Integration of the optical replica utrashort electron bunch diagnostics with the current-enhanced SASE in the LCLS

In this paper, we present a feasibility study of integrating the optical replica (OR) ultrashort electron bunch diagnostics with the current-enhanced SASE (ESASE) scheme in the LCLS. Both techniques use an external laser to energy- modulate the electron beam in a short wiggler and then convert the energy modulation to a density modulation in a dispersive section. While ESASE proposes to use the high- current spikes to enhance the FEL signal, the OR technique extracts the coherent optical radiation produced by a density modulated electron beam for frequency resolved optical gating (FROG) diagnostics. We discuss the optimization studies of combining the OR method with the ESASE after the second bunch compressor in the LCLS. Simulation results show that the OR method is capable of reproducing the expected double-horn current profile of a 200-fs bunch. The possibilities and limitations of reconstructing the longitudinal phase space profile are also explored.