From Macroscopic to Microscopic Dynamics of Superconducting Cavities

Sammanfattning: Superconducting (SC) radio frequency (RF) cavities are at the heart of many large-scale particle accelerators such as the European Spallation Source (ESS), the X-ray Free Electron Laser (XFEL), the Linac Coherent Light Source (LCLS)-II and the proposed International Linear Collider (ILC). The SC cavities are essentially resonant structures with very high intrinsic quality factors (Q0) of the order of 1010. The high Q0 of the cavities leads to increased reflection during charging of the cavities to nominal voltage because the bandwidth of the signal exceeding that of the cavity. This results in high energy losses in case of pulsed machines. In this thesis I explore and present a novel technique to optimally charge the superconducting cavities with the particular example of the spoke cavities to be used for the ESS project in Lund, Sweden. The analysis reveals that slow charging with hyperbolic sine cavity voltage profile matches the signal bandwidth to that of the cavity which leads to energy efficient filling.However, a filling rate lower than some particular value is counter-productive. The energy expended in cryogenic cooling to evacuate the heat due to ohmic losses in the cavity starts to dominate the lost energy. Such cryogenic losses are dependent on cavity Q0 through the residual resistance. The residual resistance changes with the applied electromagnetic field due to the pair-breaking mechanism of Cooper-pairs. Hence, methods for accurate measurement of the cavity Q0 are essential for accurate characterization and operation of the superconducting cavities. In this thesis I propose a novel method to accurately measure Q0 as a function of the applied electromagnetic field and present experimental results from the prototype spoke cavity in the Facility for Research Instrumentation and Accelerator Development (FREIA), at Uppsala University.The cavity quality factor (Q0) is also dependent on the material’s purity and the trapped magnetic flux in the superconducting material. Recent studies have revealed that the rate of cooling of materials through the critical temperature has an effect on the residual flux trapped in the material. In this thesis I use the time-dependent Ginzburg-Landau equations to model the process of state transition from a normal to a superconducting state. This theoretical study may allow an explanation of the experimentally observed results from the basic principles of the general theory of state transitions as proposed by Ginzburg and Landau.