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.
High performance ReBCO magnet prototypes are typically monitored and protected with voltage measurements, however a variance in safe operating limits has been observed. A potential issue arises from current redistribution phenomena associated with unidentified defects in cables composed of ReBCO tapes. In this work, a network model is developed to simulate current and voltage distributions around defects in CORC cables. The evolving network of conductor overlap is evaluated. Trends in CORC operation at 77 K are presented, and it is shown that power dissipation in an I-V curve depends strongly on a third dimension of defect magnitude. The predictive tool is then coupled with a differential evolution algorithm to recommend optimal \CORC layering topologies based on reel-to-reel tape measurements. The developed model facilitates understanding of CORC cable phenomena, and the results suggest HTS magnet protection can be improved with cable and defect characterization efforts.
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