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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 exploded view of the lower part of the variable temperature only (VTO) probe. The test fixture is bolted between the top and bottom terminals. In operation, the left direction is up. Each terminal has a thermometer and heater. The top and bottom copper shells are two halves of a cylinder. Follow the assembly arrows from the shells to the terminals to see how the two shells fit around the test fixture and terminals.
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f12-jres.118.015: Picture of exploded view of the lower part of the variable temperature only (VTO) probe. The test fixture is bolted between the top and bottom terminals. In operation, the left direction is up. Each terminal has a thermometer and heater. The top and bottom copper shells are two halves of a cylinder. Follow the assembly arrows from the shells to the terminals to see how the two shells fit around the test fixture and terminals.

Mentions: The lower part of the VTO probe is shown in Fig. 12 with the test fixture bolted between the top and bottom terminals. In operation, the left direction is up (see arrow on left end of figure). There are slots (not shown) in the terminals into which the current bus bar slips. The top and bottom terminals each have a thermometer and heater. The two heaters are aluminum-backed, low-inductance, Kapton-foil heaters with a resistance of about 29.5 Ω and dimensions of 0.27 mm × 32 mm × 57 mm. The heaters are fastened to the terminals with epoxy. We refer to the temperatures and heater powers as T1 and H1, and T2 and H2 for the top and bottom, respectively. The top and bottom copper shells are two halves of a cylinder. The top copper shell is greased (with special high-thermal conductive, cryogenic grease) and screwed (four screws) to the top terminal so that they are in electrical and thermal contact. The lower part of the top shell is screwed (two screws through two slotted holes) to two fiber-glass epoxy insulators that are connected to the bottom terminal. The bottom copper shell is connected to the terminals in the opposite manner. Follow the assembly arrows from the shells to the terminals to see how the two shells fit around the test fixture and terminals. There is a gap between the two shells so that they are not electrically connected. Polyester electrical tape is used to seal the gaps so that the gas must flow down the outside (to the right) of the two shells and then up the inside over the bottom terminal, the sample, and 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)

Picture of exploded view of the lower part of the variable temperature only (VTO) probe. The test fixture is bolted between the top and bottom terminals. In operation, the left direction is up. Each terminal has a thermometer and heater. The top and bottom copper shells are two halves of a cylinder. Follow the assembly arrows from the shells to the terminals to see how the two shells fit around the test fixture and terminals.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f12-jres.118.015: Picture of exploded view of the lower part of the variable temperature only (VTO) probe. The test fixture is bolted between the top and bottom terminals. In operation, the left direction is up. Each terminal has a thermometer and heater. The top and bottom copper shells are two halves of a cylinder. Follow the assembly arrows from the shells to the terminals to see how the two shells fit around the test fixture and terminals.
Mentions: The lower part of the VTO probe is shown in Fig. 12 with the test fixture bolted between the top and bottom terminals. In operation, the left direction is up (see arrow on left end of figure). There are slots (not shown) in the terminals into which the current bus bar slips. The top and bottom terminals each have a thermometer and heater. The two heaters are aluminum-backed, low-inductance, Kapton-foil heaters with a resistance of about 29.5 Ω and dimensions of 0.27 mm × 32 mm × 57 mm. The heaters are fastened to the terminals with epoxy. We refer to the temperatures and heater powers as T1 and H1, and T2 and H2 for the top and bottom, respectively. The top and bottom copper shells are two halves of a cylinder. The top copper shell is greased (with special high-thermal conductive, cryogenic grease) and screwed (four screws) to the top terminal so that they are in electrical and thermal contact. The lower part of the top shell is screwed (two screws through two slotted holes) to two fiber-glass epoxy insulators that are connected to the bottom terminal. The bottom copper shell is connected to the terminals in the opposite manner. Follow the assembly arrows from the shells to the terminals to see how the two shells fit around the test fixture and terminals. There is a gap between the two shells so that they are not electrically connected. Polyester electrical tape is used to seal the gaps so that the gas must flow down the outside (to the right) of the two shells and then up the inside over the bottom terminal, the sample, and 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