High-current High-temperature Superconducting Research Articles

Development of a Superconducting Wire for the Central Solenoid of a Tokamak with Reactor Technologies (TRT)

The article presents the results of the preliminary development of a superconducting wire based on the VS type design and parallel tape packages for the central solenoid of a compact tokamak with reactor technologies (TRT). One of the main problems that must be solved for successfully implementing such projects is to develop high-current high-temperature superconducting (HTS) conductors for toroidal excitation coils and central solenoid sections. The magnetic system compactness entails the need to develop a conductor with a high engineering current density up to 90 A/mm2. The operating current in the windings should be at a level of 60 kA at 15 K in a magnetic field of 15 T. The wire in the central solenoid experiences significant mechanical loads caused by Lorentz forces. In addition, in view of a significant energy stored in the magnet, there is a need to have elements in the conductor able to perform emergency evacuation of energy with an acceptable voltage across the winding and its heating that will not entail damage to its elements. The conductor structure should have enough space to accommodate stabilizing and strengthening materials, as well as cooling channels. Two design versions of the VS type conductor based on radially arranged second-generation HTS tapes and based on parallel packages are considered. The design characteristics of the proposed conductors are analyzed for various operating modes of the tokamak electromagnetic system. The results of FEM-based calculations of the magnetic field distribution in a conductor, its current carrying capacity and energy loss estimation in a varying magnetic field are presented.

Elaboration of a Promising Design of the HTS Conductor for the Central Solenoid of a Compact Thermonuclear Reactor TRT

The results of the preliminary development of the HTS conductor based on the VS-type design and parallel stacks for the central solenoid of the compact thermonuclear reactor TRT are presented. One of the main problems that need to be solved for the successful implementation of such projects is the creation of high-current high-temperature superconducting (HTS) conductors for Toroidal Field coils (TF) and Central Solenoid (CS) sections. The conductor must have a high engineering current density of at least 90 A/mm2. The induction of the magnetic field in the central solenoid reaches 14 T, which leads to the occurrence of large mechanical stresses due to the influence of Lorentz forces. Like many large magnets, CS has a lot of stored energy. For the safe withdrawal of stored energy from the magnet, it requires the inclusion of elements in the conductor that provide an acceptable level of electrical voltage and heating of the conductor insulation. Thus, a sufficient amount of stabilizing and reinforcing materials should be placed in the conductor. In addition, the “cable-in-conduit” type of conductor must have channels for pumping the refrigerant. Two fundamentally different versions of the conductor based on radially arranged REBCO tapes and on the basis of pre-assembled tape packages are considered. Based on the analysis of the magnetic field distribution in the conductor by finite element method, the design characteristics of the proposed conductors under various operating modes of the electromagnetic system (EMS) of the tokamak TRT was evaluated. The results of the evaluation of the current carrying capacity of the conductor and the estimation of energy losses in a changing magnetic field in comparison with known methods are also presented.

Fiber optic quench detection for large-scale HTS magnets demonstrated on VIPER cable during high-fidelity testing at the SULTAN facility

Fiber-optic thermometry has the potential to provide rapid and reliable quench detection for emerging large-scale, high-field superconducting magnets fabricated with high-temperature-superconductor (HTS) cables. Developing non-voltage-based quench detection schemes, such as fiber Bragg grating (FBG) technology, are particularly important for applications such as magnetic fusion devices where a high degree of induced electromagnetic noise impose significant challenges on traditional voltage-based quench detection methods. To this end, two fiber optic quench detection techniques—FBG and ultra-long FBG (ULFBG)—were incorporated into two vacuum pressure impregnated, insulated, partially transposed, extruded, and roll-formed (VIPER) high-current HTS cables and tested in the SULTAN facility, which provides high-fidelity operating conditions to large-scale superconducting magnets. During surface heater induced quench-like events under a variety of operating conditions, FBG and ULFBG demonstrated strong signal-to-noise ratios (SNRs) ranging from 4 to 32 and measured single-digit temperature excursions; both the SNR and temperature sensitivity increase with temperature. Fiber thermal response times ranged between effectively instantaneous to a few seconds depending on the operating temperature. Strain sensitivity dominates the thermal sensitivity in the conditions achievable at SULTAN; however, measurements at higher quench evolution temperatures, coupled to future work to increase the thermal-to-strain signal, show promise for quench detection capability in full-scale magnets where temperature and strain may occur simultaneously. Overall, FBG and ULFBG were proven capable to quickly and reliably detect small temperature disturbances which induced quench initiation events for high current VIPER HTS conductors in realistic operating conditions, motivating further work to develop FBG and ULFGB quench detection systems for full-scale HTS magnets.

Development and test of a 35 kA - HTS CroCo cable demonstrator

The answer to energy-efficient electric power transfer of high currents in the range of several tens of kA can be given by high temperature superconducting (HTS) cables. BSCCO and MgB2 have been used widely for such cables, reaching maximum currents of about 20 kA. REBCO coated conductors are promising for future HTS cables beyond 20 kA and allow the operation based on subcooled liquid nitrogen. Several cabling concepts based on REBCO tapes were developed world-wide to realize such cables. Using the stacked-tape concept, a scalable semi-industrial process was developed by KIT, called HTS CrossConductor (HTS CroCo). Key aspects of the conceptual design of high-current HTS cables are discussed and the design of a 35 kA DC cable demonstrator made from HTS CroCo strands is presented. Aspects regarding joints, current redistribution between individual strands and electrical stabilization are highlighted. The performance of this demonstrator cable was tested, reaching the envisaged current.

Practical Estimation of HTS Dynamo Losses

In High Temperature Superconducting (HTS) DC magnet applications, the current leads contribute a significant fraction of the cooling load. Higher current operation requires larger current leads and a larger thermal bridge into the cryogenic superconducting components. Removal of current leads into the cryogenic environment would lead to a significant reduction in cooling load and hence reduce ongoing operation costs. Superconducting dynamos are a non-contact method of energising HTS coils, capable of producing significant currents in the connected superconducting circuits that exceed usual current lead arrangements. The use of superconducting dynamos has the potential to reduce the size and mass of a complete magnet system by replacing the power source, current leads, and cryocooler. Previous work on superconducting dynamos has focused on driving larger currents, and understanding the underlying physical principles of operation, but has not investigated the AC losses associated with operation. In this work the losses of a kA class multi-stator squirrel-cage superconducting dynamo are investigated experimentally, and compared to the losses expected in the conventional copper current lead based energisation method. The thermal load associated with conduction cooled copper leads prevents practical application of kilo-amp class dc HTS magnets, however we demonstrated the feasibility of using this superconducting dynamo as an energisation method to enable feasible portable high current HTS magnet systems.

Experimental and Numerical Study of Current Distribution of Superconducting Composite Conductor With REBCO Tapes for Power Electric Applications

Superconducting power devices, such as superconducting transformer and superconducting magnetic energy storage system, are the key technologies in the power system. High-current high temperature superconducting (HTS) composite conductors have been developed for use in HTS power devices. This paper studies the electromagnetic fields of superconducting composite conductors, with high current and compact configuration, for power applications. We focus on the current distribution simulation within the tapes especially. A three-dimensional finite- element method numerical model with new boundary condition technique is proposed to analyze superconducting composite conductor using the T-A formulation. The T-A formulation is verified by experiments on a superconducting twisted stacked-tape cable composite conductor. The numerical model simulates the current redistribution among the stacked layers to show the circulation currents. According to the numerical results, it is found that the circulation current between different layers could be 45% of the peak current, which was considerably significant for a composite conductor operation. For a composite conductor with a different number of tapes, the circulation currents flow through the outmost layer and come back from the other inner layers. Comparing the circulation current between twisted and untwisted stack-tape cable, it is found that the twisting could reduce the circulation current.

Quench Simulation of REBCO Cable-in-Conduit Conductor With Twisted Stacked-Tape Cable

Quench protection is a critical issue for magnet using high-temperature superconducting (HTS) materials such as REBCO. For cable-in-conduit conductor (CICC) with twisted REBCO tapes, which is considered for high-current HTS conductor for fusion magnet, the anisotropic critical current density (Jc) of tape and complex conductor layout will further complicate the quench behavior. The CryoSoft code THEA is used to simulate the quench behavior of various subscale CICCs in view of a quench experiment in SULTAN facility. The CICCs are composed of twisted tape stack assembled into copper shell (strand). It has been found that neglecting the anisotropy of Jc in quench simulation will significantly underestimate the hot spot temperature by about 40 K. Therefore, it could leave potential risk to damage the magnet. It is also shown that the thermal contact resistance between strand and the steel jacket (the conduit) plays a very important role on quench behavior. The hot spot temperature can be reduced by more than 100 K if the thermal resistance is reduced by two order of magnitude. On the other hand, variations of interstrand electrical and/or thermal resistance have only very little influence (few kelvin).

Assessment of the performance of a 20 kA REBCO current lead

In the framework of a worldwide effort investigating the design of high-current High Temperature Superconductor (HTS) current leads (CLs) composed of REBCO, a 20 kA HTS CL developed at the Karlsruhe Institute of Technology (KIT) based on the JT-60SA CL design, but replacing the Bi-2223 tapes by brass-stabilized REBCO tapes, has been recently tested in the CuLTKa facility at KIT to investigate its steady state and transient performance. The thermal-hydraulic CURLEAD code developed at KIT, recently upgraded in a collaboration between KIT and Politecnico di Torino and validated in both steady state and transient (pulsed) operation, was applied predictively, i.e. before the tests were performed, to support the test strategy of the REBCO CL, which is presented here. The prediction was used to define the set point for the steady state tests, both with and without current, while transient simulations were used to define the number of current pulses needed to reach periodic operation conditions for given current waveforms. The comparison between predictions and experimental results is presented here, showing good agreement between the two in all operating conditions. Interpretive simulations, which took into account the experimental results of parameters, like, e.g., the contact resistance of the HTS module to both the heat exchanger and the cold end copper, are finally presented, showing as expected an even better capability to reproduce the experimental traces, giving confidence in the reliability of CURLEAD for future applications.

Magnetization Loss in REBCO Roebel Cables With Varying Strand Numbers

Assembled coated conductors are essential in many high-current high-temperature superconducting (HTS) applications. A promising option is to use Roebel cable, as it offers both low ac loss and mechanical flexibility. In this work, we measured magnetization loss at 77 K in REBCO Roebel cables with varying strand numbers from 6 to 14 to investigate the strand number dependence of ac loss characteristics of the Roebel cables. The 4-mm-wide Roebel strands were punched from nonstabilized 10-mm-wide Fujikura REBCO tapes. The applied field amplitude, frequency, and the field angle (the angle between the magnetic field and the tape wide surface) were varied. Three 10-mm-wide vertical stacks with conductor number of three, five, and seven were prepared as reference stacks using the same source Fujikura tapes. The measured magnetization loss values of the Roebel cables were compared with those of the equivalent stacks as well as with the results of COMSOL modeling of the stacks using the H -formulation. At magnetic-field amplitudes greater than the effective penetration field of the Roebel cables the cables have lower ac loss than equivalent stacks.

Universal Texturing Layer for High-Current HTSC Tapes

The inclined substrate deposition method is used to grow MgO layers applicable to set textures incorporated in high-temperature superconductor (HTSCs) tapes with high critical current density. The method makes it possible to obtain polycrystalline sublayers with pronounced textures independently of the substrate crystal structure. Simultaneously, MgO has good adhesion to substrates made of a wide spectrum of materials. This makes the method a universal means for growing texturing buffer layers incorporated in high-current HTSC tapes.

Numerical Study on AC Loss Properties of HTS Cable Consisting of YBCO Coated Conductor for HTS Power Devices

High-current high temperature superconducting (HTS) cables have been developed for use in HTS power devices. This paper presented the structures of HTS cables, including Conductor on Round Core (CORC) cable, Twisted Stacked-tape Conductor (TSTC) cable, and Double coaxial cable. Subsequently, three-dimensional finite element method numerical models were built to analyze the electromagnetic characteristics of the cables, and the critical current of the cables is about 380 Ampere @77 K, self-field. Using the T-A formulation, the numerical model assumed a sheet approximation for conductors, which shortened computational time. The T-A formulation were verified by experiments on a superconducting tape. Then HTS cables with different configurations were made, as functions of different transport current and background magnetic field, and different pitches of Double Coaxial Cable inner conducting layer. According to the results, the ac losses of Double coaxial cable and CORC cable decreased 40% than the TSTC cable with different transport current, and the Double coaxial cable ac loss decreased 20% than the CORC cable when background magnetic field was in the range of 20-60 mT. Conclusions obtained from this study will be helpful for understanding the ac loss properties of HTS cables and useful in design of HTS power devices (such as HTS transformer), using HTS cables.

Towards a 20 kA high temperature superconductor current lead module using REBCO tapes

Most of the large fusion devices presently under construction or in operation consisting of superconducting magnets like EAST, Wendelstein 7-X (W7-X), JT-60SA, and ITER, use high temperature superconductor (HTS) current leads (CL) to reduce the cryogenic load and operational cost. In all cases, the 1st generation HTS material Bi-2223 is used which is embedded in a low-conductivity matrix of AgAu. In the meantime, industry worldwide concentrates on the production of the 2nd generation HTS REBCO material because of the better field performance in particular at higher temperature. As the new material can only be produced in a multilayer thin-film structure rather than as a multi-filamentary tape, the technology developed for Bi-2223-based current leads cannot be transferred directly to REBCO. Therefore, several laboratories are presently investigating the design of high current HTS current leads made of REBCO. Karlsruhe Institute of Technology is developing a 20 kA HTS current lead using brass-stabilized REBCO tapes—as a further development to the Bi-2223 design used in the JT-60SA current leads. The same copper heat exchanger module as in the 20 kA JT-60SA current lead will be used for simplicity, which will allow a comparison of the newly developed REBCO CL with the earlier produced and investigated CL for JT-60SA. The present paper discusses the design and accompanying test of single tape and stack REBCO mock-ups. Finally, the fabrication of the HTS module using REBCO stacks is described.

High temperature superconductor cables for EU-DEMO TF-magnets

Actual progress of High Temperature Superconductor (HTS) REBCO tapes show an enormous increase of critical current density especially at highest field. These results offer new possibilities for the application of REBCO for large fusion magnets with a magnetic field well above 13T at the conductor and a large temperature margin >10K. In parallel promising new proposals for the formation of high current HTS fusion conductors from these REBCO tapes are emerging, promising the applicability for fusion magnets, e.g. for a TF magnet of an European DEMO.The actual progress of REBCO tapes and several promising HTS fusion cable proposals will be highlighted which open the path to future high field HTS magnets for fusion. Using such REBCO tape data, calculations and modeling of a TF coil for an EU-DEMO based on the PROCESS code will be presented, showing that with today's HTS performance an EU-DEMO TF conductor is feasible, which can be operated at 4.5K with a maximum field at the conductor of approximately 13.3T (magnetic field at the plasma center approximately 6.8T) with a temperature margin of approximately 12K.Special attention will be given to a new stacked HTS conductor fabrication technique proposed by KIT which is optimized for long length production of HTS strands with high engineering current density. Due to the cross shape of the soldered superconductor stacked, this type of strand is called HTS CrossConductor or HTS CroCo. The implementation of the simple, reliable and easily scalable fabrication routine will be outlined. Measurement results of first samples show the expected critical current calculated from the individual tape opening the way to a compact kA-class HTS strands. These HTS strands can be used as a basis to build a fusion cable for a TF-coil for an EU-DEMO.

Design of conduction cooling system for a high current HTS DC reactor

A DC reactor using a high temperature superconducting (HTS) magnet reduces the reactor’s size, weight, flux leakage, and electrical losses. An HTS magnet needs cryogenic cooling to achieve and maintain its superconducting state. There are two methods for doing this: one is pool boiling and the other is conduction cooling. The conduction cooling method is more effective than the pool boiling method in terms of smaller size and lighter weight. This paper discusses a design of conduction cooling system for a high current, high temperature superconducting DC reactor. Dimensions of the conduction cooling system parts including HTS magnets, bobbin structures, current leads, support bars, and thermal exchangers were calculated and drawn using a 3D CAD program. A finite element method model was built for determining the optimal design parameters and analyzing the thermo-mechanical characteristics. The operating current and inductance of the reactor magnet were 1,500 A, 400 mH, respectively. The thermal load of the HTS DC reactor was analyzed for determining the cooling capacity of the cryo-cooler. The study results can be effectively utilized for the design and fabrication of a commercial HTS DC reactor.

Critical Current of a Quasi-Isotropic HTS Strand in Magnetic Field

High-current high-temperature superconducting (HTS) cables made from HTS CCs have widely potential applications in high magnetic field and superconducting power devices. In this paper, a design of quasi-isotropic HTS strand made from REBCO coated conductors (REBCO CCs) with geometrical symmetric configuration was put forward and a sample of the strand was fabricated. It mainly consists of 72 REBCO CCs, metal filler, and sheath. In order to study the anisotropy of critical current, the magnetic field distribution on the cross-section of the strand was simulated and the critical current of the strand in the self-field and in an external magnetic field were calculated by finite element method. The experimental results show the validity of the simulation model and the calculation method.

Commissioning of HTS Adapter and Heat Exchanger for Testing of High-Current HTS Conductors

In the EDIPO test facility, superconducting cables with a current carrying capacity up to 100 kA can be tested in a background field of 12.5 T. A NbTi transformer, which is operated at ≈4.5 K, provides the sample current of up to 100 kA. The high-temperature superconductor (HTS) adapter, which is similar to an HTS current lead, connects the HTS sample under test and the NbTi transformer and limits the heat flux between them to less than 20 W at a warm end temperature of 50 K. Helium of 4.5 K and 10 bar supplied by the refrigerator is warmed up to temperatures up to 60 K, by means of heaters and a counterflow heat exchanger (HEX). The HEX ensures that the warm helium leaving the HTS sample can be returned as cold gas with less than 20 K to the refrigerator. The commissioning of the HTS adapter and the HEX has been performed together with the test of a 60-kA class HTS cable manufactured at CRPP. The results of the commissioning of the HEX and the HTS adapter are presented.

Mechanical reinforcement for RACC cables in high magnetic background fields

Operable in liquid helium, liquid hydrogen or liquid nitrogen, high temperature superconductor (HTS) cables are investigated as future alternatives to low temperature superconductor (LTS) cables in magnet applications. Different high current HTS cable concepts have been developed and optimized in the last years—each coming with its own benefits and challenges. As the Roebel assembled coated conductor (RACC) is the only fully transposed HTS cable investigated so far, it is attractive for large scale magnet and accelerator magnet applications when field quality and alternating current (AC) losses are of highest importance. However, due to its filamentary character, the RACC is very sensitive to Lorentz forces. In order to increase the mechanical strength of the RACC, each of the HTS strands was covered by an additional copper tape. After investigating the maximum applicable transverse pressure on the strand composition, the cable was clamped into a stainless steel structure to reinforce it against Lorentz forces. A comprehensive test has been carried out in the FBI facility at 4.2 K in a magnetic field of up to 12 T. This publication discusses the maximum applicable pressure as well as the behaviour of the RACC cable as a function of an external magnetic field.

Splice, Bus and High Current HTS Lead Tests for the Mu2e Solenoid System

The Muon-to-Electron conversion experiment, under development at Fermilab, utilizes a complex superconducting solenoid magnet system. Usage of high temperature superconductor (HTS) power leads and reliable superconducting bus system and splice joints are essential to the assuring safe, continuous and cost effective operation of the magnets. Left over HTS power leads from the Tevatron era combined with superconducting bus (made from aluminum-clad NbTi Rutherford cable) and three different types of splices were successfully tested in a single dewar test configuration up to 10150 A. In this paper we summarize the test results.

Electrical characterization of REBCO Roebel cables

In the quest for high-current high-temperature superconducting (HTS) cables suitable for application to high-field magnets, the Roebel cable made from (RE)-Ba2Cu3O7−δ (RE for rare earth: Y, Sm, Gd, Dy or a mixture of them) coated conductors is identified as meeting the requirements for high-current capability, compactness, transposition and good mechanical properties. In accelerator high-field magnets, Roebel cables will be operated in liquid helium at 4.2 K or lower temperatures. Previous papers have reported on the electrical characterization of Roebel cables at 77 K, but measurements at 4.2 K have not been published yet. This paper summarizes the results of the critical current measurements performed at CERN on (RE)-Ba2Cu3O7−δ Roebel cables at 4.2 K and in external fields of up to 9.6 T.

Mechanical Force Analysis in Heavy-Current HTS Transformers Based on Field and Current Nonuniformity Coupled Analysis

Mechanical stresses of magnetically induced origins are becoming more serious with the trend of high-temperature superconductor (HTS) heavy-current transformer (HCT) applications. Mechanical forces in HTS HCTs that act on the transformer windings are generated by the interaction between the current density and the leakage flux density. This paper presents the calculation of leakage flux and mechanical force distribution in high-current HTS transformer windings. This requires the need for advanced numerical techniques for simulation studies using finite-element method by adapting two auxiliary windings for the leakage flux mitigation and, thereby, mechanical forces within the main windings. A model is developed for the calculation of mechanical force considering nonuniformity of both field and current distributions in the windings, which is the main motivation for initiating this paper.