Low Joint Resistance Research Articles

Magnetometer Using Inductance Modulation of High-Critical-Temperature Superconducting Coil for Low-Frequency Field Measurement in Presence of Excitation Fields

We have developed a new type of magnetometer using a high-critical-temperature superconductor (HTS) coil. This magnetometer consists of pickup and input coils made from HTS tape. The two coils are connected with very low joint resistance, and the signal flux that is collected by the pickup coil is transferred to the input coil. The signal at the input coil is then read out using a newly developed inductance-modulation scheme. The inductance of the input coil is modulated over time using a magnetic wire that is inserted into the input coil, and the magnetic flux is converted into a signal voltage using this time-dependent inductance. It is demonstrated that this magnetometer can measure low-frequency signal fields down to 0.03 Hz without any reduction in responsivity. The noise level was measured to be 0.85 pT/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> at frequencies above 25 Hz and reached 11 pT/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> at 1 Hz when using a pickup coil with 50 turns and an average diameter of 25 mm.

MgB2 Superconducting Joint Technique Based on Mg Diffusion Method

MgB2, which has a high stransition temperature (Tc) of 40 K, has great potential for superconducting applications. Recently, significant improvements in the performance of MgB2 wires have enabled applications, such as superconducting magnets, magnetic shielding devices, flux transformers, and gradient meters. However, the problem of superconducting joints in some applications must be solved. In this study, low resistance joints between MgB2 wires were fabricated using the Mg diffusion method. The MgB2 wires were inserted into B bulks and compacted to different densities. Liquid Mg was reacted with the B bulk to form MgB2. The impact of the compaction pressure of the B bulk was studied. With the increase of compaction pressure, the Tc of the joint decreased while the critical current (Ic) of the joint increased. The highest Tc value of the joint was 39 K, and the highest Ic value of the joint at 0 T was 51 A at 20 K.

New Inspection Method of Soldering Region at Room Temperature for ITER TF Termination

For inter-double-pancake joints of ITER TF it is required to achieve sufficiently low joint resistance ( $R_{J}$ ), as 3 nΩ, to reduce Joule heating during operation with a background field of 2 T and nominal current of 68 kA. If $R_{J}$ value is measured directly, a large cryostat is required to cool down the TF coil with huge mass and size during $R_{J}$ measurement at 4 K. As alternative, authors have developed new inspection methods to obtain $R_{J}$ value. The contributions to $R_{J}$ can be divided mainly into contact resistance between the copper sole and cable ( $R_{c}$ ) and resistance of soldered region between copper soles ( $R_{sol}$ ). For the measurement of the former, the authors developed the inspection method in a past study. In this paper, the authors focus on the development of a measurement method for the latter. If there is a defect in the soldered region, a current distribution in the copper sole varies from the case of no defect, resulting in a change in the voltage profile along the copper sole. Thus, the voltage profile along the copper sole is measured at room temperature by supplying 20 A to the termination. To confirm the sensitivity to existence of a defect in soldered region, the measurements were performed for good (no-defect) and bad (defect) joints. An example of good joint was a joint sample the R J value of which was sufficiently low, while bad joint was simulated by artificially isolating a part of the soldered region. From the results of the measurements, a significant difference was observed between good and bad joint samples. Thus, it can be concluded that the developed method is practically effective for inspection of soldering quality between copper soles.

Bending properties of solder joint of YBCO coated conductors by etching copper stabilizer

The YBa2Cu3O7−x coated conductor (YBCO CC) is one of the most promising superconducting materials for magnet applications because of its high critical current (Ic) and low dependency of the Ic on the external magnetic field. For magnet applications, plenty of joints are required to meet the connection of relatively short YBCO CC tapes. The bending properties of the joints inevitably become one of the critical factors that determine the performance of the superconducting magnet. In this paper, we fabricated the joints of YBCO CC tapes by etching Cu stabilizer and studied their bending properties under various bending diameter at 77 K and self-field. Compared with the soldering joint, lower joint resistance (Rj) and preferable electrical properties under applying various bending deformation were achieved and proved by the bending strain tests and finite element simulations. This fabrication technique is a unique practicable solution to reduce the Rj and improve the bending properties for the joints of YBCO CC tapes with thicker metal stabilizing layers in second generation high temperature superconductor (2G HTS) applications.

Investigation on Current Distribution and Joint Resistance-Overlap Length Relationship for Non-Superconducting Joints

Low-resistance joint technology is crucial for high-temperature superconducting (HTS) closed-loop coils that work in persistent current mode. The non-uniform current distribution (NUCD) phenomenon in HTS joint has a significant influence on joint resistance, which may lead to a nonlinear relationship between joint resistance Rj and overlap length l (Rj-l curve) instead of following the laws of resistance. In this paper, an arched structure joint made by YBCO tapes has been designed to provide a direct evidence for the NUCD phenomenon inside solder joint. It is found that the current transports mainly at the end of the overlap area and is much different from the regular conductors. The V/I-I method is proposed to measure Rj of solder joints (less than 2 nΩ) with various overlap lengths in order to obtain more precise data. A complete Rj-l curve (l = 5-100 cm) has been successfully illustrated and a nonlinear saturation relationship between Rj and l is presented. The joint resistance tends to approach to a lower limit value about 1.5-1.6 nΩ when the overlap length exceeds 20 cm.

Test Results of ITER 52-kA HTS Current Lead Prototypes

The Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) is responsible for the construction and performance testing of the 30 pairs of the international thermonuclear experimental reactor high-temperature superconducting (HTS) current leads. The first pair of poloidal field (PF) coil type, 52-kA HTS current lead prototypes was built and cold tested in ASIPP in mid-2016. The test results demonstrated their excellent performance on low joint resistance, long loss of flow accident time, and high current-sharing temperature. The overheating time, mass flow, and heat loads to 5-K ends also meet the requirements. This paper summarizes the major test results for the PF 52-kA HTS current lead prototypes.

HTS current leads for the NICA accelerator complex

The international project “NICA” (Nuclotron-based Ion Collider fAcility) is an accelerator complex which is under construction at the Joint Institute for Nuclear Research (Dubna, Russia). Basing on the NICA project demands NRC “Kurchatov Institute” (Moscow, Russia) designed and manufactured two trial 12 kA 1G current leads (CLs) intended for powering of the JINR superconducting magnets test bench. The main requirements for the CLs were: a reliable and robust design, usage of liquid nitrogen from the NICA cooling circuits as a resistive section coolant, low resistance joints, current ramp rates up to 10 kA/s and the total heat load to the 5 K end not exceeding 0.4 W/kA per one CL. An in-situ cold test of the trial 12 kA CLs held at JINR in 2015 demonstrated the operational reliability of our technological approaches. Following the successful test results the serial production of 20 pairs of 10.5 kA HTS CLs with very few design updates for the accelerator powering started. The first 10 pairs were fabricated, tested and shipped to JINR as a part of the NICA commissioning framework in June 2017. All the CLs fulfilled the acceptance test requirements. The production of the remaining 10 pairs is to be finished in 2018. The paper gives an overview of the 12 kA and 10.5 kA HTS CLs’ design, discusses their thermal, magnetic and electrical characteristics and presents the acceptance tests results. We also compare our CLs with earlier works by other groups in the context of HTS stack-and-soldering technique, joints’ resistances, relative heat loads into the cryogenic environment etc.

Joint Resistance Characteristics in Ultrasonic Weld REBCO CC Tapes at Various Temperatures and Magnetic Fields

In coil and magnet manufacturing that requires long length 2G coated conductor (CC) tapes, the development of joining methods for CC tapes with zero or low resistance is necessary. Nowadays, although it is said that CC tapes with around 1-km length were produced, it is still inadequate to produce cables and coil windings. Therefore, the development of a stable and reliable joint for long CC tapes having a low joint resistance (R j ) and the understanding of the magnetic field and temperature dependence of R j is essential in the design of high field magnets and coils. In this study, the characteristics of joint resistance in REBCO CC tapes jointed using ultrasonic welding (UW) method were experimentally evaluated in various temperatures and magnetic field. The CC joints were also fabricated by hybrid welding, which incorporates soldering to the UW joint. The magnetic field and temperature dependencies of joint resistance of both types CC joints were measured. The acquired results showed that the joint resistance decreased with a decrease of temperature, and increased slightly as magnetic field applied increased.

3S–Superconducting DC-Busbar for High Current Applications

Within a German government funded project a superconducting (HTS) high current DC-busbar is under development. The aim is the fabrication of a demonstrator with a length of 25 m and a nominal current of 20 kA for installation and test in an industrial real-life application. The concept is a modular system with rigid superconducting elements of arbitrary length, straight or with defined bends, each equipped with own cryostat and premanufactured in the factory. These elements can easily be transported, but have to be joined on the installation site. To manufacture such elements several issues had to be addressed: The arrangement of the superconducting tapes was optimized with respect to the minimization of the magnetic self-field effects. The thermal contraction of the busbar had to be balanced and in particular low-resistance joints between the superconducting elements had to be developed. These are based on innovative comb-shaped structures with face-to-face soldered tape ends. A first prototype of such an element including two joints was built and tested at T = 77 K up to an effective current of I = 20 kA. With a resistance of less than R = 0.5 nΩ, the losses from the contacts are small compared to the cryostat losses.

Test of PF1 Coil Electrical Joint

The PF1 coil manufactured in the Russian Federation is one of the six poloidal field (PF) coils of the ITER electromagnetic system. The PF1 winding represents a stack of eight double pancakes (DPs). Each DP is wound from niobium–titanium “cable-in-conduit” conductors using the “two in hand” technique. The PF1 unidirectional winding is provided by connecting the ends of DPs into a single electric circuit by “shaking hands” low-resistance joints. An acceptance criterion for manufactured joints is an active resistance of less than 5 nΩ in the range of PF1 operating temperatures and applied magnetic fields. The initial joint design was updated on the basis of deviation requests of the ITER Organization. As a result, the previously developed techniques and tools were upgraded. To verify the modifications made, the preliminary joint sample was manufactured and tested in the NIIEFA test facility. Despite the limited current (up to 20 kA) and magnetic field, the results of the NIIEFA test verified the possibility of meeting the acceptance criterion and allowed us to start the manufacturing of the qualification joint sample. The final electrical test was performed in the “SULTAN” test facility of the Swiss Plasma Center (Switzerland). The test results are presented in this paper.

Superconducting Feeder System for ITER Central Solenoid Module Final Test Facility

General Atomics has commissioned the final test facility at the central solenoid module (CSM) manufacturing site in Poway, CA, USA. The facility includes a number of critical subsystems including the feeder system (FS), supplied by the Institute of Plasma Physics Chinese Academy of Science (ASIPP), that connects the test chamber to the cryogenic and electrical systems. It includes a coil termination box, high-temperature superconducting current leads, and a feeder duct. The FS carries the current (50 kA) and supercritical helium (4.5 K, 5.5 bara) to the CSM and its supporting structure, while monitoring and controlling the temperature, pressure, flow, and voltage drop through all elements of the superconducting components. It functions as an integral part of the system to rapidly (6-s decay time) dissipates 1-GJ energy stored in the coils, and to protect the cryogenic system. The system is complex, requiring multidisciplinary engineering including: high-temperature superconductivity, cryogenic-temperatures (77, 50, and 4.5 K), high vacuum (1.0 × 10−5 Pa), redundant quench detection (voltage, temperature), high-voltage insulation (15 kV), low thermal loads (70 W), low electrical joint resistance (2 nΩ), Paschen testing, high-voltage signal measurements, and high-current electronics. The system presented significant challenges for design and analysis, complex manufacturing assembly processes, measurement and control, and stringent quality and safety requirements.

New Inspection Method of Termination Resistance at Room Temperature for ITER TF Coil

It is important to achieve sufficiently low joint resistance ( ${\rm{R}}_{{\rm{J}}}$ ), as 3 nΩ with a background field of 2 T and nominal current of 68 kA to reduce the loss by Joule heating during operation, for interdouble-pancake joints of ITER TF coil termination. However, a large cryostat is required to cool down TF coil during ${\rm{R}}_{{\rm{J}}}$ measurement at 4 K because of huge mass and size of TF coil. Authors, therefore, developed an inspection method, which enables to detect some differences in ${\rm{R}}_{{\rm{J}}}$ at room temperature, while it is difficult to do in nano-ohm range. ${\rm{R}}_{{\rm{J}}}$ of TF coil termination is mainly determined by contact resistance between cable and copper sole of the joint box ( ${\rm{R}}_{{\rm{c}}}$ ), and contact resistance between copper sole soldering. This paper first focuses on development of inspection method for ${\rm{R}}_{{\rm{c}}}$ . ${\rm{R}}_{{\rm{c}}}$ affects current penetration length (CPL) between the cable and copper sole. We thus assumed that when a joint is fabricated with proper process, i.e., ${\rm{R}}_{{\rm{c}}}$ is sufficiently low, the behavior of CPL is repeatable. On the other hand, when there is some abnormality in the manufactured joint, we can find irregular behavior on CPL compared to the normal repeatable behavior. Thus, we propose an inspection method in which a current of 20 A is injected in the joint and the voltage profile along the joint is measured at room temperature. As a result of the measurement, good repeatability of CPL could be shown in actual all ITER TF joints tested, while a significant difference of CPL was observed in a joint sample which includes some disturbances.

10 kA Joints for HTS Roebel Cables

Future high temperature superconductor (HTS) high field magnets using multitape HTS cables need 10-kA low-resistance connections. The connections are needed between the poles of the magnets and at the terminals in a wide-operating temperature range, from 1.9–85 K. The EuCARD-WP10 Future Magnets collaboration aims at testing HTS-based Roebel cables in an accelerator magnet. Usually, low temperature superconductor (LTS) cables are jointed inside a relatively short soldered block. Powering tests at CERN have highlighted excess heating of a joint following classical LTS joint design. The HTS Roebel cables are assembled from REBCO-coated conductor tapes in a transposed configuration. Due to this, the tapes surface the cable at an angle with the cable axis. A low-resistance joint requires a sufficiently large interface area for each tape. Within one twist pitch length, each tape is located at the surface of the cable over a relatively small non-constant area. This geometry prevents making a well-controlled joint in a compact length along the cable. This paper presents a compact joint configuration for the Roebel cable overcoming these practical challenges. A new joint called fin-block is designed. The joint resistance is estimated computationally. Finally, the test results as a function of current and temperature are presented.

Mechanism of Solder Joint Cracks in Anisotropic Conductive Films Bonding and Solutions: Delaying Hot-Bar Lift-Up Time and Adding Silica Fillers

Micron sizes solder metallurgical joints have been applied in a thin film application of anisotropic conductive film and benefited three general advantages, such as lower joint resistance, higher power handling capability, and reliability, when compared with pressure based contact of metal conductor balls. Recently, flex-on-board interconnection has become more and more popular for mobile electronic applications. However, crack formation of the solder joint crack was occurred at low temperature curable acrylic polymer resins after bonding processes. In this study, the mechanism of SnBi58 solder joint crack at low temperature curable acrylic adhesive was investigated. In addition, SnBi58 solder joint cracks can be significantly removed by increasing the storage modulus of adhesives instead of coefficient of thermal expansion. The first approach of reducing the amount of polymer rebound can be achieved by using an ultrasonic bonding method to maintain a bonding pressure on the SnBi58 solder joints cooling to room temperature. The second approach is to increase storage modulus of adhesives by adding silica filler into acrylic polymer resins to prevent the solder joint from cracking. Finally, excellent acrylic based SnBi58 solder joints reliability were obtained after 1000 cycles thermal cycling test.

Research progress on the mechanical behavior of the cable in conduit conductor for the international thermonuclear experimental reactor project

The international thermonuclear experimental reactor (ITER) program is of the largest and the most influential international cooperation project, and its purpose is to verify the scientific and technical feasibility of the magnetic constraint nuclear fusion reactor. The high temperature plasma with billion degrees Celsius can be constrained in a magnetic cage, which can provide a strong magnetic field by a Tokamak device wounded by cable in conduit conductor (CICC). In 2014, Dr. Devred A, a chairman of ITER magnet project, pointed out that the ITER CICC is mainly faced with three challenges: (1) During insertion into the jacket assembly, the cable exhibits a tendency to rotate under the action of the pulling force. That may increase the twist pitch, especially for the final one. Cable twist pitches must be controlled to prevent excessive AC losses in the CICC, which is a threaten to the stability of CICC. The elongation of the twist pitch must be settled. (2) The CICC′s current sharing temperature ( T cs) showing degradation after the electro-magnetic (EM) and thermal cycling load, that means the Tokamak can only run thousands of times, less than the original design of 30000 times, raising risks at the fusion project. (3) Find the way to fabricate the high-performance superconducting wire and the low resistance joint. As we all know, the central and the toroidal field solenoid superconducting cable of the Tokamak were made of Nb3Sn superconducting strands, and the superconducting cable will working at the mechanical–thermal–electrical–magnetic fields environment. Previous studies have shown that Nb3Sn superconductivity is sensitive to the mechanical deformation. The critical current of the superconductor wires will show a significant degradation with the deformation under the loads of tension, pressure and twist, which increases the risk of ITER Tokamak directly. Therefore, it′s important for the design of the Tokamak magnet system to investigate the equivalent mechanical parameters of the cable and its mechanical behavior under the action of multiple fields. In this paper, several key mechanical problems such as the equivalent mechanical parameters of the superconducting cable, the untwisting behavior in the process of insertion, and the T cs degradation under the thermo-electromagnetic cyclic loads have been briefly reviewed. Firstly, the stress-strain curve of the triplet was analyzed based on the thin-rod model and the tensile stiffness model of the triplet was established. Secondly, on the basis of the triplet model, a complex model of the tensile stiffness and the equivalent CTE of the 3×3 strand was build, and the derivation process was provided. Thirdly, the untwisting behavior of the cable in CICC fabrication was investigated. The bending stiffness model of the petal under the wrapping was established, and the tensile untwisting model of the superconducting cable was also build. Fourthly, a mechanical model was established for the T cs degradation mechanism of TF and CS CICC conductors under the action of thermo-electromagnetic cyclic load. Based on the compatible relationship between the transverse compressive strain and the axial elongation of the superconducting cable, the impacts of axial compressive stiffness and surface friction coefficient on the axial compression strain of the cable under the thermo-electromagnetic cyclic load were studied. Finally, we summarized the existing problems and the future research points on the basis of the previous research results, which will help our researchers to catch the progress of the ITER program. In the future, the CICCs will also be used in China′s large superconducting magnets.

Low-Resistance and Strong-Adhesion Soldering of Second-Generation High-Temperature Superconductor Tapes Within a Short Time

An effective soldering process of second-generation coated conductors (CCs) with low joint resistance was presented by using home-made equipment in the present study. The benefit of preprocessing, i.e., polishing, was proved with respect to the reduced joint resistances based on the voltage-current curves. It is revealed that the joint resistance of 10 -8 Ω with an overlapping length as short as 2 cm can be achieved, while the joint resistances would further decrease with increasing the overlapping area, in agreement with Ohm's law. In the case of the overlapping length of 8 cm, the order of nΩ can be achieved for the resistances of solder joints. The heating time in joint fabrication process can be shortened to 1 min without negative effects on joint resistance, speeding up the soldering process largely. The effectiveness was evidenced further by the hard degradation in Ic through the joint part of the studied tapes and the enhanced adhesion between solder layer and Cu layer according to the cross-section observation. Temperature dependence of the joint specimen resistances suggested that the lattice constant of superconducting layer after jointing may be preferable to attain a higher critical temperature than that of original tape. The fragmentation of the superconducting layer and decohesion of the interface between hastelloy/buffer and ceramic layers, not the Cu-solder interface for the joint by external force, implied that the valid soldering was achieved using the present process.

Bending Characteristic of a Bridge-Type Mechanical Lap Joint of REBCO Tapes

This study focuses on bending characteristic of a bridge-type mechanical lap joint of Rare-Earth Barium Copper Oxide (REBCO) tapes, which has been proposed for segment fabrication of high-temperature superconducting magnet especially for a heliotron-type fusion reactor. Previous studies have developed the joint with a straight geometry, showing sufficiently low joint resistance from a view point of electrical power required to run a cryoplant for removing heat from a superconducting magnet, though conductors connected at the joint section are ideally curved to form a helical-shaped coil. In this study, we experimentally evaluated joint resistance as a function of bending strain for a bridge-type mechanical lap joint of REBCO tapes with indium foils inserted between the joint surfaces. The results showed critical bending strain, where joint resistance starts to increase, was almost the same as irreversible strain of the REBCO tape in the case of a 10-mm-long joint. The critical bending strain varied as a function of both joint resistivity and joint length; lower joint resistivity and longer joint can achieve higher critical bending strain. The fabrication process “Joining-then-bending” can be also applied to a curved joint for reactor-scaled helical coils, where 25-mm-long joint is planned to be applied, though “Bending-then-joining” would be needed for the joint of small prototype coils.

Properties of low-resistance joints between HTS tape conductors prepared by soldering

The electric resistance of contacts between superconducting tape conductors on the basis of the GdBa2Cu3O7 − x compound (2G) is investigated in an external magnetic field up to 5 T at T = 77 K. The mechanical tensile strength of both the initial conductors and the contacts is measured.

Study on the Excitation of a Bi-2223 Small Superconducting Coil by a Pulse-Type Magnetic Flux Pump

This paper presents further research on the pulse-type magnetic flux pump (MFP). The pulse-type MFP is used to excite a Bi-2223 double-pancake superconducting coil cooled by a cryocooler. The inductance of the coil is 76 μH. The cryogenic structure of the MFP is optimized by adding a cold frame outside. The MFP is powered by a dc voltage source. The excitation effects on the Bi-2223 coil are investigated by changing the input voltage. The pumping current curves have been obtained with various input voltage from 2.0 to 5.5 V. The fitting calculations show that the saturated current in Bi-2223 high-temperature superconductor (HTS) coil is 200 A when the input voltage is 5 V. The joint resistance is also numerically analyzed, and it is 0.18 μΩ. By measuring the current ripple of the Bi-2223 HTS coil, it is found that the excitation current fluctuation is ±4 A when the input voltage is 5 V. Consequently, it is expected that the pulse-type MFP could compensate the decaying current caused by the low n-index and joint resistance and enable the HTS magnet operating in persistent current mode. Further optimization on the structure and pumping process is still needed to improve the excitation properties of the pulse-type MFP.

Resistive REBCO Coated Conductor Butt-Joints Based on an Ultrasonic Welding Method

The development of low-resistance coated conductor (CC) joints is important in viewpoint of applications using power cables, coils, and magnets. In particular, a stable and reliable CC tape joint applicable to practical electrical devices should be developed. In this study, CC tape butt-joints with low joint resistance in terms of transport were fabricated by direct bonding at the interface of the Cu-Cu stabilizers in overlapped GdBCO CC tapes. Both ultrasonic welding and hybrid welding processes, which have been attempted recently to achieve low joint resistance in lap-joint CC tapes, were used. The joint quality of butt-joint CC tapes fabricated by various joining methods was evaluated and compared to that in lap-joints. The variation of joint resistance with joint length was also evaluated. The electromechanical properties of butt-joints were examined using the double-bending test. The minimum bending diameter for the retained critical current and joint resistance degradation was determined for CC butt-joints.