He, Z.G. Jia, Q.K. Wang, L. Xu, W. Zhang, S.C. MOP010 electron FEL gun linac beam-transport 2015 2015-12 We are building an infrared free-electron laser (IR-FEL) facility that will operate from 5 um to 200 um. This FEL source is drived by a linac, which is composed of a triode electron gun, a subharmonic prebuncher, a buncher, two accelerators, and a beam transport line. The linac is required to operate from 15 to 60 MeV at 1 nC charge, while delivering a transverse rms emittance of smaller than 30 mm-mrad in a 5 ps rms length, smaller than 240 keV rms energy spread bunch at the Far-infrared and Mid-infrared undulators. In this article, the preliminary Linac design studies are described.