CO.sub.2 Laser

A transverse flow gas discharge laser comprising, a closed system of ducting including a region of rectangular cross-section, means for forming an optical resonant cavity including said region, means for abstracting energy from said optical resonant cavity, means for circulating a gaseous lasing medium unidirectionally through the system of ducting, a cathode electrode situated adjacent to and spaced from one wall of the said region of ducting and extending parallel thereto, the cathode electrode also extending transversely of the direction of flow of the lasing medium and parallel to the optic axis of the optical cavity, an anode electrode at the other side of the said region of ducting opposite the cathode electrode and extending parallel thereto, the cathode electrode having a non-circular cross-section and being mounted such that the lasing medium surrounds the cathode electrode, the separation between the cathode electrode and the adjacent wall of the said region of ducting being such that, in use, distortions in the flow of the flowing lasing medium in the neighborhood of the adjacent wall of the said region of ducting interact with the cathode electrode to stabilize a cathode flare which forms in operation of the laser and hence to stabilize an associated electric discharge between the electrodes.