This letter describes an ion source using a cylindrical microwave cavity operating in a hybrid mode associated with the TE211empty cavity mode. The design principles and associated electrical systems are also discussed. Extracted beam current versus accelerating voltage, and specific energy versus extracted beam current are displayed over the range of flow rates 20–80 sccm and absorbed powers 80–150 W. The results show the feasibility of this concept. The ion source has many potential uses such as space propulsion, material processing, and neutral beam ion sources.