Abstract: A supersonic diffuser for use with a gas dynamic laser having a multichannel shock duct diffuser with outer duct walls and inner duct walls forming an inner duct channel and two outer duct channels. A retractable wall is provided in each of the outer duct channels for closing off the outer ducts after diffuser start up wherein the gas wedges forward of the retractable walls together with the inner duct channel provide diffuser operation essentially the same as a conventional multiple shock diffuser. Retractable stabilizer wedges are positioned slightly forward of the normal separation points.