USA interlaboratory comparison of superconductor simulator critical current measurements

Abstract

An interlaboratory comparison of critical current (Ic) measurements was conducted on the superconductor simulator, which is an electronic circuit that emulates the extremely nonlinear voltagecurrent characteristic of a superconductor. These simulators are high precision instruments, and are useful for establishing the integrity of part of a superconductor measurement system. This study includes measurements from participating US laboratories, with NIST as the central, organizing laboratory. This effort was designed to determine the sources of uncertainty in I, measurements due to uncertainties in the measurement apparatus, technique, or the analysis system. The participating laboratories measured the superconductor simulator with a variety of methods including dc and pulse. Thk comparison indicated the presence of systematic biases and higher variability at low voltages in the I, determinations of the measurement systems. All critical current measurements at a criterion of 10 fiV on the I, simulator were within 2% of the NIST value for nominal critical currents of 2 and 50 A. These results could significantly benefit superconductor measurement applications that require highprecision quality assurance. Current (A)I. THE HYBRID SUPERCONDUCTOR SMULATOR ne hybrid superconductor simulator emulates the extremely 1. V-I charactexistic of the 50 A superconductor simulator circuit. nonlinear voltage-current (V-I> characteristic of a superconductor along with its other major electrical properties by using passive circuit elements such as resistors and a diode [I]. The diode provides the nonlinearity necessary to generate the V-I characteristic. The simulator contains an active temperature controlled oven to maintain the diode temperature near 35°C. The term hybrid refers to the fact that each simulator consists of passive and active (only the oven) components. The simulator can be used as a sample substitution box that is measured at room temperature. Each simulator contains two separate circuits: one has an IC of 2 A, and one has an IC of 50 A . The n-value of the V-I curve is about 24 [2].The simulator can be used to test the integrity and accuracy of a complex measurement system because it has highly reproducible electrical characteristics. For example, the change in critical currents during this experiment for two out of the three simulators was less than 0.04%; the third simulator was still in circulation at the time of this writing. Fig. 1 shows a typical V-I curve for a 50 A simulator with an n-value of approximately 24. We calibrated each simulator to verify the linearity of the temperature dependence of the IC Manuscript received October 17, 1994. subject to copyright. Publication of the National Institute of Standards and Technology, notIn the calibration procedure, we obtained five V-I curves at diode tempaatures of appmximately 31, 33, 35, 37, 39, 39, 37, 35,33, a d 31°C for a total of 50 curves. We analyzed each V-I curve and determined IC at a number of criteria. Fig. 2 shows the NIST I , determinations on...

Characterisation of the Transport Critical Current Density for Conductor Applications

“…section G2.3.4), and at high E-fields, typically agree with d.c. methods to better than ~ 0.2% [55] but usually with higher noise levels. A calibrated superconducting simulator is available to assess experimental procedure [56].…”