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TY - UNPB AU - Marcus, G. ED - Ko, Kwok C. ED - Schaa, Volker R.W. ED - Tang, Chuanxiang ED - Yan, Heping TI - Soft X-ray Self-seeding at LCLS and LCLS-II: Simulation and Experiment J2 - Proc. of ICAP2015, Shanghai, China, 12-16 October 2015 C1 - Shanghai, China T2 - International Computational Accelerator Physics Conference T3 - 12 LA - english AB - X-ray free electron lasers (XFELs) have become essential tools for the investigation of dynamical systems as they have the ability to operate on the time and length scales natural to atomic and electronic motion in matter. The recent addition of soft X-ray self-seeding as an operational mode at LCLS allows for the production of narrow bandwidth FEL pulses, which is useful for a variety of advanced spectroscopic investigations. Recent experimental results, however, show the persistence of a small spectral ‘pedestal' that underpins the narrow amplified signal. Here, we report on potential sources of this pedestal that have been investigated using high-fidelity numerical particle simulations for two facilities: LCLS and LCLS-II. Furthermore, a new numerical method used to propagate the full three dimensional FEL pulse through the grating based monochromator will be presented. This method leverages a new theoretical model based on a modified Huygens-Fresnell integral that includes non-separable spatio-temporal couplings and temporal dispersive effects up to a quadratic phase. PB - JACoW CP - Geneva, Switzerland ER -