Wakefield dynamics in quasi periodic structures

The behavior of longitudinal wakefields of very short bunches, excited in multicell accelerating structures has been studied. Computations were performed in structures proposed for future linear colliders (like TESLA, SBLC and NLC) for bunch lengths down to 50 micrometers. The loss factor, which gives the global energy loss of the bunch, and the profile of the wake function, which is of major interest for bunch energy spread calculations, have been carefully studied. A strong modification of wakefields along the finite train of multicell cavities was clearly found for short bunchlengths. In particular, the wakes induced by the bunch, as it proceeds down the successive cavities, decrease in amplitude and become more linear around the bunch center, with a profile very close to the integral of the charge density. The loss factor per unit length, decreasing also with the number of cavities, becomes independent of bunch length for very short bunches and goes asymptotically towards a finite value in the permanent regime. However, any break in periodicity (or multi-periodicity) will spoil the wake distribution and hence tends to increase the wakefield amplitude