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Fast axial flow rf-excited discharges (un 13.5 MHz) in tubes with rectangular cross sections and plane electrodes have been investigated. Probing the tubes on-axis the dependence of small signal gain as well as saturation intensity on the input power density is identical for all the different cross sections.Under equal conditions the small signal gain profiles revealed significant decrease in the vicinity of the side walls with increasing tube dimensions. This is attributed to the change of discharge behavior with increasing interelectrode discharge length. It was found that the critical pressure for the onset of filamentation is reciprocal to this dimension.With this geometrical dependency the laser power of axial flow rf-excited discharges is not volumetric scalable in a straightforward way.

Industry has growing interest in lasers with high output power of high beam quality. Therefore, laser developers have to coflcentrate their efforts on the optimization of every component involved in the beam generation, e. g. the rf-power incoupling and the gas flow generation.At present CO2 lasers of very high output power are developed by increasing the dimensions of the whole system. Inevitably, a more or less pronounced reduction of beam quality and efficiency will occur, if scaling laws are not considered. In order to reach the goal of a well designed system with high performance data the knowledge of homogeneity and stability of the discharge as a function of the various parameters involved is essential. The evolution of filaments for example increases with the rf-input power. This filamentation, however, is a complex mechanism that also depends on gas temperature, gas turbulence and contamination as well as on the excitation frequency of rf-discharges. In this work, diagnostic methods for determining the degree of filamentation are presented. The results achieved are used to determine the frequency-dependency and scaling laws of rf-excited CO2 discharges.The range of parameters favourable for a stable discharge is generally not the same compared to the parameters yielding high values of small signal gain and saturation intensity. The optimal discharge geometry (length and diameter) depends on various quantities. The values for mass flow and of rf-input power are given by the gas circulating system and the rf-generator. The maximum rf-input power density depends on the limit of stable operation, which itself is a function of gas pressure and velocity and various other parameters. Taking into account the gain saturation of the medium, the optimal gas pressure itself depends on the input power. Measurement data of small signal gain and saturation intensity spatially resolved parallel to the gas flow direction as well as electro-optical efficency will be given. Based on these measurements, scaling laws are derived and will be presented.The range of rf-input power into the discharge (i. e. the input power density) is limited by the emergence of filaments. It is known, that these instabilities should be avoided because they lead to a reduction of the laser output power as well as of beam quality. For this reason, the elaboration of relations independent of the geometrical size of the discharge tube in combination with similar relations for maximum gain resp. excitation efficiency would provide a powerful means for the assessment and the comparison of different discharges and for the scalability of gas lasers.A necessary prerequisite for a maximum level of stable discharge operation is a homogeneous discharge. For this reason, an appropriate electrode shaping is necessary in order to provide a homogeneous incoupling of rf-power. A stable discharge is defined to be one without any appearance of filamentation. Hence, if the input power density is raised, the limit of stable discharge operation is reached if at leas...

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