Analyses Supporting the 2-Color Upgrade to the IR FEL at FHI Berlin

This paper provides a summary of the analyses that led to the definition of the 2-color upgrade of the IR FEL at FHI Berlin. We briefly cover several different aspects of the design, beginning with the beam dynamics of the second far-IR beamline, engineering considerations of that physics design, and the FEL physics that defined the short-Rayleigh range undulator as well as aspects of the undulator design itself. Additionally, we touch on the approach to 2-color commissioning with pulse picking, as well as considerations for the far-IR optical transport to users. The status of commissioning is described in a parallel paper at this Conference by W. Schöllkopf et al.