Nathaniel C. Allen

Electromechanical Investigation of 2G HTS Twisted Stacked-Tape Cable Conductors

Various cabling methods for HTS flat tapes have been developed in the past few years. We have been developing a REBCO tape cabling method twisting a stack of tapes (TSTC). TSTC conductors were recently tested at high magnetic fields 4 T-20 T. The test results have shown some significant degradations during high current and high field tests. This paper investigates the mechanical characteristics of a TSTC conductor. Effects of transverse loads on single tapes and TSTC conductors were studied to evaluate and compare their mechanical behaviors. A device was designed to measure the critical currents of tapes and cables at 77 K as a function of transverse load. YBCO tapes from two different suppliers were examined. The mechanical transverse load was applied to TSTC conductors to simulate a load similar to the one caused by the natural electromagnetic load during operations. Taking into account the unique geometrical structure of a TSTC conductor, the effects of localized and uniform transverse loading conditions on a TSTC conductor were investigated. Several TSTC samples including single tapes and cables were tested. The experimental results are presented and discussed in the context of use of this type of conductor for high field magnets applications.

Electromechanical Investigation of Different Type YBCO Tapes for Twisted Stacked-Tape Cabling

High-temperature superconductors (HTS), especially YBCO coated tapes, have excellent high current capabilities at high magnetic fields. It is desired to develop a large multistrand cabling method for flat HTS tapes. A cabling method of a twisted stacked-tape conductor for YBCO tapes has been developed recently. A series of tests to electromechanically characterize twisted stacked-tape cabling behaviors using two commercially available types of YBCO tapes was performed. A probe has been developed to simultaneously measure the torsion torque and the critical current as a function of twist pitch for single tapes. The torsion torques and the zero torque twist pitches of YBCO tapes from both manufacturers were determined as a function of twist pitch. The critical currents of those YBCO tapes were also measured in a wider range of twist pitches down to 50 mm. A 25-tape cable of AMSC YBCO was fabricated using the twisted stacked-tape method, and tested at 77 K in self-field. The torque measurements performed on single tapes provide useful information for this cabling process and were taken in consideration in the cable fabrication. The experimental results under torsion for the YBCO tapes from the two manufacturers showed different mechanical behaviors but similar electrical behaviors. Both tapes showed a rapid degradation of critical current for twist pitches below 70 mm. The fabrication of a twisted stacked-tape cable with AMSC tapes was harder than previous fabrication of cables with SuperPower tapes.

Present Status and Recent Developments of the Twisted Stacked-Tape Cable Conductor

The high magnetic field performance of a 40-tape twisted stacked-tape cable (TSTC) conductor made of 4-mm-width 0.1-mm-thick REBCO tapes was successfully tested. The critical current was 6.0 kA, with the n-value of 35, at 4.2 K with a background field of 17 T. No cyclic load effect was observed between 10 and 17 T with the maximum Lorentz load of 102 kN/m. Various issues, such as sample length, nonuniformity of termination resistances, and soldered joint of a coated tape cable, with regard to a TSTC conductor are discussed. Large-scale conductor designs of various scalable TSTC conductors are discussed, taking into account current densities and stabilizers.

Bending Tests of HTS Cable-In-Conduit Conductors for High-Field Magnet Applications

A high-temperature superconducting (HTS) cable-in-conduit conductor (CICC) suitable for high-field magnet applications and comprised of twisted-stacked coated-conductor tapes arranged around a helically slotted core has been recently proposed and tested, demonstrating full compatibility with existing cabling technologies. To form the desired shape of any coils for high-field magnet applications, any suitable CICC option needs to be bent. For a magnet design, it is then very important to characterize the bending behavior of the CICC and, in particular, to find the smallest bending radius achieved without performance degradation. To this aim, bending tests were carried out on 1-m-long dummy samples of five helical slots in an extruded aluminum core, in which four REBCO tapes and dummy stainless-steel tapes were mounted in each slot. A controlled bending moment has been applied to the HTS CICC samples at room temperature, and each individual superconducting tape has been electrically characterized as a function of bending radius by measuring the critical current and n-index values at 77 K in a self-field condition. Results are analyzed and explained with the help of a three-dimensional cable model implemented in ANSYS and analytical calculations. The experimental and numerical results presented in this paper will demonstrate that the twisted-stack slotted-core technology can meet the bending requirements of high-field magnet designs.

Numerical and Experimental Investigation of the Electromechanical Behavior of REBCO Tapes

To fully characterize the electromechanical behavior of a Twisted Stacked-Tape Cable (TSTC) it is important to understand the performance of the individual REBCO tapes under various loading conditions. Numerical modeling and experimentation have been used to investigate the electromechanical characteristics of two commercially available REBCO tapes (SuperPower and SuNAM). Tension and combined tension-torsion experiments on single tapes have been continued, from prior preliminary studies, to characterize their critical current behavior and mechanical strength. Additionally, structural finite element analysis was performed on single tapes under tension and combined tension-torsion to investigate the strain dependence of the critical current. The numerical results were compared to the experimental findings for validation. The SuNAM experimental data matched the numerical model very well while the SuperPower tape experienced degradation at lower stress and strain than predicted in the model. The Superpower tape also displayed greater variability in critical current between different samples as compared with the SuNAM tape.

Combined Tension-Torsion Effects on 2G REBCO Tapes for Twisted Stacked-Tape Cabling

One method of cabling flat REBCO tapes for high-current, high-field magnet applications is the twisted stacked-tape cable (TSTC) which stacks REBCO tapes between copper stabilizing strips and twist them along their axis. During fabrication and operation these TSTC conductors are subjected to various loading states including combined tension-torsion. To better characterize the electromechanical characteristics of the twisted stacked-tape cables the effect of tension on single twisted REBCO tapes was studied. A detailed structural finite-element analysis using ANSYS was done to investigate the stress-strain behavior, torque characteristics and axial elongation of REBCO tapes under pure torsion and tension-torsion loadings. The numerical strain results were combined with an analytical model to predict the critical current performance of these tapes. In addition to the numerical investigation, measurements of torque and critical current dependence on the axial load of twisted REBCO tapes at 77 K and in self-field were performed. A SuperPower sample with a 200-mm twist pitch saw 10% reversible critical current reduction under 700 MPa of tensile stress. Overall, a good agreement between the measurements and the finite element analysis was found validating the capability of the model to capture the electromechanical characteristics of REBCO tapes.

Wide range pure bending strains of Nb3Sn wires

Pure bending behavior of Nb3Sn wire over a wide range of bending has been characterized. A previously developed test device designed to apply variable bending strains to Nb3Sn strands using a beam style sample holder was used. Based on finite element and experimental investigations, two sample holder beams were developed to cover pure bending strains up to 1.25% for ITER-type Nb3Sn wires. These newly designed beams were optimized to apply consistent and uniform pure bending strains to Nb3Sn strands over the entire bending range. Their performance was evaluated by testing two ITER-type Nb3Sn wires including one internal tin and one bronze route. The internal tin strands experienced around 55% critical current degradation at 1.25% bending strain while the critical current of the bronze route strands were only reduced by 40%. Upon removal of the bending load, the internal tin wires experienced significant permanent degradation whereas the bronze route wires were completely reversible. These critical current results were evaluated and explained using an existing integrated model accounting for neutral axis shift, current transfer length, filament breakage and uniaxial strain release under pure bending loads.

Experimental Device to Electromechanically Characterize 3-Strand

Triplet cables are the basic cable element of CICC cables, and understanding their electromechanical behaviors under transverse loads is of critical importance to evaluate the electromagnetic behaviors of large cables. This paper describes a new compact experimental device to characterize electromechanically single strand and triplet cable samples as well as HTS tapes under transverse loads. The device has been built and tested with significant reduction of the cost of the experiment compared to a previously used hairpin sample device. Critical current measurements as a function of transverse mechanical load at 12 T were successfully performed on triplets using ITER Nb3Sn wire. The transversely loaded section of the samples is 110 mm in length, longer than the typical twist pitch lengths of first-stage triplets of CICC. The load is applied by using a gear system driven by a 1/2 HP motor. A wedge, vertically displaced by the motor, moves other pressing components transversely. Those parts ultimately apply transverse compression on the cable. A load cell and an extensometer are used to measure the absolute vertical load and displacement, respectively. Limitations and challenges of this compact transverse load device design are discussed together with some experimental results.