The use of X-rays to preionize gaseous laser active media has well-recognized advantages with respect to the widely used UV techniques, in terms of preionization homogeneity, discharge uniformity and longer gas mixture lifetime, in particular for high output energy and/or high repetition rate excimer lasers. In this paper we will draw the attention to more recently discovered X-rays features, like the capability to switch the pumping discharge of gas lasers and the strong influence on the final optical quality of the laser beam (e.g. divergence, brightness, etc.). The theoretical considerations will be compared with the experimental results obtained in the last few years by the High Power Laser Laboratory of the ENEA Frascati Center.
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