An improved plasma applicator uses a double-walled sapphire sleeve assembly to provide a high efficiency cooling mechanism that is adapted for use with high power applications and aggressive plasma chemistries in the generation of a plasma. Plasma contained within a highly thermally emissive first sapphire member heats the member, causing it to radiate thermal energy. The radiated thermal energy crosses a narrow gap and passes through an infrared-transparent second sapphire member. An infrared-absorbing coolant fluid that exhibits negligible microwave absorption is flowed in a second gap between the second sapphire member and a third member and absorbs most of the infrared radiation over the fluid's bulk. The use of a bulk fluid optimizes the cooling of the plasma to reduce ion and electron density and maximize reactive species output from the applicator to a vacuum process chamber.