Ferrite-free circulator for precise measurements of SRF cavities with high Q-factor

Superconducting radio frequency (SRF) accelerators at modern scientific facilities require operation in the continuous wave regime at accelerating gradients of up to 20 MV/m. This can be achieved with SRF resonators with quality factors as high as 1e11. Such high-Q cavities require a new generation of instruments for precise measurement of their RF properties, especially in the over-coupled regime. Currently the measurement precision is limited by ferrite-based circulators, as the ferrite’s material properties vary in response to the environment. Eliminating ferrite can lead to a major improvement in the circulator’s performance and accuracy of its applications. We propose to exploit optical non-linearities, realized via an asymmetric Fano resonance coupled to the Lorentzian resonance. This approach allows to surpass the fundamental restriction of non- linearity and forward transmission, where one of these properties must be sacrificed in favor of the other. In order to allow this device to work in the regime with reflected power present, we will utilize an approach based on time variation of the resonant frequencies of three resonators connected in a way to form a 3-port system. In Phase I we will perform numerical RF design and optimization of the 2- and 3-port non- reciprocal devices and demonstrate operation as an isolator. Then we will perform an experimental demonstration of non-reciprocity by fabricating a PCB prototype and measuring the S-parameters of a 3-port circulator. In Phase II, we plan to optimize the circulator’s parameters, build a full- scale prototype and test it with an actual high-Q-factor SRF cavity to demonstrate the targeted accuracy. The developed high precision diagnostic system will be a strong contender for measuring the next generation of very high Q superconducting cavities and will find application in a number of large DOE projects. The circulator can be broadly used in microwave technologies outside of accelerator applications. In addition, the proposed device can be used in quantum computer readout interfaces.