A Device for Matching RF Power into a Rapidly Varying Load

Microphonics in SRF cavities refers to the cavity frequency detune driven by external vibration sources. In CEBAF, a waveguide connects the RF source (a klystron) to the superconducting cavities. While the klystron is efficiently matched into the waveguide, minimizing reflected power, the cavity is not well matched, causing power to be reflected towards the klystron, which is protected by a circulator to deflect the power into a load. This results in a loss of RF power and therefore reduces the overall efficiency of the system. With this project, we intend to design and build a prototype active 3-stub tuner by modifying an existing CEBAF 3-stub tuner. The present proposal allows rapid tuning of the stub by replacing the copper post with a ferroelectric rod The change in tuning is achieved using an external electric field to change the permittivity of the ceramic. When the ceramic is unbiased, the RF leakage into the stub is reflected. When the ceramic is biased, the leakage field is no longer canceled, causing a discontinuity in the waveguide. It therefore acts exactly like the mechanical stub in a conventional 3-stub tuner, offering a fast and controllable tuning alternative with no moving parts. In Phase I, we will carry out a feasibility study on the use of the ferroelectric tuning (FRT) technology for the 3-stub tuner, develop its engineering design, the prototype will be fabricated and tested on the bench. In Phase II, the prototype will be tested on a superconducting cavity at Jefferson Lab and finally tested with beam at the LERF facility With this project, a prototype active ferroelectric-based 3-stub will be developed for CEBAF accelerator currently in operation at JLAB. The initial market would be for superconducting cavities in particle accelerators where the load varies with the beam current, which is usually stable over days, while microphonic vibrations cause rapid fluctuations in the cavity resonant frequency (in the range of ~10-50 Hz). CERN has recently tested the Euclid’s ferroelectric tuner with a superconducting cavity, and successfully demonstrated frequency tuning. This is a significant step forward in the development of an entirely new class of tuners that will allow electronic control of cavity frequencies by a device operating at room temperature.