Kiloampere, variable-temperature, critical-current measurements of high-field superconductors
Loren F. Goodrich, Najib Cheggour, Theodore C. Stauffer, James Filla, Xifeng Lu
We review variable-temperature, transport critical-current (Ic) measurements made on commercial superconductors over a range of critical currents from less than 0.1 A to about 1 kA. We have developed & used a number of systems to make these measurements over the last 15 years. Two exemplary variable-temperature systems with coil sample geometries will be described: a probe that is only variable-temperature & a probe that is variable-temperature & variable-strain. The most significant challenge for these measurements is temperature stability, since large amounts of heat can be generated by the flow of high current through the resistive sample fixture. Therefore, a significant portion of this review is focused on the reduction of temperature errors to less than or equal to} 0.05 K in such measurements. A key feature of our system is a pre-regulator that coverts a flow of liquid helium to gas & heats the gas to a temperature close to the target sample temperature. The pre-regulator is not in close proximity to the sample & it is controlled independently of the sample temperature. This allows us to independently control the total cooling power, & thereby fine tune the sample cooling power, at any sample temperature. The same general temperature control philosophy is used in all of our variable-temperature systems, but the addition of another variable, such as strain, forces compromises in design & results in some differences in operation & protocol. These aspects are analyzed to assess the extent to which the protocols for our systems might be generalized to other systems at other laboratories. Our approach to variable- temperature measurements is also placed in the general context of measurement system design, & the perceived advantages & disadvantages of design choices are presented. To verify the accuracy of the variable-temperature measurements, we compared critical-current values obtained on a specimen immersed in liquid helium ("liquid" or liq K).
, Cheggour, N.
, Stauffer, T.
, Filla, J.
and Lu, X.
Kiloampere, variable-temperature, critical-current measurements of high-field superconductors, Journal of Research (NIST JRES), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://doi.org/10.6028/jres.118.015
(Accessed December 7, 2021)