Collective Effects in the Hadron Future Circular Collider

Abstract

The instability of a beam in hadron circular accelerators due to the beam coupling impedance and electron clouds is investigated. The Future hadron-hadron Circular Collider (FCC-hh) is a potential successor of the Large Hadron Collider (LHC). FCC-hh will operate at 100 TeV collision energy in a tunnel with a circumference of 100 km, in comparison to 14 TeV collision energy and 27 km circumference in the LHC. Similarly to the LHC, in the FCC-hh the beam screen can be a limiting factor for beam stability. The electromagnetic interaction of the beam and beam screen is described by a beam coupling impedance, whereas the interaction between the beam and free electrons in the beam pipe leads to the formation of the electron cloud. As a result, machine performance can be affected by beam instabilities, beam losses, heat load, and vacuum degradation. Absolute predictions of collective effects require the most realistic impedance database. Because of uncertainties in the electromagnetic material properties and missing component details at the present design stage of the beam screen, the absolute estimations would carry large error bars. Instead, this thesis focuses on the scaling of collective effects with beam energy and beam screen geometry from LHC to FCC-hh, as this can relate actual observations from an existing collider to a new one, with many similarities. The impedances are computed for the detailed transverse cross-section for the FCC-hh and the LHC beam screens. The current design of the FCC-hh beam screen is found to make a significant contribution to impedance. Based on an analysis of impedance-induced effects, this thesis work proposes modifications in the beam screen design to mitigate its impedance contribution. An electron cloud buildup study is also performed for the detailed transverse cross-section for the FCC-hh and the LHC beam screens. As a result of these computations, one can conclude that an additional coating on the FCC-hh beam screen is required to mitigate the effects driven by the electron cloud. Further, to investigate the impact on the effects induced by electron clouds due to the higher beam energy and smaller aperture of FCC-hh beam screen in comparison to LHC, a study of the heat load with beam energy and pipe radius is performed.