Wehnelt cylinder

It is named after Arthur Rudolph Berthold Wehnelt, a German physicist, who invented it during the years 1902 and 1903.

The bottom side of the cylinder has an aperture (through hole) located at its center, with a diameter that typically ranges from 200 to 1200 μm.

The anode is biased to a high positive voltage (typically +1 kV to +30 kV) relative to the emitter so as to accelerate electrons from the emitter towards the anode, thus creating an electron beam that passes through the Wehnelt aperture.

The Wehnelt is biased to a negative voltage (typically −200V to −300V) relative to the emitter, which is usually a tungsten filament or Lanthanum hexaboride (LaB6) hot cathode with a V-shaped (or otherwise pointed) tip.

This bias voltage creates a repulsive electrostatic field that suppresses emission of electrons from most areas of the cathode.