Luisa Chiesa

HTS twisted stacked-tape cable conductor

The feasibility of high field magnet applications of the twisted stacked-tape cabling method with 2G YBCO tapes has been investigated. An analysis of torsional twist strains of a thin HTS tape has been carried out taking into account the internal shortening compressive strains accompanied with the lengthening tensile strains due to the torsional twist. The model is benchmarked against experimental tests using YBCO tapes. The critical current degradation and current distribution of a four-tape conductor was evaluated by taking account of the twist strain, the self-field and the termination resistances. The critical current degradation for the tested YBCO cables can be explained by the perpendicular self-field effect. It is shown that the critical current of a twisted stacked-tape conductor with a four-tape cable does not degrade with a twist pitch length as short as 120 mm. Current distribution among tapes and hysteresis losses are also investigated. A compact joint termination method for a 2G YBCO tape cable has been developed. The twisted stacked-tape conductor method may be an attractive means for the fabrication of highly compact, high current cables from multiple flat HTS tapes.

Conductor Characterization of YBCO Twisted Stacked-Tape Cables

Cable performances of a flat high temperature superconductor (HTS) tape cable fabricated by twisted stacked-tape cable (TSTC) method were experimentally evaluated under various bending diameters at 77 and 4.2 K in self-field. The straight conductor was made with 32 YBCO tapes and twisted using the TSTC method. The 32-tape YBCO TSTC straight conductor was soldered, and then the critical currents were tested at various bending diameters between 1 m and 140 mm. The critical current at a bending diameter of 140 mm was about 6% lower than the straight cable. The same cable with a bending diameter of 250 mm was charged to 10 kA in one second in liquid helium. A new magnet winding technique, stacked-tape twist-wind (STTW), of YBCO tapes using the TSTC concept has been developed, and a 2.3-m, 50-tape YBCO tape coil fabricated with this method was tested. Its critical current was 4.0 kA at 100 μV/m at 19.7 T in liquid helium. It was observed that for an electromagnetic Lorentz force of 50 kN/m the soldered YBCO cable started degrading. A curved saddle coil winding using a STTW technique was demonstrated.

Cabling Method for High Current Conductors Made of HTS Tapes

A small-scale test of a twisted stacked-tape conductor made of coated YBCO tapes was performed using four-tape cable. The critical current degradation and current distribution of this four-tape conductor was evaluated by taking account the twist-strain, the self-field and the termination resistance. The critical current degradation for the tested YBCO tape may be explained by the perpendicular self-field effect solo. The critical currents of the twisted stacked-tape conductor with four-tape cable have been confirmed not to degrade up to 120 mm twist pitch length. This type of conductor design is proposed to make it possible to fabricate highly compact, high current cables from multiple flat HTS tapes.

Pure bending strains of Nb3Sn wires

This paper presents in detail recent pure bending strain investigations of ITER Nb3Sn wires including a pure bending device development, bending effects on the critical currents and their model analyses. The pure bending test device allows applying a uniform bending over a large range of bending strain on a strand sample under a large electromagnetic Lorentz force. Pure bending characteristics of five different Nb3Sn wires including three internal tin and two bronze wires were tested and evaluated with a newly developed integrated model that accounts for neutral axis shift, current transfer length, mechanical filament breakage and uniaxial strain release due to the applied bending load.

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.

Development of Termination Methods for 2G HTS Tape Cable Conductors

Two types of mechanical contact joint termination methods for a 2G YBCO cable have been developed. Each YBCO tape of a cable is connected to a BSCCO tape while the other ends of BSCCO tapes are bundled together and soldered inside a copper tube. Another termination method uses only YBCO tapes. It has been confirmed that both joint termination methods are very useful for 2G HTS cables. Mechanical contact joint resistances between YBCO-BSCCO tapes and YBCO-YBCO tapes have been measured under various contact pressures and contact surface conditions at 77 K. Joint resistance distribution among the tapes in the cable have been also evaluated for both termination methods. These contact joint methods can be used for a demountable joint. Joint resistance is greatly reduced with increasing applied pressure especially for YBCO-YBCO joints. However, in the case of a YBCO-BSCCO joint, the BSCCO tapes may degrade at high pressure. The contact pressure should be optimized to obtain low joint resistance.

Finite Element Analysis of

Currently, very few experimental results describing the behavior of Nb3Sn subcables under transverse load are available. Those results are of importance for predicting how a full-sized cable-in-conduit conductor behaves during operations. Current experimental devices used to study the effect of transverse load on the electrical performance of cables utilize concave plates to apply mechanical loads and contain the sample and subject it to mechanical loads that mimic the electromagnetic loads of full-sized cables during operation. From finite element analysis, it is found that the strain state in the strands of a triplet is greatly affected by the shape of the pressing element contact surface. We will discuss the strain state within the strands from the simulations using two pressing configurations: concave and flat plates. The strain state in each strand in a twisted triplet is investigated by considering two cross-sections of a triplet along the length of the cable. Those results can provide useful information on the electrical performance of each strand based on its location along the axis. It is verified that the load distribution is very different depending on the shape of the pressing plates.

\hbox{Nb}_{3}\hbox{Sn}

An analytical model to evaluate bending strains of a twisted stack-tape cable (TSTC) conductor has been developed. Through a comparison with experimental results obtained for a soldered 32-tape YBCO TSTC conductor, it has been found that a Perfect-Slip Model (PSM) taking into account the slipping between tapes in a stacked-tape cable during bending gives much better estimation of the bending performance compared to a No-Slip Model (NSM). In the PSM case the tapes can slip so that the internal longitudinal axial strain can be released. The longitudinal strains of compression and tension regions along the tape are balanced in one twist-pitch and cancel out evenly in a long cable. Therefore, in a cable the strains due to bending can be minimized. This is an important advantage of a TSTC conductor. The effect of the cable diameter size on the bending strain is also expected to be minor, and all tapes composing a TSTC conductor have the same strain response under bending, therefore the cable critical current can be characterized from a single tape behaviour.