A plasma tube for a gas laser includes a series of heat webs and insulative spacers between the heat webs, with tungsten bore insert members supported by the heat webs and having aligned apertures defining a laser discharge path. To produce the plasma tube, the heat webs, spacers and other connected components are assembled in a vertical stack outside the ceramic tube of the laser, with a stacking gauge which helps assure that the heat webs are correctly spaced apart within close tolerances. The heat webs lie adjacent to annular metallized areas on the inside surface of the ceramic tube when the assembly is inserted into the tube, which is precision-formed ceramic tubing. The tube assembly is heated in vertical orientation to expand the heat webs diametrically so that they engage outwardly against and are brazed to the metallized areas of the ceramic tube. The heat webs are of a special configuration that substantially eliminates hoop stress in the ceramic tube under conditions of rising and falling temperature in the discharge tube. For adding gas to the discharge tube, a special fill system is connected to the discharge tube for metering small and precise volumes of gas into the discharge tube when needed. The construction of the fill system also aids in heat-cleaning steps in the assembly of the laser, and in the initial gas fill. A cooling system for the discharge tube, surrounding the ceramic tube, includes a variable pitch helical flow diverter for effecting more efficient and uniform cooling of the discharge tube.