Effect of transverse compressive monotonic and cyclic loading on the performance of superconducting CORC^® cables and wires

Abstract

Superconducting CORC ® cables and wires have become practical conductors for use in high-field magnets for fusion machines and particle accelerators by demonstrating their ability to carry very high currents in background magnetic fields of up to 20 T. The high mechanical stresses that develop on the CORC ® conductor during such operation could result in permanent degradation of the conductor critical current. Transverse compressive stress is one of the predominant mechanical stresses when CORC ® cables or wires are bundled into CICCs that allow fusion and detector magnets to operate at currents as high as 100 kA. The effect of transverse compressive load on the critical current of CORC ® cables and wires has been investigated at 76 K to determine their irreversible load limit under monotonic loading and load cycling up to 100 000 cycles. The results show a clear effect of the CORC ® conductor layout with respect to gap spacing between tapes, thickness of the copper plating surrounding the tapes and thickness of the former onto which the tapes are wound. CORC ® cables and wires have demonstrated a remarkable resilience to transverse compressive load cycling where their critical current decreased by no more than a few percent after 100 000 load cycles at peak loads that resulted in an initial decrease in critical current of less than 5%. The results indicate that no significant degradation of CORC ® cable and wire performance due to transverse compressive load is expected in large magnets after 100 000 load cycles, as long as the peak load on the conductor does not exceed the irreversible load limit defined at 95% retention in critical current. The irreversible load limit of CORC ® conductors could be increased further by increasing the size or hardness of the former that makes up the conductor's core.

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“…The design allows the incoming REBCO conductor to enter the groove with minimum obstacle. The tensile stress (1.3 MPa) due to the winding tension is well below the irreversible tension stress measured on CORC R wires with 27 or 30 tapes [60].…”

“…This stress level is sufficient for a 20 T block REBCO dipole design [67]. Measurements on commercial CORC R wires, without impregnation, showed an estimated irreversible stress limit between 99 and 241 MPa at 76 K, defined at a 3% I c degradation criterion [60].…”

“…In addition to the external loads, what is the impact of Lorentz force (I × B) as an internal load to the multi-tape REBCO conductors? Measurements similar to those performed on REBCO fusion conductors [133,134] To improve the conductor stress/strain limit, one can optimize conductor architecture such as introducing a stronger core material for the round REBCO wires [60]. It may also be necessary to support individual tapes in the conductor through impregnation to better distribute the stresses as discussed in Section 3.1.…”

“…The available measurement data, though limited in statistics, indeed show non-negligible degradation in high-current REBCO conductors after cycling the Lorentz load [133,134,[136][137][138]. On the other hand, cycling tests on CORC R conductors in liquid nitrogen with external mechanical loads show that the current-carrying capability does not degrade over 10 5 cycles when the load is below the irreversible limit [56,60].…”

“…• C N/A 665 [27] 892 [50] N/A Irreversible tensile strain limit ->1% [51] 0.4% [52] 0.3% [53,54] 0.36% [55] 0.85% [56] TBD Irreversible transverse stress limit MPa >200 [24] 150-260 [57,58] 60-TBD [59] 370-440 [37] 99 k [60] TBD Stabilizer fraction -62% [24] 55% [27] 78% [61] 20% [37] 57% l [38] 17% l [34] Stabilizer RRR -200 [24] ≥150 [27] 90-440 [61,62] 13-69 m [63,64] g When the magnetic flux lines are in parallel with the broad surface of the REBCO tape, the REBCO layer thickness becomes the characteristic length scale for an effective filamnet size.…”