Vanadium oxide coatings to self-regulate current sharing in high-temperature superconducting cables and magnets

Abstract

High-temperature superconductors such as REBa2Cu3O7-δ (REBCO, RE = rare earth) enable high-current cables and high-field magnets. By removing the turn-to-turn insulation in a magnet application, recent experiments demonstrated that REBCO magnets can self-protect against catastrophic damage during a superconducting-tonormal transition (quench), i.e. when the stored magnetic energy rapidly converts to heat. The current can bypass the hot spot during a quench, thereby reducing the localized heat dissipation. The removal of the insulation between turns, however, leads to excessive eddy currents during current ramping, thereby forcing a muchprolonged magnet charging time. To address this issue, we investigate vanadium oxide (VOx) coatings as a temperature-dependent self-switching medium that automatically manages current sharing. VOx coatings (with 1.70 ≤ x ≤ to 2.07) were deposited by reactive cathodic arc deposition, initially on insulating glass to determine the electrical properties, and later on commercial REBCO tapes. The coatings are Xray amorphous but with short-range crystalline ordering according to Raman spectrometry. The resistivity of VOx decreased by at least three orders of magnitude when the temperature increased from 80 to 300 K. The coating process is compatible with commercial REBCO tapes as evidenced by the negligible change of the critical current caused by the coating process. The results from current sharing experiments and circuit analysis suggest that the VOx coating can effectively self-regulate current sharing in REBCO magnets, suppress excessive eddy currents and enable selfprotection during quenches.

Development and performance of a 2.9 Tesla dipole magnet using high-temperature superconducting CORC® wires

“…If the inter-tape coupling currents lead to the ramp-rate dependence in the C1 and C2 magnets, it would suggest that the contact resistances between the as-received REBCO tapes in a CORC ® wire can be low enough to allow current flowing between tapes, consistent with the turn-to-turn coupling currents in non-insulated REBCO coils [57][58][59]. Then, further reduction of contact resistance by using, for example, pretinned tapes, can lead to more pronounced and undesired ramp-rate dependence [60]; and control of inter-tape contact resistance may be necessary [61][62][63]. More detailed A c c e p t e d M a n u s c r i p t studies are required to clarify the source of the observed ramp-rate dependence and its implications on conductor optimization.…”

Analysis of Defect Irrelevancy in a Non-Insulated REBCO Pancake Coil Using an Electric Network Model

“…In addition, heat generation due to current sharing in a no-insulation coil or cable should be considered, in particular for conduction-cooled magnet applications [22]. The model may be used to study the impact of electrical contact resistances between conductor turns to regulate current sharing [18].…”

“…Here, we follow the idea of an equivalent electrical circuit [8]- [16] in the form of a rectilinear schematic [12] and expand our previous work on a stack of tapes [17], [18] by adding a Fig. 1.…”