Thermionic and Field X-ray emission technologies. (a) Images of micro-coil and rotating anode thermionic emission (TE) electron sources. (b) Schematic depiction of the operation principles of a thermionic emission (TE) electron emitter based X-ray source. Not the active anode cooling. (c) Band diagram illustrating the routes to thermionic, photo, and field emission as means to stimulate an electron beam by overcoming the surface potential barrier. (d) Schematic depiction of the operation of a field emission (FE) X-ray source.