A Method for ¿in Situ¿ Measurements of the Unloaded High-Quality Factor of an SRF Resonator Installed at a Cryomodule

Accelerator projects such as LCLS-II naturally require low-loss superconducting (SRF) cavities. Due to strong demand for improving intrinsic quality factor (Q0), importance of accurate cavity characterization, including Q0 itself, increases. SRF R&D, where a precise knowledge of Q0 is critical, includes studies of different cool-down rates, cavity material grain structure, effects of thermal treatment, and others. Although SRF resonators have extremely high Q-factors (106-107 loaded quality factor, and typical values of 1010-1011 for the intrinsic Q), there are well developed methods to measure these values. However, problems arise when a SRF resonator, installed in its cryogenic module, is connected with a RF feed source via a fixed RF coupler. In this case, there are no direct ways to measure the degradation of Q0 in situ. In this project, Euclid Beamlabs proposes to solve the mentioned problem using phase measurements of an amplitude-modulated signal reflected by SRF resonator near its resonant frequency. Such a signal can be synthesized as a beat-wave composed of two highly stabilized frequencies that are close to the resonant frequency. Analyzing the envelope of the reflected signal, one can find the phase shift for two chosen frequencies, and use them to compute the loaded Q as well as Q0. In Phase I, we will develop the concept by performing computer simulations of the proposed measurement technique, which will allow us to choose the best measurement procedure, from the viewpoint of reduction of the systematic measurement uncertainties. We also plan to elaborate the technical requirements for RF components, and to finalize the design of the RF circuit and electronic components needed for the measurements. We plan to test RF electronic components via Q-factor measurements for a room temperature test resonator. This project aims to develop a novel scientific methodology and apparatus to solve an issue for SRF accelerator facilities. The potential market for the method and the device to measure frequencies and fields of high-Q SRF resonators is significant, with anticipated future uses for CEBAF cavities, and other large SRF based accelerators for fundamental research, such as PIP-II, eRHIC, LCLS-II, SRF linac based FELs for industrial applications, and for compact SRF industrial electron accelerators for security, medical, and manufacturing purposes.