Nuclear lasing of Ar 10% Xe at 760 Torr with a thermal neutron flux of 3.5×1016n/cm2 s has produced an output power of 50 W. Lasing occurred at 2.6 &mgr;m in Xe‐I. The argon buffer gas is shown to efficiently stop fission fragments at reasonable pressures emitted from235U3O8coatings. A unique folded optical path was used to increase the excited gas volume which in turn increases laser output.