Transportation of far-IR beams for use in plasma diagnostics

In this paper we discuss certain aspects concerning the transport and focusing of a high power far-IR laser beam into a plasma for Thomson scattering measurements. To attain the required megawatt power level in a D2O laser, it is important to use efficiently the volume of the optically pumped vapor. The best known method to achieve this is use of an unstable resonator which produces a beam with an annular intensity profile. This has a negative effect on the quality of the focused beam and on the intensity profile incident on the beam dump. A system utilizing a Cassegrainian telescope and spatial filtering techniques is found to be a good solution