Abstract: A microwave resonant cavity for a spectroscopic light source includes a housing having therein a chamber formed by side walls and a cylindrical outer wall. The side walls having aligned openings therethrough which are on the longitudinal axis of the outer wall. A refractory tube which is adapted to contain a gaseous plasma extends through the aligned openings and across the chamber in the housing. The portion of the side walls of the chamber adjacent the openings are closer together than the remaining portions of the side walls so that the chamber has a first portion around the refractory tube which is narrower than a second portion of the chamber around the first portion. A coupling loop is electrically coupled to a side wall of the chamber within the second portion of the chamber and is connector to a coaxial connector which extends through the outer wall of the housing to deliver microwave power to the chamber.