Abstract: The invention relates to a method for cooling a plasma processing system for processing a mixture of particularly gaseous fluids, which comprises a means for coupling a source of microwave power to a mixture of particularly gaseous fluids flowing in a dielectric tube at the level of the coupling means adapted for transferring a portion of the microwave energy to the fluid mixture in order to generate therein a plasma for initiating the breaking of at least some chemical bonds of the fluid molecules, wherein said dielectric tube is at least partially cooled down by a coolant flow in thermal contact with the outer wall of the tube to be cooled.