Hydrogen gas production includes supplying a hydrocarbon fluid to a gap between a pair of electrodes, applying a voltage across the electrodes to induce an electrical arc, wherein the electrical arc contacts the hydrocarbon to form a plasma and produces a gaseous product comprising hydrogen gas and a solid product comprising carbon, and dynamically adjusting the gap length to control at least one parameter of the plasma. Preferably, the gap length is decreased during plasma initiation or reformation and increased to increase the hydrogen gas production rate. The method preferably includes dynamically adjusting the spatial separation of the electrodes and rotating at least one electrode while generating hydrogen gas to reduce adherence of solids to the electrodes. Furthermore, the polarity of the electrodes may be periodically reversed, primarily to reduce adherence of solids. If the hydrocarbon fluid is a liquid, the method may include controlling the level of the hydrocarbon liquid relative to the pair of electrodes.
 This invention was made with government support under contract numbers F09650-02-M-0523, F09650-03-C-0036, FA8501-05-M-0163 awarded by the United States Air Force, under contract number DE-FG02-05ER84240 awarded by Department of Energy (DOE) and under contract numbers NNG05CA63C and NNC06CA35C awarded by the National Aeronautics and Space Administration (NASA). The government has certain rights in this invention.