High-temperature Superconducting Tapes Research Articles

Charging an HTS Coil: Flux Pump With an HTS Square Bridge

In recent years, along with the unceasing development of High-temperature Superconductor (HTS) technology, the performance of HTS tape has been improved significantly, including the higher critical current, lower joint resistance, and longer tape length etc. To use HTS instead of Low-temperature Superconductor (LTS) in Magnetic Resonance Imaging (MRI) applications, the flux pumping method with travelling wave has been investigated. In this paper, our new experimental results were regarding to the frequency, amplitude, and wavelength characteristics of charging an HTS coil with an HTS square bridge, using an electromagnetic flux pump designed at the University of Cambridge.

Quench Protection Modeling of an HTS Magnet for MRI System

The High-temperature Superconducting (HTS) magnet is one of the most promising technologies to accommodate various high field applications. With the high magnetic field, the quench problem of HTS material should be considered with extra care, and the specific quench protection scheme should be projected. In this article, the modeling of the quench protection of an HTS magnet for the MRI system is presented. The modeling strategy was composed of two stages: Stage 1 the HTS magnet was in the normal operation circuit, and Stage 2 the HTS magnet was switched to the heat dissipation circuit. The 1st stage was based on the finite element method (FEM) using the H-formulation together with the thermal-coupled model, which simulated the current distribution of typical HTS tapes and showed the process of temperature increase when the hot source occurred. The 2nd stage was based on the analytical model to investigate the reasonable damping resistance and the hot spot temperature of the HTS magnet.

Low-resistance joint development for segment-fabrication of high-temperature superconducting fusion magnets

Two designs have been proposed for segment-fabrication of the high-temperature superconducting (HTS) helical coils in the FFHR-d1 helical fusion reactor, joint-winding of the HTS coils wound by connecting conductor segments, and the ‘remountable’ HTS magnet (here ‘remountable’ means being able to mount and demount repeatedly) assembled from coil segments with demountable joints. A bridge-type mechanical lap joint and mechanical edge joint are planned to be applied to those two designs, respectively. This paper presents the progress in electrical and mechanical performances of the mechanical joints of HTS conductors and methods to evaluate their quality for segment-fabrication of HTS helical coils. R&D of joint performance has progressed during this decade and it shows acceptable performance for the HTS helical coils. The contact-probing current transfer length method and x-ray computer tomography scanning are promising for quality assessment of the joints and HTS tapes.

Cryogenic testing of microphones and preamplifiers for MEMS quench detection in superconducting magnets

High Temperature Superconducting (HTS) tapes are needed for high field magnets in high-energy physics and nuclear fusion. However, quench events, in which the superconductor loses its superconductivity, can cause significant and costly damage to the device if allowed to propagate. Consequently, rapid quench detection methods are crucial. Existing methods such as voltage taps can be less effective in HTS applications, as the normal zone propagation velocities of HTS devices are slow, drastically increasing quench detection time (Iwasa, Cryogenics, 2003). We propose an alternative method that utilizes a linear array of MEMS microphones embedded in the cryogenic coolant channel (Takayasu, IEEE Transactions on Applied Superconductivity, 2019). The system requires amplifiers and acoustic sensors capable of operation at cryogenic temperatures as low as 4.2 K, although operation at 20 K may be sufficient. We report on cryogenic testing of the Vesper microphone preamplifier ASIC, as well as opamp based preamplifiers using the OPA838, LT1464, AD8591, and AD8057 ICs. The amplifiers are being used with the Vesper VM4 MEMS microphone to achieve acoustic measurements in gaseous helium at temperatures as low as 8 K. A charge amp scheme using the AD8591 achieved the best results to date. [Support from DOE STTR DE-SC0019905.]

Analysis of AC Transport Loss in Conductor on Round Core Cables

The conductor on round core (CORC) cable wound with second-generation high-temperature superconducting (HTS) tapes is a promising cable candidate with superiority in current capacity and mechanical strength. The composing superconductors and the former are tightly assembled, resulting in a strong electro-magnetic interaction between them. Correspondingly, the AC loss is influenced by the cable structure. In this paper, a 3D finite-element model of the CORC cable is first built, and it includes the complex geometry, the angular dependence of critical current and the periodic settings. The modelling is verified by the measurements conducted for the transport loss of a two-layer CORC cable. Subsequently, the simulated results show that the primary transport loss shifts from the former to the superconductors as the current increases. Meanwhile, the loss exhibited in the outer layer is larger than that of the inner layer, which is caused by the shielding effect among layers and the former. This also leads to the current inhomogeneity in CORC cables. In contrast with the two-layer case, the simulated single-layer structure indicates stronger frequency dependence because the eddy current loss in the copper former is always dominant without the cancellation of the opposite-wound layers. The core eddy current of the single structure is denser on the outer surface. Finally, the AC transport losses among a straight HTS tape, a two-layer cable and a single-layer cable are compared. The two-layer structure is confirmed to minimise the loss, meaning an even-numbered arrangement makes better use of the cable space and superconducting materials. Having illustrated the electro-magnetic behaviour inside the CORC cable, this work is an essential reference for the structure design of CORC cables.

Current transfer length in individual superconducting tape and quasi-isotropic superconducting strand

In the practical applications, superconducting cable should be connected to the power supply by copper terminals. The lap joint between the superconducting cable and copper terminals has some significant effects on the current transfer length (CTL). Firstly, three kinds of soldering methods for an individual high temperature superconducting (HTS) tape are modeled and its CTL is calculated. Then, the distributions of electric field and current along a quasi-isotropic superconducting strand are simulated. Lastly, the CTL of a quasi-isotropic strand is measured by the distribution of electric potential along the strand. The results of simulation and experiment indicate different soldering methods have great influence on the CTL of the superconducting tape/strand.

Effects of twisting pitch on critical current of HTS tapes in Quasi-isotropic strand

With the development of the second generation (2G) high temperature superconducting (HTS) tape, a several types of high current-carrying conductors have been proposed, among which the quasi-isotropic strand (Q-IS) can effectively improve critical current anisotropy. Twisting technology is essential in practical applications as it can effectively improve current uniformity and stability, reducing AC loss. Aiming at the structural characteristics of the Q-IS, this paper establishes a longitudinal strain model to describe the torsional strain of the tapes in the strand and studies the dependence of critical current characteristics on twisting pitch by theory and experiment. A set of twisting devices were designed, experiments were carried out on the tapes at three different positions in the Q-IS, and their critical currents under different twisting pitches were measured at 77 K. After comparing experiments, the results show that the longitudinal strain model can well describe the characteristics of the critical current of the twisting tapes in a Q-IS.

Research on DC overcurrent characteristics and life prediction of YBCO tapes

The repeatability of superconducting DC limiters has not been introduced by other literature. Understanding the performance degradation characteristics of high-temperature superconducting (HTS) tapes under the overcurrent is of great significance for popularizing the application of a resistive-type DC superconducting fault current limiter. In this paper, the threshold experiment and repeated DC overcurrent experiment of HTS tape were carried out. With the critical current decay of the tapes under repeated DC overcurrent recorded, the current threshold under the DC overcurrent was obtained. And the performance degradation characteristics of the tapes under different repeated overcurrents were analysed, the resistance characteristics of the samples during different currents were compared. The results show that the DC overcurrent threshold of the copper-encapsulated second-generation (2G) YBCO tapes samples of Suzhou Advanced Materials Research Institute is 2.3/4 ms/3700 A. Under the repeated overcurrent, the performance degradation of the samples is abrupt, and the critical value is between 2.3/4 ms/3500 A and 2.3/4 ms/3550 A. The overcurrent has a limited influence on the magnitude of the resistance and will not affect the change trend of the resistance. The resistance at the falling edge of the current was greater than that at the rising edge of the current. The simulation was carried out by using COMSOL Multiphysics, and the simulation results matched the experimental results. The life prediction model of the samples was established and it was found that there is a custom equation relationship between the reliability of superconducting tapes and the number of the overcurrent, and the performance decline of this kind of samples is catastrophic.

Current Transfer Length and Interfacial Resistance Between Superconductors and Metals in Commercial REBCO Tapes and Cables

High-temperature superconductors (HTSs) are used in a growing number of applications. Technical HTSs usually consist of superconducting and metallic parts. HTS conductors are characterized by their critical current density and critical temperature defined by the superconducting layer, but other quantities such as the current transfer length (CTL) and the interfacial resistance between the superconducting part and surrounding metals are also important characteristics. Since the CTL and the interfacial resistance cannot be measured directly, a method is known to evaluate these parameters indirectly from the characterization of the spatial variation of the surface voltage of the metallic surrounding that is in direct contact with the superconducting material. In this article, we present the details of the experimental method developed to determine the CTL and the interfacial resistance of commercial second-generation HTS conductors of any type of architecture. We apply this method to quantify the interfacial resistance of ten HTS tapes from six major tape manufacturers. Also, since the architecture of a commercial HTS tape is usually more complicated than an “idealized” superposition of two flat layers (i.e., a metallic layer and a superconducting layer), recourse to three-dimensional numerical modeling has been required in order to understand in details the current transfer mechanisms taking place within the tape. We also show that the method can be applied directly to more complicated structures encountered in HTS cables, namely to determine the barrier resistance between a metallic structure in contact with a superconducting tape, providing important information required in numerical modeling when designing superconducting devices.

High-temperature superconducting screens for magnetic field-error cancellation in accelerator magnets

Accelerators magnets must have minimal magnetic field imperfections to reduce particle-beam instabilities. In the case of coils made of high-temperature superconducting (HTS) tapes, the magnetization due to persistent currents adds an undesired field contribution, potentially degrading the magnetic field quality. In this paper we study the use of superconducting screens based on HTS tapes for reducing the magnetic field imperfections in accelerator magnets. The screens exploit the magnetization by persistent currents to cancel out the magnetic field error. The screens are aligned with the main field component, such that only the undesired field components are compensated. The screens are self-regulating, and do not require any externally applied source of energy. Measurements in liquid nitrogen at show for dipole-field configurations a significant reduction of the magnetic field error up to a factor of four. The residual error is explained via numerical simulations accounting for the geometric defects in the HTS screens, achieving satisfactory agreement with experimental results. Simulations show that if screens are increased in width and thickness, and operated at , field errors may be eliminated almost entirely for the typical excitation cycles of accelerator magnets.

Optimization Design of a 40-kJ HTSPPT Module for Inductive Pulsed Power Supply

The topology category based on high-temperature superconducting pulse power transformer (HTSPPT) is an important branch of inductive pulse power supply (IPPS) for rail guns. Optimization design of the HTSPPT can improve the overall performance of the IPPS system. The purpose of this article is to design a 40-kJ air-core HTSPPT module with a high-temperature superconducting (HTS) primary winding and a normally conducting secondary winding. According to the application characteristics of the HTSPPT, under the constraints of the three main performance indicators of critical current, stored energy, and coupling coefficient, the amount of HTS tapes and the volume of the entire HTSPPT are reduced as much as possible. The design of the secondary winding combines the electromagnetic launch requirements for the current pulse in the approximate flat waveform and steep pulse rising edge. With this optimized design, a 43-kJ HTSPPT module with a primary inductance of 79.4 mH, a secondary inductance of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$255.4~\mu \text{H}$ </tex-math></inline-formula> , and a coupling coefficient of 0.97 was obtained. Finally, the maximum stress and strain of the designed HTSPPT in the energy storage state are analyzed through the electromagnetic (EM) structure coupling field.

Quench recovery time of an optical fiber encapsulated resistance-type DC superconducting fault current limiter coil determined by a Raman-based distributed temperature sensing system

The superconducting fault current limiter (SFCL), as a very important power facility for high voltage DC system, can consume fault current energy and make DC breakers cut off fault transmission lines successfully. For the moment of reclosing DC breakers, the quench recovery time of the SFCL must be determined in advance. In this article, according to the design requirement of a 160 kV/1 kA resistance type DC SFCL in the Nan’ao multi-terminal flexible HVDC transmission demonstration project, the quench recovery time of a SCFL coil wound by two optical fiber encapsulated high temperature superconducting (OFE-HTS) tapes was checked by our optimized Raman-based distributed temperature sensing (Rm-DTS) system. The experimental results show that the used OFE-HTS tapes can withstand a 1500 A/100 ms AC short circuit current impact and are qualified with the SFCL. Furthermore, the quench recovery time of the SFCL coil measured by the Rm-DTS was 2930 ms. During the quench, the highest temperature zone occurred near the center of the SFCL coil.

Experimental and Numerical Analysis of High-Temperature Superconducting Tapes Modified by Composite Thermal Stabilization Subjected to Thermomechanical Loading.

The strain behavior of SiC/Stycast 2850 FT composites under thermomechanical loading using a finite element analysis (FEA) was studied. These composites can serve as thermal stabilizers of high-temperature superconducting (HTS) tapes during limitation event in resistive superconducting fault current limiter (R-SCFCL) applications. For this purpose, the thermomechanical properties of four composite systems with different filler content were studied experimentally. The FEA was calculated using an ANSYS software and it delivered useful information about the strain distribution in the composite coating, as well as in particular layers of the modified HTS tapes. The tapes were subjected to bending over a 25 cm core, cooled in a liquid nitrogen (LN2) bath, and finally, quenched from this temperature to various temperatures up to 150 °C for a very short time, simulating real limitation conditions. The outputs from simulations were also correlated with the experiments. The most promising of all investigated systems was SB11-SiC20 composite in form of 100 µm thick coating, withstanding a temperature change from LN2 up to 120 °C.

‘Defect-irrelevant-winding’ no-insulation (RE)Ba2Cu3O7 − x pancake coil in conduction-cooling operation

This paper reports experiment and simulation results of a ‘defect-irrelevant-winding (DIW)’ no-insulation (NI) (RE)Ba2Cu3O7 − x pancake coil operated in a conduction-cooling environment at temperatures lower than 77 K. The key idea of the DIW high-temperature superconductor (HTS) winding technique, firstly proposed in 2016, is to wind an NI pancake coil with HTS tapes, on purpose, containing multiple ‘defects.’ Using the defect (flawed) HTS tapes, we expect to significantly lower the construction cost of a DIW HTS magnet in marginal sacrifice of the magnet performance. In this study, charging tests of a DIW coil were performed in a conduction cooling system by controlling the temperature of the second stage in a range of 20–65 K, to determine the maximum operable current that avoids thermal runaway of the test coil at a given temperature. Coil terminal voltages, center fields, local temperatures, and power supply currents were carefully monitored during the tests, and the trend for each parameter was obtained. Also, by use of an equivalent circuit model, the test results were numerically analyzed to evaluate the effect of local defects on the overall performance of the test coil in order to validate the feasibility of the DIW HTS coil in operation under the conduction-cooling environment at temperatures lower than 77 K.

AC Loss Evaluation of a Superconducting Pancake Coil with Coated Conductors using an Extended A-V Formulation

In this study, an extended A-V formulation is used to determine the alternating current (AC) losses of a coil made of high temperature superconducting (HTS) tape. The current is the same at each turn by balancing the voltages of each turn. By this modification, this method can be applied in a wide application area. This method also provides the chance to determine the calculations at currents higher than the critical current values in a more stable way. The AC loss characteristics of the pancake coil are examined in detail by changing the current value and frequency. Also, the losses at individual turns (1st, 5th, and 10th) are presented. It was seen that the most outer and inner turns have higher losses than that of the other turns. The AC loss results obtained from the extended A-V formulation used in this study are compared with experimental and H-formulation-based results and are in good agreement. The frequency dependence of the pancake coil's loss is revealed at 5 frequency values. The results show that the loss is frequency and current amplitude dependent as expected. Similarly, the calculated losses for each frequency are in good agreement with that obtained from the H- formulation. The current distributions of a single tape and the entire coil are also presented, and all results are discussed in detail.

Short Circuit and Recovery Time Tests in a helical bifilar R-SFCL module

Superconducting Fault Current Limiter (SFCL) modules with distinct arrangements have been investigated, seeking design and performance enhancements. In this context, this work aims to propose a design for resistive SFCL (R-SFCL) modules, using RE-Ba-Cu-O high-temperature superconducting tapes (HTS) wound on an alternative support material. An R-SFCL bench prototype was designed for 400 A and 500 V. Two pieces of 2G tapes, arranged in a bifilar antiparallel configuration were wound on Acrylonitrile Butadiene Styrene (ABS) tubes, which were made by means of a 3D printer. During the experiments, the SFCL was immersed in an open bath cryostat with liquid nitrogen and several measurements were carried out. The average limited current is about three times lower than the prospective one. To test the SFCL recovery time, the following procedure was adopted: The fault is induced during three cycles, and after, the current is reduced to zero. The SFCL is reconnected after some time considering that the fault was extinct, and the voltage level returned to normal operation. The results showed promising levels of limitation and recovery time for further developments.

Analysis of Overcurrent Performance of YBCO Tape Considering Different Heat Exchange Conductions

The performance degradation of High Temperature Superconducting (HTS) tape, which may be influenced by various heat exchange conditions due to different cooling methods, is of significant for HTS devices under repeated overcurrent. In this article, considering different heat exchange conditions, a series of repeated overcurrent experiments were carried on YBCO tapes under 50Hz sinusoidal alternating current. The degradation of critical current of YBCO tapes after repeated overcurrent was recorded. the resistance characteristics of YBCO tapes under different cooling conditions were analyzed. Through finite element simulation, the temperature and current distribution of YBCO tape under different heat exchange conditions were simulated. The experimental results show that the overcurrent capability of the conductor under conduction cooling is far inferior to the liquid nitrogen immersion method.

Imaging of Strong Nanoscale Vortex Pinning in GdBaCuO High-Temperature Superconducting Tapes.

The high critical current density of second-generation high-temperature superconducting (2G-HTS) tapes is the result of the systematic optimisation of the pinning landscape for superconducting vortices through careful engineering of the size and density of defects and non-superconducting second phases. Here, we use scanning Hall probe microscopy to conduct a vortex-resolved study of commercial GdBaCuO tapes in low fields for the first time and complement this work with “local” magnetisation and transport measurements. Magnetic imaging reveals highly disordered vortex patterns reflecting the presence of strong pinning from a dense distribution of nanoscale Gd2O3 second-phase inclusions in the superconducting film. However, we find that the measured vortex profiles are unexpectedly broad, with full-width-half-maxima typically of 6 μm, and exhibit almost no temperature dependence in the range 10–85 K. Since the lateral displacements of pinned vortex cores are not expected to exceed the superconducting layer thickness, this suggests that the observed broadening is caused by the disruption of the circulating supercurrents due to the high density of nanoscale pinning sites. Deviations of our local magnetisation data from an accepted 2D Bean critical state model also indicate that critical state profiles relax quite rapidly by flux creep. Our measurements provide important information about the role second-phase defects play in enhancing the critical current in these tapes and demonstrate the power of magnetic imaging as a complementary tool in the optimisation of vortex pinning phenomena in 2G-HTS tapes.

Preliminary Conceptual Design Study on HTS Toroidal Field Coil for Compact High Magnetic Field Tokamak

By rapid development of high temperature superconductors (HTS), the HTS magnet has become an enabling technology for compact, high magnetic field tokamaks requiring larger current densities and higher magnetic fields where low temperature superconductors (LTS) have limitations. Thus, a preliminary conceptual design study on HTS magnet for toroidal field (TF) coil, a major component of compact and high field tokamak, is presented to investigate the possibility of the use of an HTS. General features relevant to a large-scale superconducting magnet are discussed such as electromagnetics, mechanical stress, protection, and cryogenics. Two types of HTS tapes are used to design TF coils for a tokamak that targets 9.2 T at its major radius 3.3 m: 1) Design A, using an HTS tape used in previous high field HTS magnets; and 2) Design B, using the same HTS tape with twice the performance. The results convey the possibility of HTS TF coil with currently available technology, but with the expense of a large amount of HTS tapes inferring that a reduction in the HTS tape cost is required. Improvement on the HTS TF coil design is needed using the unique characteristics of HTS. Hence, some of the challenges for HTS fusion magnet are addressed including performance degradation of conductors, quench detection and protection method, and joint for demountable TF coil.

Calculation of CORC Cable Loss Using a Coupled Electromagnetic-Thermal T-A Formulation Model

High-temperature superconducting (HTS) potential solution of solving the urban demand of large load and high-density power. However, the ac loss within the cable body is one of the primary factors limiting the large-scale HTS cable application. Due to the complex spiral structure of CORC (conductor on round core) cable and the high aspect ratio of conductors, modeling and calculation of CORC cable are pretty challenging. To better assess the cable loss, this article illustrates an improved model based on T-A formulation. Combined with a low-dimensional thermal model, the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cBT</sub> (B,T) characteristics of HTS tape are applied in simulation. With a global parameter control equation, the current redistribution of the tape can be analyzed by the model. The loss of a three-phase coaxial (tri-axial) HTS cable under different electric and thermal conditions has been calculated. This article provides a method based on T-A formulation to study the current redistribution and quench process.