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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

Pre-regulator heater power versus Ic for a set of measurements on a Nb-Ti sample at 5 K, in gas, and on the VTS probe. The solid circle symbols indicate the average heater power before each V-I curve was taken and the open circle symbols indicate the average heater power during the time that the sample current was near Ic. T bias (T1−T2) was set to 60 mK.
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f46-jres.118.015: Pre-regulator heater power versus Ic for a set of measurements on a Nb-Ti sample at 5 K, in gas, and on the VTS probe. The solid circle symbols indicate the average heater power before each V-I curve was taken and the open circle symbols indicate the average heater power during the time that the sample current was near Ic. T bias (T1−T2) was set to 60 mK.

Mentions: To lower the obtainable H2, we need a T bias that increases the cooling power for H1. Figure 46 shows the average Pre-Reg power just prior to the V-I curve (solid circles) and the average power during the time when the current is near Ic (open circles) as a function of Ic. The T bias (T1−T2) was set to 60 mK for these measurements. T bias did not have a noticeable change to Pre-Reg power. Figure 47 shows the average H1 and H2 powers just prior to the V-I curve (solid symbols) and the average powers during the time when the current is near Ic (open symbols) as a function of Ic with T bias = 60 mK. This T bias allowed H1 to be close to or above H2 just prior to the curve and allowed H2 to be low without H1 going to zero when the current was on, as can be seen by comparing H2 values on Figs 45 and 47. H1 dropped more when the current was on than H2 did in both figures, which was due to a higher heat load from the sample current on the top terminal.


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)

Pre-regulator heater power versus Ic for a set of measurements on a Nb-Ti sample at 5 K, in gas, and on the VTS probe. The solid circle symbols indicate the average heater power before each V-I curve was taken and the open circle symbols indicate the average heater power during the time that the sample current was near Ic. T bias (T1−T2) was set to 60 mK.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f46-jres.118.015: Pre-regulator heater power versus Ic for a set of measurements on a Nb-Ti sample at 5 K, in gas, and on the VTS probe. The solid circle symbols indicate the average heater power before each V-I curve was taken and the open circle symbols indicate the average heater power during the time that the sample current was near Ic. T bias (T1−T2) was set to 60 mK.
Mentions: To lower the obtainable H2, we need a T bias that increases the cooling power for H1. Figure 46 shows the average Pre-Reg power just prior to the V-I curve (solid circles) and the average power during the time when the current is near Ic (open circles) as a function of Ic. The T bias (T1−T2) was set to 60 mK for these measurements. T bias did not have a noticeable change to Pre-Reg power. Figure 47 shows the average H1 and H2 powers just prior to the V-I curve (solid symbols) and the average powers during the time when the current is near Ic (open symbols) as a function of Ic with T bias = 60 mK. This T bias allowed H1 to be close to or above H2 just prior to the curve and allowed H2 to be low without H1 going to zero when the current was on, as can be seen by comparing H2 values on Figs 45 and 47. H1 dropped more when the current was on than H2 did in both figures, which was due to a higher heat load from the sample current on the top terminal.

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