Metamaterial-Based Absorbers for the Reduction of Accelerator Beam-Coupling Impedance

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 article investigates the use of metamaterial-based absorbers for sensibly reducing or nearly canceling the beam-coupling impedance. We design and fabricate subwavelength 2-D 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 have been fabricated and measured in a 'test model' pillbox cavity. Experimental results, combined with full-wave electromagnetic simulations, prove the efficiency of the SRR-based metamaterials. This article 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 beamline, 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.