IEC 61788-17:2021 pdf free download – Superconductivity
5.1 Measurement equipment
Figure 1 shows a schematic diagram of a typical electric circuit used for the third-harmonicvoltage measurements. This circuit is comprised of a signal generator, power amplifier, digitalmultimeter (DMM) to measure the coil current, band-ejection filter to reduce the fundamentalwave signals and lock-in amplifier to measure the third-harmonic signals. It involves the single-coil approach in which the coil is used to generate an AC magnetic field and detect the inductivevoltage.This method can also be applied to double-sided superconducting thin films with noobstacles. In the methods proposed here, however,there is an additional system to reduceharmonic noise voltages generated from the signal generator and the power amplifier [14]. Inan example of Figure 1, a cancel coil of specification being the same as the sample coil is usedfor cancelling. The sample coil is mounted just above the superconducting film,and asuperconducting film with a Jd sufficiently larger than that of the sample film is placed belowthe cancel coil to adjust its inductance to that of the sample coil.Note that the inductance ofthe sample coil decreases by 20 % to 30 % due to the superconducting shielding current whenit is mounted on a superconducting film.Both coils and superconducting films are immersed inliquid nitrogen (a broken line in Figure 1).Other optional measurement systems are describedin Annex B.
NOTE In this circuit, coil currents of about 0,1 A(RMS) and power source voltages of > 6 V(RMS) are needed tomeasure the superconducting film of J,d = 10 kA/m while using coil 1 or 2 of Table 2.A precision power amplifierwith sufficiently high power is used to supply such large currents and voltages.
5.2.1coils
Currently available large-area HTS films are deposited on areas as large as about 25 cm indiameter，while films about 5 cm in diameter are commercially used to prepare microwavefilters [22]. Larger YBa,Cug07(YBCO) films, about 10 cm in diameter and 2,7 cm x 20 cm, wereused to fabricate fault current limiter modules [3][4][5].For the J。 measurements of such films,the appropriate outer diameter of the sample coils ranges from 2 mm to 5 mm.The requirementfor the sample coil is to generate as high a magnetic field as possible at the upper surface ofthe superconducting film, for which flat coil geometry is suitable.Typical specifications are asfollows.
5.2.2Spacer film
Typically, a polyimide film with a thickness of 50 um to 125 um is used to protect the HTS films.The coil has generally some protection layer below the coil winding，which also insulates thethin film from Joule heat in the coil.The typical thickness is 100 um to 150 um, and the coil-to-film distance Z is kept to be 200 um.
5.2.3 Mechanism for the set-up of the coil
To maintain a prescribed value for the spacing Z between the bottom of the coil winding andthe film surface, the sample coil should be pressed to the film with sufficient pressure, typicallyexceeding about 0,2 MPa [18].Techniques to achieve this are to use a weight or spring, asshown in Figure 2.The system schematically shown in the figure left is used to scan a widearea of the film. Before the U measurement the coil is initially raised up to some distance,moved laterally to the target position,and then lowered down and pressed to the film.Anappropriate pressure should be determined so that too high pressure does not damage thebobbin, coil, HTS thin film or the substrate. It is reported that the YBCO deposited on biaxially-textured pure Ni substrate was degraded by transverse compressive stress of about 20 MPa[23].