This paper reports on an extensive parametric investigation of a TEA CO2 laser operated in an unstable confocal resonator. The coupling mirror diameter and the cavity magnification, M, which give the better use of the laser medium for different applications, are experimentally determined. These results are compared to the diffractional and geometrical theories of unstable resonators. It is found that, in our experimental conditions, the energy density in the focal plane of a lens depends only weakly on the value of M while the cavities having a magnification smaller than 2 are the most efficient in extracting the energy out of the lasing medium.
