Tuned Decoherence Damping Applied to the CLIC Drive Linac

Numerical simulations of beam transport in the CLIC drive linac, have always included decoherence of betatron phase between and within bunches. This played an essential part in avoiding multi-bunch beam breakup, BBU, due to the weak, but non-negligible, resistive-wall and synchronous transverse wakes. In early studies, the initial energy variation along each bunch train, which was a part of the design, provided adequate decoherence. For the more recent isochronous ring source of the drive beam train, with no bunch-to-bunch energy variation, passive damping of transverse wakes within the transfer structures and betatron phase decoherence along the linac must be added to ensure beam stability. The latter can be achieved by varying the energy of alternate bunches, by an amount that depends on the lattice, and is an effective way of delaying the onset of BBU.