Very Fast Losses of the Circulating LHC Beam, their Mitigation and Machine Protection

The Large Hadron Collider (LHC) has a nominal energy of 362MJ stored in each of its two counter-rotating beams - over two orders of magnitude more than any previous accelerator and enough to melt 880kg of copper. Therefore, in case of abnormal conditions comprehensive machine protection systems extract the beams safely from the LHC within not more than three turns $\approx$270$\mu$s. The first years of LHC operation demonstrated a remarkable reliability of the major machine protection systems. However, they also showed that the LHC is vulnerable to losses of the circulating beams on very fast timescales, which are too fast to ensure an active protection. Very fast equipment failures, in particular of normal-conducting dipole magnets and the transverse damper can lead to such beam losses. Whereas these failures were already studied in the past, other unexpected beam loss mechanisms were observed after the LHC start-up: so-called (un)identified falling objects (UFOs), which are believed to be micrometer-sized macro particles, lead to beam losses with a duration of a few LHC turns when interacting with the beams. UFOs have significantly affected the LHC availability and may become a major performance limitation for future LHC operation. Another unconsidered beam loss mechanism is the sudden absence of the long-range beam-beam interactions after the extraction of a single beam. This leads to significant losses of the counter-rotating beam within a single turn. In the long-term future, crab cavities will be installed in the LHC with the High-Luminosity LHC (HL-LHC) upgrade in 2022/23. However, in a failure case, they can lead to large beam trajectory perturbations and related beam losses within the reaction time of the machine protection systems. The focus of this thesis is on very fast losses of the circulating LHC beams, their mitigation and the related protection of the LHC. The operational experience from the first LHC run, dedicated studies and the state of knowledge are summarized. Extrapolations for mid-term and long-term future are presented and mitigation strategies and optimizations of the machine protection systems are discussed.