Device Technology Based on High Temperature Superconductors and Microwave Breakdown in Gases

Abstract: This thesis deals with two separate fields of microwave technology. The first section treats microwave characterization and applications of high temperature superconducting (HTS) thin films, while the second section treats microwave breakdown in transmit-receive (TR) switches and in air.

Within the first section, four main topics are adressed. The first topic deals with non-destructive characterization of the surface impedance of HTS films. Two methods are investigated: the dielectric rod resonator method and the parallel-plate resonator method. Calibration measurements show that surface resistance measurements can be performed at 10 GHz in the range from 15 µ.OMEGA. to 100 µ.OMEGA. using both methods. Good agreement is obtained between a microstrip resonator method and the two non-destructive methods. The parallel-plate resonator is used as a fast non-destructive microwave characterization method for YBa2Cu3O7 (YBCO) films grown on (001) yttria-stabilized ZrO2 substrates, in order to optimize film deposition parameters.

The second topic involves an analytical formulation for calculation of the quasi-static propagation parameters of HTS coplanar waveguides and striplines. The resistance and kinetic inductance per unit length of the line is calculated for HTS film thicknesses comparable to the penetration depth.

The third topic deals with a YBCO microstrip modulator, which is based on the transition from the superconductive (S) state to the normal (N) state of current controlled YBCO film elements. The elements are used as terminations of a 3 dB hybrid coupler and work as shorts or terminations depending on state. The actual material parameters of the used YBCO films differ most likely from those used in the simulations, which can be deduced from the small measured difference in insertion loss between the S- and N-states.

The fourth topic involves measurements and modeling of the frequency and quality factor of high quality YBCO thin film coplanar resonators as function of dc bias in the center strip conductor. Agreement between experiments and the results of a current-dependent coupled-grain model has been obtained.

Within the second section, two topics are investigated. The first topic deals with a detailed theoretical and experimental investigation of the TR switch. It is shown that the characteristic physical phenomena which appear in connection with microwave breakdown of the filling gas as well as the subsequent interaction between the microwave and the breakdown plasma, can be described very well in terms of qualitative and quantitative physical models. The theoretical results are compared with experimental data from a number of experiments involving continuous and pulsed incident powers. The second topic of this section involves an investigation of the breakdown properties of high-power microwave pulses propagating in air. A simple analytical modeling of the breakdown-induced pulse erosion phenomenon is made by use of empirical approximations for the ionization frequency of the microwave. The theoretical predictions are compared and found to be in good agreement with experimental results.

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