Quench Detection in REBCO Using the Tape Itself for Microwave Transmission Line Excitation and Sensing

We present progress in the use of rf/microwave standing waves in REBCO tapes to develop a nondestructive (NDE) technique for quench detection. REBCO tape can be treated as a transmission line for electromagnetic waves at high frequencies, or alternatively, as a capacitor at low frequencies. In principle, standing waves in a transmission line can be excited either capacitively or inductively, which provides great flexibility and multitude of choices for circuit optimization to form a system that is most sensitive to current and temperature disturbances. EM standing waves in transmission lines are particularly sensitive to any change in uniformity along its length, either in its geometry or in material property changes such as permittivity or of conductivity of the layers. Upon quench initiation, the standing waves formed in the transmission line are immediately disturbed, which can be readily sensed by a network analyzer and in general leads to formation of new peaks, change in Q factor and frequency shifts of existing peaks, and these signal changes will be sensed almost instantaneously. We present simulation and experimental results on short segments of REBCO tape subjected to transport dc current and simultaneously coupled as a transmission line. Heat pulses were applied to the tapes to increase tape's temperature and collect and analyze transmission line fingerprint changes. The results are analyzed and discussed in terms of the magnitude of sensitivity to quench.