An electron gun for heating, fusing and vaporizing having a high voltage portion, an electron emitting cathode and at least one beam forming electrode associated with the cathode. An accelerating anode, a beam guiding tube extending in the direction of the beam path and surrounded by a jacket tube and an electromatic lens are also provided. One or more deflection systems are positioned in the space formed between the beam guiding tube and the jacket tube. The beam guiding tube, the jacket tube, the electromatic lens and the deflection systems are contained in a single, replaceable unit which is joined by a plurality of hollow posts parallel to the gun axis to the high-voltage portion of the gun. The space for the electromagnetic lens and the deflection systems communicates with the atmosphere through at least one of the hollow posts.