PURPOSE:To repeat the stable discharge excitation of laser medium gas, by ionizing the laser medium gas by preliminary discharge, and thereafter generating excitation by main discharge. CONSTITUTION:Laser medium gas is inputted into a supersonic nozzle 1 and reaches a sound speed at a throat part 17A. The gas undergoes adiabatic expansion and becomes a supersonic stream. The gas is ionized by ultraviolet rays, which are emitted by the discharge between preliminary electrodes 4 and 5. The electron density, at which avalanche discharge is possible by main discharge, is reached. The ionization density is high at a part, where the ultraviolet rays between the electrodes 4 and 5 is intense, and uneven in a space. Therefore, a delay time is provided by a time length so that electrons are diffused uniformly in the space. After the delay time, the main discharge is started. When the main discharge is started, the avalanche discharge is generated between main electrodes 2 and 3, and the laser medium is excited. Laser light 19 is obtained in the direction perpendicular to the discharge direction and the flowing direction of the medium gas. In this way, the number of repetitions of the ionization can be increased in comparison with the ionization of the medium gas by electron beams, which are used in a conventional method.
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