• R.B. Fiorito, M. Cornacchia, A.G. Shkvarunets, J.C.T. Thangaraj
  UMD, College Park, Maryland
• S. Di Mitri, M. Veronese
  ELETTRA, Basovizza
• H. Loos, J. Wu
  SLAC, Menlo Park, California

Recent observations of coherent optical transition radiation (COTR) at LCLS and other laboratories have been recognized as a signature of theμbunching instability, which affects the longitudinal phase space of the electron beam and ultimately the performance of the Free Electron Laser. In addition, the COTR emission limits the utility of OTR screens as beam profiling diagnostics. In an effort to understand and predict the extent of COTR emission and to help specify required instrumentation for new FELs, we have developed codes at UMD and SLAC-LCLS that use the output from the ELEGANT particle tracking code to predict the emission of COTR at specific wavelengths or within a band width. The COTR codes provides plots of the intensity patterns in the transverse plane, simulating a virtual OTR screen. Both incoherent and coherent intensities are produced thus providing an estimate of theμbunching gain at the observed wavelengths. Since the ELEGANT simulation of microbunching strongly depends on the number of particles, efforts have been carried out to speed up the COTR code analysis. The results of these codes applied to the LCLS and FERMI@elettra linac FELs are presented.