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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 only (VTO) probe. The active 3 turns of the coil sample are wound on the oxidized Ti-6Al-4V mandrel (center 0.95 cm long) with Cu lugs on each end and current bus bars. In operation, the left direction is up.
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f11-jres.118.015: Picture of sample test fixture for variable temperature only (VTO) probe. The active 3 turns of the coil sample are wound on the oxidized Ti-6Al-4V mandrel (center 0.95 cm long) with Cu lugs on each end and current bus bars. In operation, the left direction is up.

Mentions: The sample is mounted on a thin-walled, coil mandrel with OFHC copper current-contact lugs on each end, as shown in Fig. 11. The coil mandrel can be made of either a non-magnetic stainless steel or an oxidized Ti-6Al-4V (percent by mass, Ti-6-4, also known as Grade 5 Titanium) alloy. If the mandrel is stainless steel, we have the option of soldering the sample to the mandrel with non-superconducting solder to keep the specimen from moving and to provide some extra thermal stability. For both mandrel types, the mandrel diameter is about 32 mm with a 90° spiral groove with a pitch of 3.2 mm and 3 turns between the current contacts. Two bolts clamp the current lugs to the coil mandrel. The bolts are insulated from one lug so that they do not provide a parallel electrical path for the sample current. The Ic measured when the sample is normal is less than 0.03 A, indicating that the shunted current in the mandrel and solder is quite low. Three twisted pairs of voltage-tap wires are soldered along the sample to provide the main sample V-I characteristics. Each of the main sample voltage-tap pairs measure adjacent, 8-cm segments of the sample. Additional pairs of voltage taps are placed along the entire current path to allowing monitoring of the voltage drop along all portions of the sample and current leads within the probe. The current bus bars are connected to each copper lug so that the sample can be soldered to this sample test fixture separate from the VTO probe. The bus bar that delivers current to the right (bottom when in use) lug goes through and is insulated from the left lug. This bottom lead has an extra joint (extra resistance) as it transitions to the surface of the right lug. The test fixture can be bolted into the probe. The current contacts are made between the probe’s current leads and the sample current bus bars at a distance selected so that the test fixture does not need to be heated again.


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 only (VTO) probe. The active 3 turns of the coil sample are wound on the oxidized Ti-6Al-4V mandrel (center 0.95 cm long) with Cu lugs on each end and current bus bars. 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

f11-jres.118.015: Picture of sample test fixture for variable temperature only (VTO) probe. The active 3 turns of the coil sample are wound on the oxidized Ti-6Al-4V mandrel (center 0.95 cm long) with Cu lugs on each end and current bus bars. In operation, the left direction is up.
Mentions: The sample is mounted on a thin-walled, coil mandrel with OFHC copper current-contact lugs on each end, as shown in Fig. 11. The coil mandrel can be made of either a non-magnetic stainless steel or an oxidized Ti-6Al-4V (percent by mass, Ti-6-4, also known as Grade 5 Titanium) alloy. If the mandrel is stainless steel, we have the option of soldering the sample to the mandrel with non-superconducting solder to keep the specimen from moving and to provide some extra thermal stability. For both mandrel types, the mandrel diameter is about 32 mm with a 90° spiral groove with a pitch of 3.2 mm and 3 turns between the current contacts. Two bolts clamp the current lugs to the coil mandrel. The bolts are insulated from one lug so that they do not provide a parallel electrical path for the sample current. The Ic measured when the sample is normal is less than 0.03 A, indicating that the shunted current in the mandrel and solder is quite low. Three twisted pairs of voltage-tap wires are soldered along the sample to provide the main sample V-I characteristics. Each of the main sample voltage-tap pairs measure adjacent, 8-cm segments of the sample. Additional pairs of voltage taps are placed along the entire current path to allowing monitoring of the voltage drop along all portions of the sample and current leads within the probe. The current bus bars are connected to each copper lug so that the sample can be soldered to this sample test fixture separate from the VTO probe. The bus bar that delivers current to the right (bottom when in use) lug goes through and is insulated from the left lug. This bottom lead has an extra joint (extra resistance) as it transitions to the surface of the right lug. The test fixture can be bolted into the probe. The current contacts are made between the probe’s current leads and the sample current bus bars at a distance selected so that the test fixture does not need to be heated again.

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