A N. Srivastava

n-value and second derivative of the superconductor voltage-current characteristic

The authors studied the n-value (V varies as I/sup n/) and second derivative (d/sup 2/V/dI/sup 2/) of the voltage-current curve of high- and low-temperature superconductors and superconductor simulators. They used these parameters for diagnosing problems with sample heating and data acquisition, and as indicators of the superconducting-to-normal state transition. The superconductor simulator may be useful in testing the measurement system integrity and reducing measurement variability since its characteristics are highly repeatable.<<ETX>>

First VAMAS USA interlaboratory comparison of high temperature superconductor critical current measurements

We conducted an interlaboratory comparison of critical current (I/sub c/) measurements on Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ tapes (2223). This study includes measurements from six participating US laboratories, with NIST as the central, organizing laboratory. A number of specimens were prepared with different degrees of instrumentation to isolate sources of variability. Most of the specimens were pre-measured by NIST to reduce uncertainties due to sample variability. Different specimen routing patterns among the laboratories were implemented to isolate sources of variability due to the specimens measurement history. This study is similar to other VAMAS (Versailles Project on Advanced Materials and Standards) intercomparisons being performed in Japan and Europe and is the first internationally cooperative interlaboratory comparison of HTS (high temperature superconductors) I/sub c/ measurements. These are the first steps towards developing standard measurement procedures for HTS.<<ETX>>

Standard reference devices for high temperature superconductor critical current measurements

Abstract Obtaining repeatable critical current measurements for a high temperature superconductor (HTS) is a challenging task, since HTSs are highly susceptible to degradation due to mechanical stress, moisture, thermal cycling and aging. This paper discusses the development of a high temperature superconducting standard reference device (SRD) to address these measurement concerns and gives preliminary data on its characteristics. An SRD is an HTS specimen that has had its critical current lc non-destructively evaluated. Because HTSs are sensitive to mechanical alterations, minor changes in sample preparation or mounting procedure could yield large changes in the measured critical current. Preliminary data on SRDs made using Bi-based oxide tapes (2212) with an Ag substrate are presented. Differences between two consecutive measurements of lc can typically change by 40%; these deviations have been reduced to ≈ 4%.

Simulators of Superconductor Critical Current: Design, Characteristics, and Applications

The superconductor simulator is an electronic circuit that emulates the extremely nonlinear voltage-current characteristic (the basis of a critical-current measurement) of a superconductor along with its other major electrical properties. Three different types of simulators have been constructed: the passive, active, and hybrid simulator. The passive simulator has the fewest circuit components and offers the least amount of versatility, while the active and hybrid simulators offer more versatility and consequently have more components. Design, characteristics, and applications of the superconductor simulator along with a summary of features are presented. These simulators are high precision instruments, and are thus useful for establishing the integrity of part of a superconductor measurement system. They are potentially useful for testing the measurement method and data acquisition and analysis routines. The 50 A simulator provides critical-current precision of 0.1% at a 1 μV signal. This is significantly higher than the precision of a superconducting standard reference material. The superconductor simulator could significantly benefit superconductor measurement applications that require high-precision quality assurance.

High current pressure contacts to Ag pads on thin film superconductors

High current, low resistance, nonmagnetic, and nondestructive pressure contacts to Ag pads on YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin film superconductors were developed in this study. The contact resistance reported here includes the resistance of the current lead/Ag pad interface, the Ag pad/YBCO interface, and the bulk resistance of the contact material. This total contact resistance is the relevant parameter which determines power dissipation during critical-current measurements. It was found that regardless of the optimization of the Ag pad/YBCO interface through annealing, a pressure contact can yield a lower total resistance than a soldered contact. The lowest resistance obtained with pressure contacts was 3 /spl mu//spl Omega/ (for a 2/spl times/4 mm/sup 2/ contact). These contacts may be useful for many different high temperature superconductor (HTS) studies where high-current contacts with low heating are needed.<<ETX>>

Comparing the accuracy of critical-current measurements using the voltage-current simulator

A passive voltage-current simulator developed by the National Institute of Standards and Technology (NIST) is used to compare the accuracy of critical current measurements and the power-law behavior of high temperature superconductors (HTS). In this study, critical current measurements made from four data acquisition and analysis systems are compared with those carried out at NIST. This paper also discusses various measurement techniques, methods of calculating critical current, and n-values. The V-I simulator is believed to be an advancement towards defining the standards for critical current measurements and ensuring the traceability of results at different test facilities. >

Software Techniques to Improve Data Reliability in Superconductor and Low-Resistance Measurements

Software techniques have been developed to take low-amplitude data in various patterns, assign a figure of merit to a set of data readings, edit data for erroneous readings (or other experimental variations), and to alert the experimenter if the detected errors are beyond the scope of the software. Erroneous voltage readings from digital voltmeters, intermittent electrical connections, and an array of similar variations in data have been detected through the use of a data editor. The fixed-limit data editor removes readings that are inconsistent with the distribution of the majority of the data readings. The frequency of erroneous readings from a particular digital voltmeter ranges from 1 error per 100 000 readings to 1 error per 100 readings. The magnitude of the error can be as large as 3% of full scale with a zero volt input to the voltmeter. It may be necessary to have multiple meters measuring voltages in the same circuit in order to generate these erroneous readings. A systematic study was performed on the occurrence of the internally-generated erroneous voltmeter readings, and it was determined that the amount that a reading was in error scaled with one of a few parameters. The software techniques described here have been used in a variety of measurements, such as resistance-versus-temperature measurements made on cryoconductors or superconductors, and voltage-versus-current measurements made on superconductors to determine the critical current.