Proceedings of the 8th Workshop on Mechanical and Electromagnetic Properties of Composite Superconductors (MEM 2016)

The International Workshop on Mechanical and Electromechanical Properties of Composite Superconductors (MEM) was first held in 2001 in Kyoto, Japan. Fifteen years later, the 8th MEM16 was held from 21–23 March 2016 in Tallahassee, Florida. A total of 64 scientists from Europe, the United States, Asia and New Zealand attended the workshop. The workshop covered a wide range of topics on the mechanical and electromechanical characteristics of both low-temperature superconductors (LTSs) and high-temperature superconductors (HTSs). Forty-five presentations covered topics ranging from the basic science of strain effects in superconductors, strain-scaling relations, stress-strain relations of superconducting tapes and wires, development of standards, all the way to stress-strain relations of superconducting cables and magnets. This special issue contains 11 full papers submitted by the attendees of the MEM16.

Starting with Nb₃Sn, Brown et al describe the correlation between filament distortion and RRR in drawn and rolled PIT and RRP Nb₃Sn wires that are being developed for the accelerator magnet upgrades of the high-luminosity LHC [1]. The study found that excessive distortion of the filaments could result in diffusion barrier breaks, followed by Sn leakage during reaction and finally an increase in RRR degradation. Sanabria et al describe metallographic autopsies of full-scale ITER prototype cable-in-conduit-conductors after full cyclic testing in SULTAN [2]. The authors found that reduction in current sharing temperature under transverse load was likely caused by an overall increase in compressive strain due to longitudinal movement of the strands, instead of a few (and very specific) instances of increased tension (or bending caused by transverse movement of the strands), as earlier proposed. A high surface roughness of strands in one particular Toroidal Field cable was found to limit the longitudinal movement of the strands within the cable, therefore effectively preventing degradation of the properties under test. Oguro et al describe the performance of a 14 T CuNb/Nb₃Sn Rutherford coil with 300 mm wide cold bore [3] constructed for the outsert of a 25 T LTS/HTS superconducting magnet. The Rutherford coil reached its target field after a 1 h ramp without any training quench.

Continuing with MgB₂, a paper by Konstantopoulou et al describes the electro-mechanical characterization of MgB₂ wire for the Superconducting Link Project at CERN [4]. The electrical and mechanical properties of ex situ processed MgB₂ wires were tested in an effort to optimize the layout of 18-strand cables that would be used in the superconducting links. Wires extracted from cables were also tested to determine the level of degradation after cabling.

Moving on to oxide high-temperature superconductors, Osamura et al described the reversible stress and strain limits of the critical current of practical RE-Ba₂Cu₃O_7−δ (REBCO) and Ba₂Sr₂Ca₂Cu₃O_x (BSCCO) wires [5]. The definition of reversible stress and strain limit on the critical current was discussed. It was concluded that the reversible limits could be defined as a recovery in critical current of 99% after the stress or strain was released, but that a recovery of only 95%, which is often used as definition of the reversible limit, is invalid.

Efforts in cabling REBCO coated conductors for use in power transmission cables and high-current magnets have increased significantly during recent years and studying the electromechanical behavior of REBCO tapes under stresses typical in REBCO cables has developed in a fruitful area of interest. Grether et al studied the electromechanical behavior of REBCO tape lap splices under transverse compressive loading as experienced by tapes in Roebel cables that operate at high magnetic fields [6]. The splice resistance and critical current of soldered splices up to 250 MPa transverse stress, while the contact resistance between unsoldered tapes depends strongly on the transverse pressure. Gorospe et al describe the degradation behavior of the critical current in REBCO tapes in pure tension and combined tension-torsion modes [7]. Conductors containing electroplated copper and those with external laminations were characterized to assist in the design of twisted stacked tape cables in which tapes experience torsion during cabling and operation of magnets wound from these cables.

The development of high-current REBCO cables is also moving forward at a rapid pace. Bykovsky et al report on the performance evaluation of 60 kA HTS cable prototypes that were tested in EDIPO [8]. These high-current cables were developed for use in fusion magnets, and their dc performance at fields between 8 and 12 T was found to be close to the expected value, while significant degradation after 1000 load cycles was reported. Many high-field magnets require high operating current densities. Sundaram et al report on the development of REBCO coated conductors with 30 mm thick Hastelloy substrates in an effort to reduce the overall thickness of the conductor to allow for higher current densities [9]. The paper outlines the electrical and mechanical properties of these extremely thin tapes. Weiss et al introduce highly flexible, round REBCO CORC^® wires for high-field magnet and power transmission applications [10]. REBCO tapes with 30 mm thick Hastelloy substrates enable these thin CORC^® wires. The thinner substrates limit the bending strain in the REBCO film, allowing them to be wound onto much thinner cable formers. Another approach to develop REBCO cables with high current densities is described by Solovyov et al who describe exfoliated REBCO filaments for REBCO superconducting cables [11]. The substrate is removed from the REBCO tapes during this process and replaced by a stronger metal film. That paper reports 90% retention in critical current and full retention of pinning strength of the REBCO film after exfoliation.