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Integrating ferroelectric fast reactive tuners (FE-FRTs) into SRF cavities offers a promising route to mitigate microphonics in LINAC cavities. This work reports the design and characterization of an FE-FRT coupled to a 1.3 GHz two-cell cavity, supported by 3D CST simulations and an extended lumped element circuit model. This extended model improves upon conventional frameworks by directly incorporating the FPC, alongside a lossy, arbitrary-length transmission line that accounts for both electric (probe) and magnetic (loop) coupling to accurately derive FPC specifications and RF power requirements, including power loss estimates to identify optimal working points. By appropriate resonant frequency choice, monopole higher-order modes (HOMs) in the tuner can be kept away from harmonics of the fundamental mode; however, the permittivity variation in the FE wafers during operation could shift a monopole HOM close to the third harmonic at 3.9 GHz, potentially affecting the stable operating range. In our example design, the two observed monopole HOMs exhibited fields mainly on the sapphire ring rather than on the FE wafers, thereby strongly reducing direct excitation by the fundamental field and mitigating associated risks for LINAC operation.
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