Excitation of C02-N2-He gas mixtures by means of microwave discharges (ii = 2.45 GHz) leading to powerful laser action has been reported to work successfully. Experiments with fast axial flow systems were carried out using three different discharge tube diameters d at a fixed discharge length of L =0.3 m. In the case of d = 0.016 m an optimum cw output of PL = 400 W in the TEM01 mode at a discharge pressure of p = 3.8 kPa for cw excitation or p = 4.2 kPa for pulsed excitation was reached. Calculations of the electrical field within the waveguide filled with plasma and dielectric are compared with the results of invasive probe measurements. Using d = 0.024 m a cw laser output of more than 1 kW at a discharge pressure of p = 5 kPa was achieved. The design of a CO2 laser amplifier requires large and homogeneously excited discharge cross sections to minimize saturation effects and to enlarge the energy stored per length in the discharge volume. Therefore a discharge tube diameter of d = 0.051 m was chosen. One discharge unit was carefully studied with respect to the small signal gain 7o and the saturation intensity 1s using different types of cw or pulsed power supplies for magnetron operation.