe damage to the vessel and inner peripherals of any tokamak and its safe operation, a robust technique for the mitigation of runaway electrons (REs) is required. The REs in ADITYA tokamak are effectively mitigated by an application of local vertical magnetic field (LVF) perturbation. The LVF perturbation is applied using a pair of electromagnetic coils placed at the top and bottom of the ADITYA vacuum vessel in a Helmholtz configuration at one toroidal location. Powered by a capacitor bank power supply, these coils can produce a localized vertical magnetic field at the plasma center in the range of ∼150 G–260 G for a variable duration of 5–20 ms. The LVF pulse is first applied at the breakdown/current-ramp phase, where the REs are generated in the discharges initiated by the conventional ohmic breakdown in ADITYA. With the application of LVF pulse the REs are significantly reduced as indicated by the reduction in the REs generated hard x-ray flux. It has been observed that to extract the REs efficiently, an LVF pulse of magnitude at least ∼1% of the toroidal magnetic field with a minimum duration of ∼5 ms should be applied. The LVF perturbation is applied at different times into the discharge, i.e. during the breakdown/current ramp-up phase and current flat-top phase. The REs are significantly reduced in all the phases and improved discharge consistency. The LVF acts as an error field and a short-pulse of the LVF influences the REs more in comparison to the thermal electrons due to the faster velocities of the REs.

An alternating current calibration method for Helmholtz coil constant based on orthogonal calculation principle

An alternating current calibration method for Helmholtz coil constant based on orthogonal calculation principle

Decomposition of the induced magnetism degaussing problem for fast determination of currents in demagnetization coils wrapped outside an object under arbitrary external field conditions

Safe passage of ships in the presence of sea mines can be ensured by limiting or reducing the ship’s magnetic footprint. For vessels with plastic hulls, the main component that requires magnetic damping is the engine. Demagnetization of such an object can be achieved by wrapping it with coils and setting the direct current appropriately. For each specific geographic location, the currents in the coils can be determined iteratively from measurements of the magnetic signature in the cardinal directions. In this paper the magnetic signatures are calculated using decomposition-based approach for each coil and each component of the external field separately. Hence the overall magnetic signature of the object can be reproduced in arbitrary external magnetic field (i.e. anywhere on the Earth). Knowing the influence of each coil, it is possible to formulate the optimization task (signature minimization) and determine the currents. The presented method is verified in FEM software with the use of engine models of both symmetrical and asymmetrical shapes. Since the determination of the currents takes place as a result of solving the optimization problem, the effectiveness of obtaining the results, the speed of convergence and the dependence on initial conditions is under investigation. The effect of the model mesh size on the quality of object signature reduction is also analyzed. The developed method can be used for a real object. In that case acquiring the data then requires the measurements of the object placed inside the Helmholtz coils.

Design and Testing of a Radiofrequency Ellipsoidal Helmholtz Coil in Contrast with a Circular Pair for Nuclear Magnetic Resonance

The design and testing of a Helmholtz prototype probe with a relatively homogeneous radiofrequency field that can be used for MRI are presented in this paper. It consists of two coaxial separately tuned ellipsoidal rings of wire in a symmetric arrangement. The developed ellipsoidal structure is tested experimentally in free space and is compared with the results obtained with an equivalent circular standard Helmholtz coil. Complete electrical modeling of the coils taking into account all couplings (inductive and capacitive) is studied. The proposed ellipsoidal Helmholtz coil provides better performance in terms of field homogeneity, efficiency, and quality factor. The width of the lobe at 10% for the ellipsoidal coil is 20% bigger. Moreover, a significant improvement in the quality factor is observed in free space. Also, the efficiency is increased by 37%. Finally, the Signal to Noise Ratio (SNR) of the ellipsoidal coil is higher than that of the circular coil.

Magnetic Interference Compensation Using the Adaptive LMS Algorithm

The aim of this study is to evaluate the method for the compensation of magnetic field interference from the mains, which consists primarily of components with a frequency of 50 (60) Hz and its multiples. The setup compensates for the interfering field using three Helmholtz coils, magnetic field sensors, a digital processing unit, and power amplifiers. The processing unit implements an adaptive least-mean-square (LMS) algorithm to calculate the currents through coils that are necessary for compensation. The prototype fully compensates for interfering with the magnetic field at 50, 150, 250, and 350 Hz down to the noise level of the sensor arrangement within 20 s and reduces higher harmonics. The overall reduction in the rms value of the interfering field is better than 28 dB, and a significantly better reduction is achieved in the frequency range from 0 Hz to 400 Hz.

Optimization Design Method of Spherical Magnetic Field Generation Coil Based on Differential Evolution Algorithm

In a coil magnetometer, the size and uniformity of the bias magnetic field generated by the Helmholtz coil directly determine the accuracy of the solution of the geomagnetic direction. The design of traditional spherical coils relies heavily on the manual experience or mathematical derivation, making it difficult to obtain optimal parameters or requiring larger spherical coils. To address the problem, first, a coaxial symmetrical spherical coil model that improves space utilization was established. Second, an optimal design method for the spherical magnetic field generation coil based on a differential evolution algorithm was proposed. Third, the optimal bias magnetic field was obtained without increasing the volume of the coil. The verification results showed that the magnetic non-uniformity and magnetic gradient of the bias field generated by the optimized coil were reduced by 63.2% and 82.8%, respectively.

Magnetostatic interaction between Bloch point nanospheres

Three-dimensional topological textures have become a topic of intense interest in recent years. This work uses analytical and numerical calculations to determine the magnetostatic field produced by a Bloch point (BP) singularity confined in a magnetic nanosphere. It is observed that BPs hosted in a nanosphere generate magnetic fields with quadrupolar nature. This finding is interesting because it shows the possibility of obtaining quadrupole magnetic fields with just one magnetic particle, unlike other propositions considering arrays of magnetic elements to generate this kind of field. The obtained magnetostatic field allows us to determine the interaction between two BPs as a function of the relative orientation of their polarities and the distance between them. It is shown that depending on the rotation of one BP related to the other, the magnetostatic interaction varies in strength and character, being attractive or repulsive. The obtained results reveal that the BP interaction has a complex behavior beyond topological charge-mediated interaction.