Abstract: Injector for an annular beam of monokinetic electrons in helical orbits having a high inclination angle relative to the axis of the helix of the type having an annular electron gun in a revolving vacuum enclosure. It comprises electrical coils which are able to create the cyclotron effect by a static magnetic field varying in progressive and continuous manner in accordance with the axis of the injector from a value B.sub.1 in the actual gun zone up to a value B.sub.3 in the contracted outlet zone of the injector passing through an intermediate value B.sub.2 in the convergent connection zone between said gun zone and said outlet zone, and a correcting electrode located so as to be movable in accordance with the axis of the injector and which is raised to an electrical potential differing from that of the suction or extraction anode of said electron gun.