Optical replica based longitudinal diagnostic for CLIC

The performance of the CLIC main linac critically depends on the beam properties at its entrance. We discuss a method, based on practical experience gathered in the optical replica experiment in FLASH at DESY, that allows to diagnose longitudinal properties of individual bunches within a bunch train at several places in the transfer line between the damping ring and the main linac. Apart from the possibility to obtain single-shot non-invasive longitudinal bunch profiles this method allows to analyze the longitudinal-transverse correlations (beam-tails) within a bunch.