  We consider the collective beam dynamics at the injector section of the
European XFEL. The results of the measurements for the longitudinal phase space
(LPS) of the electron beam are compared with those obtained from numerical
modeling. A new approach is proposed for the analysis of the self-field effects
in the LPS measurements. It allows to determine accurately the synchronous RF
phase in an accelerating module and to subtract properly the RF curvature
imprinted in the LPS. A further incorporation with the simulation made it
possible to separate the collective effects originating from different sources
and thus to quantify, individually, the impact of these effects on the beam
dynamics. This includes the space-charge dominated beam dynamics in the RF gun
as well as the collective effects dominated by the wake fields after the gun up
to the end of the injector section. A new analytical model is also proposed for
the short-range wake function of a finite chain of RF cavities and verified by
both numerical analysis and experimental results. It is shown that the physical
models used in the simulation of the beam dynamics after the gun allow to
reduce the absolute error in the modeling of the correlated energy chirp by
order of magnitude in comparison with the case when the collective effects
after the gun are neglected.
