A discharge tube of glass, filled with a halogen/noble-gas mix, which passes through a ½ lambda wave guide of alumina at an aperture ¼ lambda from one end. The wave guide is silver plated to establish resonance between its opposed ends. An antenna/probe is provided in another aperture, driven via a matching circuit from an amplifier. The discharge tube has a length greater than twice the thickness of the wave guide, extending from the wave guide on at least one side thereof.