Stacks Of Coated Conductors Research Articles

Modified Interconductor Contact Resistivity in Coated Conductor Stacks and Roebel Cables

Interconductor contact resistivity ( ICR ) is a key property in determining the stability and current sharing of coated conductor cables. Most coated conductor cables have relatively high contact resistivity and low current sharing as fabricated because of surface roughness and an oxide layer that forms on the Cu-stabilizer. Here we work to quantify the differences in using three methods to modify ICR : sample diffusion bonding, deposition surface modification, and thin conformable inserts. At first a stack of two coated conductors was used to simulate a cable. This stack was put under transverse pressure and exposed to moderate temperatures to promote diffusion bonding via the removal of the unstable Cu-oxide layer ( ICR measurements were performed on stacks before and after diffusion bonding, for stacks with deposited surface layers, and for stacks with a smart material insert. In addition, Roebel cables were prepared with deposited layers on the individual strands and atmospherically controlled diffusion bonding, and ICR was compared between the as-received and modified Roebel cables as well as coated conductor stacks.

Fabrication of a Compact High-field Magnet by Coated Conductor Stacks

A bulk magnet was fabricated by stacking coated conductors with enhanced Jc by introducing nanorods. The fabricated CC stack consists of 100 pieces of CCs, and it was magnetized by field-cooling method in an applied field of 14.5 T down to 4.2 K. By reducing the magnetic field down to zero at a ramp of rate 1 T/min, we have succeeded in trapping 12.6 T at the center of the CC stack. We also confirmed that the CC stack can sustain the high trapped field for more than 20 min, which demonstrates the practical usefulness of such a staked CC magnet.

The influence of cyclical lateral displacements on levitation and guidance force for the system of coated conductor stacks and permanent magnets

For the development of levitation bearings and transportation systems based on coated conductor (CC) tapes it is important to know not only information on levitation force at vertical displacement, but also the data on stability of the system during lateral displacements of superconductor relative to permanent magnets. In this paper, we present new experimental results on investigation of both levitation and guidance forces of CC-tapes stacks subjected to cyclical lateral displacements relative to the position of permanent magnet. For the measurements we used 12 mm wide commercially available CC-tape manufactured by SuperOx. The long tape was cut into pieces 12 mm × 12 mm which then put to the stack. Total number of the single tape in the stack N was varied from 5 to 100. Under lateral displacement we measured the dependencies of levitation and guidance forces on value of displacement for various number of N and found that both levitation and guidance forces tend to decrease as a number of periodical cycles of displacements increase. The rate of reducing of forces was observed to depend on the number of tapes in the stack. We observed that dependencies of levitation and lateral forces on lateral displacements have a hysteresis. The area of hysteresis, which corresponds to the magnitude of energy losses, was found to decrease with increasing of number of elements in the stack. Experimental results are in a good agreement with the theoretical calculations of lateral force.

Magnetizing high-Tc superconducting coated conductor stacks using a transformer–rectifier flux pumping method

High-Tc superconducting (HTS) coated conductor stacks have been demonstrated as very promising superconducting permanent magnets (PMs), to be used in applications like rotary motors and levitation devices. In this paper, we present a novel magnetization method to address a key problem in the application of HTS stacks, which is effective magnetization with compact, light and efficient setups. The proposed method, transformer–rectifier flux pumping, uses the mechanism developed for HTS coils to magnetize the modified HTS stacks. The accumulation of flux with time is achieved in the HTS stacks. The results show that the flux pump can effectively magnetize long HTS stacks with a magnetic field applied to a much smaller area. Operational characteristics of the flux pump are experimentally studied and some explanations are proposed for the distinct behaviors observed in the experiments. This study heralds a new direction in the magnetization of superconducting PMs and has the potential to be a practical method for industrial applications.

Trapping Large Magnetic Field by Suppression of Thermomagnetic Instability in Coated Conductor Stacks

A new type of bulk magnet can be fabricated by simply stacking short segments of coated conductors (CCs) of 123-type superconductors. To enhance the trapping field of the CC stack, we irradiated 800 MeV Xe ions to CCs. The fabricated CC stack consists of 80 pieces of irradiated CCs, and it was magnetized by field-cooling method in an applied field 9 T down to 5 K. When we tried to trap a large field by reducing the field at ramp rates faster than 0.5 mT/s, flux jumps occurred, which abruptly suppressed the trapped field. By reducing the magnetic field down to zero at a ramp rate slower than 0.5 mT/s, we have succeeded in trapping 9 T at the center of the CC stack. We also confirmed that the CC stack can sustain the high trapped field for more than 20 min, which demonstrates the practical usefulness of such a staked CC magnet.

Experiment and simulation of superconducting magnetic levitation with REBCO coated conductor stacks

Three superconducting stacks made of 120 REBCO coated conductor tapes were each fabricated and assembled to obtain several REBCO modules. Their levitation responses over two different permanent magnet (PM) guideways were investigated by experiment and finite element simulation. For the experiment, a test rig was developed that can measure the force in the three directions for any given relative movement between the REBCO stacks and the PM guideway. For the finite element simulation, a 2D H-formulation was adopted. To treat the high aspect ratio of REBCO tapes, an anisotropic homogenization technique was used. The agreement between the measurements and the simulations is good, thus validating the modeling methodology. It was observed from the experiment and simulation results that the perpendicular field contributes to the levitation force whereas the parallel field is responsible for the guidance force, as a result of the existence of anisotropy on the local magnetic stimulation. Based on that, promising REBCO modules including both longitudinal and transverse arrangements of REBCO stacks were proposed and tested, in terms of providing a significant levitation force with the lateral stability preserved. Moreover, a pre-load process able to suppress the relaxation of the levitation force was put forward. To conclude, this study outlines explicit principles to obtain an appropriate layout of coated conductor stacks that could be effective for practical magnetic levitation operation.

Simulation and experiments of stacks of high temperature superconducting coated conductors magnetized by pulsed field magnetization with multi-pulse technique

High temperature superconducting bulks or stacks of coated conductors (CCs) can be magnetized to become trapped field magnets (TFMs). The magnetic fields of such TFMs can break the limitation of conventional magnets (<2 T), so they show potential for improving the performance of many electrical applications that use permanent magnets like rotating machines. Towards practical or commercial use of TFMs, effective in situ magnetization is one of the key issues. The pulsed field magnetization (PFM) is among the most promising magnetization methods in virtue of its compactness, mobility and low cost. However, due to the heat generation during the magnetization, the trapped field and flux acquired by PFM usually cannot achieve the full potential of a sample (acquired by the field cooling or zero field cooling method). The multi-pulse technique was found to effectively improve the trapped field by PFM in practice. In this work, a systematic study on the PFM with successive pulses is presented. A 2D electromagnetic-thermal coupled model with comprehensive temperature dependent parameters is used to simulate a stack of CCs magnetized by successive magnetic pulses. An overall picture is built to show how the trapped field and flux evolve with different pulse sequences and the evolution patterns are analyzed. Based on the discussion, an operable magnetization strategy of PFM with successive pulses is suggested to provide more trapped field and flux. Finally, experimental results of a stack of CCs magnetized by typical pulse sequences are presented for demonstration.

Optimisation of composite superconducting bulks made from (RE)BCO coated conductor stacks using pulsed field magnetization modelling

Coated conductors, although designed to carry transport current, are ideally suited to carrying persistent current and can therefore be cut and stacked to form a type of composite bulk which has superior thermal properties compared to existing bulks despite having less than 2 % superconductor by volume. The magneto-thermal modelling reported follows on from previous experimental work on pulsed magnetization of a 12 mm square tape stack. The magnitude of the applied field has a strong effect on the trapped field and flux. The optimum applied field depends on sample height and the maximum trapped field and flux saturates as the height reaches the diameter of the stack. The nature of a composite bulk made from a stack of tapes gives complete control over the height of the stack which needs to be optimised for pulsed magnetization.