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Cooling efficiency and thermal stability is strictly demanding for practical applications of superconductors operating at current values close to the critical current, such as superconducting detectors. Indeed, a thermally unstable device can show premature quench, i.e. it can suddenly switch from the superconducting state to the normal one at a current value lower than the expected one, which can result in false counts. Cooling by direct contact with a liquid He bath is considered the best way to obtain thermal stability in a superconducting device. Other, cheaper cooling techniques can be suitable to achieve satisfactory working conditions. In this work, we evaluate the impact of three different cooling environments, namely liquid He in a standard cryostat and both dynamic and static He gas in a cryogen-free cryostat, on current voltage characteristics (CVCs) acquired in ultra-thin superconducting microbridges suitable for detectors. In particular, we use the Flux-Flow Instability phenomenon as a tool to analyze voltage stability in CVCs in the three different environments and we find that cryogen-free techniques have performance comparable to liquid He cooling.

Abstracb This paper presents the fabrication of Ta Superconducting Tunnel Junction detector working at 0.2 K to be used for photon counting instruments in astronomical applications. We would like to operate this type of detectors up to 2.5 pm with a moderate energy resolution in order to offer innovative instrumental perspectives to the astronomical community. The Ta junction fabrication and characterization as well as photon counting experiments in the near-infrared are presented. Fabrication process improvements are discussed at the end of this paper.

A frequency dependent inductance estimation has been carried out for Rapid-Single-Flux Quantum (RSFQ) applications in NbN multilayer technology up to frequencies above the NbN gap frequency. For this purpose an extension of the fullwave modified transverse resonance method (MTRM) has been used, in which each superconducting layer is characterized by its complex conductivity according to the BCS derived model of Mattis and Bardeen. Our estimation shows significant influence of the gap frequency on the NbN multilayer structure inductance at high frequencies.