IEC 61788-7:2020 pdf free download – Superconductivity
5.2 Measurement apparatus for R,
Figure 2 shows a schematic of a typical measurement apparatus (closed type resonator) forthe Rs of HTS films deposited on a substrate with a flat surface.The upper HTs film is presseddown by a spring,which is made of phosphor bronze.The plate type spring should be used forthe improvement of measurement uncertainty.This type of spring reduces the friction betweenthe spring and the other part of the apparatus,and allows the smooth movement ofsuperconductor films due to the thermal expansion of the dielectric rod. In order to minimizethe measurement uncertainty, the sapphire rod and the copper ring shall be arranged coaxially.
Two semi-rigid cables for measuring transmission characteristics of the resonator shall beattached on both sides of the resonator in an axial symmetrical position (g = 0 and T, where pis the rotational angle around the central axis of the sapphire rod).Each of the two semi-rigidcables shall have a small loop at the ends.The plane of the loop shall be set parallel to that ofthe superconductor films in order to suppress the unwanted Transverse Magnetic Wave Modes(TMmno modes).The coupling loops shall be carefully checked for cracks in the spot weld jointthat may have developed upon repeated thermal cycling.These cables can move right and leftto adjust the insertion attenuation (IA). In this adjustment,coupling of unwanted cavity modesto the interested dielectric resonance mode shall be suppressed. Unwanted,parasitic couplingto the other modes reduces the high Q-value of the Transverse Electro-Magnetic Mode(TE mode) resonator.For suppressing the parasitic coupling, special attention shall be paid todesigning high-Q resonators.Two other types of resonators along with the closed type shownin Figure 2 can be used.They are explained in A.4.
6.1Specimen preparation
From uncertainty estimation, the film diameter shall be about three times larger than that of thesapphire rods. in this configuration，the increase in uncertainty of Rs due to the differentradiation losses between TE1 and TE013 mode can be considered negligible, given the targetrelative combined standard uncertainty of 20 %.The film thickness shall be about two timeslarger than the magnetic-field penetration depth value at the maximum temperature in themeasurement temperature range. lf the film thickness is much less than two times the magnetic-field penetration depth, the measured Rshould mean the effective surface resistance.
6.2Set-up
Set up the measurement equipment as shown in Figure 1.All of the measurement apparatus,standard sapphire rods, and superconductor films shall be kept in a clean and dry state as highhumidity may degrade the unloaded Q-value.The specimen and the measurement apparatusshall be fixed in a temperature-controlled cryocooler. The specimen chamber shall be totallyevacuated.The temperatures of the superconductor films and standard sapphire rods shall bemeasured by a diode thermometer, or a thermocouple.The temperatures of the upper and lowersuperconductor films and standard sapphire rods shall not differ by more than 0,5 K.This canbe achieved by covering the measurement apparatus with aluminium foil or filling the specimenchamber with helium gas.
6.3Measurement of reference level
The level of full transmission power (reference level) shall be measured first.Fix the outputpower of the synthesized sweeper below 10 mW because the measurement uncertaintydepends on the measuring signal level.Connect the reference line of semi-rigid cable betweenthe input and output connectors. Then,measure the transmission power level over the entiremeasurement frequency and temperature range. The reference level can change severaldecibels when temperature of the apparatus is changed from room temperature to the lowestmeasurement temperature.Therefore, the temperature dependence of the reference level mustbe taken into account.