Sito, Leonardo (2026) Metamaterial absorbers for beam-coupling impedance mitigation. [Tesi di dottorato]

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In circular particle accelerators, high-intensity beams can excite electromagnetic resonant modes in geometric discontinuities of the vacuum chamber. This beam–structure interaction, described in the frequency domain by the beam-coupling impedance, poses significant challenges to beam stability and can lead to severe heating of accelerator components. While conventional impedance mitigation approaches rely on higher-order mode couplers or lossy material insertions, novel approaches leveraging metamaterials could offer promising alternatives for mode damping. This work investigates the use of metamaterial structures for selective impedance mode suppression. To support this study, a comprehensive simulation and experimental framework for characterising beam-coupling impedance and power deposition has been developed and validated. The methodology integrates well-established tools with emerging ones; moreover, it includes newly developed techniques and numerical codes designed to overcome limitations in current approaches. In combination with the design, fabrication, and characterisation of suitable absorbing metamaterial slabs, this framework enables an extensive study that leads to the demonstration of frequency-selective impedance resonance reduction in a dedicated test structure. Studies of the associated beam-induced power dissipation, spatial energy deposition, and thermal distribution are also presented, along with more robust design proposals for bulk metamaterial absorbers. Finally, the feasibility of applying metamaterials for impedance and beam-induced heating mitigation in an operational accelerator device is assessed. The work provides the first comprehensive demonstration of metamaterial based impedance mitigation in an accelerator context.

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