Abstract: An X-ray tube, a medical X-ray device comprising such X-raytube and a method for operating such X-ray tube are proposed. The X-ray tube (1) comprises an electron emitter (3) with a substrate (4) having an electron emission surface (5). The electron emission surface (5) is adapted for field emission of electrons therefrom by providing a substantial roughness Such roughness may be obtained by applying carbon nano-tubes (19) onto the electron emission surface (5). A field generator (7) is provided for generating an electrical field adjacent to the electron emission surface (5) for inducing field emission of electrons therefrom. Furthermore, a heater arrangement (15) is provided and adapted for heating the electron emission surface (5) contemporaneous with the field emission of electrons. Accordingly, while electrons are emitted from the electron emission surface (5) due to a field effect, this electron emission surface (5) may also be heated to substantial temperatures of between 100 and 1000° C.