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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

A schematic diagram of the lower part of our variable temperature measurement system shown with variable-temperature, variable-strain probe. The horizontal gaps between many components are exaggerated for clarity.
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f7-jres.118.015: A schematic diagram of the lower part of our variable temperature measurement system shown with variable-temperature, variable-strain probe. The horizontal gaps between many components are exaggerated for clarity.

Mentions: A schematic diagram of the lower part of our variable temperature measurement system is shown in Fig. 7. The sample probe is inserted into a re-entrant dewar (also known as a variable-temperature insert), which is inserted into a dewar with a superconducting magnet. Some of the system is built into the re-entrant dewar and it is used by multiple sample probes. This schematic is shown with the VTS probe. The differences between the VTO and VTS probes will be described later. The horizontal gaps between many components are exaggerated for clarity. The re-entrant dewar has a vacuum and super-insulated space with a liquid helium transfer tube that goes from the outside to the inside. The transfer tube is a thin-walled stainless-steel tube with an outer diameter of about 3.2 mm. It is coiled for one turn around the vacuum space of the re-entrant dewar, which accommodates thermal contraction and is easier to weld in place than a bellows feed-through that was used in an earlier design. The transfer tube allows liquid helium to flow from the magnet dewar to the middle of the re-entrant dewar.


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)

A schematic diagram of the lower part of our variable temperature measurement system shown with variable-temperature, variable-strain probe. The horizontal gaps between many components are exaggerated for clarity.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f7-jres.118.015: A schematic diagram of the lower part of our variable temperature measurement system shown with variable-temperature, variable-strain probe. The horizontal gaps between many components are exaggerated for clarity.
Mentions: A schematic diagram of the lower part of our variable temperature measurement system is shown in Fig. 7. The sample probe is inserted into a re-entrant dewar (also known as a variable-temperature insert), which is inserted into a dewar with a superconducting magnet. Some of the system is built into the re-entrant dewar and it is used by multiple sample probes. This schematic is shown with the VTS probe. The differences between the VTO and VTS probes will be described later. The horizontal gaps between many components are exaggerated for clarity. The re-entrant dewar has a vacuum and super-insulated space with a liquid helium transfer tube that goes from the outside to the inside. The transfer tube is a thin-walled stainless-steel tube with an outer diameter of about 3.2 mm. It is coiled for one turn around the vacuum space of the re-entrant dewar, which accommodates thermal contraction and is easier to weld in place than a bellows feed-through that was used in an earlier design. The transfer tube allows liquid helium to flow from the magnet dewar to the middle of the re-entrant dewar.

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