Whispering-Gallery-Mode Sapphire Resonators in the Forms of Cylindrical Disc and Cone for Millimeter-Wave Resistance Measurements of HTS Films

The technique using WGM sapphire disc dielectric resonators allows one to measure the HTS films surface resistance Rs in the interval from 4.2 to Tc with a high sensitivity and accuracy in mm wave band. The evaluation of measurement sensitivity at very low temperatures indicates possibility to study microwave residual resistance problem. However this approach to individual characterization of films is rather impractical. A new type of high g-factor sapphire WGM resonators in the form of a truncated cone with conducting endplate, was proposed for Rs measurements. The resonators were studied experimentally in the millimeter wavelength range. It was demonstrated experimentally that at alpha = 24.1deg sapphire cone resonator can be used to study the surface impedance properties of HTS films. As compared with a cylindrical WGM resonator, it demands only one endplate, which simplifies considerably the measurement process. In comparison to a hemisphere sapphire resonator the cone one is significantly easier to fabricate. Moreover, the resonators enable the control of the interaction of microwave field and HTS film by varying the cone angle a . This opens perspectives for application of this resonator technique in mm-wave range.