Coaxial multimode cavities for fundamental superconducting rf research in an unprecedented parameter space

Recent developments in superconducting radio-frequency (SRF) research have resulted in significant increases of cavity performance. Advances have been made in both reducing rf surface resistance and pushing the readily achievable accelerating gradient by using novel SRF cavity treatments including surface processing, custom heat treatments, and flux expulsion. These developments have been largely driven by specific tests on single-cell elliptical cavities, while TEM cavities have typically been designed and developed as specific to a project application and not as standard test articles that could be replicated across various labs. To address this, two purpose-built research cavities, one quarter-wave and one half-wave resonator, have been designed and built to allow characterization of TEM-mode cavities with standard and novel surface treatments. The cavities are intended as the TEM-mode equivalent to the 1.3 GHz single-cell cavity, which is the essential tool for high-frequency cavity research. Given their coaxial structure, the cavities allow testing at the fundamental mode and higher harmonics, giving unique insight into the role of rf frequency on fundamental loss mechanisms from intrinsic and extrinsic sources. In this paper, the cavities and testing infrastructure are described, and the first performance measurements of both cavities are presented. Temperature-dependent surface resistance data are analyzed to extract both the temperature-dependent and temperature-independent components and their dependence on the rf field and frequency. In particular, the temperature-dependent component was found to be at low fields ∝ω^{1.8(1)} at 4.2 K and ∝ω^{1.7(2)} at 2 K, agreeing fairly well with the theoretical model. The growth of the temperature-dependent surface resistance with increasing field amplitude matches both exponential and quadratic growth models fairly well in the examined range. The independent component is determined to be ∝ω^{0.7(1)}, matching roughly with anomalous losses, while no clear field dependence was determined. In addition, first measurements of a 120 °C baking treatment and of the external magnetic field sensitivity are presented.