Research On The Measurement Methods Of Microwave Surface Resistance Distribution And The Nonlinearity Of HTS Thin Films

The investigations on the nonlinearity of high temperature superconductor(HTS)are very important,and they are the hot spots of the research on HTS,since the nonlinearity of HTS influences the applications of the devices based on HTS under high power,and it is the basis of the research on the novel devices based on the nonlinearity.The nonlinearity of HTS depends on lots of parameters,and the mechanism is not clear,therefore many methods to measure the nonlinear parameters still limited with appearance measurement,even there is no one with the determinate error limit can be chosen as national standard or accepted by all.To solve the problem mentioned above,this thesis proposes the solutions and technology:1.The evanescent bandpass filter with broad stopbands and a novel notch filter having two stopbands are proposed,which are suitable for broadband applications.With the evanescent bandpass filter technology,the ratio of the frequency of the first harmonic and the center frequency of the passband is up to 14.With the novel notch system,two stopbands with arbitrary characters can be located at any frequencies in DC-18 GHz.They are utilized to eliminate the nonlinear interferences to improve the measurement accuracy of the measurement platform which is used to measure the nonlinear parameters of HTS thin films.2.A novel method based on image dielectric resonator and the focus function from a metal ring to measure the surface resistance distribution of HTS thin films is proposed.The resolution ratio of this novel method is a circle whose diameter is 5 mm and area is 19.6 mm~2,which is 10 times lager than traditional method's resolution ratio(diameter is 16 mm and area is 201.1 mm~2),and the relative error is1.8%.The resonant frequency of this novel method is under 35 GHz,which is 1/5of the traditional method based on quasi optical cavity resonator.The measured partial surface resistance of HTS thin film without etching with this novel method is a contrastive parameter for that with resonator.3.A novel measurement method based on balanced type single loaded micro-bridge resonator to measure the nonlinearity of HTS thin films is proposed.This method is not only suitable to measure the partial surface resistance,but also for nonlinearity measurement with the modified measurement platform,when the power lever is low or medium.The relative error limit of the measurement method is 2.3%.The novel measurement method can be used to measure the microwave critical current of HTS thin film at different frequencies,accurately,since the error caused by the difference of thin film can be eliminated than other methods which use different resonator on different thin films to form the resonators with different resonant frequencies.The relationships between equivalent parameters of different modes are analyze and decided,it is a benefit to explore the relationship between nonlinear parameters and frequency.4.When the novel method mentioned above was used to measure the microwave critical current of a HTS thin film,we found that the microwave critical current density are 5.135 MA/cm~2?5.435 MA/cm~2 and 9.620 MA/cm~2 at 3 GHz? 9GHz and 15 GHz,respectively,they are larger than the dc critical current density of the HTS thin film(2.2 MA/cm~2).Then we re-measured the microwave critical current with a HTS tapered coplanar waveguide transmission line whose narrowest width of center conductor is 4 m,and we found that the abrupt change of the transmission parameter disappear.Finally,we proposed a model based on the time-varying character of microwave current and the current distribution and the variation of current distribution,and explain the observed phenomena.