Source of electron emission

The invention relates to a source (12) emission of electrons characterized in that the grid (222) and the cathode (220) are arranged relative to each other to that the gate (222) passes all of the electrons emitted by the cathode (220) when the potential difference between the cathode (220) and the gate (222) is less than the threshold value and that the first source (224) establishes a potential difference between the cathode (220) and the anode (218).