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Integrating ferroelectric fast reactive tuners (FE-FRTs) into SRF cavities offers a promising route to mitigate microphonics. This work reports the design and characterisation of an FE-FRT coupled to a 1.3 GHz two-cell cavity, supported by a lumped element equivalent circuit model and 3D CST simulations. From this framework, external Q-factors, FPC specifications, and generator power requirements are derived, including power loss estimates to identify optimal working points. The tuner’s non-linear response to RF current is characterised, accounting for higher-order modes (HOMs) in both the cavity and the tuner resonator. By appropriate resonant frequency choice, monopole 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 HOM close to the third harmonic at 3.9 GHz, potentially coercing the stable operating range. In our example design, the two monopole HOMs below the fourth harmonic exhibit fields mainly on the sapphire window 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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