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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 for Nb-Ti and Nb3Sn #1 samples that were soldered and not soldered to the sample mandrel of the VTO probe. Determining the appropriate H2 values as a function of Ic is an important part of the protocol to obtain the correct gas measurements.
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f31-jres.118.015: Plot of H2 where ΔT = 0 versus Ic for Nb-Ti and Nb3Sn #1 samples that were soldered and not soldered to the sample mandrel of the VTO probe. Determining the appropriate H2 values as a function of Ic is an important part of the protocol to obtain the correct gas measurements.

Mentions: A key value from each curve in Fig. 26 and 30 is the H2 value where ΔT is zero. The value depends on Ic and, if this comparison is reproducible, shows what H2 value will give the correct Ic when measuring in gas. The target H2(Ic) is an important part of the correct protocol. A plot of H2 where the ΔT is zero versus Ic for the four cases are shown in Fig. 31. The two Nb3Sn #1 samples had the same heat treatment and were measured at magnetic fields from 5 T (soldered) and 5.5 T (not-soldered) to 16 T. The values of Ic at 16 T and 5 K (soldered 79 A and not-soldered 107 A) were somewhat different for the two Nb3Sn #1 samples because they had different strain states due to the different mandrel materials and possibly variations from sample to sample. The results on the two Nb-Ti samples are very close to each other. The curve for the Nb3Sn sample that was soldered is also close to that of the Nb-Ti samples. The curve for the Nb3Sn sample that was not soldered is shifted (by ~0.03 W) to slightly higher H2 values. Using the slope of −0.45 K/W, this power shift is equivalent to a T shift of about 0.014 K. We have seen slight changes in Ic, of segments of Nb3Sn samples that are not soldered to the measurement mandrel, with thermal cycling, which was attributed to changes in the strain state of various segments of the sample and the strain sensitivity of Ic in Nb3Sn. Thus, we think that the slight shift to higher H2 values is due to changes in the strain state of the sample between the liquid and gas measurements, which included violent heating (10 to 15 W) to boil away the remaining liquid helium and T cycling to regain T control back at 5 K. These results demonstrate that reproducibility and appropriate H2 values are unrelated to sample type and whether or not the sample was soldered to a mandrel.


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 for Nb-Ti and Nb3Sn #1 samples that were soldered and not soldered to the sample mandrel of the VTO probe. Determining the appropriate H2 values as a function of Ic is an important part of the protocol to obtain the correct gas measurements.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f31-jres.118.015: Plot of H2 where ΔT = 0 versus Ic for Nb-Ti and Nb3Sn #1 samples that were soldered and not soldered to the sample mandrel of the VTO probe. Determining the appropriate H2 values as a function of Ic is an important part of the protocol to obtain the correct gas measurements.
Mentions: A key value from each curve in Fig. 26 and 30 is the H2 value where ΔT is zero. The value depends on Ic and, if this comparison is reproducible, shows what H2 value will give the correct Ic when measuring in gas. The target H2(Ic) is an important part of the correct protocol. A plot of H2 where the ΔT is zero versus Ic for the four cases are shown in Fig. 31. The two Nb3Sn #1 samples had the same heat treatment and were measured at magnetic fields from 5 T (soldered) and 5.5 T (not-soldered) to 16 T. The values of Ic at 16 T and 5 K (soldered 79 A and not-soldered 107 A) were somewhat different for the two Nb3Sn #1 samples because they had different strain states due to the different mandrel materials and possibly variations from sample to sample. The results on the two Nb-Ti samples are very close to each other. The curve for the Nb3Sn sample that was soldered is also close to that of the Nb-Ti samples. The curve for the Nb3Sn sample that was not soldered is shifted (by ~0.03 W) to slightly higher H2 values. Using the slope of −0.45 K/W, this power shift is equivalent to a T shift of about 0.014 K. We have seen slight changes in Ic, of segments of Nb3Sn samples that are not soldered to the measurement mandrel, with thermal cycling, which was attributed to changes in the strain state of various segments of the sample and the strain sensitivity of Ic in Nb3Sn. Thus, we think that the slight shift to higher H2 values is due to changes in the strain state of the sample between the liquid and gas measurements, which included violent heating (10 to 15 W) to boil away the remaining liquid helium and T cycling to regain T control back at 5 K. These results demonstrate that reproducibility and appropriate H2 values are unrelated to sample type and whether or not the sample was soldered to a mandrel.

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