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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 H2 where ΔT = 0 versus Ic at 5 K for a Nb-Ti wire soldered to a stainless-steel mandrel on the VTO probe. The T bias (T1−T2) was set to 0, 10, 20, and 30 mK. The weighted temperature Tw = 0.5 T1+0.5 T2 was used to determine the correct Ic.
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f41-jres.118.015: Plot of H2 where ΔT = 0 versus Ic at 5 K for a Nb-Ti wire soldered to a stainless-steel mandrel on the VTO probe. The T bias (T1−T2) was set to 0, 10, 20, and 30 mK. The weighted temperature Tw = 0.5 T1+0.5 T2 was used to determine the correct Ic.

Mentions: Measurements of ΔT versus H2 were also made at a number of magnetic fields/critical currents and T biases as summarized in Fig. 41. The figure shows that H2 where ΔT is zero has a similar dependence on T bias over the whole range of Ic values with the simple average Tw equation. When Tw = 0.14 T1+0.86 T2 was used to determine the correct Ic, as shown in Fig. 42, effectively all of the Fig. 41 data collapsed onto one curve, indicating that the scaling optimized on 143 A also works from 4 to 981 A. Figure 43 shows H1 where ΔT is zero versus Ic with the same Tw equation (Tw = 0.14 T1+0.86 T2), illustrating that the value of H1 where the correct Ic is determined can be systematically varied by changing T bias. In general, we did not need to operate the VTO probe using a non-zero T bias, as can be seen in Fig. 43, because the values of H1 where ΔT = 0 are all positive or very close to zero for all critical currents.


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 H2 where ΔT = 0 versus Ic at 5 K for a Nb-Ti wire soldered to a stainless-steel mandrel on the VTO probe. The T bias (T1−T2) was set to 0, 10, 20, and 30 mK. The weighted temperature Tw = 0.5 T1+0.5 T2 was used to determine the correct Ic.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f41-jres.118.015: Plot of H2 where ΔT = 0 versus Ic at 5 K for a Nb-Ti wire soldered to a stainless-steel mandrel on the VTO probe. The T bias (T1−T2) was set to 0, 10, 20, and 30 mK. The weighted temperature Tw = 0.5 T1+0.5 T2 was used to determine the correct Ic.
Mentions: Measurements of ΔT versus H2 were also made at a number of magnetic fields/critical currents and T biases as summarized in Fig. 41. The figure shows that H2 where ΔT is zero has a similar dependence on T bias over the whole range of Ic values with the simple average Tw equation. When Tw = 0.14 T1+0.86 T2 was used to determine the correct Ic, as shown in Fig. 42, effectively all of the Fig. 41 data collapsed onto one curve, indicating that the scaling optimized on 143 A also works from 4 to 981 A. Figure 43 shows H1 where ΔT is zero versus Ic with the same Tw equation (Tw = 0.14 T1+0.86 T2), illustrating that the value of H1 where the correct Ic is determined can be systematically varied by changing T bias. In general, we did not need to operate the VTO probe using a non-zero T bias, as can be seen in Fig. 43, because the values of H1 where ΔT = 0 are all positive or very close to zero for all critical currents.

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