Impact of a hybrid coating REBCO-CC-Cu to the resistive wall beam impedance of the FCC-hh beam screen

Abstract

The Future Circular Hadron Collider design studies proposed a novel dual chamber beam screen consisting of copper and stainless steel. However, one concern about the current design is the inherent resistive wall beam impedance of the beam screen, which may not be low enough to guarantee stable beam operation especially critical on the vertical plane. In order to reduce the resistive wall beam impedance as much as possible while keeping the dipole field quality within specifications, a hybrid beam screen consisting of REBCO-CC and Cu is proposed for the inner chamber of the beam screen. We performed a comprehensive position and REBCO-CC content study, leading to an optimum configuration for a REBCO-CC-Cu hybrid design. These studies utilized measured values of REBCO-CC surface impedance obtained under realistic FCC-hh conditions. The calculations were carried out by combining numerical simulations and beam coupling impedance theory for general beam pipe cross sections, where we found a substantial decrease in the vertical resistive wall beam impedance by about an order of magnitude compared to the nominal beam screen design made of copper. Limitations of the proposed design and possible mitigation actions are also discussed.