A processing apparatus, such as a microwave-based processing apparatus. The apparatus includes a vessel having an inner surface defining a chamber configured to hold a reaction mixture, a guide, and a launch coupled to the guide. The guide can be at least partially disposed in the vessel, and is configured to propagate electromagnetic energy. The launch is configured to couple at least a portion of the electromagnetic energy from the guide to the reaction mixture. Example launches include a dielectric window and a projection. Example projections include a metallic projection and a dielectric projection.