Calculations and Mitigation of THz Mirror Heating at the Jefferson Lab FEL

Short bunches of electrons in the Jefferson Lab FEL emit multiparticle coherent edge radiation as they enter the dipole prior to the outcoupler mirror. This light is more collimated than synchrotron light and furthermore is modified by interference from the last chicane magnet after the high reflector. This light provides an additional heat load on the outcoupler in a wavelength range it was not designed to handle. We have performed calculations of this effect using a new extension of the Synchrotron Radiation Workshop code which, importantly, takes into account both acceleration and velocity (or Coulomb) terms of the emitted electric field. We have also measured THz properties of some of the mirrors. We show how the addition of a decompression chicane mitigates these problems.