IEC 61788-4-2020 SUPERCONDUCTIVITY – Part 4: Residual resistance ratio measurement – Residual resistance ratio of Nb-Ti and Nb 3 Sn composite superconductors.
5.1 Material of measurement mandrel or of measurement base plate
Material of the measurement mandrel for a coiled Nb-Ti specimen or of the measurement base plate for a straight Nb-Ti or Nb 3 Sn specimen shall be copper, aluminium, silver, or the like whose thermal conductivity is equal to or better than 1 00 W/(m·K) at liquid helium temperature (4,2 K). The surface of the material shall be covered with an insulating layer (tape or a layer made of polyethylene terephthalate, polyester, polytetrafluoroethylene, etc.) whose thickness is 0,1 mm or less.
5.2 Diameter of the measurement mandrel and length of the measurement base plate The diameter of the measurement mandrel shall be large enough to keep the bending strain of the specimen less than or equal to 2 % for the Nb-Ti specimen. The Nb 3 Sn specimen on a base plate shall be measured in a strain-free condition or a condition with allowable thermal strain. The measurement base plate shall be at least 30 mm long in one dimension.
5.3 Cryostat for the resistance ( 2 R ) measurement The cryostat shall include a specimen support structure and a liquid helium reservoir for measurement of the resistance 2 R . The specimen support structure shall allow the specimen, which is mounted on a measurement mandrel or a measurement base plate, to be lowered into and raised out of a liquid helium bath. In addition, the specimen support structure shall be made so that a current can flow through the specimen and the resulting voltage generated along the specimen can be measured.
6 Specimen preparation The test specimen shall have no joints or splices with a length of 30 mm or longer. The specimen shall be instrumented with current contacts near each of its ends and a pair of voltage contacts over its central portion. The distance between two voltage taps ( L ) shall be 1 5 mm or longer. A thermometer for measuring cryogenic temperature shall be attached near the specimen. Some mechanical method shall be used to hold the specimen against the insulated layer of the measurement mandrel or base plate. Special care should be taken during instrumentation and installation of the specimen on the measurement mandrel or base plate so that no excessive force, which may cause undesired bending strain or tensile strain, would be applied to the specimen. Ideally, the Nb 3 Sn specimen is intended to be as straight as possible; however, this is not always the case, thus care should be taken to measure the specimen in its as received condition. The specimen shall be mounted on a measurement mandrel or on a measurement base plate for these measurements. Both resistance measurements, 1 R and 2 R , shall be made on the same specimen and the same mounting.
7 Data acquisition and analysis
7.1 Resistance ( 1 R ) at room temperature The mounted specimen shall be measured at room temperature ( m T (K)), where m T satisfies the following condition: 273 K ≤ m T ≤ 308 K. A specimen current ( 1 I (A)) shall be applied so that the current density is in the range of 0,1 A/mm 2 to 2 A/mm 2 based on the total wire cross- sectional area, and the resulting voltage ( 1 U (V)), 1 I and m T shall be recorded. Formula (2) below shall be used to calculate the resistance ( m R ) at room temperature. The resistance ( 1 R ) at 293 K (20 °C ) shall be calculated using Formula (3) for a wire with Cu matrix. For wires that do not contain a pure Cu component, the value of 1 R shall be set equal to m R , without any temperature correction.IEC 61788-4 pdf download.