2G YBCO Tapes Research Articles

A Cyclotron Magnet Case Study: Would Replacing the LTS Coils With HTS Coils Make Sense?

The cyclotron gas-stopper magnet (CGSM) at Michigan State University (MSU) was used as model for a study for whether HTS conductor is feasible for use in cyclotrons. The outside diameter of the CGSM split warm iron magnet yoke is 4 m, with a pole radius of 1.1 m. The desired field shape is obtained by shaping the iron pole profile. Each superconducting coil is in a separate cryostat. The two coils are connected in series through the warm electrical connection. Each coil is wound and potted within an 80 mm × 80 mm cross section and mounted within a 304 stainless steel helium vessel that is designed to carry magnetic forces to the magnet cold mass supports without excessive deflection. The iron is split, so that the cyclotron chamber and RF extraction system can be mounted between the two poles. This magnet has been successfully cooled down and tested at MSU to its 180-A operating current. One can ask the following question, “If one built a cyclotron magnet the size of the MSU cyclotron gas-stopper magnet, would replacing the existing LTS coils with HTS coils using a standard second generation HTS tape make technical sense or economic sense?” The report will explore the technical and economic issues of replacing an existing LTS coil with an HTS coil of the same size and shape, using 2G YBCO tape conductor commercially available in 2015 in place of the Nb-Ti used in the magnet.

Quench Characteristic and Minimum Quench Energy of 2G YBCO Tapes

Due to a much extended current sharing regime and to the power law like E-J characteristics instead of the critical state, the quench characteristics of high temperature superconductors (HTSs) potentially differs from LTS. The understanding of thermal stability is therefore of crucial importance to meet the performance requirements set up by applications. In this work the quench characteristic and MQE measurements were carried out on 2G YBCO tape in self-field and field (0.1 T) scenario, in a wide range of temperature (40-80 K), where the conductors have a modest power exponent n ~ 20-30. The study focuses in particular on high current in the vicinity of Ic (j = I/Ic ~ 1), where power law current sharing dominates. Instead of vanishing with (1 - j), as expected for the critical state, the measured MQE is much higher near Ic. The high MQE and the heat generation of a non-(1 - j) scaling were constantly found in the temperature range 40 K-80 K of measurements. Such an enhanced MQE in the high current regime could be significant for HTS applications and suggests the possibility of sufficient stability very close to Ic. We show that the results can be explained by approximating the non-linear current sharing power-law heat generation with temperature by an equivalent critical state at a higher current sharing temperature. While the power-law and critical state converge at low j regions, the non-linear temperature dependence of thermal properties needs to be considered due to elevated quench/current sharing temperatures.

HTS Coils Fabrication From Commercial 2G YBCO Tapes: Measurements and Simulation

In this paper, YBCO-based commercial wire (tape) has been used for fabricating coils in doubled pancake configuration assembly. The tape has been wound on a circular-shaped support; lap joints between coils have been realized with particular attention to resistance minimization. Each high-temperature superconducting coil by itself, and the assembly afterward, has been powered and the current-voltage curves at different temperatures have been measured using a four-probe technique. Critical currents have been computed by using the electric field criteria. Applied magnetic field was both in perpendicular and parallel configuration. A magneto-static finite element model has been developed by using commercial software, in order to check the agreement with experimental results.

Flux pinning distribution and E-J characteristics of 2G YBCO Tapes

E-J characteristics of SuperPower YBCO 2G tapes have been measured in the temperature range 15K-80K. It was found that the E-J characteristics deviate significantly from the standard power-law behaviour with apparent power exponent decreasing continuously at high voltage with increasing current. The deviation of E-J characteristic from the standard power-law was found to be consistent with a Weibull distribution of critical current. The identical scaling of E-J characteristics above 40K suggests a common critical current distribution. At lower temperature the critical current distribution becomes narrower to give higher apparent power exponent at lower temperature. With the critical current distribution model it is shown that the dissipation can be correlated directly to the flux flow resistance of Bardeen. The distribution of the critical current can be associated with a distribution of pinning potential of collective pinning and the headline pinning range is obtained as function of temperature.

Ybco Superconductor Characterization under Shear Strain

YBCO based high temperature superconductors in practical applications are subjected to shear strain. `Critical current characteristics’ of such high temperature superconductors are known to get degraded in strain state. In this work, shear stresses resulting from the finite twisting of HTS tape having varying widths have been modeled using FEA analysis for different twisting angles. Supporting the findings of this model, an American Superconductor Corporation (AMSC) 2G YBCO tape of a given width has been twisted and experimentally investigated in self field for a given current ramp-rate. Under uniform twist of the YBCO tape at 77 K, the degradation in the current carrying ability up to 30% was observed. The irreversibility in the current carrying ability of HTS tape was also observed beyond the twisting angle per unit length of 25 degree/cm. The superconductor to resistive transition index, `n’ is found to behave in an identical manner to the critical current as a function of twisting angle. Such degradation is largely attributed to the torsional shear strain resulting from the twisting. High temperature second generation (2G) commercial grade YBCO coated conductors are available now-a-days in long length for extensive uses in electrical power cable (Rutherford cable) in the form of stacked twisted HTS conductors [1-3]. Even the proposal of coated conductor in conduit cable (CCICC) employs the idea of stacked HTS conductor twisted over different transposition lengths [4]. In such applications individual HTS tape subjects to twist induced shear stress/strain. The strain beyond a certain limit can critically affect the transport property as well as can modify the super-current carrying path in the coated conductor [5]. Hence, it has become necessary to ensure the maximum tolerable shear stress/strain applied to HTS which could induce recoverable damage. Previous works on the torsoinal strain dependence of Bi-2223, YBCO coated conductor shows experimental findings on Ic degradation behavior. Finite element analyses along with theoretical co-relation also have been surveyed on torsion experiments of high temperature superconducting tapes [6-8]. In case of pure torsion, the strain induced on the tape cross-section is non-uniform and complex. However, the maximum torsional shear strain could be determined at the midpoint of the width side of the cross-section using the formula (et = tθ/L) [8-9]. In the present study, FEA modeling for shear stress developed on 2G YBCO tape of different widths has been considered. Further, the shear strain dependence of the critical current of coated conductor has been investigated experimentally for a high current ramp-rate. In this paper, the FEA analysis is discussed in section-II. The experimental results and conclusion are discussed in section-III & section-IV respectively.

E-J Characteristic of 2G YBCO Coated Conductor Tapes at Different Temperatures

Abstract The E-J characteristics of High temperature superconducting (HTS) composite are not only fundamental to the understanding of flux dynamics and flux pinning but also the quench behaviour of HTS at an extended temperature range. The present work present the E-J characteristics of state-of-the-art 2G YBCO Tapes of SuperPowerTM measured at different temperatures between 40K and 80K. The results revealed a appreciable deviation from the power-law E-J characteristics, an established framework for 1G 2223 conductors. In contrast, the equivalent power exponent n was found to decrease with increasing current, resulting in a reduced rate for the non-linear increase of voltage at higher current. The softening of E-J characteristic remain unchanged as the temperature is lowered. Furthermore the power exponent n at the critical current appears to be constant increasing at lower temperatures. Analysis presented in the work shows that the collective pinning of vortex glass gives a satisfactory quantitative description of the E-J characteristics of 2G YBCO tapes. A constant collective pinning potential U / k B T ∼ 15 and vortex glass exponent μ ∼ 2 were found for temperatures between 45 K and 80 K.

HTS twisted stacked-tape cable conductor

The feasibility of high field magnet applications of the twisted stacked-tape cablingmethod with 2G YBCO tapes has been investigated. An analysis of torsional twiststrains of a thin HTS tape has been carried out taking into account the internalshortening compressive strains accompanied with the lengthening tensile strainsdue to the torsional twist. The model is benchmarked against experimental testsusing YBCO tapes. The critical current degradation and current distribution ofa four-tape conductor was evaluated by taking account of the twist strain, theself-field and the termination resistances. The critical current degradation for thetested YBCO cables can be explained by the perpendicular self-field effect. Itis shown that the critical current of a twisted stacked-tape conductor with afour-tape cable does not degrade with a twist pitch length as short as 120 mm.Current distribution among tapes and hysteresis losses are also investigated. Acompact joint termination method for a 2G YBCO tape cable has been developed.The twisted stacked-tape conductor method may be an attractive means for thefabrication of highly compact, high current cables from multiple flat HTS tapes.

Design and Results of a YBCO Magnet for a 95 GHz Gyrotron Application

The use of high-temperature superconductors (HTS) for cryogen-free gyrotron magnets will greatly reduce the power requirement and overall physical size of the system. Because some gyrotron systems are mounted on a vehicle and need to be mobile, the refrigeration system must be as lightweight and compact as possible. Cryomagnetics previously built a BSCCO magnet based on the design of a NbTi magnet. The limiting factors with this magnet were the cost of the BSCCO material and the excessive weight due to the necessity of field-shaping iron. Since it is projected that the cost of 2G YBCO tape will become significantly lower than BSCCO, and the critical current will also increase, the next generation of these magnets will mitigate those problems. The first objective was to study quench behavior in HTS coils. Cryomagnetics has also designed, built and successfully tested a 3.57 T magnet wound with 2G YBCO tape. This is the first full-scale magnet of its kind built using 2G YBCO tape. It consists of 10 double-pancakes operating at 140 A and 20 K. A single-stage cryocooler is used to cool the magnet. Because the performance of YBCO material is improving quickly, future magnets will be able to operate at lower current, higher temperature and will be lighter weight.

Numerical and Experimental Analysis of ${\rm I}_{\rm c}$ and AC Loss for Bent 2G HTS Wires Used in an Electric Machine

Application of the second generation High Temperature Superconducting (HTS) YBCO tapes has been increasingly popular since the low-cost superconducting materials were discovered. This paper mainly presents the properties of two types of 2G YBCO tapes from American Superconductor Corporation (AMSC) and SuperPower, Inc, respectively. All superconducting materials are cooled by liquid nitrogen (77 K). At first, we will introduce the basic design strategy for synchronous motor and characteristics of superconducting materials. Then, the basic principle and algorithm for calculation and measurement of Ic and AC transport loss will be explained. Secondly, the novel measurement system using a high-precision digital lock-in amplifier is presented. Finally, we present and analyze simulation and experimental results for two types of YBCO tapes in bending condition.

${\rm I}_{\rm c}$ and AC Loss of 2G YBCO Tape Measurement for Designing and Fabrication of an HTS Motor

This paper presents a new synchronous motor whose stator uses 2 High Temperature Superconducting (HTS) pancake-coils wound from 2G HTS tapes and 4 conventional copper coils, while the rotor contains 75 bulk HTS elements. All superconducting materials are cooled by liquid nitrogen (77 K). The HTS and copper armature includes 3 phase circuit and is mounted in the slots of a nonmagnetic and insulating material. Each phase winding, fabricated in the form of a pancake-coil, is distributed in slots of 60-degree intervals. YBCO melt-textured pucks were incorporated into the rotor and the reluctance was tested. In addition, a unique cooling system using a copper conductor is designed, which is similar to that of a superconducting flywheel. Finally, we present and analyse results on measuring DC critical current and self-field AC loss in HTS tape supplied by American Superconductor. The measurement systems are described. The results on the AC loss measurement of YBCO samples are presented and analyzed within the framework of the critical state model.

Magnetisation ac-loss measurements on YBa2Cu3O7 tapes with weakly-ferromagnetic NiW substrates

The commonly-used NiW substrate for RaBiTS-process 2G conductors is weakly-ferromagnetic, so that the interaction between the superconductor and the magnetic material is important, particularly for AC applications. We have studied the AC losses in a model system of a high-current 2G YBCO tape that has been fabricated on a non-ferromagnetic substrate (Hastelloy), with and without adjacent weakly-ferromagnetic Ni-5at%W tape(s). This approach allows us to explore fully the interactions between substrate and superconductor. The sample architectures examined were YBCO, NiW-YBCO, NiW-YBCO-NiW, YBCO-NiW and NiW in magnetic fields applied perpendicular, parallel and at 45° to the tape normal. The power losses were measured in applied ac-magnetic fields of up to 100 mT rms at close to power line frequencies and at liquid nitrogen temperature. In addition, complementary magnetisation measurements were made on some architectures. Below the penetration field of the superconducting tape, the overall AC losses in the perpendicular field orientation are reduced significantly by a single adjacent NiW layer, with a further slight decrease with the addition of a second NiW layer. Above the penetration field the losses are not altered greatly. At very low fields the dominant loss mechanism appears to be the ferromagnetic hysteresis loss.

Numerical Simulation of AC Current Limiting by Bulk Bi-2212 Ceramic and 2G YBCO Tapes

Second generation YBCO tapes have extremely high (~10000) geometric aspect ratios (width to thickness), which require huge computer resources and thus renders the simulation of fault current limiters (FCLs) based on them very difficult. In this paper we offer a solution to the aforementioned problem by assuming a linear scaling of the thermal and electrical properties with the geometrical size. The results obtained in proprietary software are in very good agreement with those from commercial FEM software. We also compare the limiting properties of YBCO and BSCCO, and find that in order to enable BSCCO to carry the same rated current as YBCO, one needs more than 40 times larger volume of BSCCO. In addition, YBCO limits the current quicker and much more efficiently than BSCCO. Finally, we study the influence of defects on the current limiting properties of HTS tapes and find that defects lead to stronger heating and more non-uniform current and temperature distributions, especially in BSCCO.

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.