A Comprehensive Analysis of Performance Degradation in Niobium Thin Film Radio-Frequency Cavities

Niobium thin film radio-frequency (RF) cavities have historically shown performance degradation as the RF field progressively increases, posing limitations on their use in particle accelerators where the real-estate gradient has to be maximized. This issue, often referred to as the medium-field <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i>-slope problem, has not yet been fully understood and is the subject of ongoing research. This study analyzed the RF performance of several niobium thin film cavities reported in the literature. These cavities, with resonance frequencies ranging from 100 MHz to 1.5 GHz, were produced using a variety of coating techniques and manufacturing processes. Despite these notable differences, the field-dependent increase in surface resistance, when normalized by the resonance frequency, is consistently similar across all the niobium thin film cavities analyzed. Consequently, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i>-slope problem might not be strictly influenced by the specific treatments or coating techniques applied. Instead, it appears to be intrinsically associated with the interaction between the RF field and the superconductor, with the frequency of the field playing a significant role.