Electron gun (101, 201) comprising - at least one electron source, said at least one electron source having an electron emission surface configured to emit electrons, - at least one first electrode configured to control a path of electrons emitted from said electron emission surface, said at least one first electrode being located at a given distance to said at least one electron source and comprising a first side and a second side, said first side and said second side being opposite to each other, - at least one second electrode configured to suppress emissions of electrons from a side surface of said at least one electron source, - at least one third electrode configured to accelerate electrons emitted from said at least one electron source, wherein - said first side of said at least one first electrode faces said at least one second electrode, - said second side of said at least one first electrode faces said at least one third electrode, - said at least one first electrode has a first electrode potential, wherein a first voltage is a first potential difference between said first electrode potential of said at least one first electrode and an electron source potential of said at least one electron source, wherein said first voltage is adjustable to at least a first value and a second value, - said at least one second electrode has a second electrode potential, wherein a second voltage is a second potential difference between said second electrode potential of said at least one second electrode and said electron source potential of said at least one electron source, wherein said second voltage is adjustable to at least a third value and a fourth value, - said at least one third electrode has a third electrode potential, wherein a third voltage is a third potential difference between said third electrode potential of said at least one third electrode and said electron source potential of said at least one electron source, said third voltage is adjustable to at least a fifth value and a sixth value, and wherein - in use of the electron gun said first voltage, said second voltage and said third voltage are adjusted to avoid a crossover of electrons emitted from said electron emission surface in an area between said at least one electron source and said at least one third electrode, characterized in that - the at least one electron source is a thermionic source, wherein the thermionic source is heated to emit thermally excited electrons. - at least one electron source, said at least one electron source having an electron emission surface configured to emit electrons, - at least one first electrode configured to control a path of electrons emitted from said electron emission surface, said at least one first electrode being located at a given distance to said at least one electron source and comprising a first side and a second side, said first side and said second side being opposite to each other, - at least one second electrode configured to suppress emissions of electrons from a side surface of said at least one electron source, - at least one third electrode configured to accelerate electrons emitted from said at least one electron source, - said first side of said at least one first electrode faces said at least one second electrode, - said second side of said at least one first electrode faces said at least one third electrode, - said at least one first electrode has a first electrode potential, wherein a first voltage is a first potential difference between said first electrode potential of said at least one first electrode and an electron source potential of said at least one electron source, wherein said first voltage is adjustable to at least a first value and a second value, - said at least one second electrode has a second electrode potential, wherein a second voltage is a second potential difference between said second electrode potential of said at least one second electrode and said electron source potential of said at least one electron source, wherein said second voltage is adjustable to at least a third value and a fourth value, - said at least one third electrode has a third electrode potential, wherein a third voltage is a third potential difference between said third electrode potential of said at least one third electrode and said electron source potential of said at least one electron source, said third voltage is adjustable to at least a fifth value and a sixth value, and wherein - in use of the electron gun said first voltage, said second voltage and said third voltage are adjusted to avoid a crossover of electrons emitted from said electron emission surface in an area between said at least one electron source and said at least one third electrode, - the at least one electron source is a thermionic source, wherein the thermionic source is heated to emit thermally excited electrons.