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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 full-logarithmic scale plot of V-I curves for three voltage taps at two temperatures in liquid helium measured on a Nb-Ti sample at 3 T using the VTO probe.
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f16-jres.118.015: A full-logarithmic scale plot of V-I curves for three voltage taps at two temperatures in liquid helium measured on a Nb-Ti sample at 3 T using the VTO probe.

Mentions: A full-logarithmic scale plot of V-I curves is shown in Fig. 16 for three voltage taps at two temperatures. These curves are fairly linear over small regions indicating an approximately constant n-value. The V-I curves below 0.1 μV can be influenced by voltage noise and other subtle effects that create interfering voltage [3] including negative voltages [15]. The V-I curves are fairly stable to voltages of 3 to 4 μV at these currents with the sample in liquid helium. We use a linear fit of three V-I points to determine Ic at a given criterion. For example, the voltage equivalent to Ec = 0.1 μV/cm is 0.8 μV (8 cm tap spacing), which is indicated by the horizontal reference line in Fig. 16. Ic values at 5 K for each of the three taps, V1, V2, and V3 are 534.6 A, 536.2 A, and 538.6 A, respectively. The n-values near 0.1 μV/cm for the three taps are 53.8, 54.0, and 57.0, respectively. The slight Ic differences among the taps (a range of about 0.75 %) are very systematic as shown by the relative differences at 4.8 K being the same as at 5 K to within about 0.01 %. In some cases over an extended range of temperatures or magnetic fields, the relative order of the three sample segments can change in a smooth, continuous manner. Tap V1 covers the centermost portion of the sample and had the lowest Ic in this case, but did not seem to be damaged, as suggested by the n-value and low range of Ic values. So, we will just focus on Ic for tap V1.


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 full-logarithmic scale plot of V-I curves for three voltage taps at two temperatures in liquid helium measured on a Nb-Ti sample at 3 T using the VTO probe.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f16-jres.118.015: A full-logarithmic scale plot of V-I curves for three voltage taps at two temperatures in liquid helium measured on a Nb-Ti sample at 3 T using the VTO probe.
Mentions: A full-logarithmic scale plot of V-I curves is shown in Fig. 16 for three voltage taps at two temperatures. These curves are fairly linear over small regions indicating an approximately constant n-value. The V-I curves below 0.1 μV can be influenced by voltage noise and other subtle effects that create interfering voltage [3] including negative voltages [15]. The V-I curves are fairly stable to voltages of 3 to 4 μV at these currents with the sample in liquid helium. We use a linear fit of three V-I points to determine Ic at a given criterion. For example, the voltage equivalent to Ec = 0.1 μV/cm is 0.8 μV (8 cm tap spacing), which is indicated by the horizontal reference line in Fig. 16. Ic values at 5 K for each of the three taps, V1, V2, and V3 are 534.6 A, 536.2 A, and 538.6 A, respectively. The n-values near 0.1 μV/cm for the three taps are 53.8, 54.0, and 57.0, respectively. The slight Ic differences among the taps (a range of about 0.75 %) are very systematic as shown by the relative differences at 4.8 K being the same as at 5 K to within about 0.01 %. In some cases over an extended range of temperatures or magnetic fields, the relative order of the three sample segments can change in a smooth, continuous manner. Tap V1 covers the centermost portion of the sample and had the lowest Ic in this case, but did not seem to be damaged, as suggested by the n-value and low range of Ic values. So, we will just focus on Ic for tap V1.

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