Further studies of a laser quenched superconducting opening switch

NbN films were prepared on planar sapphire and on small-diameter cylindrical quartz and sapphire substrates. Films prepared on cylindrical substrates had a greater critical current density than those prepared on planar substrates. The highest current densities of 1×10⁷ A/cm² were achieved on cylindrical sapphire substrates. Measurements of the laser power required to drive the switch into the normal state were made as a function of current being carried by the NbN-clad sapphire rod. These experiments indicated that near J_c very small amounts of laser energy were needed to drive the sample normal as expected. At current values well below J_c, much larger energies were required. These large energies, which are roughly 10³ larger than the condensation energy, were attributed to heating of the sapphire rod necessary to raise the NbN above its transition temperature. The desirable properties of a superconducting switch are the ability to carry large currents in the superconducting state and have a high resistance in the normal state. Considerations for using the high-T _c oxides in a superconducting switch are examined