Optimizing high frequency response of thin films deposited on composite substrates

The authors consider the thermal limitations placed on the high-frequency response of a superconducting device (switch) composed of a thin film (YBa₂Cu₃O₇) deposited on magnesium, sapphire, or silver substrates. In applying successive current pulses to the film, the heat generated during the quench must be transmitted from the film (source) through the substrate and dissipated in the coolant bath. The heat transfer from the film to the coolant medium is modeled in stages using plane wave theory. The optimum high-frequency response is obtained by matching the acoustic impedance of the multilayered structure to that of the liquid coolant (load). In general, a 20-80% improvement in the transmission coefficient for the generated heat could be gained by the addition of the matching layers