The opportunities and advantages of an innovative cylindrical thermionic converter for solar power and propulsion systems are discussed. The converter consists of coaxial cylinders, where the emitter is located on the exterior cylinder and the collector on the interior, resulting in an inverted design compared to traditional thermionic converters. Design considerations and parameters for a solar thermal power system led to the basic concept of the cylindrical inverted thermionic converter. The materials and design of an experimental prototype, consisting of a single-cell inverted thermionic converter are described. Results of calculations on output parameters, emitter temperatures during operation, and efficiency are presented. Fabrication and testing of this device are in progress.