1,049,832. Lasers. NATIONAL RESEARCH DEVELOPMENT CORPORATION. June 5, 1964 [April 10, 1963], No. 14353/63. Heading H3B. A laser comprises a gas discharge tube filled with a gaseous mixture of helium at a pressure of at least 10 mm. of mercury and another gas at a lower pressure, the tube being provided with electrodes for pulsed excitation of the gas discharge. The gaseous active medium may be a helium/neon mixture at pressures of 10 to 100 mm. mercury and 1 to 5 mm. mercury respectively. Alternative gaseous mixtures are helium/nitrogen and helium/carbon monoxide. Excitation is by means of D.C. pulses of several kv. applied to electrodes in the form of metal rings around the centre and ends of the tube. If the pulse repetition frequency is high enough the operation of the laser is practically C.W. The electrodes may be arranged in groups along the tube and pulsed sequentially. The resonant cavity is formed by multilayer dielectric concave mirrors.