Magnetic shielding behavior of YBCO cylinders— A universal scaling property

Magnetic shielding studies have been carried out to investigate the usefulness of high-T _c YBCO ceramics for EMS applications. The starting materials were single-phase YBCO powder. The cylinders were prepared by uniaxial compresssion and isostatic compression, with and without Ag additive. The shielding fieldH ^*, defined as the value of the applied field at which a detectable field will be seen inside the superconducting shield, was 12 G for the shield prepared by uniaxial compression and around 17 G for shields prepared by isostatic compression. Measurements were done in both ac and dc fields. The field inside the shield for the sinusoidal ac applied field is highly distorted, showing the presence of three higher harmonics. The penetrated waveform was Fourier-analyzed to yield the rms value ofB _in. When the normalized value ofB was plotted against the normalized value ofH with respect toH ^*, all the points corresponding to different shields fall on the same curve.     

Performance of Whipple shields at impact velocities above 9 km/s

The results of 18 impact tests performed on Whipple shields were compared to the predicted ballistic limits of the shields in the region where the impact velocity of the threatening particle was high enough to produce melting and incipient vaporization of the particle. Ballistic limit equations developed at NASA Johnson Space Center were used to determine nominal failure thresholds for two configurations of all-aluminum Whipple shields. In the tests, 2017-T4 aluminum spheres with diameters ranging from 1.40 to 6.35 mm were used to impact the shields at impact velocities ranging from 6.94 to 9.89 km/s. Two different aluminum alloys were used for the rear walls of a simple Whipple shield. The results of 13 tests using these simple Whipple shields showed they offered better-than-predicted capability as impact velocity increased and that the strength of the rear wall material appeared to have a smaller-than-predicted effect on the shield performance. The results of five tests using three configurations of a scaled Space Station shield - a plain shield at 0 degrees, two shields with multilayer insulation in the space between the bumper and the rear wall (also at 0 degrees), and two tests with the plain shield at 45 degrees obliquity - showed that these shields met their predicted capabilities.     

Magnetic shielding using high-temperature superconductors

Magnetic shields of various high-temperature superconductors, YBa₂Cu₃O_7−x (YBCO), YBa₂Cu₃O_7−x -Ag composites (random inclusions as well as non-random coatings) and Bi₂Sr₂Ca₂Cu₃O _x (BSCCO) were prepared by uniaxial as well as isostatic compression with various dimensions. The shielding properties were measured at 77 K for dc and ac magnetic fields in the range of frequencies from 100 Hz to 10 kHz. The critical penetration field (CPF), defined as the value of the applied magnetic field at which a detectable field was observed inside the cylinder, varied from cylinder to cylinder and also with the ageing of the cylinders in the case of YBCO shields. The highest value of CPF was 16 G at 77 K for YBCO shield prepared by isostatic compression. Even though the stability of BSCCO shields with respect to ageing is good, the CPF values are very low compared to those for YBCO. Detailed studies were performed in the case of YBCO shields. The CPF decreased as a function of time over a period of 90 days. The CPF decreased as the frequency of the applied field was increased. The wave form of the field inside the pot for a sinusoidal applied field was highly distorted and showed the presence of higher harmonics with appreciable amplitude. The wave form was Fourier-analysed to yield the field inside the shield along with the harmonics. The shields with Ag addition seem to give better performance at high fields.     

Shielding Effectiveness Measurements for Ferromagnetic Shields

This paper proposes a new way to measure the shielding effectiveness of ferromagnetic shields. The procedure combines an experimental characterization of the shielding material and numerical simulations. In a first case, magnetic hysteresis is reduced to a series of equivalent $B$ –$H$ linear relations through an optimization procedure that is applied to a measured set of symmetric minor loops, and an equivalent multilayer linear medium is defined through a subsequent iterative procedure. In a second case, magnetic hysteresis is reduced to a simple $B$–$H$ nonlinear relation, obtaining an equivalent nonlinear shield. In both cases, the obtained results are compared with the direct measurements of the shielding effectiveness at different operating frequencies, showing good agreement and the validity of the proposed method. A comparison and a discussion on the harmonic content of the attenuated field for the investigated shield geometry are reported. The same analysis is carried out for a basic diffusion problem, comparing the results with those obtained by the use of a Preisach model.      

应用于铯喷泉钟的磁屏蔽系统的设计

高质量磁屏蔽系统是铯喷泉钟的重要部件之一。首先给出评定磁屏蔽效果的3个指标,即磁场均匀区长度、磁场均匀区起点位置和磁屏蔽效率。在此基础上,讨论了利用有限元计算3层磁屏蔽系统的层间径向间距、轴向间距、端盖孔套管长度和端盖连接方式对磁屏蔽的影响,给出了相应的优化参数,并对比了按优化参数设计的3层磁屏蔽与原有4层磁屏蔽系统用有限元计算的磁屏蔽效果。有限元计算结果为中国计量科学研究院新铯喷泉钟磁屏蔽系统的技术设计提供了理论指导。     

Cryogenic design and operation of liquid helium in an electron bubble chamber towards low energy solar neutrino detectors

We are developing a new cryogenic neutrino detector: electron bubble chamber, using liquid helium as the detecting medium, for the detection of low energy p-p reaction neutrinos (<420 keV), from the Sun. The program focuses in particular on the interactions of neutrinos scattering off atomic electrons in the detecting medium of liquid helium, resulting in recoil electrons which can be measured. We designed and constructed a small test chamber with 1.5 L active volume to start the detector R&D, and performed experimental proofs of the operation principle. The test chamber is a stainless steel cylinder equipped with five optical windows and ten high voltage cables. To shield the liquid helium chamber against the external heat loads, the chamber is made of double-walled jacket cooled by a pumped helium bath and is built into a LN₂/LHe cryostat, equipped with 80 K and 4 K radiation shields. A needle valve for vapor helium cooling was used to provide a 1.7-4.5 K low temperature environments. The cryogenic test chamber has been successfully operated to test the performance of Gas Electron Multipliers (GEMs) in He and He + H₂ at temperatures in the range of 3-293 K. This paper will give an introduction on the cryogenic solar neutrino detector using electron bubbles in liquid helium, then present the cryogenic design and operation of liquid helium in the small test chamber. The general principles of a full-scale electron bubble detector for the detection of low energy solar neutrinos are also proposed.     

Improvement of YBCO Superconductor Magnetic Shielding by Using Multiple Bulks

HTS bulks present a high critical current density which can be used as magnetic shields. Previous works showed that BSCCO bulks can screen magnetic fields up to 0.1 T. For large scale applications like electrical machines, stronger magnetic field is usually needed. In so doing, (RE)BCO materials are more suitable since they can shield much higher magnetic fields. Another key issue concerns the size of the bulks. Nowadays, it is possible to manufacture 150-mm diameter class cylindrical YBCO bulk. In order to get larger magnetic shielding areas, multiple bulk superconductors should be arrayed and stacked in layers. This paper presents experimental results on screening performances of layered YBCO pellets. These results are compared with 2D simulations. The experiments are carried out at 77 K under external magnetic fields of 150 mT. Different topologies are considered: single-layer configuration with 9 square pellets and double-layer configuration with respectively 9 and 4 square pellets. Experimental and simulation results show that a checkerboard of one layer configuration does not fully shield the external magnetic field. Improved shielding properties are obtained when the double-layer configuration is used.     

Design, development and experiment of a 1.5 T MgB2 superconducting test magnet

Superconducting magnets using MgB₂ tapes are potentially applicable in many areas, such as medical magnetic resonance imaging and fault current limiting. Under conduction cooling environments, the magnets can work at 15-20 K. In this work, a solenoid structured magnet with ∅ 100 mm bore is designed, built and tested. The maximum field at its center is up to 1.5 T. The field homogeneity, the thermal stability and the quench characteristics in the magnet are also investigated.     

Optimizing active and passive magnetic shields in induction heating by a genetic algorithm

This paper presents a method for designing optimal passive and active shields for axisymmetric induction heaters. Such shields are needed to protect human operators and external electronic equipment from stray magnetic fields. The method uses a genetic algorithm (GA) to minimize an objective function. This function reduces the magnetic field in the target area, the power dissipation in the active and passive shields, and the influence of the shields on the heating process. The GA returns the position and height of the passive shield, the optimal current for the active shield, and the number of turns of all coils. The paper describes two optimization modes: 1) optimization of only the active shield with fixed passive shield and 2) global optimization of both active and passive shields. Several passive shields are studied: electrically conductive shields and both electrically and magnetically conductive shields. The field reduction depends on the optimization mode and the passive shield properties, but always exceeds 25 dB for combined active and passive shields. Finally, the paper compares the results of the simulations to experimental measurements.     

Observations on dual-ended readout of 100 mm long LYSO crystals

We are investigating using dual-ended readout of axially oriented long thin scintillator crystals in detectors for a compact geometry, small ring diameter animal PET system. The axial position of interaction is determined from the light sharing between two photodetectors at opposite ends of the crystal. We examine the light output, energy resolution and axial spatial resolution of 1.5-5×2×100 mm³ polished LYSO crystals by irradiating with an electronically collimated beam of 511 keV photons oriented perpendicular to the long axis and read out at either end by position sensitive photomultiplier tubes (PSPMTs). Three reflector materials, namely Teflon, 3 M enhanced specular reflector (ESR) and black paint are examined for the 2×2×100 mm³ crystal size. The light output increases and energy resolution improves with the crystal cross-section. Generally, the spatial resolution worsens with increase in crystal cross-section. For the 2×2×100 mm³ crystal size, the mean energy resolutions of the photopeak over the nine irradiation positions were 14.4±0.4%, 16.0±1.2% and 28.3±2.1% with mean spatial resolutions of 7.0±1.0, 9.4±3.3 and 26.0±5.0 mm using ESR, Teflon and black paint, respectively. ESR reflector gave the best light output, energy and axial spatial resolutions. These characterization results of PSPMT-based dual-ended long LYSO crystals will be useful in the design of detector modules for a highly compact geometry preclinical PET system using this detector technology.     

An accurate method of calculating multilayer, multimaterial cylindrical magnetic shields

The basic concepts of shielding theory have existed since the last century [1,2]. There have been many publications on the subject of magnetic shielding, treating the case of shielding apparatus from static fields by means of multiple concentric shields and deriving several principles of fundamental importance. Unfortunately, however, theory has been applied to only the most ideal shield configurations, for the case of constant permeability [3-5]. This paper covers the analysis of shielding effectiveness of variable Permeability cylindrical shielded enclosures for the DC magnetic field case. When the permeability is a function of magnetic induction, the simple boundary solution for spherical or cylindrical shields can no longer be applied since the induction, through the permeability, is caused to vary as much as two orders of magnitude, causing nonuniformity in the field in the cavity and inside the shield. Thus, the permeability of the shielding material is considered as a function of the induction, and a significantly improved method of estimating the induction and permeability of the shield is presented. The effects of a multiple-shell geometry are treated in the equations of this analysis. This method gives fast, accurate results and can be run on a small computer for shielding optimization. Results of these magnetic field calculations allowed the selection of optimization criteria and showed how system requirements could be met by choosing a suitable shell structure arrangement. Experimental measurements on real materials for various shell structures confirmed the accuracy of this method.     

Microstructure and magnetic properties of plasma-sprayed Fe73.5Cu1Nb3Si13.5B9 coatings

Amorphous and nanocrystalline Fe_73.5Cu₁Nb₃Si_13.5B₉ coatings were formed by plasma-spraying micron-sized powders onto H62 brass substrates and aluminum pipes. The coatings are about 0.2-0.3 mm in thickness with fully dense and low porosity. The microstructure of the coatings is classified into two regions, namely, a full amorphous phase region and homogeneous dispersion of α-Fe nanoscale particles with a scale of 30-70 nm. The hardness of the amorphous and nanocrystalline coatings is about 960 HV_100g. Coercivity (Hc), saturation induction (B₈₀₀), and initial relative permeability (μ_i) of the coatings are 144 A/m, 0.27 T, 249, respectively, under 800 A/m direct current (DC) magnetic field. The magnetic shielding performance is good under DC magnetic field and its magnetic shielding effectiveness (SE) is 10-12 dB at coating thickness of 0.45 mm under static magnetic field of 2-40 Oe. The SE increases by increasing the coating thickness when the magnetic field frequencies are 50, 100 and 200 Hz with an intensity of 0.85 Oe. The results indicate that the amorphous and nanocrystalline alloy coatings can be good for some magnetic shielding applications.     

Performance of a photomultiplier tube with transmissive dynodes in a high magnetic field

The performance of a photomultiplier tube with transmissive dynodes was examined. We found that the amplification gain was higher than 10⁴ even in a magnetic field of 7.8 kG. The energy resolution for 662 keV gamma rays from ¹³⁷Cs was 12% fwhm measured using a NaI crystal at 7.8 kG. The time resolution was as good as about 200 ps at 7.8 kG for the light generated by 2 GeV pions when they passed through a 1 cm thick scintillator mounted on the tube.     

Multipole shaking field for magnetic shielding

The authors propose a novel method of magnetic shaking for enhancing the performance of ferromagnetic shielding. The method employs a new type of planar coil which generates higher-order multipole fields. Ferromagnetic shielding walls placed close to the planar coil can be suitably shaken by its field, whereas unwanted leakage of the shaking field into the shielded space can be avoided due to the highly localized nature of the multipole field. The configuration of the novel planar coil is similar to a square mesh in which alternating cells are clockwise or counterclockwise current loops. The shaking effect on the shielding performance and the leakage of the shaking field are evaluated using cylindrical shields and compared with results obtained with toroidal shaking coils     

Numerical analysis of the AC losses of 500-m HTS power cable in Super-ACE project

The alternating current (AC) losses of 500-m high-T_C superconducting (HTS) power cable in Super-ACE project are calculated by using an electric-circuit model. The cable core is consisted of a former (copper-stranded wire conductor), HTS conductor (Ag/Bi-2223 tapes, 1 layer), electrical insulation (LN₂-impregnated laminated paper), HTS shield (Ag/Bi-2223 tapes, 1 layer) and protection (copper-braided wire). Numerically calculated AC loss (the total loss) is obtained by a sum of the self-field loss and external-field loss (hysteresis losses) consumed at the HTS conductor and HTS shield, the ohmic loss caused by a resistance of copper of the former and protection, and the eddy-current loss caused by an axial field at the former. The calculations are compared with experimental results measured by a calorimetric method. The calculations are almost equal to the measurements at wide transport-current range. At transporting 1 kA_rms to the cable, the calculation indicates that the total loss of 1.29 W m⁻¹ (the measurement is 1.3 W m⁻¹) is obtained by a sum of 0.89 W m⁻¹ as the self-field loss, 0.32 W m⁻¹ as the external-field loss, 0.06 W m⁻¹ as the ohmic loss and 0.02 W m⁻¹ as the eddy-current loss. On the other hand, Central Research Institute of Electric Power Industry (CRIEPI) has estimated that the AC loss is obtained by a sum of 0.5 W m⁻¹ consumed at the HTS conductor and HTS shield as the hysteresis loss, and 0.8 W m⁻¹ consumed at the former as the eddy-current loss. The author’s result contradicts CRIEPI’s estimation. It is considered that the difference is caused by an overestimation of the axial field at the former in CRIEPI’s loss calculation.     

Build Axial Graident Field by Using Axial Magnetized Permanent Rings

Axial magnetic field produced by an axial magnetized permanent ring was studied.For two permanent rings,if they are magnetized in the same directions,a nearly uniform axial field can be produced.If they are magnetized in opposite direction,an axial gradient magnetic field can be generated,with the field range changing from-B0 to B0.Apermanent magnet with a high axial gradient field was fabricated,the measured results agree with the PANDIRA calculation very well.For wider usage,it is desirable for the field gradient to be changed.Some methods to produce the variable gradient field are presented.These kinds of axial gradient magnetic field can also be used as a beam focusing for linear accelerator if the periodic field can be produced along the beam trajectory.The axial magnetic field is something like a solenoid,large stray field will leak to the outside environment if no method is taken to control them.In this paper,one method is illustrated to shield off the outside leakage field.     

Method to improve high-frequency magnetic characteristics of Fe80Ni20-O alloy films by introducing low-dose oxygen

In this letter, we propose a method to fabricate Fe₈₀Ni₂₀-O film with improved static and high-frequency, magnetic and electrical properties. Fe₈₀Ni₂₀-O alloy films were prepared by direct current magnetron sputtering at room temperature. The results show that the in-plane uniaxial magnetic anisotropy fields can be adjusted in a broad range by solely adding a very low dose of oxygen into Fe₈₀Ni₂₀ alloy films without applying any inducing field on substrates during deposition. By increasing the oxygen flow ratio from 0.75% to 3%, Fe₈₀Ni₂₀-O alloy films could be achieved with an adjustable ferromagnetic resonance frequency f_r (from 2.2 to 5.9 GHz), a large saturation magnetization 4πMs (from 16.7 to 15.2 kG), and a high resistivity ρ (from 56.7 to 108 μΩ cm).     

Fast timing with plastic scintillators for in-beam heavy-ion spectroscopy

The design, R&D, and testing of a new plastic-scintillator detector for Time-of-Flight measurements with relativistic heavy-ion beams are presented. A design approach using 32 independent precise timing measurements of the same physical event is followed. This is different from the conventional scheme, which aims at two or four high-precision measurements. A circular, 27 cm in diameter, BC-420 plastic-scintillator sheet is read-out by 32 photomultiplier tubes in order to achieve an intrinsic detector resolution on the order of 10 ps root mean square.     

Giant magnetostriction in magnetite nanoparticles

Typically the value of the magnetostrictive coefficient (λ) observed for bulk magnetic materials such as cubic ferrites is 10⁻⁶. However, giant magnetostriction (λ ≤ 10⁻³) is only observed in a few bulk intermetallic materials based on alloys of rare earth and iron such as TbFe, TbFe₂, DyFe₂ and Terefenol-D. While giant magnetostriction is known in nanostructured films, we show for the first time, this phenomenon occurs in magnetic nanoparticles. By using in-field small angle X-ray scattering (SAXS) as a tool, we demonstrate that a 4% relative change in dimension of the particle can be observed in 5.0 nm Fe₃O₄ nanoparticles at room temperature with 1 kG magnetic field. Also, we propose that the observed values are due to interaction effects and magnetoelastic coupling of particle magnetic moments and external magnetic field.