A causes a current through the hot cathode. The cathode starts glowing. Because of the emission some can leave the wire and build a cloud of so called around the glowing wire. To produce a fine electron beam these electrons must be forced to the center of the glowing wire. Therefor a is used. The cylinder is charged and forces the electrons in the center of the cylinder. In addition a high voltage is applied between the filament and the circular . Here the filament is charged negative, the anode . This voltage causes an field between hot cathode and anode. This field accelerates the electrons and causes a motion towards the anodee. In the center of the Anode is a small , so the electrons can pass through the anode. A fine electron beam is created. When passing throug the anode the electrons leave the electric field between filament and anode. So there is no more acceleration and the electrons move forward with velocity.