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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

Picture of sample test fixture for variable-temperature and variable-strain (VTS) probe. The Cu-Be Walters spring is 7.2 cm tall and 2.5 cm in diameter with 4 active turns in the center. Cu lugs are screwed, pinned, and soldered to each end of the spring. Each lug has a current bus bar. Each end of the coil sample laps onto each lug and is soldered along its entire length. In operation, the left direction is up.
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f13-jres.118.015: Picture of sample test fixture for variable-temperature and variable-strain (VTS) probe. The Cu-Be Walters spring is 7.2 cm tall and 2.5 cm in diameter with 4 active turns in the center. Cu lugs are screwed, pinned, and soldered to each end of the spring. Each lug has a current bus bar. Each end of the coil sample laps onto each lug and is soldered along its entire length. In operation, the left direction is up.

Mentions: The Cu-Be spring, OFHC Cu current contacts (Cu lugs), and current bus bar form a modular sample test fixture shown in Fig. 13. The Cu lugs are mechanically held to the Cu-Be spring with sets of pins, screws, and solder. The spring is 7.2 cm tall and 2.5 cm in diameter with 4 active turns in the center. Each end of the coil sample laps onto each lug and is soldered along its entire length. We typically add another coil sample segment in parallel with each end of the sample, outside the active portion of the spring. These extra coil segments act as splices around any potentially damaged portions of the sample ends and help carry the current to the active portion of the sample. Only the T-section of the spring is electrically and mechanically in parallel with the sample in the active region of the spring. The Ic measured when the sample is normal is less than 0.03 A, indicating that the shunted current in the spring and solder is quite low. The sample test fixture is soldered and instrumented before it is installed on the apparatus. The current bus bars are made of Cu strip and a number of layers of YBCO-coated conductors. The bus bars are imbedded in the Cu lugs and the current transfers from the YBCO to the helical sample over the length of the Cu lugs. The portion of the bus bar that extends past the Cu lug allows for the final current connection to be made between the current bus bars and bus bars in the sample probe. Two set screws for the tube that is connected to the protractor can be seen on the left end of the spring in Fig. 13. Two set screws for the rod that is connected to the pointer can be seen on the right end of the spring. The protractor tube and the pointer rod are not shown in Fig. 7, but they are inside the inner torque shaft that is shown. The protractor tube and the pointer rod each have an electrical isolation break along their lengths.


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)

Picture of sample test fixture for variable-temperature and variable-strain (VTS) probe. The Cu-Be Walters spring is 7.2 cm tall and 2.5 cm in diameter with 4 active turns in the center. Cu lugs are screwed, pinned, and soldered to each end of the spring. Each lug has a current bus bar. Each end of the coil sample laps onto each lug and is soldered along its entire length. In operation, the left direction is up.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f13-jres.118.015: Picture of sample test fixture for variable-temperature and variable-strain (VTS) probe. The Cu-Be Walters spring is 7.2 cm tall and 2.5 cm in diameter with 4 active turns in the center. Cu lugs are screwed, pinned, and soldered to each end of the spring. Each lug has a current bus bar. Each end of the coil sample laps onto each lug and is soldered along its entire length. In operation, the left direction is up.
Mentions: The Cu-Be spring, OFHC Cu current contacts (Cu lugs), and current bus bar form a modular sample test fixture shown in Fig. 13. The Cu lugs are mechanically held to the Cu-Be spring with sets of pins, screws, and solder. The spring is 7.2 cm tall and 2.5 cm in diameter with 4 active turns in the center. Each end of the coil sample laps onto each lug and is soldered along its entire length. We typically add another coil sample segment in parallel with each end of the sample, outside the active portion of the spring. These extra coil segments act as splices around any potentially damaged portions of the sample ends and help carry the current to the active portion of the sample. Only the T-section of the spring is electrically and mechanically in parallel with the sample in the active region of the spring. The Ic measured when the sample is normal is less than 0.03 A, indicating that the shunted current in the spring and solder is quite low. The sample test fixture is soldered and instrumented before it is installed on the apparatus. The current bus bars are made of Cu strip and a number of layers of YBCO-coated conductors. The bus bars are imbedded in the Cu lugs and the current transfers from the YBCO to the helical sample over the length of the Cu lugs. The portion of the bus bar that extends past the Cu lug allows for the final current connection to be made between the current bus bars and bus bars in the sample probe. Two set screws for the tube that is connected to the protractor can be seen on the left end of the spring in Fig. 13. Two set screws for the rod that is connected to the pointer can be seen on the right end of the spring. The protractor tube and the pointer rod are not shown in Fig. 7, but they are inside the inner torque shaft that is shown. The protractor tube and the pointer rod each have an electrical isolation break along their lengths.

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