Superior critical current of Symmetric Tape Round (STAR) REBCO wires in ultra-high background fields up to 31.2 T

Abstract

Symmetric Tape Round (STAR) wire fabricated with an ultra-thin RE-Ba-Cu-O (REBCO, RE=rare earth) coated conductor has a high engineering current density (J e ) and remarkable flexibility. It is a competitive candidate for future canted cosine theta magnets with smaller winding radii that operate at fields above 20 T at 4.2 K. STAR wires consisting of six or eight REBCO coated conductor layers have been fabricated with an outer diameter smaller than 2 mm. We performed in-field tests of the critical current of STAR wires at 4.2 K in background fields up to 31.2 T. At a 15 mm bending radius, J e values of 438 A mm −2 at 20 T and 299 A mm −2 at 31.2 T were obtained. These are the highest J e values measured in any reported round-geometry REBCO wires at such ultra-high background magnetic fields. This exhibits the potential of STAR wires in future compact accelerator magnets.

Progress in scale-up of REBCO STAR™ wire for canted cosine theta coils and future strategies with enhanced flexibility

“…The downside is that increasing the gap size reduces tape support, making the outer layers more susceptible to deformation due to Lorentz force. This is different from previous STAR™ wires where a constant tape geometry was used in each layer [32,33].…”

“…An optimal shield was used to deposit copper stabilizer primarily on the REBCO film side to fabricate the symmetric tapes. In these copper-plated symmetric tapes, REBCO film is positioned near the neutral plane and experiences negligible strain while bending to a small bend radius [28][29][30][31][32][33]. As per our earlier reported elastoplastic model, the thickness of the electroplated copper stabilizer is individually adjusted for each tape layer in order to minimize bending strain as a function of layer radius [30].…”

“…Oxygen-free high-conductivity (OFHC) copper wires with diameters of 1.02 mm (18 AWG) and 0.81 mm (20 AWG) were selected as formers for the 2 m and 10 m long STAR™ wires, respectively, and 0.81 mm and 0.64 mm diameter (22 AWG) copper wires were selected as formers for the 25 cm long STAR™ wires. We had previously shown that this former is strong enough to withstand huge Lorentz forces (36 kN m −1 ) in operation while flexible enough to bend the STAR™ wire to a small diameter [31][32][33]. The former had no insulation layer to provide current-sharing ability.…”

“…A challenge with REBCO coated conductors as compared to Nb-Ti, Nb 3 Sn and Bi-2212 wires is associated with their flat rather than round geometry and wide (~12 mm) profile rather than multifilamentary architecture. This challenge has been overcome using symmetric REBCO tapes wherein the REBCO film is positioned at the neutral plane [27][28][29][30][31][32][33]. Due to the excellent bend tolerance of the REBCO symmetric tapes, the flat REBCO tape geometry can be converted to round wire by the helical winding of narrow tapes on a round former as small as 0.51-1.02 mm [27][28][29][30].…”

Small REBCO isotropic round wire (STAR): an important step from performant material to practical conductor in high field HTS magnets

“…Bearing this in mind, two cable layouts have emerged with a good potential to reach high current density while giving the desired isotopic behaviour: the Conductor On Round Core (CORC ® ) [7,8] and the Symmetric Tape Round (STAR) discussed in this paper. This layout by Luo et al is the subject of the most recent paper [9] published by the Selvamanickam team at the Texas Center for Superconductivity of the University of Houston, following many years of successful development and improvement of the STAR wires [10,11].…”