Abstract: A laser oscillator that has a resonator cavity defined by oppositely disposed discharge electrodes as well as a plurality of total reflecting mirrors and a partial reflecting mirror, the resonator cavity volume containing the path of a laser beam generated by the oscillator. A laser gas medium is used to control the temperature of the oscillator and is directed in a main flow generally transverse to the path of the laser. A side flow is taken from the main flow and is directed by a duct to a position in the vicinity of the partial reflecting mirror. In order to avoid excess heating of the gas medium in the vicinity of the partial reflecting mirror and to avoid directly impacting the mirror with the gas medium, a U-shaped structure is disposed at the exit of the side flow duct, proximate to the mirror, for directing the side flow in the vicinity of but away from the mirror.