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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

Absolute value of the slope of critical current versus temperature near 5 K plotted versus critical current at 5 K on a semi-logarithmic scale for various samples and applied strains.
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f6-jres.118.015: Absolute value of the slope of critical current versus temperature near 5 K plotted versus critical current at 5 K on a semi-logarithmic scale for various samples and applied strains.

Mentions: It is useful to compare the slope of Ic(T) for all of the samples near 5 K since this is where we will be making liquid/gas comparisons. Figure 5 shows the slope of Ic(T) in units of A/K versus Ic at 5 K for all four samples. For Nb3Sn wires Ic(H, T) also changes significantly with applied strain [1], so data are shown for two different applied strains. A semi-logarithmic plot of the absolute value of the slope of Ic(T) in units of %Ic/K versus Ic at 5 K is shown in Fig. 6. The transition temperature is one parameter that determines the Ic sensitivity to T, which is why the Nb-Ti sample is more sensitive than the other two types of superconductors. The two Nb3Sn wires have nearly the same transition temperature, but Nb3Sn #2 has a higher critical-current density than Nb3Sn #1, which causes a bigger change in Ic with T. Because the transition temperature of MgB2 is about 39 K, the slope of Ic(T) is much lower at all measured critical currents. HTS materials such as YBa2Cu3O7-δ (YBCO) would have even lower slopes of Ic(T) than MgB2 at 5 K. Because of these lower slopes and, in some cases, critical currents that vary more with time and magnetic field history, HTS samples are not as appropriate for making liquid/gas comparisons at 5 K.


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)

Absolute value of the slope of critical current versus temperature near 5 K plotted versus critical current at 5 K on a semi-logarithmic scale for various samples and applied strains.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f6-jres.118.015: Absolute value of the slope of critical current versus temperature near 5 K plotted versus critical current at 5 K on a semi-logarithmic scale for various samples and applied strains.
Mentions: It is useful to compare the slope of Ic(T) for all of the samples near 5 K since this is where we will be making liquid/gas comparisons. Figure 5 shows the slope of Ic(T) in units of A/K versus Ic at 5 K for all four samples. For Nb3Sn wires Ic(H, T) also changes significantly with applied strain [1], so data are shown for two different applied strains. A semi-logarithmic plot of the absolute value of the slope of Ic(T) in units of %Ic/K versus Ic at 5 K is shown in Fig. 6. The transition temperature is one parameter that determines the Ic sensitivity to T, which is why the Nb-Ti sample is more sensitive than the other two types of superconductors. The two Nb3Sn wires have nearly the same transition temperature, but Nb3Sn #2 has a higher critical-current density than Nb3Sn #1, which causes a bigger change in Ic with T. Because the transition temperature of MgB2 is about 39 K, the slope of Ic(T) is much lower at all measured critical currents. HTS materials such as YBa2Cu3O7-δ (YBCO) would have even lower slopes of Ic(T) than MgB2 at 5 K. Because of these lower slopes and, in some cases, critical currents that vary more with time and magnetic field history, HTS samples are not as appropriate for making liquid/gas comparisons at 5 K.

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