Electron microscope, electron source for electron microscope, and methods of operating an electron microscope

An electron microscope (100) is described. The electron microscope comprises an electron source (110) for generating an electron beam, a condenser lens (130) for collimating the electron beam downstream of the electron source, and an objective lens (140) for focusing the electron beam onto a specimen (16). The electron source comprises a cold field emitter with an emission tip (112), an extractor electrode (114) for extracting the electron beam (105) from the cold field emitter for propagation along an optical axis (A), the extractor electrode having a first opening (115) configured as a first beam limiting aperture, a first cleaning arrangement (121) for cleaning the emission tip (112) by heating the emission tip, and a second cleaning arrangement (122) for cleaning the extractor electrode (114) by heating the extractor electrode. Further described is a method of operating such an electron microscope.