Saddle-shaped Coil Research Articles

Transport AC Loss Properties of a Bi-2223 Superconducting Coil From 0.1 Hz to 10 Hz

We propose a method to evaluate transport AC losses in Bi-2223 superconducting coils by integrating the local external-field losses of the stacked taped wires over the coil's windings. We measured the transport AC losses of a 16 double-pancake coil of twisted Bi-2223 Ag-sheathed tapes, in the frequency range from 0.1 to 10 Hz at liquid nitrogen temperature. We also measured the external-field losses of short specimens of 6-stack Bi-2223 wires both in perpendicular and parallel field configurations through a saddle-shaped pickup coil method. The external losses are mainly due to hysteresis losses in the superconducting filaments and coupling losses among the filaments. The local external-field loss in the test coil is approximately calculated as the sum of perpendicular- and parallel-field losses of the short specimen. In the calculation, geometrical effects because of the neighboring turns in the coil are also considered, in addition to the magnitude and direction distributions of the local magnetic field. We compared the numerically evaluated external-field losses to the measured transport AC losses, and discussed the applicability of the proposed method.

Magnetic Catheter Manipulation in the Interventional MR Imaging Environment

PurposeTo evaluate deflection capability of a prototype endovascular catheter, which is remotely magnetically steerable, for use in the interventional magnetic resonance (MR) imaging environment. Materials and MethodsCopper coils were mounted on the tips of commercially available 2.3–3.0-F microcatheters. The coils were fabricated in a novel manner by plasma vapor deposition of a copper layer followed by laser lithography of the layer into coils. Orthogonal helical (ie, solenoid) and saddle-shaped (ie, Helmholtz) coils were mounted on a single catheter tip. Microcatheters were tested in water bath phantoms in a 1.5-T clinical MR scanner, with variable simultaneous currents applied to the coils. Catheter tip deflection was imaged in the axial plane by using a “real-time” steady-state free precession MR imaging sequence. Degree of deflection and catheter tip orientation were measured for each current application. ResultsThe catheter tip was clearly visible in the longitudinal and axial planes. Magnetic field artifacts were visible when the orthogonal coils at the catheter tip were energized. Variable amounts of current applied to a single coil demonstrated consistent catheter deflection in all water bath experiments. Changing current polarity reversed the observed direction of deflection, whereas current applied to two different coils resulted in deflection represented by the composite vector of individual coil activations. Microcatheter navigation through the vascular phantom was successful through control of applied current to one or more coils. ConclusionsControlled catheter deflection is possible with laser lithographed multiaxis coil-tipped catheters in the MR imaging environment.

Calculation and error analysis for magnetic field parameters of a standard saddle coil

The calculation and measurement of field parameters and their derivatives play a very important role in designing and constructing saddle-shaped bipolar-magnetic coils [1]. In this article, new distribution function Fn and its derivatives along z-axis are defined, formularized and presented. Calculation and error analysis for the magnetic field parameters of a standard saddle coil are conducted and presented using the formulas. It is shown that the method and formulas can be used in calculating the field parameters, derivatives and the magnetic vector H for the bipolar-magnetic coils.

AC Loss Properties of Laser-Scribed Multi-Filamentary GdBCO Coated Conductors with Artificial Pinning Centres

RE 1 Ba 2 Cu 3 O 7-∂ (RE: Rare Earth, Gd, Y and so on) coated conductors have a large aspect ratio and also anisotropy in critical current density. For practical application of REBCO tapes, we evaluated J c and ac loss properties in a wide range of temperature and magnetic field. The induced ac losses due to the perpendicular magnetic field to a tape face are considerably huge. Therefore, in case that REBCO coated conductors are applied electric power machines and devices, ac loss induced in the windings should amount to a great part of the total heat load. We proposed an ac loss reduction method, which is combined with a laser scribing technique and a special winding process into a multi–filamentary structure. Additionally, we are trying to enhance critical current density J C, by doping artificial pinning centre. In this study we investigated the ac loss properties of multi-filament structural GdBCO tapes with BZO nano–lods as artificial pinning centers. We measured ac losses of 5, 10, 20-filament GdBCO tapes by using a saddle-shaped pickup coil. Consequently we confirmed the ac loss decreased in inverse proportion to the number of filaments even in the GdBCO tapes with enhanced J C by introducing artificial pinning centers.

5T級 Bi-2223 超伝導コイルの通電損失の評価

This study considers a method to evaluate AC transport current losses in Bi-2223 superconducting coils through integrating local external field losses of stacked, taped wires over the winding part of the coils. We measured the transport current losses of two kinds of pancake superconducting coils of Bi-2223 Ag-sheathed tapes, which were composed of 8 or 16 doublepancake (DP) coils, at 0.008 and 0.04 Hz. We also measured the AC external field losses of short specimens of six-layer Bi-2223 wires in both perpendicular and parallel field configurations using a method based on saddle-shaped pickup coils. The approximate local external field loss was calculated as the sum of perpendicular and parallel field losses of the short specimen. In the calculation, a demagnetization effect was considered in order to introduce the geometrical situation of winding in the coil, in addition to the distributions of the perpendicular and parallel magnetic-field components. The comparison between the experimental results of the transport current loss and the numerical results revealed that there is good agreement in the lower amplitude region of the transport current, and deviation in the higher amplitude region. The former finding indicates that this method is applicable as a general means to estimate the transport current losses in superconducting coils. The latter finding suggests that the effects of the transport current may cause significant changes to the AC loss properties of the superconducting coils.

The signal-to-noise ratio of the nuclear magnetic resonance experiment

A fresh approach to the calculation of signal-to-noise ratio, using the Principle of Reciprocity, is formulated. The method is shown, for a solenoidal receiving coil, to give the same results as the traditional method of calculation, but its advantage lies in its ability to predict the ratio for other coil configurations. Particular attention is paid to the poor performance of a saddle-shaped (or Helmholtz) coil. Some of the practical problems involved are also discussed, including the error of matching the probe to the input impedance of the preamplifier.

Transport and AC loss properties of the repaired multifilamentary REBCO superconducting tapes

For near-future applications of REBa2Cu3O7 (REBCO) coated conductors to electric power cables, transformers and Superconducting Magnetic Energy Storage (SMES), the long taped wires with high performance in the transport properties have been designed and fabricated. Moreover, in order to drastically reduce AC losses in perpendicular field configuration, advanced multifilament YBCO coated conductors (MFYCCs) fabricated with technique of a laser scribing process have been also developed. In the present study, from engineering viewpoints to utilize such advanced conductors, we evaluated the transport and AC loss properties of short MFYCCs with a repaired part or a joint by a diffusion joint technique with the saddle-shaped pickup coil method.

Force–pain relationship in functional magnetic and electrical stimulation of subjects with paresis and preserved sensation

Using "painless" magnetic stimulation (FMS) to support the cycling of paretic subjects with preserved sensation is possible and potentially superior to electrical stimulation (FES). We investigated the dependence of the torque and the pain evoked by FMS and FES on stimulation conditions in order to optimize magnetic stimulation. Torque and pain induced by quadriceps stimulation in 13 subjects with paresis and preserved sensation (due to multiple sclerosis) were compared under the conditions: (1) small vs large stimulated surfaces of the thigh, (2) varying contraction velocities of the muscle (isometric vs 15 and 30 rpm isokinetic speed), (3) FMS vs FES modalities, and (4) varying magnetic coil locations. Torque and pain significantly depended on the amount of surface and location of stimulation during FMS, on the stimulation modality, and on the muscle contraction velocity during FES and FMS. FMS with a saddle-shaped coil produced more torque (p<0.05) than any other stimulation modality, even at 30 rpm velocity. To support leg cycling of subjects with preserved sensation, the application of FMS stimulation with a large-surface saddle-shaped coil and the focusing of stimulation on the lateral-frontal surface of the thigh produces greater torque and less pain than FES. Optimized magnetic stimulation is a superior alternative to electrical stimulation in the rehabilitation of subjects with preserved sensation.

Sensitivity of Cryogenic Solenoidal Probe for Split-Pair Superconducting Magnet With Cross-Bore

By using a split-pair superconducting magnet with cross-bore, one can implement a solenoidal radio frequency (RF) coil for solution nuclear magnetic resonance (NMR). We expect the solenoidal RF coil to give us higher sensitivity than an ordinary saddle-shaped RF coil. We have developed both room temperature and cryogenic NMR probes with a solenoidal RF coil for a split-pair superconducting magnet. The probes and the solenoidal RF coils were evaluated using a sensitivity test with standard 0.1% ethylbenzene. The sensitivity tests of the room temperature probes indicated that the signal-to-noise ratio (SNR) of the solenoidal RF coil was higher than that of an ordinary saddle-shaped RF coil. In the sensitivity test of the cryogenic probe, we compared a four-turn solenoidal RF coil with a two-turn one. As a result, the SNR of the one of two-turn RF coil was higher than that of the one of the four-turn one. It is believed that the reason is that the SNR depends on the quality factor of the specimen Qs and, furthermore, Qs depends on the shape of the RF coil.

Development of Low-AC-Loss Bi-2223 Superconducting Multifilamentary Wires

We have been designing and developing advanced Ag-sheathed Bi-2223 multifilamentary wires for the future applications to superconducting transformers of electric rolling stocks. Trial wires were twisted without remarkable degradation in the transport property for suppressing AC losses in a perpendicular magnetic field configuration. The AC losses were measured by a saddle-shaped pickup coil in a wide range of frequency up to a commercial frequency. The experimental results show that the AC loss per cycle has linear frequency dependence in a region of the practical amplitude of AC magnetic field for the transformers, where the excess loss may come from an electromagnetic coupling between the filaments. The hysteresis loss evaluated as a low frequency limit of the AC loss was also reduced with the decrease in the twist pitch, which may be considered as a similar effect to the so-called ldquotwist effectrdquo observed in in-situ Nb3Sn wires.

Selective passive shielding and the Faraday bracelet

We describe a general approach for the design of passive conducting structures that preferentially shield the ac magnetic field produced by a specific coil geometry. We demonstrate the utility of selective passive shielding with the Faraday bracelet, an open-access shield for saddle-shaped rf coils. Our data also corroborate the poorly known fact that effective shielding can occur when the ratio of shield thickness to skin depth is much less than unity.

A round robin test for pre-standardization of a saddle-shaped pickup coil method to measure AC losses in Bi-2223 Ag-sheathed tapes

We performed a round robin test to measure AC losses in Bi-2223 Ag-sheathed tapes with saddle-shaped pickup coils in an alternating perpendicular magnetic field at liquid nitrogen temperature. Two laboratories participated in the round robin test with each system of the pickup coil and the measurement circuit. Short specimens of a Bi-2223 tape and a copper magnet to generate the alternating magnetic field were used for common conditions in the test. The saddle-shaped pickup coils were made under recommended conditions for the structure and the sizes that give a calibration coefficient around unity. The signals of the magnetic moment from the pickup coils were calibrated by using a standard hairpin copper tape. AC losses of the common specimen of the Bi-2223 tape were measured in the round robin test in the ranges of frequency up to 60 Hz and the amplitude up to 0.2 T of perpendicular magnetic field. While we had only two participants in the test, we obtained the following results in a pre-standardization process of the AC loss measurement with the saddle-shaped pickup coil (i) it was suggested from the calibration procedure using hairpin copper tape that the deviations from numerical simulation for an ideal pickup coil and a sufficiently long specimen is within 3% on an average, (ii) the values of COV for the AC losses measured at 1 Hz and 10 Hz are less than about 3% in the range of the field amplitude between 0.02 T and 0.2 T.

Basic AC loss properties of IBAD/CVD-YBCO tapes for pancake-type coils

We are experimentally studying basic AC loss properties of IBAD/CVD-YBCO coated conductors with a copper layer for stabilizing, especially the temperature dependence of perpendicular field loss in alternating electromagnetic environments. We prepared two types of short specimens with and without a copper layer and measured AC losses by a saddle-shaped pickup coil in an alternating magnetic field perpendicular to the wide surfaces at liquid helium temperature. In the ranges of the amplitude up to 4 T and the frequency up to 0.2 Hz, the AC losses both of the two specimens are hardly dependent upon the frequency. The results show that hysteresis loss is a major component of the AC loss in the specimens and the effects of the copper layer can be negligible. We also measured AC losses for the specimens with the copper layer at liquid nitrogen temperature to estimate the dependence on measurement temperature. The results suggested that the AC loss vs. the amplitude of applied field can be scaled by a critical current at a zero magnetic field.

Development of cryogenic probe system for high-sensitive NMR spectroscopy

Utilizing a cryogenic probe system is a promising approach to improve signal-to-noise-ratio (SNR) in the nuclear magnetic resonance (NMR) spectrometer. We have been developing an NMR spectrometer based on a superconducting-split-pair magnet. This unique structure of the magnet allows using solenoid-shaped antenna coil, which ensures about 2 times higher sensitivity than the conventional saddle-shaped antenna coil. The goal of the present study is to realize higher sensitivity by implementing the superconducting-split-pair magnet and solenoidal-shaped antenna coil. Our immediate objective is to establish the cryogenic probe system for the superconducting-split-pair magnet. In this paper, the basic idea and design of the system and some preliminary results are described.

Open-Type Magnetically Shielded Room Using Only Canceling Coil Without a Ferromagnetic Wall for Magnetic Resonance Imaging

In order to improve amenities for patients in hospitals, a new open type of magnetically shielded room (MSR) for magnetic resonance imaging (MRI), which uses only canceling coils without a ferromagnetic wall, has been developed. The device was designed by three-dimensional magnetic field analysis and experimentally investigated using an actual MRI of 1.5 T. First, a coil and ferromagnetic material were modeled on an actual MRI, so that the distribution of the leakage magnetic flux vectors was in agreement with that measured. Second, the optimal shaped canceling coil, a saddle-shaped coil which can reduce the leakage flux from the edge plane to the side plane of the room, was designed using the modeled MRI, and realized. The effectiveness of this coil for shielding and its influence on MRI imaging were confirmed experimentally using an actual MRI. In addition, an optimal U-shaped enclosure that can sufficiently shield the leakage flux generated by the coil itself was designed; its effectiveness was confirmed experimentally by using an actual canceling coil.

Ac loss properties of YBCO superconducting tapes fabricated by IBAD-PLD technique

Temperature dependence of the ac loss in YBCO superconducting tapes fabricated by IBAD-PLD technique was investigated. The ac loss was measured with a saddle-shaped pickup coil by applying magnetic field perpendicular to the tape. Temperature dependence of Ic-B characteristics was estimated from the observed magnetization curves. The Ic-B curves normalized by the zero-field Ic at the respective temperature coincided with one master curve regardless of temperature. Ic-B characteristics of YBCO superconducting tapes are scaled with temperature. According, when the magnetization curves and the ac loss vs field amplitude curves are normalized by using the zero field Ic at the respective temperature, they coincided with one master curve for any temperature from 4.2 to 77 K. It is verified that the magnetization and the ac loss are scaled with temperature.

Temperature scaling of ac loss in YBCO superconducting tapes fabricated by theIBAD-PLD technique

Temperature dependence of the ac loss in YBCO superconducting tapesfabricated by the IBAD-PLD technique was investigated. The ac loss wasmeasured with a saddle-shaped pickup coil by applying a magnetic fieldperpendicular to the wide surface. Temperature dependence of the critical current,Ic, versus themagnetic field, B, characteristics was estimated from the magnetization curves observed. By definingBb(T) as a specific valueof B at the breakingpoint of the Ic–B curve in logarithmic scale at the respective temperature,T,Ic for0≤B<Bb was nearly constantindependent of B, and Ic forB>Bb decreased inproportion to the (1−γ)th power of B, where γ is a pin parameter of the Irie–Yamafuji model and0<γ<1. In addition, the temperature dependencies ofBb and zero fieldIc were almostidentical and γ had no temperature dependence. We found that the normalizedIc–B curves by the zerofield Ic at the respectivetemperature, Ic0(T), coincide with one master curve regardless of temperature. This means that theIc–B characteristics of YBCO superconducting tapes are scaled with temperature. Consequentlyit was clarified that the magnetization and the ac loss are scaled with temperature. Themagnetization curves and the ac loss versus field amplitude curves, which are normalized by usingIc 0(T), coincided with one master curve for any temperature from 4.2 to 77 K. This fact is veryuseful in the design of YBCO superconducting systems because it is possible to estimatethe ac loss at any temperature only if we know the field amplitude dependence of the acloss at some temperature, e.g., 77 K, and the temperature dependence of zero fieldIc.

Active shielding of cylindrical saddle-shaped coils: Application to wire-wound RF coils for very low field NMR and MRI

We provide an exact expression for the magnetic field produced by cylindrical saddle-shaped coils and their ideal shield currents in the low-frequency limit. The stream function associated with the shield surface current is also determined. The results of the analysis are useful for the design of actively shielded radio-frequency (RF) coils. Examples pertinent to very low field nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are presented and discussed.

Ac loss characteristics of YBCO superconducting tapes fabricated by TFA-MOD technique

We investigated the ac loss properties of a YBCO superconducting tape fabricated by TFA-MOD technique. The thickness of the YBCO layer is 1.2μm. A 6-tape stack of 50mm in length and 10mm in width was inserted into a saddle-shaped pickup coil. The ac loss was measured at 35–77K by applying an external ac magnetic field. The magnetic field angle was changed by rotating the sample stack around its longitudinal axis. The effective penetration field, which corresponds to the breaking point of an ac loss curve, decreased with increasing field angle though it was much smaller than that which was estimated for a superconducting slab with the same thickness as the width of a tape. As a result, the ac loss increased monotonically with an increasing field angle against the wide surface for any amplitude. The observed field angular dependence of the ac loss agreed with the theoretically predicted one by using the observed ac losses in perpendicular magnetic field. Anyway the ac loss for a larger amplitude than the effective penetration field was proportional to the critical current density and the projective width of a tape in the direction of the applied magnetic field for any field angle and any temperature as well known. In addition we estimated the magnetic field, B, dependence of the critical current, Ic, from the observed magnetization curves. It was shown that zero-field Ic appeared to be a linear function of temperature and Ic–B characteristics was scaled with zero-field Ic. We also discussed a difference in Ic–B characteristics and its temperature dependence between TFA-MOD tapes and IBAD-PLD ones.

AC Loss Properties of YBCO Superconducting Tapes Exposed to External Ac Magnetic Field

We investigated the ac loss properties of YBCO superconducting tapes fabricated by IBAD-PLD technique. The thickness of a YBCO superconducting layer was 0.5 and 1 /spl mu/m. Sample tapes of 60 mm in length and 10 mm in width were stacked into multilayers and inserted into a saddle-shaped pickup coil. The ac loss was measured at 64 K and 77 K by applying external ac magnetic field. The magnetic field angle was changed by rotating the sample stack around its longitudinal axis. The ac loss for the smaller amplitude than the effective penetration field strongly depended on the number of stacked tapes owing to the demagnetization effect. On the other hand, the ac loss for the much larger amplitude than that was independent of the number of tapes. In addition the ac loss decreased monotonically with decreasing field angle against the wide surface for any amplitude. We proposed the theoretical expression to estimate the ac losses for any field angle by using the observed ac losses in perpendicular magnetic field and confirmed the applicability by good agreement between theory and experiment.