IEC 61788-17-2021 pdf download.Superconductivity – Part 17: Electronic characteristic measurements – Local critical current density and its distribution in large-area superconducting films.
7 Uncertainty in the test method
7.1 Major sources of systematic effects that affect the U 3 measurement The most significant systematic effect on the U 3 measurement is due to the deviation of the coil-to-film distance Z 1 from the prescribed value. Because the measured value J c d in this technique is directly proportional to the magnetic field at the upper surface of the superconducting film, the deviation of the spacing Z 1 directly affects the measurement. The key origins of the uncertainty are listed in
a) to c) below. a) Inadequate pressing of the coil to the film As the measurement is performed in liquid nitrogen, the polyimide film placed above the HTS thin film becomes brittle and liquid nitrogen may enter the space between the polyimide and HTS films. Thus, sufficient pressure is needed to keep the polyimide film flat and avoid the deviation of Z 1 . An experiment has shown that the required pressure is about 0,2 MPa [1 8]. Here it is to be noted that thermal contraction of polyimide films at the liquid nitrogen temperature is less than 0,008 × (300 − 77) ≈ 1 ,78 %, which leads to negligible values of 0,9 μm to 4,5 μm compared with the total coil-to-film distance (about 200 µm) [26].
b) Ice layer formed between the coil and polyimide film The liquid nitrogen inevitably contains powder-like ice. If the sample coil is moved to scan the large-area HTS film area for an extended period, an ice layer is often formed between the polyimide film and the sample coil, which increases the coil-to-film distance Z 1 from the prescribed value. As shown later in 7.2, this effect reduces coil coefficients (k and k′), and the use of uncorrected k′ results in an overestimate in J c . Special care should be taken to keep the measurement environment as dry as possible. If the measurement system is set in an open (ambient) environment, the J c values measured after an extended period of time become sometimes greater than those measured before, and the overestimation was as large as 6 % when measured after one hour. If the measurement system is set in almost closed environment and the ambient humidity is kept less than about 5 %, such effect of ice layers can be avoided. We can check this effect by confirming reproducibility. If the same J c values are obtained after an extended period, it proves that there is negligible effect of ice layers. These two systematic effects (a) and b)) are not considered in the estimate of the uncertainty in the experimental coil coefficient k′ in 7.3 and Clause C.1 , because they can be eliminated by careful measurements.
c) Underestimation of the induced electric field E by a simple Bean model The calculation of average induced electric fields 3 avg-U E in the superconducting film via Equation (4) is sufficiently accurate provided the magnetic-field penetration below the bottom of the film can be neglected. However, considerable magnetic fields penetrate below the film when the experimental threshold current I th is determined and detectable U 3 has emerged. It was pointed out that the rapid magnetic-field penetration below the film at I 0 = I th may cause a considerable increase of the induced electric field and that the E calculated by Equation (4) might be significantly underestimated [27]. However, several experimental results have shown that the relative standard uncertainty from this effect is usually less than 5 %. The detail is described in Clause C.2.IEC 61788-17 pdf download.