Methods and apparatus are provided for igniting, modulating, and sustaining a plasma for various plasma processes and treatments. Such treatments include cleaning and sterilizing parts. In some embodiments, a plasma is ignited by subjecting a gas in a multi-mode processing cavity to electromagnetic radiation having a frequency between about 1 MHz and about 333 GHz in the presence of a plasma catalyst. A part can be cleaned by, for example, inserting hydrogen into the plasma and exposing the part to the hydrogen-enriched plasma. A part can be sterilized by heating the part with the plasma.