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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

Percentage difference (Ic gas − Ic liq)/Ic liq versus the average heater power of control loop #2 (H2) at various magnetic fields where Ic varies from 79 to 770 A at 5 K for Nb3Sn #1 probe soldered to a stainless-steel mandrel and measured on the VTO.
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f29-jres.118.015: Percentage difference (Ic gas − Ic liq)/Ic liq versus the average heater power of control loop #2 (H2) at various magnetic fields where Ic varies from 79 to 770 A at 5 K for Nb3Sn #1 probe soldered to a stainless-steel mandrel and measured on the VTO.

Mentions: A liquid/gas comparison on the VTO probe with Nb3Sn #1 sample that was soldered to a stainless-steel mandrel is shown in the next three figures. A plot of ΔIc(A) versus H2 is shown in Fig. 28 and a plot of ΔIc(%) versus H2 is shown in Fig. 29 for various magnetic fields where Ic varies from 79 A at 16 T to 770 A at 5 T. A plot of ΔT versus H2 is shown in Fig. 30 for the same measurements as shown in Figs. 28 and 29. The ΔT versus H2 plots for Nb3Sn #1 and the Nb-Ti (Fig. 26) samples are nearly the same. This indicates that this relationship among ΔT, H2, and Ic is an important property of the VTM system is independent of sample type. The ΔIc plots for these same two samples have scales that are different by about a factor of 3 because the temperature dependence of the Nb3Sn sample is much less than that of the Nb-Ti sample at the same Ic. Notice that at the highest field (16 T), the Nb3Sn sample still had an Ic of 79 A. The ΔIc(%) plot for the Nb3Sn sample illustrates that VTM are much more sensitive to heater power at the lower currents than at the higher currents, which is also what was observed on the Nb-Ti sample.


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)

Percentage difference (Ic gas − Ic liq)/Ic liq versus the average heater power of control loop #2 (H2) at various magnetic fields where Ic varies from 79 to 770 A at 5 K for Nb3Sn #1 probe soldered to a stainless-steel mandrel and measured on the VTO.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f29-jres.118.015: Percentage difference (Ic gas − Ic liq)/Ic liq versus the average heater power of control loop #2 (H2) at various magnetic fields where Ic varies from 79 to 770 A at 5 K for Nb3Sn #1 probe soldered to a stainless-steel mandrel and measured on the VTO.
Mentions: A liquid/gas comparison on the VTO probe with Nb3Sn #1 sample that was soldered to a stainless-steel mandrel is shown in the next three figures. A plot of ΔIc(A) versus H2 is shown in Fig. 28 and a plot of ΔIc(%) versus H2 is shown in Fig. 29 for various magnetic fields where Ic varies from 79 A at 16 T to 770 A at 5 T. A plot of ΔT versus H2 is shown in Fig. 30 for the same measurements as shown in Figs. 28 and 29. The ΔT versus H2 plots for Nb3Sn #1 and the Nb-Ti (Fig. 26) samples are nearly the same. This indicates that this relationship among ΔT, H2, and Ic is an important property of the VTM system is independent of sample type. The ΔIc plots for these same two samples have scales that are different by about a factor of 3 because the temperature dependence of the Nb3Sn sample is much less than that of the Nb-Ti sample at the same Ic. Notice that at the highest field (16 T), the Nb3Sn sample still had an Ic of 79 A. The ΔIc(%) plot for the Nb3Sn sample illustrates that VTM are much more sensitive to heater power at the lower currents than at the higher currents, which is also what was observed on the Nb-Ti sample.

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