Limits...
Kiloampere, Variable-Temperature, Critical-Current Measurements of High-Field Superconductors.

Goodrich LF, Cheggour N, Stauffer TC, Filla BJ, Lu XF - J Res Natl Inst Stand Technol (2013)

Bottom Line: Therefore, a significant portion of this review is focused on the reduction of temperature errors to less than ±0.05 K in such measurements.We also calibrated the magnetoresistance effect of resistive thermometers for temperatures from 4 K to 35 K and magnetic fields from 0 T to 16 T.This calibration reduces systematic errors in the variable-temperature data, but it does not affect the liquid/gas comparison since the same thermometers are used in both cases.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Colorado, Boulder, CO 80309 ; National Institute of Standards and Technology, Boulder, CO 80305.

ABSTRACT
We review variable-temperature, transport critical-current (I c) measurements made on commercial superconductors over a range of critical currents from less than 0.1 A to about 1 kA. We have developed and 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 and a probe that is variable-temperature and 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 ±0.05 K in such measurements. A key feature of our system is a pre-regulator that converts a flow of liquid helium to gas and heats the gas to a temperature close to the target sample temperature. The pre-regulator is not in close proximity to the sample and it is controlled independently of the sample temperature. This allows us to independently control the total cooling power, and 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 and results in some differences in operation and 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, and the perceived advantages and 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 I c liq) at 5 K to those measured on the same specimen in flowing helium gas ("gas" or I c gas) at the same temperature. These comparisons indicate the temperature control is effective over the superconducting wire length between the voltage taps, and this condition is valid for all types of sample investigated, including Nb-Ti, Nb3Sn, and MgB2 wires. The liquid/gas comparisons are used to study the variable-temperature measurement protocol that was necessary to obtain the "correct" critical current, which was assumed to be the I c liq. We also calibrated the magnetoresistance effect of resistive thermometers for temperatures from 4 K to 35 K and magnetic fields from 0 T to 16 T. This calibration reduces systematic errors in the variable-temperature data, but it does not affect the liquid/gas comparison since the same thermometers are used in both cases.

No MeSH data available.


Related in: MedlinePlus

Pre-regulator heater power versus critical current for a set of measurements on a Nb-Ti sample at 5 K, in gas, and on the VTO probe. The solid circle symbols indicate the average heater power before each V-I curve was taken and the open circle symbols indicate the average heater power during the time that the sample current was near Ic.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4487313&req=5

f20-jres.118.015: Pre-regulator heater power versus critical current for a set of measurements on a Nb-Ti sample at 5 K, in gas, and on the VTO probe. The solid circle symbols indicate the average heater power before each V-I curve was taken and the open circle symbols indicate the average heater power during the time that the sample current was near Ic.

Mentions: Figures 15 and 16 (liquid) and the next 11 figures contain data that were taken using the VTO probe on the same tap V1 (except for Fig. 16 that shows all three taps), the same Nb-Ti wire mounted on a Ti-6-4 coil mandrel, and the sample was not soldered to the mandrel. We focus on this one set of data to provide the reader with a more complete comparison of liquid and gas measurements. We will start with temporal plots like Fig. 15, then look at V-I curves like Fig. 16, and finally compare Ic values. The next 11 plots, Figs. 17–27, will show that the thermometer readings, sample temperature, sample voltage, and V-I curves in gas are well defined and the resulting Ic values are not merely a product of chance, data selection, or clever averaging or fitting methods.


Kiloampere, Variable-Temperature, Critical-Current Measurements of High-Field Superconductors.

Goodrich LF, Cheggour N, Stauffer TC, Filla BJ, Lu XF - J Res Natl Inst Stand Technol (2013)

Pre-regulator heater power versus critical current for a set of measurements on a Nb-Ti sample at 5 K, in gas, and on the VTO probe. The solid circle symbols indicate the average heater power before each V-I curve was taken and the open circle symbols indicate the average heater power during the time that the sample current was near Ic.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4487313&req=5

f20-jres.118.015: Pre-regulator heater power versus critical current for a set of measurements on a Nb-Ti sample at 5 K, in gas, and on the VTO probe. The solid circle symbols indicate the average heater power before each V-I curve was taken and the open circle symbols indicate the average heater power during the time that the sample current was near Ic.
Mentions: Figures 15 and 16 (liquid) and the next 11 figures contain data that were taken using the VTO probe on the same tap V1 (except for Fig. 16 that shows all three taps), the same Nb-Ti wire mounted on a Ti-6-4 coil mandrel, and the sample was not soldered to the mandrel. We focus on this one set of data to provide the reader with a more complete comparison of liquid and gas measurements. We will start with temporal plots like Fig. 15, then look at V-I curves like Fig. 16, and finally compare Ic values. The next 11 plots, Figs. 17–27, will show that the thermometer readings, sample temperature, sample voltage, and V-I curves in gas are well defined and the resulting Ic values are not merely a product of chance, data selection, or clever averaging or fitting methods.

Bottom Line: Therefore, a significant portion of this review is focused on the reduction of temperature errors to less than ±0.05 K in such measurements.We also calibrated the magnetoresistance effect of resistive thermometers for temperatures from 4 K to 35 K and magnetic fields from 0 T to 16 T.This calibration reduces systematic errors in the variable-temperature data, but it does not affect the liquid/gas comparison since the same thermometers are used in both cases.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Colorado, Boulder, CO 80309 ; National Institute of Standards and Technology, Boulder, CO 80305.

ABSTRACT
We review variable-temperature, transport critical-current (I c) measurements made on commercial superconductors over a range of critical currents from less than 0.1 A to about 1 kA. We have developed and 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 and a probe that is variable-temperature and 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 ±0.05 K in such measurements. A key feature of our system is a pre-regulator that converts a flow of liquid helium to gas and heats the gas to a temperature close to the target sample temperature. The pre-regulator is not in close proximity to the sample and it is controlled independently of the sample temperature. This allows us to independently control the total cooling power, and 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 and results in some differences in operation and 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, and the perceived advantages and 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 I c liq) at 5 K to those measured on the same specimen in flowing helium gas ("gas" or I c gas) at the same temperature. These comparisons indicate the temperature control is effective over the superconducting wire length between the voltage taps, and this condition is valid for all types of sample investigated, including Nb-Ti, Nb3Sn, and MgB2 wires. The liquid/gas comparisons are used to study the variable-temperature measurement protocol that was necessary to obtain the "correct" critical current, which was assumed to be the I c liq. We also calibrated the magnetoresistance effect of resistive thermometers for temperatures from 4 K to 35 K and magnetic fields from 0 T to 16 T. This calibration reduces systematic errors in the variable-temperature data, but it does not affect the liquid/gas comparison since the same thermometers are used in both cases.

No MeSH data available.


Related in: MedlinePlus