Report from LHC MD 2171: Dynamic aperture at 6.5 TeV

Throughout the LHC’s first and second operational runs, increasing emphasis has been placed on the study of nonlinear single-particle beam dynamics. Of particular interest is the beam dynamics at top-energy, end-of-squeeze, where errors in low-β insertions can create strong nonlinear perturbations of the beams. Dynamic aperture (DA) is one of the key observables for nonlinear beam dynamics, however the classical method (based upon single kicks) is not viable at top energy in the LHC due to insufficient strength of the kicker magnets, the risk to machine protection, and the beam-destructive nature of the measurements. Two alternative methods have been proposed to study dynamic aperture at top energy in colliders. The first examines long-term dynamic aperture of free oscillations, via observed beam-losses of bunches which have been slowly heated to large emittance with a transverse damper. The long-term free DA is expected to be a critical parameter for successful exploitation of the High-Luminosity LHC. The second method studies short-term dynamic aperture of forced oscillations driven by an AC-dipole. While this ‘forced DA’ represents a different quantity from the classical ‘free DA’, and there is no de facto correlation between the two observables, it provides a useful tool to analyse the beam dynamics and is of direct relevance to the successful commissioning of the HL-LHC, which will rely on driven oscillations. Both of these methods have undergone initial testing at injection, as well as some parasitic studies at 6.5 TeV. However, this MD represents the first detailed tests of these approaches at top energy