Cylindrical Shield Research Articles

Magnetic field design in a cylindrical high-permeability shield: The combination of simple building blocks and a genetic algorithm

Magnetically sensitive experiments and newly developed quantum technologies with integrated high-permeability magnetic shields require increasing control of their magnetic field environment and reductions in size, weight, power, and cost. However, magnetic fields generated by active components are distorted by high-permeability magnetic shielding, particularly when they are close to the shield’s surface. Here, we present an efficient design methodology for creating desired static magnetic field profiles by using discrete coils electromagnetically coupled to a cylindrical passive magnetic shield. We utilize a modified Green’s function solution that accounts for the interior boundary conditions on a closed finite-length high-permeability cylindrical magnetic shield and determine simplified expressions when a cylindrical coil approaches the interior surface of the shield. We use an analytic formulation of simple discrete building blocks to provide a complete discrete coil basis to generate any physically attainable magnetic field inside the shield. We then use a genetic algorithm to find optimized discrete coil structures composed of this basis. We use our methodology to generate an improved linear axial gradient field, dBz/dz, and a transverse bias field, Bx. These optimized structures generate the desired fields with less than 1% error in volumes seven and three times greater in spatial extent than equivalent unoptimized standard configurations. This coil design method can be used to optimize active–passive magnetic field shaping systems that are compact and simple to manufacture, enabling accurate control of magnetic field changes in spatially confined experiments at low cost.

Hybrid shield for microwave single-photon counter based on a flux qubit

A scenario of shielding and stabilization of magnetic and electromagnetic fields in the measuring volume occupied by a superconducting flux qubit is considered. The qubit is used as an artificial macroscopic atom with discrete energy levels in a counter of single photons of the microwave range. It is shown that a decrease in the amplitude of variations of the external magnetic field inside the 3-layer hybrid cylindrical shield, composed of superconducting and ferromagnetic cylinders with the diameter-to-length ratio of 1:5, provides high stability of the magnetic field. The absolute value of the magnetic field at the sample location is determined mainly by the magnetic flux captured by the superconductor shields during their superconducting transition. Although the magnetic field stability is more important than the field itself for the photon counter, the paper also discusses experimental methods for reducing the absolute field value in the hybrid shield.

Design of Highly Uniform Field Coils Based on the Magnetic Field Coupling Model and Improved PSO Algorithm in Atomic Sensors

The coupling between the coil and the cylindrical magnetic shield (CMS) in atomic sensors not only results in the inconsistency of the coil constant with the theory but also deteriorates the uniformity, which affects the sensor performance. To solve the above deficiencies, a design method of highly uniform field coils inside the CMS based on the coupling model with finite permeability and thickness and particle swarm optimization (PSO) with dynamic inertia weights is proposed for the first time. The coupling model of the coil inside the CMS is established by Green’s function method (GFM). Then it is combined with the nonlinear optimization method, and the coil configuration is optimized by the dynamic inertia weight PSO to obtain the optimal parameters. It not only reduces the calculation error of the model, but also improves the uniformity. Comparing this work with the Lee-Whiting coil inside a CMS, the theoretical uniformity is further improved by 2 orders of magnitude, and measured uniformity errors in the target region reach 1.404×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> , and 2.317×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> , respectively. The correctness and effectiveness are proved by finite element simulation and experimental results. The accuracy and uniformity of coils can be considerably improved, which is beneficial to further improve the measurement sensitivity of instruments related to magnetic field uniformity.

Analysis and Improvement of the Uniformity of Magnetic Field Coil Based on the Cylindrical Magnetic Shield in Atomic Magnetometers

Analysis and Improvement of the Uniformity of Magnetic Field Coil Based on the Cylindrical Magnetic Shield in Atomic Magnetometers

Analytical and experimental studies on 245 kV gas insulated surge arrester

The voltage distribution across gas insulated surge arrester (GISA) is highly non-uniform as compared to conventional porcelain-clad surge arresters. The aim of the present paper is to optimize design of grading shield in such a way that the non-uniformity voltage factor across GISA must be less than ±10%. To achieve this order of non-uniformity factors, special grading shields like window based cylindrical and conical shields are proposed. In the present paper, 245 kV GISA model has been considered to analyze voltage distribution across the metal-oxide (MO) blocks. The electrostatic field and voltage distribution analyses are carried out on surge arrester by considering various shield configurations using numerical techniques. Further, design of shield and development is verified by performing dielectric duties on GISA assembly without MO blocks as per IEC 62,271–203. The lightning and switching impulse discharge current tests were carried out on the developed 245 kV GISA residual voltages were recorded. These voltage levels for above discharge currents are found to be in the order of 2.8 – 3.2 p. u. and are at acceptable levels as per IEC 60,099–4. Finally, protection level or V-I characteristics of GISA is established by measuring residual voltages for the entire current range.

Detection Efficiency of a NaI (Tl) Gamma Spectrometry System for Measurement of Low Level Radioactivity

Qualitative analysis of radionuclides requires the use of reliable gamma-ray detection system. The NaI(Tl) detector has been widely used and still one of the most used detectors today. It is therefore imperative to validate the reliability of the 5x5 cm2 NaI(Tl) gamma spectrometry system used in carrying out gamma-ray analysis of soil samples in the Radiation and Health Laboratory, Federal University of Agriculture Abeokuta, Nigeria. The gamma ray spectrometer is housed in a 5 cm thick cylindrical lead shield. Calibration was executed using standard materials produced under the auspices of the International Atomic Energy Agency (IAEA). Resolution and detection limit (LD) of the detector were determined using full width at half the maximum of the energy peak of 137Cs and background signal level of the reference materials respectively. Counting efficiencies of the detector was calculated using energies of 1460 keV, 1764keV and 2615 keV for 40K, 226Ra and 232Th respectively. Secondary samples, RGMIX1 and RGMIX2 were formulated and counted to calculate activity concentrations using the NaI(Tl) detector. Resolution of the detector was calculated to be 7.8% of 137Cs, which is good for a NaI(Tl) detector. The counting efficiency of the detector is seen to depend on the gamma ray energy. The results from this work shows that the detector system is suitable gamma spectrometry, and will give quality measurements when used for quantitative determination of radionuclides in environmental samples. The efficiency and resolution of the NaI(Tl) detector should also be determined using photon energies obtained from other radioactive sources.

An actively compensated 8 nT-level magnetic shielding system for 10-m atom interferometer.

We design and develop a high-performance magnetic shielding system for a long baseline fountain-type atom interferometer. The shielding system is achieved by a combination of passive shielding using permalloy and active compensation with coils. An 11.4 m-long three-layer cylindrical shield is completed by the process of welding, local annealing, and entire annealing. The active compensations compress the residual magnetic field to 8.0 nT max-to-min and the corresponding gradient below 30 nT/m over 10m along the axial direction in which external compensation, internal compensation, and constant magnetic field (C-field) compensation reduce the inhomogeneities to 25.0, 12.6, and 1.7 nT (standard deviation) sequentially. We estimate that this system could reduce the systematic error of the quadratic Zeeman shift to the 10-13 level for the weak equivalence principle test with a simultaneous 85Rb-87Rb dual-species atom interferometer.

Planar Coil Optimization in a Magnetically Shielded Cylinder

Hybrid magnetic shields with both active field generating components and high-permeability magnetic shielding are increasingly needed for various technologies and experiments that require precision-controlled magnetic field environments. However, the fields generated by the active components interact with the passive magnetic shield, distorting the desired field profiles. Consequently, optimization of the active components needed to generate user-specified target fields must include coupling to the high-permeability passive components. Here, we consider the optimization of planar active systems, on which an arbitrary static current flows, coupled to a closed high-permeability cylindrical shield. We modify the Green's function for the magnetic vector potential to match boundary conditions on the shield's interior surface, enabling us to construct an inverse optimization problem to design planar coils that generate user-specified magnetic fields inside high-permeability shields. We validate our methodology by designing two biplanar hybrid active-passive systems, which generate a constant transverse field, $\mathbf{B}=\stackrel{^}{\mathbf{x}}$, and a linear field gradient, $\mathbf{B}=(\ensuremath{-}x\stackrel{^}{\mathbf{x}}\ensuremath{-}y\stackrel{^}{\mathbf{y}}+2z\stackrel{^}{\mathbf{z}})$, respectively. For both systems, the inverse-optimized magnetic field profiles agree well with forward numerical simulations. Our design methodology is accurate and flexible, facilitating the miniaturization of high-performance hybrid magnetic field generating technologies with strict design constraints and spatial limitations.

Effect of gaps on magnetic noise of cylindrical ferrite shield

Cylindrical ferrite shields consisting of multiple annuli can provide low-magnetic-noise environments. However, the gaps between different parts of the shield adversely affect the shielding performance. In this study, we numerically study the influence of the gaps on both the axial and radial magnetic noise of the multi-annular ferrite shield with different gap widths, permeabilities, and aspect ratios using the generalized Nyquist relation and the finite element method. Our study indicates that as the gap width increases, the axial magnetic noise decreases, while the radial magnetic noise increases. For the shield with a gap width of 0.10 mm and an aspect ratio of one in our study, the axial magnetic noise decreases by 5.2%, and the radial magnetic noise increases by 8.3%–33.5%. The difference in the increase is related to the permeability, and gaps have smaller effect on the radial magnetic noise of the shield with lower permeabilities. Therefore, compared with the no-gap shield, the multi-annular ferrite shield can provide an even better low-magnetic-noise environment along the axial direction. Although the gaps can induce an increase of the radial magnetic noise, this detrimental effect can be suppressed significantly by using a ferrite material with lower permeability and reducing the gap width. Our findings can aid in the application and optimization of low-noise ferrite shields.

3D position and radioactivity estimation of radiation source by a simple directional radiation detector combined with structure from motion

It is important to visualize radioactive substances' position and distribution and estimate their radioactivity levels to reduce the exposure dose of workers in radioactive areas (such as decommissioning worksites of nuclear power stations) and improve nuclear security functions. To visualize the radioactive substance's three-dimensional (3D) location, a directional radiation detector with a cylindrical shield on a simple single-pixel gamma-ray detector was applied to the structure from motion (SfM) technology using an ordinary digital camera. Verification was performed by a system that combines SfM with a CdTe sensor probe having narrow directivity. 241Am radiation source's position was visualized by drawing the radiation source's image acquired by the gamma-ray detector on the work area 3D model reconstructed through SfM. Furthermore, as SfM is a simultaneous localization and mapping technology, the system measures the gamma rays while measuring the gamma-ray detector's dynamic position and posture information. The measurements can be acquired while the gamma-ray detector is freely moving in the work area. These methods visualized the radiation source's position and quantitatively estimated the radiation source's radioactivity.

Bubble Formation by Short Plunging Jet in a Continuous Casting Tundish

A short plunging jet technique was developed to produce small bubbles in continuous casting tundish, with argon sealing, in order to promote the removal of inclusions smaller than 50 μm. The liquid steel coming out of the ladle shroud is accelerated and vibrated by gravity, leading to gas entrainment. This novel approach is free from bubbles growing along the nozzle surface due to the poor wetting condition, which is applicable to producing small bubbles in liquid steel. Water modeling was carried out to investigate the impact of the free-fall length on gas entrainment by a short plunging jet. The results show that gas can be entrained into the liquid bath with a free fall longer than 15 mm. Part of the entrained gas is separated from the gas sheath by the rough surface of the inflow stream, forming initial bubbles. These initial bubbles are further refined into small ones of 0.4~2.5 mm due to the turbulent flow in the pouring region. The cylindrical shield can effectively isolate the surface fluctuation caused by the short plunging jet; thereby, a stable slag layer in the tundish can be maintained during gas entrainment.

A 20 m wide-field diffraction-limited telescope.

A 20 m space telescope is described with an unvignetted 1° field of view—a hundred times larger in area than fields of existing space telescopes. Its diffraction-limited images are a hundred times sharper than from wide-field ground-based telescopes and extend over much if not all the field, 40 arcmin diameter at 500 nm wavelength, for example. The optical system yielding a 1°, 1.36 m diameter image at f/3.9 has relatively small central obscuration, 9% by area on axis, and is fully baffled. Several carousel-mounted instruments can each access directly the full image. The initial instrument complement includes a 400 gigapixel silicon imager with 2 µm pixels (0.005 arcsec), and a 60 gigapixel HgCdTe imager with 5 µm pixels (0.012 arcsec). A multi-object spectrograph with 10 000 fibres will allow spectroscopy with 0.02 arcsec resolution. Direct imaging and spectroscopy of exoplanets can take advantage of the un-aberrated, on-axis image (5 nm RMS wavefront error). While this telescope could be built for operation in free space, a site accessible to a human outpost at the Moon's south pole would be advantageous, for assembly and repairs. The lunar site would allow also for the installation of new instruments to keep up with evolving scientific priorities and advancing technology. Cooling to less than 100E K would be achieved with a surrounding cylindrical thermal shield.This article is part of a discussion meeting issue ‘Astronomy from the Moon: the next decades’.

Optimal Inverse Design of Magnetic Field Profiles in a Magnetically Shielded Cylinder

Magnetic shields that use both active and passive components to enable the generation of a tailored low-field environment are required for many applications in science, engineering, and medical imaging. Until now, accurate field nulling, or field generation, has only been possible over a small fraction of the overall volume of the shield. This is due to the interaction between the active field-generating components and the surrounding high-permeability passive shielding material. In this paper, we formulate the interaction between an arbitrary static current flow on a cylinder and an exterior closed high-permeability cylinder. We modify the Green's function for the magnetic vector potential and match boundary conditions on the shield's interior surface to calculate the total magnetic field generated by the system. We cast this formulation into an inverse optimization problem to design active--passive magnetic field shaping systems that accurately generate any physical static magnetic field in the interior of a closed cylindrical passive shield. We illustrate this method by designing hybrid systems that generate a range of magnetic field profiles to high accuracy over large interior volumes, and simulate them in real-world shields whose passive components have finite permeability, thickness, and axial entry holes. Our optimization procedure can be adapted to design active--passive magnetic field shaping systems that accurately generate any physical user-specified static magnetic field in the interior of a closed cylindrical shield of any length, enabling the development and miniaturization of systems that require accurate magnetic shielding and control.

A Monte Carlo study on the gamma-ray buildup factors for the linear sources embedded in a cylindrical shield

While the studies on the photon buildup factors for point gamma sources have been widely made, studies for linear gamma sources, for which the spent nuclear fuel rods and/or brachytherapy seeds could be a relatively good approximation, have become extremely scarce. Buildup factors for linear gamma sources are computed here by means of the MCNPX Monte Carlo code. The exposure buildup factors were calculated for isotropic linear gamma sources with different lengths (1, 10, and 100 cm) and also for different energies (0.1, 0.5, 1, 5, and 10 MeV) in a cylindrical concrete shield up to depths of 10 mean free paths. A number of 45 simulations were done to reach the appropriate results for the linear gamma sources and 5 more simulations were performed for point gamma sources to be compared with the linear sources of photons at the same energies. For validation, we calculated the total mass attenuation coefficients, mass energy absorption coefficients, and uncollided photon flux for some photon energy in concrete and we showed that the simulation results have a good agreement with NIST-XCOM database and theory. Finally, comparisons are shown for the exposure buildup factors of the linear source in terms of the gamma-rays energy, of the length of the linear source and of the cylindrical shield, which can be useful in the design of a suitable shield.

Analyses and verifications of magnetic shielding of long pipelines aiming for pipeline orientation measurements

So far, none of conventional magnetic shielding models can model a field long pipeline consisting of many welded short sections with different permeabilities. This paper demonstrates the magnetic shielding model of long pipelines via simulation analyses based on the finite element method (FEM) and experimental verifications of the magnetic shielding of short pipes, torus pipes, and field long pipelines. The radial and axial shielding factors of a finite pipe can be accurately calculated by the FEM. As the pipe length increases, the axial factor rapidly converges to a value less than 1, and the convergence value is independent of the pipe length because of the magnetic charges at the two ends. For a long pipeline consisting of many short sections, the averaged shielding factors over a couple of adjacent sections are equal to that of an ideal infinite pipeline, and the axial factor is always equal to 1. An “average” strategy for field pipeline orientations that employs the averaged magnetic fields and shielding factors over several sections is then proposed and demonstrated. As the geomagnetic field is taken as the absolute reference, this orientation measurement method has no risk of divergence over a long time and distance. Index Terms—cylindrical magnetic shield, magnetic fields, pipeline orientation.

Magnetic field enhancement technique in the fluid flow gap of a single coil twin tube Magnetorheological damper using magnetic shields

Smart dampers in the automobile suspension system bring a precise balance between the ride comfort and stability through a controllable damping coefficient. Energy absorbed by a Magnetorheological (MR) damper is a dependent function of flux density in the fluid flow gap. In this paper, magnetic field enhancement technique in the form of a single cylindrical shield and sandwich cylindrical shield is incorporated in a twin tube single coil MR damper. The field strength in different configurations of MR damper having various type of shield configuration is computationally investigated. Further, the effect of shield thickness on field strength is investigated. A significant overall improvement in the magnetic field strength is observed in the MR damper configuration having copper alloy shield.

Numerical and Workbench Design of 2.35 T Double-Tuned (¹H/²³Na) Nested RF Birdcage Coils Suitable for Animal Size MRI.

The birdcage Radio Frequency (RF) coil is one of the most used configurations in Magnetic Resonance Imaging (MRI) scanners for the detection of the proton (1H) signal over a large homogeneous volume. More recently, birdcage RF coils have been successfully used also in the field of X-nuclei MRI, where the signal of a second nucleus (e.g. 13C, 23Na, 31P, and many others) needs to be detected with high sensitivity and spatial homogeneity. To this purpose several technical solutions have been adopted to design Double Tuned (DT) volume RF coils, including the recent configuration of the nested birdcage RF coils. One of the main problems in the design of DT RF coils is the decoupling between the 1H and X channels, and a number of solutions have been adopted over the years. In this work, based on numerical and workbench methods, we report the decoupling optimization of DT (1H/23Na) nested RF birdcage coils suitable for 2.35 T MRI scanners encompassing an inner Low-Pass (LP) birdcage used for X-nuclei, an outer High-Pass (HP) birdcage for 1H and an external cylindrical RF shield. We show that a suitable geometrical selection of the two coaxial RF birdcage coils (relative angular orientation, diameters and lengths) and RF shield (diameter, length) allows a significant decoupling optimization. We also provide valuable information about the RF B1+ field homogeneity and efficiency. Our approach was validated both with numerical simulations and workbench testing using DT nested RF coil prototypes.

Approximate expressions for the magnetic field created by circular coils inside a closed cylindrical shield of finite thickness and permeability

The common approach to create a uniform magnetic field in the μT range is placing a coil set inside a magnetic shield. One of the main obstacles to achieving the requested uniformity can be a field distortion caused by the shield. We derive an approximate expression for the magnetic field generated by circular coils enclosed in ferromagnetic cylinders, the most used shield geometry. The main advantage of the expression over the known approximation is considering the finite permeability and the finite thickness of the cylindrical shield. To model the cylinder end caps, we propose the finite thickness mirror method, which extends the method of mirror image to the case of finite thickness. The cylinder barrel is modeled as an infinite tube and its effect on the field is analytically solved. The error of the expression and its dependency on the observation point and the shield geometry are discussed. Compared with finite-element simulations our analytic method is consistent within a relative error of less than 0.3% for practical shielding setups. The expression enables the rapid optimization of shield-coupled coil sets with a custom-designed objective function. To show the applicability of our method, we present simulations for the direct selection of the optimum spacing of a coil pair to match the given dimensions of a cylindrical shield.

Local Accumulation of Ferromagnetic Particles by Controlling Frequency of Rotating Magnetic Field

A novel cancer treatment with low side effect and less invasiveness by blocking newborn blood vessels around the diseased part was proposed. Ferromagnetic particles administered into the body are accumulated in the newborn blood vessels using a rotating magnetic field (RMF), and then are aggregated to block the blood vessels. We proposed a system consisting of four superconducting magnets and a cylindrical magnetic shielding material with a slit to apply the RMF to the human body. The purpose of this study was to accumulate particles locally within a spherical range of 10 mm in diameter. The results of fundamental experiments with neodymium magnets and particle trajectory calculations showed that the range of particle accumulation could be controlled by adjusting the frequency of the RMF.

Design and Optimization of Multilayer Cylindrical Magnetic Shield for SERF Atomic Magnetometer Application

A set of five-layer cylindrical magnetic shields is designed and constructed to attenuate external geomagnetic field to less than 10 nT (ideally zero). We aim to achieve a shield design having high shielding factor, low magnetic noise, and small volume. However, magnetic noise is incompatible with shielding factor and volume. Thus, there is a compromise. This paper presents an optimization method to acquire a sufficiently effective, low-noise, and lightweight multi-layer shield. The optimization objective and constraints are provided in the form of analytic expressions. The mathematical model is constructed in MATLAB. The optimization is executed in iSIGHT software integrated MATLAB. By optimizing the nonlinear programming quadratic line (NLPQL) algorithm, theoretically the axial shielding factor SAtot can be increased by a factor of about 390.3, on the premise that the magnetic noise and volume have increased negligibly. Apart from better shielding performance, a combination of optimum parameters is obtained. This optimization method has good universality. As long as certain shielding requirements are determined, the corresponding optimum parameters can be acquired accurately. This method is of great significance for future improvements to shield design.