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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

Plot of slope of the ΔT(H2) versus Ic for Nb-Ti (T = 5 and 6.5 K) and Nb3Sn #1 (T = 5, 8, and 12 K) samples that were soldered and not soldered to the sample mandrel of the VTO probe.
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f33-jres.118.015: Plot of slope of the ΔT(H2) versus Ic for Nb-Ti (T = 5 and 6.5 K) and Nb3Sn #1 (T = 5, 8, and 12 K) samples that were soldered and not soldered to the sample mandrel of the VTO probe.

Mentions: We cannot do a liquid/gas comparison at temperatures above 5 K, so this leaves the question somewhat open as to whether the same protocol leads to different results at higher temperatures where gas flow rates, thermal conductivity, and heat capacity are different. However, we can still measure Ic gas above 5 K versus heater power and see how it varies. We cannot know the correct Ic, but we can measure the approximate T dependence at two different temperatures and nearly the same heater power and use an arbitrary Ic gas at the lowest heater power as the correct value. Then, we can make plots that look very similar to Figs. 26 and 27, except the curves are shifted arbitrarily so that ΔT = 0 at the lowest heater power. The slope of ΔT versus heater power at any T is not arbitrary and gives some information about the effect of heater power at higher temperatures. A plot of the slope of ΔT(H2) in units of K/W versus Ic is shown in Fig. 33 for the four cases (two samples, two solder conditions) and various temperatures from 5 K to 12 K. All of the data at 5 K (all black symbols) are based on liquid/gas comparisons. As before, we ignore the slopes at very low currents. Solid symbols indicate that the sample was soldered to the mandrel. Red symbols indicate that T was above 5 K. All of these slopes are within a narrow band of values (range about 0.2 K/W) and are relatively independent of Ic. The average slope is −0.41 K/W at 5 K and −0.39 K/W for T greater than 5 K, suggesting that a protocol based on heater power may remove the other effects of temperature. Thus, we expect the protocol to remain similar for a reasonable range of temperatures.


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)

Plot of slope of the ΔT(H2) versus Ic for Nb-Ti (T = 5 and 6.5 K) and Nb3Sn #1 (T = 5, 8, and 12 K) samples that were soldered and not soldered to the sample mandrel of the VTO probe.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f33-jres.118.015: Plot of slope of the ΔT(H2) versus Ic for Nb-Ti (T = 5 and 6.5 K) and Nb3Sn #1 (T = 5, 8, and 12 K) samples that were soldered and not soldered to the sample mandrel of the VTO probe.
Mentions: We cannot do a liquid/gas comparison at temperatures above 5 K, so this leaves the question somewhat open as to whether the same protocol leads to different results at higher temperatures where gas flow rates, thermal conductivity, and heat capacity are different. However, we can still measure Ic gas above 5 K versus heater power and see how it varies. We cannot know the correct Ic, but we can measure the approximate T dependence at two different temperatures and nearly the same heater power and use an arbitrary Ic gas at the lowest heater power as the correct value. Then, we can make plots that look very similar to Figs. 26 and 27, except the curves are shifted arbitrarily so that ΔT = 0 at the lowest heater power. The slope of ΔT versus heater power at any T is not arbitrary and gives some information about the effect of heater power at higher temperatures. A plot of the slope of ΔT(H2) in units of K/W versus Ic is shown in Fig. 33 for the four cases (two samples, two solder conditions) and various temperatures from 5 K to 12 K. All of the data at 5 K (all black symbols) are based on liquid/gas comparisons. As before, we ignore the slopes at very low currents. Solid symbols indicate that the sample was soldered to the mandrel. Red symbols indicate that T was above 5 K. All of these slopes are within a narrow band of values (range about 0.2 K/W) and are relatively independent of Ic. The average slope is −0.41 K/W at 5 K and −0.39 K/W for T greater than 5 K, suggesting that a protocol based on heater power may remove the other effects of temperature. Thus, we expect the protocol to remain similar for a reasonable range of temperatures.

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