A model for hot spots and Q-slope in granular Niobium thin film superconducting RF cavities

Abstract: We propose a model to explain power dissipation leading to the formation of hot spots in the inner walls of niobium thin film superconducting RF cavities. The physical mechanism that we explore is due to the constriction of surface electrical current flow at grain interface boundaries. This constriction creates an additional electrical contact resistance which induces localized punctual heat dissipation. The temperature at these spots is derived; and the electrical contact resistance is shown to depend on the magnetic field, on the gain contact size over which dissipation occurs, and on other key parameters, including the effective London penetration depth and the frequency. The surface resistance and the quality factors are determined using our model and are shown to be in excellent agreement with experimental data.