IMPACT OF REALISTIC BUNCH PROFILES AND TIMING JITTER ON THE OUTPUT OF THE BESSY LOW ENERGY FEL LINE

In present FEL designs, the undulators are often optimised for an electron bunch with properties constant along the length of the bunch. The mean energy, emittance and other parameters are assumed not to vary from slice to slice. This paper investigates the impact of more realistic bunch properties, extracted from start-to-end simulations of the BESSY FEL [1]. The energy chirp needed for the bunch compression, and the impact of the passed structure imprint typical parameter profiles on the bunch at the end of the linac. Especially in high gain harmonic generation (HGHG) structures, that use consecutive parts of a long electron bunch, each stage must be adjusted to the expected bunch parameters. Due to this individual adjustment, synchronisation between the bunch and the seeding radiation becomes an issue. Even assuming an on-time seed laser pulse, the changing properties along the bunch in combination with jitter in its arrival time cause varying conditions for the interaction of the electron bunch with the seed laser radiation for every shot. This paper investigates the impact of the expected timing jitter and the realistic bunch profile on the BESSY low energy FEL at = 10nm and outlines counter measures.