Abstract: A temperature controller unit for use in a gas laser resonator structure using a gas lasing medium excited within a resonator cavity having reflective optical surfaces for producing a beam of coherent radiation, and the gas lasing medium being circulated through a heat exchanger to maintain a predetermined lasing temperature range. The temperature controller unit includes a tubular member of substantial length forming a primary chamber for conducting said gas lasing medium through the heat exchanger in a non-contaminating environment to reduce gas contaminate buildup on the optical surfaces in the resonator cavity, end caps sealably closing the ends of the tubular member and receiving gas lasing medium from the resonator structure, and an outlet tube having an open end positioned within the primary chamber for conveying the gas lasing medium for recirculation to the resonator structure.