Critical current retention of potted and unpotted REBCO Roebel cables under transverse pressure and thermal cycling

Abstract

Coated conductor Roebel cables are an effective way to create a high current density, fully transposed cable. However, despite REBCO tapes being robust against transverse stress, the Roebel architecture can concentrate transverse stress in non-trivial and random patterns depending on the exact arrangement of strands. If stands are embedded in a solid media which consolidates all strands then a transverse stress concentration will not occur. We tested this idea through mechanical and thermo-cycling tests on 5 strand Roebel cables. For non-impregnated cable irreversible degradation in critical currents is initiated at transverse pressures in a range of 4–34 MPa. Optical examination of the cables shows stress concentration patterns beyond those predicted by thickness variations. For cables impregnated with epoxy filled with SiO2 nanopowder, which has a similar thermal expansion coefficient to the metallic substrate of the strands, the irreversibility point is increased above our highest experimentally available pressure of 270 MPa. Thermo-cycling experiments confirmed a closely matched thermal expansion coefficient between the embedding media and metallic substrate is critical to avoid wire failures.

Dipole Magnets above 20 Tesla: Research Needs for a Path via High-Temperature Superconducting REBCO Conductors

“…First, Lorentz forces can cause deformation and stress concentration in tapes and joints that are not well supported and lead to I c degradation [140,141]. One solution is to fully impregnate the coil [141], consistent with the significantly improved resilience to the transverse pressure in impregnated Roebel cables [37,111,112]. On the other hand, solder-filled REBCO fusion conductors still show degradation [19,142].…”

“…Impregnation enables cables to distribute the stresses and to operate with high transverse pressure. For instance, the transverse pressure that a Roebel cable can withstand increases from below 40 MPa to at least 170 MPa after impregnation [37,111,112]. Meanwhile, epoxy in contact with REBCO tapes can substantially degrade the tape I c due to the low delamination stress in REBCO tapes (about 10 MPa) [113,114].…”

“…Meanwhile, epoxy in contact with REBCO tapes can substantially degrade the tape I c due to the low delamination stress in REBCO tapes (about 10 MPa) [113,114]. One approach to address this issue is to tailor the mechanical properties of the epoxy so as to mitigate the tape degradation as much as possible [112,[115][116][117][118], although non-negligible I c degradation still occurs in some cases when the epoxy touches the REBCO tapes [37,111]. A most effective approach appears to be completely shielding REBCO tapes from epoxy [15,116,119].…”

A review of commercial high temperature superconducting materials for large magnets: from wires and tapes to cables and conductors

“…Today all these limitations have been overcome: Cu electroplating is applied after punching and it efficiently reduces the delamination; an optimization of the cable layout made the cable tolerant to large transverse pressure [84] and impregnation further increases the resilience to transverse compression [85]; there is virtually no length limitation thanks to a computer controlled feedback system for punching and cabling.…”

Quench and stability of Roebel cables at 77 K and self-field: Minimum quench power, cold end cooling, and cable cooling efficiency

“…Numerous high-field magnets and insert magnets have been demonstrated using HTS conductors and a few high-field HTS-hybrid research magnets are nearly complete for user facilities [3][4][5][6]. Bi:2212 cables will likely require a high-pressure, atmospherically-controlled, wind-and-react, and epoxy impregnated magnet manufacturing process while Rare-Earth Barium Copper Oxide (ReBCO) will be react-and-wind, have simpler insulation options, and may not require epoxy impregnation depending on the stresses and cable type [7][8][9][10]. The structure of Bi:2212 Rutherford cable is similar to the NbTi and Nb3Sn Rutherford cables that are presently used, but ReBCO cables are quite different.…”