Left: FLAIR (the FLUKA graphical interface [223]) representation of the 1.3 GHz electric field in LCLS-II cavities at SLAC (magnetic field alternates at 90°), as coded in FLUKA for simulation of field-emission and/or transport of charged particles. Right: trajectories of field-emitted electrons from the inner surface of an LCLS-II Nb cavity, and propagation to neighboring cavities. The green track represents an electron that is eventually extracted to the down beam end of the 8-cavity cryomodule, while the blue track will be part of the backwards “captured current”.