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Description
Resistive-wall impedance constitutes a significant percentage of the total beam-coupling impedance budget of an accelerator. A number of different reduction techniques have been proposed during the years depending on the specific applications, ranging from higher order modes damping to solutions entailing high electrical-conductivity coatings of the pipe. This paper investigates the use of metamaterial-based absorbers for sensibly reducing or nearly cancelling resistive-wall impedance. We design and fabricate sub-wavelength two-dimensional metallic resonant structures based on the split ring resonator (SRR) geometry that can be employed as mode dampers in accelerating structures. A number of prototypes are fabricated and measured in a “test model” pillbox cavity. Experimental results agree well with full wave electromagnetic simulations and with the constitutive effective parameters of the SRR-based metamaterials retrieved using a numerical analysis. This study opens up to the possibility of considering metamaterials as a valid alternative to other devices for impedance mitigation in experimental setups commonly operating along a particle beam line, such as accelerating cavities or collimators, and more in general for the development of filters with a large out-of-band signal rejection in specific applications.
