Operation
A Wehnelt acts as a control grid and it also serves as a convergent electrostatic lens. An electron emitter is positioned immediately above the Wehnelt aperture, and an anode is located below the Wehnelt. The anode is biased to a high positive voltage (typically +1 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. The Wehnelt bias voltage creates a repulsive electrostatic field that condenses the cloud of primary electrons produced by the filament. The filament tip is formed into a "v" shape and located near the aperture so that the beam is emitted from a small area that is, effectively, a circular emission region.
Read more about this topic: Wehnelt Cylinder
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