Optical modeling of beamline for transport of infrared beam to user area for IR FEL at RRCAT

We present an optical modeling of the IR FEL beamline to be installed at RRCAT. The output beam which is tunable at around 30 to 50 μm wave length , has to propagate a distance of 45 m, through several bends, to reach the experimental hall. Since the IR beams have relatively higher diffraction it is necessary to design the transport system for limiting the beam size over longer distances. The output beam is assumed to be Gaussian beam with waist located at the out-coupling hole of the resonator. Using Gaussian optics the beam sizes at various mirror locations have been calculated theoretically. A mirror box with a spherical mirror for focussing the beam at the end station has been designed and studied. An optical model for the IR FEL beamline has been developed and results of its simulation with optical simulation software FINESSE are presented in this paper. (author)