Abstract:

The High-Luminosity Large Hadron Collider will push the performance of the Large Hadron Collider to unprecedented levels, introducing new challenges in beam dynamics. A key ingredient of this upgrade is the use of crab cavities, which couple longitudinal and transverse motion to recover geometric luminosity loss. These new devices require a careful understanding of their impact on beam dynamics, in particular during commissioning. This seminar presents a set of studies exploring the consequences of this coupling through a combination of theory, tracking simulations, and dedicated experiments in the Super Proton Synchrotron and the LHC. Both coherent and incoherent effects are investigated, including the first observation of proton crabbing induced purely by beam–beam interactions in the LHC, and the detailed characterization of beam distribution evolution under crab cavity amplitude noise in the SPS, where both emittance growth and tail population are quantitatively reproduced. The interplay between longitudinal–transverse coupling and nonlinear dynamics is further examined, highlighting the emergence of chaotic structures in phase space and their potential impact on long-term beam stability. Overall, this work provides a framework to interpret measurements, validate diagnostics, and guide operational choices during the HL-LHC era.