A cold field emission electron source comprising: an electron emitter, the emitter being a wire having a sharpened end forming an emitter tip (103; 403; 503; 703; 803; 903), an axis of the emitter defining a source emission axis; and an extractor electrode (108; 408; 508; 708; 808; 908), located at a front side of the emitter tip, the extractor electrode having a first hole (450; 550) approximately centered on the source emission axis; the source further including: an emitter enclosure electrode (552; 752; 852; 952), located at a rear side of the emitter tip, the emitter enclosure electrode having a second hole approximately centered on the source emission axis, the second hole diameter being larger than the diameter of the emitter; and wherein the wire protrudes through the second hole; a cleaning filament positioned between the emitter enclosure electrode and the extractor electrode and capable of electron bombarding and heating the inner surfaces of the emitter enclosure electrode and extractor electrode so as to clean them by at least one of electron bombardment and radiative heating; wherein the emitter enclosure electrode and extractor electrode are configured to produce a confinement space containing the emitter tip, the confinement space limiting the paths of electrons backscattered from the extraction electrode and/or reducing the flow of gas into the confinement space; the electron source further comprising a cleaning filament current supply, configured to supply a current through the cleaning filament to ohmically heat the cleaning filament; and a cleaning filament biasing voltage supply, configured to apply a voltage between the cleaning filament and both the emitter enclosure electrode and the extractor electrode, to induce thermionic emission of electrons from the cleaning filament to bombard the surfaces of the emitter enclosure electrode and extractor electrode facing the emitter wire, the electron bombardment stimulating electron impact desorption of molecules from the emitter enclosure electrode and extractor electrode. an electron emitter, the emitter being a wire having a sharpened end forming an emitter tip (103; 403; 503; 703; 803; 903), an axis of the emitter defining a source emission axis; and an extractor electrode (108; 408; 508; 708; 808; 908), located at a front side of the emitter tip, the extractor electrode having a first hole (450; 550) approximately centered on the source emission axis; the source further including: an emitter enclosure electrode (552; 752; 852; 952), located at a rear side of the emitter tip, the emitter enclosure electrode having a second hole approximately centered on the source emission axis, the second hole diameter being larger than the diameter of the emitter; and wherein the wire protrudes through the second hole; a cleaning filament positioned between the emitter enclosure electrode and the extractor electrode and capable of electron bombarding and heating the inner surfaces of the emitter enclosure electrode and extractor electrode so as to clean them by at least one of electron bombardment and radiative heating; wherein the emitter enclosure electrode and extractor electrode are configured to produce a confinement space containing the emitter tip, the confinement space limiting the paths of electrons backscattered from the extraction electrode and/or reducing the flow of gas into the confinement space; the electron source further comprising a cleaning filament current supply, configured to supply a current through the cleaning filament to ohmically heat the cleaning filament; and a cleaning filament biasing voltage supply, configured to apply a voltage between the cleaning filament and both the emitter enclosure electrode and the extractor electrode, to induce thermionic emission of electrons from the cleaning filament to bombard the surfaces of the emitter enclosure electrode and extractor electrode facing the emitter wire, the electron bombardment stimulating electron impact desorption of molecules from the emitter enclosure electrode and extractor electrode. an emitter enclosure electrode (552; 752; 852; 952), located at a rear side of the emitter tip, the emitter enclosure electrode having a second hole approximately centered on the source emission axis, the second hole diameter being larger than the diameter of the emitter; and wherein the wire protrudes through the second hole; a cleaning filament positioned between the emitter enclosure electrode and the extractor electrode and capable of electron bombarding and heating the inner surfaces of the emitter enclosure electrode and extractor electrode so as to clean them by at least one of electron bombardment and radiative heating; wherein the emitter enclosure electrode and extractor electrode are configured to produce a confinement space containing the emitter tip, the confinement space limiting the paths of electrons backscattered from the extraction electrode and/or reducing the flow of gas into the confinement space; the electron source further comprising a cleaning filament current supply, configured to supply a current through the cleaning filament to ohmically heat the cleaning filament; and a cleaning filament biasing voltage supply, configured to apply a voltage between the cleaning filament and both the emitter enclosure electrode and the extractor electrode, to induce thermionic emission of electrons from the cleaning filament to bombard the surfaces of the emitter enclosure electrode and extractor electrode facing the emitter wire, the electron bombardment stimulating electron impact desorption of molecules from the emitter enclosure electrode and extractor electrode.