BS 61788-8:2010 pdf download – Superconductivity Part 8: AC loss measurements — Total AC loss measurement of round superconducting wires exposed to a transverse alternating magnetic field at liquid helium temperature by a pickup coil method
5.1 Testing apparatus The testing apparatus shall be constructed such that the pickup coils and a coiled specimen are arranged in a uniform alternating magnetic field applied by a superconducting magnet. The coils of the testing apparatus are arranged as described below. Typically, the main pickup and compensation coils are coaxially positioned on the outside and inside of the coiled specimen, respectively. The applied alternating magnetic field shall have a high uniformity as shown in 7.1 .5. The testing apparatus has a sub-system that calculates the magnetization and the AC loss of the specimen by integrating the signal of the pickup coils. A typical electrical circuit for the AC loss measurement is given in Figure 2.
5.2 Pickup coils Pickup coils shall be made of very fine insulated wire, such as insulated copper wire with a diameter of 0,1 mm, to avoid eddy currents at low temperatures. The pickup coil formers shall be made of non-metallic and non-magnetic material such as glass fiber reinforced plastic, phenol resin, etc. The main pickup coil shall be arranged coaxially and adjusted concentrically outside the compensation coil. The standard arrangement is shown schematically in Figure 1 , where the height of the compensation coil is the same as that of the main pickup coil. The number of turns in the compensation coil shall be usually adjusted to be a little larger than the balance level in which the total interlinkage flux of the applied magnetic field into the compensation coil is equal to that into the main pickup coil. The pickup coil system shall be constructed so that the coiled specimen can be taken in and out easily from the system. The pickup coil method has geometrical errors in relation with the arrangement of the coiled specimen and the pickup coils. The geometrical error is mentioned briefly in Annex C. To achieve a low uncertainty due to geometrical effects of less than 1 %, the following arrangement for the coiled specimen and the two pickup coils shall be the standard one; a height of 30 mm for the coiled specimen, a height of 1 0 mm for the pickup coils, a coil radius of 1 8 mm for the specimen, and a 2 mm difference between the radii of the specimen and each pickup coil. In the case where the arrangement of the specimen and pickup coils are a little different from the above standard one, the geometrical error in the arrangement shall be estimated, as shown in Annex C. If the geometrical error cannot be estimated quantitatively, the calibration indicated in Annex D may need to be performed.
5.3 Compensation circuit The total interlinkage flux of the applied field in the compensation coil is usually a little larger than that in the main pickup coil by adjusting the number of turns. The signal from the main pickup coil is counterbalanced against a reduced signal of the compensation coil by means of the compensation circuit. For delicate adjustment of the reduction ratio, called the compensation coefficient, the compensation circuit usually has the structure of a resistive potential divider with a wide adjustable range of four or five digits, namely minimum adjustable unit of 1 part in 1 0 4 or 1 part in 1 0 5 . The delicate adjustment using the wide range of the circuit results in a full compensation to almost remove the tilt in the magnetization loop in accordance with the procedures in 7.4.1 . The number of digits for the compensation circuit is designed with the condition that the minimum adjustable unit is sufficiently fine in comparison with the ratio of the moment-related component to the field-related one in the signal from the main pickup coil.