NOVELTY - The method involves feeding process gas from a process gas source (107) into a reaction tube (103). Microwaves in a waveguide (113) are received from a microwave generator (111). Energy from the waveguide is propagated to the reaction tube to cause a hydrocarbon feedstock and the process gas to react and form into a product stream comprising a fuel product. Cleaning gas is periodically delivered into the reaction tube without stopping propagation of the energy and without pausing feeding of the feedstock and the process gas into the reaction tube.    USE - Method for operating a hydrocarbon material processing system for processing naturally occurring hydrocarbon-based materials e.g. fossil hydrocarbons such as coal, bitumen, oil sands, tar sands, oil shale, resins, petroleum resids, asphaltenes and preasphaltenes and biomass. Can also be used for processing municipal waste and sludge, and hydrocarbon gases e.g. natural gas, methane, propane, ethylene.    ADVANTAGE - The method enables converting and upgrading hydrocarbon fuel products in an improved manner. The method allows an entry port including a window to prevent plasma and chamber gases from entering the waveguide. The method enables operating a hydrocarbon material processing system for processing naturally occurring hydrocarbon-based materials in an effective manner.    DETAILED DESCRIPTION - The process gas source comprises sources of gases selected from a group consisting of helium, argon, krypton, neon, xenon, methane, propane, butane, ethane, acetylene, propylene, butylene, ethylene, carbon monoxide, carbon dioxide, water vapor, hydrogen and nitrogen. The cleaning gas comprises oxygen, argon and steam. The waveguide is a waveguide surfatron, a surfatron and a surfaguide. An INDEPENDENT CLAIM is also included for a system for processing hydrocarbon materials.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a system for processing hydrocarbons.    Reaction chamber (101)    Reaction tube (103)    Process gas source (107)    Microwave generator (111)    Waveguide (113)