Design of high gradient, high repetition rate damped C-band rf structures
The gamma beam system of the European Extreme Light Infrastructure–Nuclear Physics project
foresees the use of a multibunch train colliding with a high intensity recirculated laser pulse. The linac
energy booster is composed of 12 traveling wave C-band structures, 1.8 m long with a field phase
advance per cell of 2?=3 and a repetition rate of 100 Hz. Because of the multibunch operation, the
structures have been designed with a dipole higher order mode (HOM) damping system to avoid beam
breakup (BBU). They are quasiconstant gradient structures with symmetric input couplers and a very
effective damping of the HOMs in each cell based on silicon carbide (SiC) rf absorbers coupled to each
cell through waveguides. An optimization of the electromagnetic and mechanical design has been done to
simplify the fabrication and to reduce the cost of the structures. In the paper, after a review of the beam
dynamics issues related to the BBU effects, we discuss the electromagnetic and thermomechanic design
criteria of the structures. We also illustrate the criteria to compensate the beam loading and the rf
measurements that show the effectiveness of the HOM damping.