Electron beam source and the use of the same

An apparatus for forming a three-dimensional article through successively depositing individual layers of powder material that are fused together so as to form the article wherein the apparatus comprises:

• an electron beam source for generating an electron beam, the electron beam source comprising a cathode, an anode, and a grid for regulating an electron beam current, the grid comprising a grid aperture;
• and
• a control unit,
• wherein:
• the cathode comprises a base and a protrusion with sidewalls and a top surface,
• the base has:
• a base surface which is essentially parallel with the top surface;
• and
• freely exposed sidewall surfaces extending away from the base surface in a direction away from the protrusion,
• the base surface and the top surface are essentially flat and facing towards the anode,
• the base surface and the top surface are arranged at a predetermined distance from each other, wherein the predetermined distance of the top surface and the base surface is 0.2-2 mm,
• the base surface has a circumference that is larger than a circumference of the top surface and is less than or equal to a circumference of the grid aperture,
• the sidewalls are essentially perpendicular to the base surface and the top surface, and
• the control unit is configured to switch a spot size of the electron beam on a target surface between at least a first spot size corresponding to emission from the top surface of the cathode only by applying a first grid voltage and to a second spot size corresponding to emission from the top surface and the base surface of the cathode by applying a second grid voltage, wherein a voltage variation between the first grid voltage and the second grid voltage corresponds to the predetermined distance between the top surface and the base surface.