YBCO Coil Research Articles

Performance Improvement of YBCO Coil for High-Field HTS-SMES Based on Homogenized Distribution of Magnetically-Mechanically Influenced Critical Current

Generally speaking for a HTS coil, perpendicular magnetic field to conductor's broad surface should be suppressed as small as possible in relation to the magnetic anisotropy. This is a reason why toroidal coil with relatively many elementary coils is expected for HTS-SMES. On the other hand, from the point of view of the homogenization of critical current distribution in the coil, perpendicular field and parallel field should be balanced corresponding to the ratio of the magnetic anisotropy. This means that a certain level of the perpendicular field is effective to reduce local heat generation in the coil. Furthermore, this concept is especially reasonable for a high-field coil with usual winding method (flat-wise winding) because the perpendicular field does not induce hoop stress which decreases the critical current. In this paper, we show these findings through an optimal design of a MOCVD-YBCO toroidal coil for 2 GJ class SMES taking account of magnetically and mechanically influenced J - E characteristics.

A Criterion for Determining Stabilizer Thickness of YBCO Coated Conductors Based on Coil Protection

The purpose of this study is to propose a criterion for determining stabilizer thickness of YBCO coated conductors. The permissible disturbance energy density in high-temperature superconductors (HTSs) is two or three orders of magnitude greater than that in low-temperature superconductors (LTSs). On the other hand, to enhance the reliability and safety of HTS coils, an establishment of a protection scheme assuming a quench caused by a failure in power supply, cooling system or similar problem is also required for HTS coil systems. Quenching produces excessive temperature or voltage in the coil winding; overheating may cause the conductor to meltdown, or high voltage may punch a hole in the winding. To avoid these types of damages in YBCO coils, a suitable stabilizer thickness must be determined for the sophisticated structure of YBCO coated conductors, which are composed of highly textured thin-film on a flat flexible substrate. Therefore, we investigate the appropriate structure for YBCO coated conductors from the perspective of energy dumping (i.e., quench protection). Based on the protection scheme that the stored magnetic energy in the coils is dumped by an external dump resistance prepared as a quench protection, we derive a function that examines the heat balance of a quenching YBCO coil in an adiabatic condition and propose a criterion using the function to determine an appropriate stabilizer thickness for YBCO coated conductors.

Current-Voltage Measurements in a 2G YBCO Coil

The Oak Ridge National Laboratory in collaboration with American Superconductor Corporation and Cryomagnetics Inc. has designed, fabricated, and tested an HTS coil wound with second-generation (2G) YBCO coated conductor tape. The purpose of the HTS coil project was to study the quench characteristics in 2G YBCO coils at 77 K and lower temperatures (~30-45 K). These quench characteristics were investigated in both a pool boiling LN2 environment and in a conduction cooled configuration at ~30 K and 45 K. Transport critical current (Ic) measurements taken on the very first thermal cycle of the YBCO coil in pool boiling LN2 showed an Ic~31 A corresponding to a central magnetic field of 0.32 T. The measured Ic value was consistent with the calculated value using the calculated maximum perpendicular B-field component and the measured short sample Ic at 77 K. Subsequent Ic measurements taken in the conduction cooling configuration at 34 K and 45 K, showed a steady-state Ic~45-49 A and 38-44 A, respectively. These Ic values were significantly lower than the calculated value assuming a literature derived temperature dependent Ic of the 2G YBCO tape. A steady degradation was observed in the Ic of the coil with each successive thermal cycle. In addition, the coil was also pulse tested up to 1-T in non-steady state transient conditions and for ramp rates varying between 0.01 and 5 A/s. The issues and limitations encountered during testing of this new type of 2G coil are briefly discussed.

GdBaCuO and YBaCuO Long Coated Conductors by IBAD-PLD Method—Enhancement of Production Speed and Critical Current in a Magnetic Field

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We have been developing long coated conductors with high critical current, <formula formulatype="inline"><tex>$I_{\rm c}$</tex></formula>, by multi-plume and multi-turn pulsed laser deposition (MPMT-PLD) method. In addition to <formula formulatype="inline"><tex>${\hbox{YBa}}_{2}{\hbox{Cu}}_{3}{\hbox{O}}_{7-{\rm X}}$</tex></formula> (YBCO) coated conductors, we have recently succeeded in fabricating long <formula formulatype="inline"><tex>${\hbox{GdBa}}_{2}{\hbox{Cu}}_{3}{\hbox{O}}_{7-{\rm X}}$</tex></formula> (GdBCO) coated conductors. The 30 m long and 60 m long GdBCO conductors showed higher <formula formulatype="inline"><tex>$J_{\rm c}$</tex></formula> values in a magnetic field and lower anisotropy than the conventional long YBCO conductor. Critical current, <formula formulatype="inline"> <tex>$I_{\rm c}$</tex></formula>, of the GdBCO at 3 T and 77 K was more than 20 A, which was three times larger than that of the YBCO long coated conductor. To demonstrate the high performance of GdBCO, we fabricated GdBCO coils and compared it with the YBCO coil. The GdBCO coil was successfully energized up to large <formula formulatype="inline"><tex>$I_{\rm c}$</tex></formula>'s of 74.2 A and 184.8 A at 77 K and 64 K, which generated magnetic field of 0.5 T and 1.2 T at 64 K and 77 K. We confirmed that the GdBCO conductor is more promising than the YBCO conductor for the application because its superior transport properties in a magnetic field. </para>

High Field and High Temperature Characteristics of Small Test Coil Using CVD-YBCO Tape for SMES

Magnetic field dependencies of the I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the IBAD/CVD-YBCO short tape sample and its small coil sample were measured in high fields, up to 18 T at 77 K. Compared with the Ic of the tape sample, the Ic of the coil sample at 0.1 muV/cm showed the same tendency in high fields. If YBCO tape is applied to a high-field coil application, the application should be operated at a temperature which is lower than 77 K. Using long CVD-YBCO tape, six stacked pancake coils were fabricated. Various current tests were conducted using one of these stacked coils. In AC current tests, thermal stability of the YBCO coil was estimated. When the peak values of AC current were 1.2 times higher than the maximum DC current in a thermal stable state, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dcmax</sub> , and the average electric field of the coil at the first triangular wave was about 10 times higher than 1 muV/cm criterion, the peak values of the built-up voltage did not tend to increase even after the 99th triangular wave current, and thermal run-away in the coil was not observed. In DC current with overlapped pulse current tests, the maximum peak current of the coil in a thermal stable state was obtained as a function of DC current and sweep time. It was 1.3 times higher than Ic and 1.4 times higher than I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dcmax</sub> in a test condition. These results indicate that the YBCO coil has high potential in short-time, over-current operations at high temperatures. In cases where built-up voltages did not disappear, they began to increase just after the coil currents reverted to the initial DC currents. It was found that DC current influenced the increasing speed of built-up voltages once the pulse current had decreased to zero.

Fabrication Test of YBCO Coil and Multi-Tape Conductor

A testing coil composed of six pancake-coils with a YBCO coated conductor has been fabricated and its superconducting properties have been investigated. The resulting coil has generated magnetic fields up to 0.65 T at 65 K in zero external magnetic field. In order to enhance capability for transporting current of YBCO coil, superconducting behavior of the multi-tape YBCO conductor has also been investigated. The critical current of the conductor that is formed by bundling two tapes has practically achieved the sum of the critical currents.

Protection and Quench Detection of YBCO Coils Results With Small Test Coil Assemblies

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents the results of an experimental and theoretical study of the stability and protection of small test coil assemblies wound with second-generation (2G) High-Temperature Superconducting (HTS) wires. The study, conducted as a part of the efforts to improve the design and applicability of YBCO wire technology to real-life power devices, such as magnets, focused on two specific issues: 1) internal voltage developed within the winding resulting from a spatially nonuniform normal zone in a coil that is effectively short-circuited across its terminals; and 2) detection of a localized “hot spot” with Acoustic Emission (AE) signals to complement the usual resistive voltage technique. Each test coil assembly, a stack of four single YBCO pancakes electrically connected in series and shunted by a “persistent-mode” switch, simulated a power-supply driven magnet a moment after a quench zone appeared within the winding. A heater was incorporated into each pancake coil to drive 25% (one pancake), 50%, 75%, or 100% (four pancakes) of the test coil assembly. Measured internal voltages arising from different quench sizes and distributions are compared with the results of the simulation. “Hot-spot” induced AE signals are also presented on a single coil to determine if AE signals may be useful to facilitate the early detection of a normal zone in HTS windings. </para>

Nitrogen boil-off method of measuring AC losses in YBCO coils

The nitrogen boil-off method, which is categorized as one of the calorimetric methods, has been developed as a practical method for measuring AC losses in high temperature superconducting (HTS) coils. The boil-off method offers direct and convenient measurement compared to the electric method. The measurement sensitivity of about 0.5W was confirmed by the improvement of sample vessel structure and the control of the heater power. This paper reports the results of a comparison of the AC losses in superconducting coils using YBCO tapes measured with the boil-off method and those obtained by the electric method and by analysis.

GdBCO and YBCO long coated conductors and coils

We developed long coated conductors and a coil of GdBa2Cu3O7−x (GdBCO). The GdBCO coated conductor exhibited higher critical current density (Jc) in a magnetic field property than that of the YBa2Cu3O7−x (YBCO) coated conductor. The GdBCO coil was wound using a 53.5m long GdBCO coated conductor. The coil was a 4 turn-83 layered solenoid with a bore diameter of 28.0mm, inner diameter of 30.0mm, outer diameter of 70.9mm, and height of 44.3mm. The coil was successfully energized to a current of 74.2A at 77K in a liquid nitrogen experiment, and 895A at 4.2K in a liquid helium experiment. Then, the generated central magnetic fields (B0) were 0.48T, and 5.68T, respectively. Thus, high current operation and high magnetic field generation were first occurred for a coated conductor coil. The maximum hoop stress of the coated conductor in the coil was 67MPa in the liquid helium experiment. In terms of 0.2% proof strength of coated conductors, the tolerable stress for GdBCO conductors can be increased. The GdBCO coated conductor is promising for magnetic field application.

Ac losses in a YBa2Cu3O7−x coil

The properties of a small pancake coil made with a 10mm wide copper-stabilized YBa2Cu3O7−x (YBCO) coated conductor were investigated. The radial component of the magnetic field was mapped at the coil edge in both the dc and ac regimes and differs significantly from that calculated assuming a uniform current distribution. The observed hysteresis indicates the strong influence of the ferromagnetic properties of the substrate. The ac losses of the coil were measured for ac frequencies between 60 and 1000Hz. The differences in properties of the YBCO coil and a similarly prepared copper coil are discussed.

Properties of a YBCO Pancake Coil Operating With AC Current at Frequencies up to 1000&lt;tex&gt;$rm Hz$&lt;/tex&gt;

A small pancake coil was wound using 1.2 m long, 10 mm wide YBCO coated superconductor tape with impregnation by epoxy resin. The coil was immersed in liquid nitrogen and tested in several regimes. In the DC regime, we measured the I-V curve, the hysteresis of the magnetic field-current curve at liquid nitrogen temperature and the radial component of the coil field at the coil edges. The critical currents of a short sample at 77 K were also measured and compared with those of the coil. The AC losses measured in the frequency range from 60 Hz to 1000 Hz are compared with those of a similar coil wound with copper tape. The coil heating due to AC losses was monitored. At 60 Hz, the losses of the YBCO coil were nearly two orders of magnitude lower than those in the Cu coil. With increasing frequency, this difference becomes smaller, but the YBCO coil still exhibited lower losses at 1000 Hz

Electromechanical studies of YBaCuO tape fabricated by metal-organic chemical vapor deposition for coil applications

YBCO coated conductor has high prospects for coils used in high magnetic fields, but not only its higher transport characteristics but also its adequate workability are required for coil applications. In these studies, mechanical properties of YBCO tape fabricated by multistage chemical vapor deposition were investigated by flat-wise and edgewise bend strain tests. After estimation of influences of its bend strain and its self magnetic field, a small coil of the long YBCO tape could be manufactured. Basic characteristics of the coil were investigated and maximum magnetic field of 0.4 T class was achieved in decompressed liquid nitrogen by the small YBCO coil.

Tests of a Bi/Y Transformer

In the context of a European project (READY) we designed, constructed and tested a single phase transformer. The primary winding uses a PIT-Bi-2223 tape. The secondary is wound with two lengths (2 /spl times/ 8 m) of YBCO coated conductor. With these two lengths in series, the secondary characteristics are 75 V-50 A at 77 K. We designed the transformer with an integrated refrigerator. The cooler is a high cooling capacity (100 W at 80 K) single stage coaxial pulse tube cooler (PTC) developed by Air Liquide. We chose a cold magnetic circuit with low iron loss FeSi sheets. The cryostat is then very simple with metallic vessels and only one interface with the PTC, and the cooling fluid is helium in order to investigate a wide temperature range (40-80 K). About 30 hours are required to reach 70 K from 300 K using only the PTC. This paper presents the transformer layout and some electrical tests (no load and short circuit tests). The ac losses are high in the PIT coil, but in good agreement with numerical simulations. In the YBCO coil the ac losses are lower and in good agreement with the Norris formula.

Surface YBa/sub 2/Cu/sub 3/O/sub 7/ receive coils for low field MRI

The authors present results from magnetic resonance imaging experiments using superconducting thick film YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) receive coils. The coils' properties and image qualities are compared with identical copper and silver It was found that the SNR (at 77 K) and image quality obtained using the YBCO coil is significantly increased relative to metal coils of identical geometry at 77 K.

YBaCuO input coils with low T c and high T c SQUIDs

We have efficiently coupled single-level YBaCuO (YBCO) input coils to superconducting quantum interference devices (SQUIDs) using coils and SQUIDs made on separate substrates and sandwiched together. At 4.2 K, the coupling efficiency α to a low noise Nb-based SQUID was ∼0.76, with no increase in flux noise. At 77 K, the coupling efficiency of a YBCO coil to a TlBaCaCuO SQUID was ∼0.81. The magnetic field sensitivity of the coupled high Tc SQUID at 77 K was 9 pT/√Hz at 1000 Hz, a five times improvement compared to the bare SQUID.