Tailored metamaterial-based absorbers for high order mode damping

In modern particle accelerators, satisfying the desired beam properties (high currents, high luminosities, etc.) increasingly implies limiting the total beam coupling impedance, which depends on the machine and the parameters of the circulating beams. A number of different reduction techniques have been proposed during the years depending on the specific applications, ranging from higher order modes (HOM) 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 the HOM contribution to the 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. Experimental results inside a pillbox cavity well agree with full wave electromagnetic simulations. In this work we present the first results on a SRR geometry tailored for LHC collimators. We show how metamaterials can be a valid alternative for impedance mitigation in experimental devices commonly operating along a particle beam line.