This paper presents recent advances in the application of binary-divided 1 V array, consisting of 8192 intrinsically shunted SNIS overdamped Josephson junctions (JJs), for the synthesis of stepwise waves with quantum accuracy. The maximum output voltage is ensured by opportunely driving the subsections of the SNIS array by means of three states biascurrent setpoints to the Shapiro steps n = 0, ±1 or n = ±2, respectively. Reconfigurable digital modular electronics has been designed to bias individually each of the 13 subsections of the SNIS array. A two-stage closed cycle refrigerator equipped with LF and RF electrical lines is employed for cooling-down the SNIS array for temperatures ranging from 3.6 K to above 7 K. Stepwise sine waves with rms amplitude ranging from 1 V to 2 V using the first (n=1) and second (n=2) Shapiro steps, different temperatures and bias-current setpoints have been synthesized up to the kHz range. The synthesized waves have been recorded and analyzed by a high-precision differential sampling system. We report the results of the first characterizations carried out with the new multi-bit current source and an improved version of the sample holder designed to optimize the heat dissipation of the SNIS array for operation in cryocooler setups.
Cryogen-free operation is essential to expand applications of superconductivity, is unavoidable in some special cases and, in perspective, provides a solution to the expected shortages of liquid helium. In electrical metrology applications, high-temperature operation to reduce the refrigerator size and complexity is not yet possible since arrays of Josephson junctions for voltage standard applications made with high-temperature superconductors are not presently available. The superconductor-normal metal-insulator-superconductor (SNIS) technology developed at INRIM uses low-temperature superconductors but allows operation at temperatures close to 6 K. Thus, it is interesting for the cryocooler operation of a programmable voltage standard. We report on measurements of SNIS devices cooled with a closed-cycle refrigerator and in liquid helium to test the electrical behavior and its dependence on specific fabrication parameters that can be used to optimize the temperature stability
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