The invention relates the field of electron emitter of an X-ray tube. More specifically the invention relates to flat thermionic emitters to be used in X-ray systems with variable focus spot size and shape. The emitter provides two main terminals (3, 5) which form current conductors and which support at least two emitting portions (7, 9). The emitting portions are structured in a way so that they are electron optical identical or nearly identical increasing the emergency operating options in case of emitter damage.