Development Of Superconducting Rf Sample Host Cavities And Study Of Pit-Induced Cavity Quench

Superconducting rf (SRF) cavities made of niobium are now approaching their theoretical superheating field limit. Alternative materials such as Nb3 Sn and MgB2 are predicted to have significant higher fields and are very interesting for next generation srf cavities. A high field and high sensitivity sample host cavity will be an ideal tool for studying various field-dependent loss phenomena and to explore the ultimate performance of these new types of rf superconductors. In this thesis, I will present my developments of two TE-type sample host niobium cavity systems which recently have reached among the highest magnetic field ever achieved (~ 60 mT) on the sample surface with nΩ sensitivity in rf surface resistance. The rf design, fabrication, surface treatments, input coupler development and rf testing results both with baseline niobium and with a Nb3 Sn sample plate will be presented in detail. Methods of improving sample surface magnetic field up to 100 mT will be presented. Surface defects such as pits have been identified as some of the main sources of limitations of srf cavity performance. I have made a single cell cavity with 30 artificial pits in the high magnetic field region to gain new insight in how pits limit the cavity performance. The test of the pit cavity showed clear evidence that the edges of two of the largest radius pits transitioned into the normal conducting state at field just below the quench field of the cavity, and that the quench was indeed induced by these two pits. The pit geometrical informa- tion measured by laser confocal microscopy combined with a numerical finite element ring-type defect model will be compared with temperature mapping results. Insights about quench and non-linear rf resistances will be presented