Test Coil Research Articles

Mode of entry of a vaporized pyrethroid knockdown agent into the body of the housefly, Musca domestica (Diptera: Muscidae).

We investigated the mode of entry of pyrethroids into the insect body using adult housefly, Musca domestica L., as an insect model. The wings of adult female houseflies were removed, and empenthrin was applied topically to three different sites: the mesothoracic spiracle, the ventral mesothorax, and the dorsal mesothorax. Among these treatments, the application of the compound to the mesothoracic spiracle led to the quickest knockdown of the flies. To determine the importance of the spiracle as a primary entry site for the pyrethroid, knockdown times were compared between houseflies with blocked and non-blocked spiracles, using two bioassays: a vapor action test using technical grade empenthrin, and a mosquito coil test using empenthrin-impregnated coils. In both tests, the times required for 50 % knockdown of spiracle-blocked houseflies were significantly higher than those required for the non-blocked flies. However, the mortality rates of the two groups were nearly identical, suggesting that spiracles play an important role in the knockdown of houseflies. These results also suggest that the rate of pyrethroid uptake through the spiracles was decreased due to the blocking of the mesothoracic spiracle. Therefore, the spiracle may be considered the main entry site for vaporized pyrethroids.

‘Defect-irrelevant’ behavior of a no-insulation pancake coil wound with REBCO tapes containing multiple defects

A no-insulation (NI) pancake coil was wound with a 23 m long REBCO tape containing multiple ‘defects’, at which local critical currents are substantially lower (<80%) than the tape’s lengthwise average. During a charging test up to 60 A in a bath of liquid nitrogen at 77 K, the test coil showed ‘defect-irrelevant’ behaviors, i.e. coil terminal voltages and center fields in steady-state operations were barely discernible from those of an ideal ‘defect-free’ coil. Also, the critical current (Ic) of the coil was measured to be 68 A, close to 72 A estimated from the measured Ic angular dependency data of a defect-free sample tape that was used for construction of the coil. The results demonstrate a potential to build a pancake coil with REBCO tapes regardless of their defects, which may lead to a substantial reduction in the construction cost of high field NI REBCO magnets.

Bioaerosol deposition on an air-conditioning cooling coil

This study is concerned with the role of a fin-and-tube heat exchanger in modifying microbial indoor air quality. Specifically, depositional losses of ambient bioaerosols and particles onto dry (not cooled) and wet (cool) coil surfaces were measured for different airspeeds passing through the test coil. Total, bacterial and fungal DNA concentrations in condensate water produced by a wet coil were also quantified by means of fluorescent dsDNA-binding dye and qPCR assays. Results revealed that the deposition of bioaerosols and total particles is substantial on coil surfaces, especially when wet and cool. The average deposition fraction was 0.14 for total DNA, 0.18 for bacterial DNA and 0.22 for fungal DNA on the dry coil, increasing to 0.51 for total DNA, 0.50 for bacterial DNA and 0.68 for fungal DNA on the wet coil. Overall, as expected, deposition fractions increased with increasing particle size and increasing airspeed. Deposited DNA was removed from the cooling coil surfaces through the flow of condensing water at a rate comparable to the rate of direct deposition from air. A downward trend of bacterial and fungal DNA measured in condensate water over time provides suggestive evidence of biological growth on heat exchangers during nonoperational times of a ventilation system. This investigation provides new information about bioaerosol deposition onto a conventional fin-and-tube cooling coil, a potentially important factor influencing indoor exposure to microbial aerosols in air-conditioned buildings.

Progress in the Design and Testing of In-Vessel Magnetic Pickup Coils for ITER

This paper presents the results of the R&D activities recently performed to achieve the preliminary design of electromagnetic transducers for in-vessel local magnetic field measurements at low frequency (<30 kHz) for ITER. The study is based on thermoelectrical and electromagnetic bench tests performed on a batch of 40 sensor prototypes manufactured with the low temperature cofired ceramic (LTCC) technology. Also, the outcomes of a first neutron irradiation test on LTCC sensors are presented. The development of an alternative concept of a magnetic sensor for ITER, based on mineral insulated cables winding is also reported, with suitable thermomechanical and electromagnetic numerical analyses performed to assess the reliability of such sensors within the ITER operating conditions.

Design of a conduction-cooled 9.4 T REBCO magnet for whole-body MRI systems

A project on the development of REBa2Cu3O7−δ (REBCO) magnets for ultra-high-field magnetic resonance imaging (MRI) was started in 2013. Since REBCO-coated conductors feature high mechanical strength under tensile stress and high critical current density, use of REBCO coils would allow superconducting magnets to be made smaller and lighter than conventional ones. In addition, a conduction-cooled superconducting magnet is simpler to use than one cooled by a liquid helium bath because the operation and maintenance of the cryogenic system become simpler, without the need to handle cryogenic fluid. Superconducting magnets for MRI require homogeneous, stable magnetic fields. The homogeneity of the magnetic field is highly dependent on the coil shape and position. Moreover, in REBCO magnets, the screening-current-induced magnetic field, which changes the magnetic field distribution of the magnet, is one of the critical issues. In order to evaluate the magnetic field homogeneity and the screening-current-induced magnetic field, a 1 T model magnet and some test coils were fabricated. From an evaluation of the 1 T model magnet, it was found that the main reason for magnetic field inhomogeneity was the tolerances in the z-axis positions of the coils, and therefore, it is important to control the gap between the single pancakes. In addition, we have already demonstrated the generation of an 8.27 T central magnetic field at 10 K with a small test coil. The screening-current-induced magnetic field was 0.43 T and was predictable by using an electromagnetic field simulation program. These results were reflected in the design of a conduction-cooled 9.4 T REBCO magnet for whole-body MRI systems. The magnet was composed of six main coils and two active shield coils. The total conductor length was 581 km, and the stored energy was 293 kJ. The field inhomogeneity was 24 ppm peak to peak and 3 ppm volume-root-mean-square (VRMS) for a 500 mm diameter spherical volume (DSV). The axial and radial 5 gauss line locations were less than 5 m and 4.2 m respectively.

New Method for Quick Decay of Shielding Current in REBCO Superconducting Coils

We are developing superconducting magnets for MRI with $\mbox{Re}_{1}\mathrm{Ba}_{2}\mbox{Cu}_{3}\mbox{O}_{7-\delta}$ (Re: Rare Earth, Y, Gd, Eu, REBCO) superconducting tapes. We intend to operate them in subcooled liquid nitrogen at 65 K. Superconducting magnets for MRI need uniformity of magnetic field in time and space. The disturbance should be mainly caused by the shielding current induced in the wound tapes. To investigate this, we made small test coils with a laser-scribed four-filament EuBCO tape and a nonscribed GdBCO tape. After cooling down to 65 K with subcooled liquid nitrogen and applying a dc bias magnetic field, transport current was applied step by step to the test coil and the magnetic field variation at the coil center was observed. The actual magnetic field followed the variation of transport current with a delay. We derived the fitting expression for the observed field variations. The magnitude and the decay time constant of the shielding current increased with transport current. However, when further increasing transport current close to the critical current, the shielding current decayed quickly. Using this tremendous knowledge, we proposed a new operating method so that the magnetic field reached to the target one in a short time. In addition, the comparison between both coils demonstrated that not only the magnitude of shielding current but also the decay time constant was drastically reduced by scribing.

Study on AC Loss Reduction in HTS Coil for Armature Winding of AC Rotating Machines

The fundamental study on the ac loss reduction in high-temperature superconducting (HTS) coils for armature windings of ac rotational machines is presented in this paper. In this paper, a simplified model that simulates a part of the distributed armature winding of ac rotational machines is used. For the ac loss reduction, we study a method to reduce the perpendicular magnetic field applied to the tape surface in the coil by arranging a magnetic material around the coil winding. The numerical magnetic field calculation shows that the perpendicular magnetic field clearly decreases by arranging the magnetic material around the coil winding in the slot of the iron core. To investigate the effect of the insertion of the magnetic material on the ac loss reduction, we fabricate the measurement setup using the test HTS coils wound by a Bi2223 tape and a YBCO tape, combined with the laminated iron core. The transport ac losses in the test coils are measured by changing the arrangement of the magnetic material. The measurement results demonstrate well the ac loss reduction by use of the magnetic material around the coil winding in the slot of the iron core.

Development of a Test Conduction-Cooled MgB2Coil

We are now developing conduction-cooled MgB 2 coils toward the realization of magnets with 1-MJ magnetic energy. We devised a new fabrication method, which is based on the wind and react method. A bobbin in the react process is made of stainless steel. To improve cooling performance, one of the stainless steel flanges of the bobbin is replaced with an oxygen-free copper (OFC) one after the react process. To prevent winding collapse, these flanges consists of several pieces, and these pieces are replaced in sequence. The OFC flange and the coil winding are bonded with epoxy impregnation. To validate this new method, we fabricated a test coil with an inner diameter of 100 mm and a self-inductance of 8.3 mH by using a multistrand MgB 2 wire. We cooled the test coil by a two-stage GM cryocooler and measured its critical current (I c ) at 25 K. The difference between the temperature of the OFC flange and that of the winding surface was within 2 K. The I c was about 263 A, which had been expected to be 275.8 A in accordance with its load line and short-sample measurements. These results support the validity of our fabrication method.

A Size-Constrained 3-T REBCO Insert Coil for a 21-T LTS Magnet: Mechanical Investigations, Conductor Selection, Coil Design, and First Coil Tests

Recent progress in second-generation (2G) rare-earth-Ba2Cu3O7-x (REBCO) coated conductors (CCs) has paved the way for the development of all-superconducting magnets generating fields well above 23.5 T, i.e., beyond the limits of the present technology based on Nb3Sn. However, the design of REBCO coils still poses several fundamental and engineering challenges. In particular, the tape geometry and the highly anisotropic electromechanical properties require new approaches for coils. In REBCO magnets, the inner windings' layer changes (passing from the nth layer to the (n + 1)th layer) are particularly critical from the electromechanical point of view as in-plane bending of the tapes results in additional strain that adds up to the bending strain and adds to the out-of-plane winding strain and the strain induced by the Lorentz forces. With the goal of building an all-superconducting 24-T magnet consisting of a 3-T REBCO insert coil in a 21-T magnet, we have determined the mechanical and electromechanical application limits of various industrial 2-G high-temperature superconductor tapes. In a first step, we have investigated the influence of longitudinal mechanical strain on the electrical transport properties of industrial REBCO CCs at the insert's operating conditions. In a second step, we have investigated for these CCs the impact of in-plane bending in different layer-change configurations (layer change in one turn and in two turns). These experiments are accompanied by bending strain calculations linking layer-change configurations and in-plane bending with the tapes' longitudinal strain dependence of their critical current. We present the superconductor characterization, the material selection, the design, and the first tests of this size-constrained 3-T REBCO insert coil for a 21-T LTS magnet.

R&amp;D Project on HTS Magnets for Ultrahigh-Field MRI Systems

An RD $\text{RE}=\text{rare earth}$ ) wires for ultrahigh-field (UHF) magnetic resonance imaging (MRI) systems is described. Our targets are 9.4-T MRI systems for whole-body imaging and brain imaging. REBCO wires are promising components for UHF-MRI because REBCO wires have high critical current density in high magnetic fields and high strength against hoop stresses, which allows MRI magnets to be smaller and lighter than conventional ones. The aim of the project is to establish basic magnet technologies for adapting REBCO coils for UHF-MRI. The project term is three years, and this year is the final year. We have already demonstrated the generation of an 8.27-T magnetic field with a small test coil composed of 22 REBCO pancake coils. A magnetic field spatial distribution with inhomogeneity of several hundreds of parts per million within 100-mm diameter spherical volume (DSV) was demonstrated with a 1-T model magnet. A stable magnetic field of a few parts per million per hour was also demonstrated with the 1-T model magnet. The targets of the project, to be achieved by March 2016, are to demonstrate the generation of a 9.4-T field with the small REBCO coil, and to demonstrate a homogeneous magnetic field in 200-mm DSV with a 1.5-T magnet having three pairs of split coils. Imaging will be performed with the 1.5-T magnet.

Decay of Shielding Current in Stacked REBCO Superconducting Tapes

We are developing superconducting magnets for MRI with $\mbox{Re}_{1}\mbox{Ba}_{2}\mbox{Cu}_{3}\mbox{O}_{7-\delta}$ (Re: rare earth, Y, Gd, Eu, REBCO) tapes. Superconducting magnets for MRI need the uniformity of the produced magnetic field in time and space. The disturbance is mainly caused by the induced shielding current in the wound superconducting tapes, which corresponds to the magnetization of the REBCO superconducting tapes. We have made a test coil with REBCO tapes and evaluated the influence of the shielding current on the produced magnetic field. In this paper, applying external magnetic field perpendicular to the EuBCO superconducting tape face, we investigated the magnetization and its relaxation properties of the tapes at 20–40 K using a saddle-shaped pickup coil. Increasing an applied magnetic field and interrupting the field sweep temporarily at 0, 0.5, 1, 1.5, 2, and 2.5 T, we observed the variation of the magnetization for each step of magnetic field. The magnetization decayed at a speed of a few to tens of percent per minute. It was faster beyond expectation. The relaxation rate of the magnetization depended on magnetic field, temperature, and the number of stacked tapes. It became faster with increasing temperature and magnetic field and with decreasing number of stacked tapes.

Development of HTS Sextupole Magnet for SuperKEKB Interaction Region

The SuperKEKB project underway at KEK will increase the KEKB luminosity by 40 times using nanobeam-interaction-region focusing optics. To achieve this high luminosity, the chromaticity correction of the collision beams is crucial; thus, special sextupole magnets, which can simultaneously produce normal and skew sextupole fields, are required in the straight sections around the interaction point. As a candidate for the sextupole, a cryocooled high-temperature superconducting magnet was proposed; development has begun. Here, the basic magnet design and the results of a small coil test are presented. The test studied the quench characteristics of the REBCO-impregnated coil.

Superconducting Dipole Magnet for Strangeness Physics Program at J-PARC

A large-acceptance superconducting dipole magnet is currently under construction for extensive strangeness nuclear physics programs at Japan Proton Accelerator Research Complex (J-PARC), Japan. The conduction-cooled superconducting magnet is composed of two circular coils with a radius of 500 mm, vertically 500 mm apart from each other. This Helmholtz-type magnet was designed to keep the field uniformity Br/By at the level of 1% over the inner region of 500 mm in diameter and 500 mm in height. Our main detector is a time projection chamber that uses parallel E and B fields. If there is a transverse component, drifting electrons would take helical paths due to the E × B effect. As a result, the field uniformity is crucial in the design of the magnet. The maximum field strength is 1.5 T. We report our design study and coil test results using a test cryochamber.

Experimental Results of Screening-Current Field With 10-T Class Small REBCO Coil

A REBCO-coated conductor has a tape shape and is suitable for forming a stack of single pancakes for generating a high magnetic field. However, the magnetic flux going through the tape surface causes a screening current, which reduces the magnetic field in conventional coil designs. Magnetic resonance imaging systems need high uniformity of the magnetic field, both spatially and temporally, and thus, the screening field resulting from the screening current is one of the critical issues. In this paper, a 10-T class small test coil using REBCO-coated conductors was fabricated and tested in a conduction-cooled configuration from 10 to 60 K for investigating the effect of the screening field on the generated magnetic field in the inner space of the coil. The coil was composed of a stack of 22 single pancakes using 4-mm-wide REBCO tapes with an inner diameter of 50 mm, an outer diameter of 129 mm, and a height of 104 mm. For different stacking orders based on the coil critical currents, the measured values of the central magnetic field at 10 K were 7.66 and 8.27 T, which were lower than the calculated values of 8.07 and 8.70 T obtained without taking account of the screening-current fields.

A Longitudinal Mode Electromagnetic Acoustic Transducer (EMAT) Based on a Permanent Magnet Chain for Pipe Inspection.

A new electromagnetic acoustic transducer (EMAT) design, employing a special structure of the permanent magnet chain, is proposed to generate and receive longitudinal guided waves for pipe inspection based on the magnetostriction mechanism. Firstly, a quantitative analysis of the excitation forces shows the influence of the radial component can be ignored. Furthermore, as the axial component of the static magnetic field is dominant, a method of solenoid testing coils connected in series is adopted to increase the signal amplitude. Then, two EMAT configurations are developed to generate and receive the L(0,2) guided wave mode. The experimental results show the circumferential notch can be identified and located successfully. Finally, a detailed investigation of the performance of the proposed EMATs is given. Compared to the conventional EMAT configuration, the proposed configurations have the advantages of small volume, light weight, easy installation and portability, which is helpful to improve inspection efficiency.

Performance correlation between YBa2Cu3O7−δ coils and short samples for coil technology development

A robust fabrication technology is critical to achieve the high performance in YBa2Cu3O (YBCO) coils as the critical current of the brittle YBCO layer is subject to the strain-induced degradation during coil fabrication. The expected current-carrying capability of the magnet and its temperature dependence are two key inputs to the coil technology development. However, the expected magnet performance is not straightforward to determine because the short-sample critical current depends on both the amplitude and orientation of the applied magnetic field with respect to the broad surface of the tape-form conductor. In this paper, we present an approach to calculate the self-field performance limit for YBCO racetrack coils at 77 and 4.2 K. Critical current of short YBCO samples was measured as a function of the applied field perpendicular to the conductor surface from 0 to 15 T. This field direction limited the conductor critical current. Two double-layer racetrack coils, one with three turns and the other with 10 turns, were wound and tested at 77 and 4.2 K. The test coils reached at least 80% of the expected critical current. The ratio between the coil critical currents at 77 and 4.2 K agreed well with the calculation. We conclude that the presented approach can determine the performance limit in YBCO racetrack coils based on the short-sample critical current and provide a useful guideline for assessing the coil performance and fabrication technology. The correlation of the coil critical current between 77 K and 4.2 K was also observed, allowing the 77 K test to be a cost-effective tool for the development of coil technology.

Relaxation of Shielding Current in Test Coils for MRI With REBCO Superconducting Scribed Tapes

Superconducting magnets for MRI need uniformity of produced magnetic field within ppm order in time and space. To develop the superconducting magnets for MRI operating in subcooled liquid nitrogen at 65 K, two pieces of small test coils were fabricated with Re <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> (REBCO, Re: Rare Earth such as Y, Gd, and Eu) superconducting tapes. One was wound with a nonscribed tape and the other was with a laser-scribed tape into a four-filament structure. Each test coil was inserted inside a NbTi superconducting magnet. After bath cooling, the test coil with subcooled liquid nitrogen and applying a dc bias magnetic field of 0.5-2 T, transport current was applied step by step, and the variation of magnetic field at the coil center was observed by a Hall probe with high accuracy. Although magnetic field followed the variation of transport current with a delay in the respective coils, the drastic difference between them was revealed. It was demonstrated that the influence of the induced shielding current on the produced magnetic field was drastically reduced by scribing. Not only the magnitude of shielding current but also the decay time constant was reduced by scribing. The produced magnetic field of the test coil with the scribed tape reached to a target one in a short time as programmed. Scribing REBCO tapes brought about faster relaxation of magnetization.

Quench Protection of YBCO Coils: Co-Winding Detection Method and Limits to Hot-Spot Temperature

A quench detection method of a high-temperature superconducting (HTS) coil using a cowinding method is studied. It is experimentally shown using a small YBCO test coil that the cowinding method is highly effective in canceling the inductive voltage component and can detect a quench occurring in an HTS coil with high sensitivity, compared with a commonly used bridge-balance method. An HTS coil is damaged by a quench when hot-spot temperature exceeds a certain threshold. The authors also experimentally investigated conditions to keep the hot-spot temperature of coils wound of YBCO tapes below a safe level, using the highly sensitive cowinding quench detection method.

Factory Acceptance Tests: A Comparative Analysis to Establish a Baseline for Testing Data

Factory acceptances tests (FATs) of a new high-voltage (HV) rotating machine include comprehensive diagnostic tests of the finished stator winding insulation system. From these tests, the end user obtains the baseline data for the condition monitoring performed during the machine?s operating life. The design geometry of the stator windings and manufacturing processes used to produce them will affect the dielectric test results. There is a significant challenge in comparing the test results from different stators to obtain a statistically useful sample for an acceptance test database. This article compares the dielectric test results from the 14.4-kV windings of several new 1,800-r/min, 26.1-MW (35,000-hp) centrifugal compressor synchronous motor stators of identical design, built for two U.S. chemical plants. The results are compared with those from accompanying sacrificial coils. The stator diagnostic tests include the dissipation factor (DF) or tan delta, power factor tip up (PFTU) or delta tan delta, offline partial discharge (PD), corona inspection with an ultraviolet (UV) analyzer, and online PD. Sacrificial coils produced and processed alongside each stator receive the specified American Petroleum Institute (API) coil acceptance tests and corona inspection with a UV analyzer. The PD measurements on the sample coils were repeated at an elevated voltage, and the results were compared with the corona inspection observations. This article shares the comparative analysis of the stator and coil test results as an excellent example of the baseline FAT data.

An 800-MHz all-REBCO Insert for the 1.3-GHz LTS/HTS NMR Magnet Program-A Progress Report.

A critical component of the 1.3-GHz nuclear magnetic resonance magnet (1.3 G) program, currently ongoing at the Francis Bitter Magnet Laboratory, Plasma Science and Fusion Center, Massachusetts Institute of Technology, and now approaching its final stage, is the all high-temperature superconductor 800-MHz insert (H800). The insert consists of three nested double-pancake (DP) coils fabricated with 6-mm-wide REBCO conductor. Coil 1, the innermost coil of H800, has already been fabricated and tested at 77 and 4.2 K. In addition, one third of the DPs for Coil 2 have been wound and each DP individually fully tested. Work described here includes details of Coil 1 fabrication: DP winding, DP testing, assembling, joint performance, overbanding, and coil testing; winding details of DPs for Coil 2 and their testing are also included.