Dynamic Modes of Current-Carrying Elements of REBCO Tapes with no Copper Stabilizer at 77 and 4.2 K

In this paper, we present the results of the experimental study of the current-carrying capacity for two high-temperature superconducting samples made of either one 12-mm bare REBCO tape or a soldered stack of five such tapes on current growth rates of up to 350 kA/s. For samples in the form of rings with junctions, the transformer method of current injection was used. Experiments at 77 K showed that the currents of thermal equilibrium with the coolant achieved in the samples very weakly depend on the rate of change in the current and are close to critical ones. At 4.2 K, single mechanical defects of the superconducting layer appeared in the samples with a significant decrease in the local critical current. Current charging at different rates were accompanied by local transitions to the normal state caused by Joule heating of the silver coating when the current bypassed a defect with a resistance of ~1 μΩ at 4.2 K. Repeated measurements at 77 K showed that the resistance of these defects increased to 4 μΩ. This resistance determined the achievable currents at low induction voltages in the samples (low rates of current input). However, when passing through 200 kA/s, the currents reached during repeated measurements at 77 K practically equaled the currents of the first measurements at 77 K before the appearance of defects in the samples in liquid helium.