Focusing Electrode (Wehnelt Cylinder)

The Focusing Electrode, commonly known as the Wehnelt Cylinder, is a critical component in electron gun assemblies used in electron microscopes, cathode ray tubes, and electron beam lithography systems. This cylindrical electrode surrounds the cathode and applies a negative bias voltage relative to it, creating an electrostatic field that shapes and focuses the emitted electron beam while regulating beam current. Its precise geometry and voltage control determine the beam's convergence angle, spot size, and stability, directly impacting imaging resolution and system performance.

The Wehnelt Cylinder operates on electrostatic lens principles. By applying a negative voltage relative to the cathode, it creates an equipotential surface that repels electrons, forming a converging electrostatic field. This field focuses diverging electrons from the cathode into a crossover point, controlling both beam convergence and emission current through space-charge effects. The electrode's cylindrical shape and precise positioning optimize field symmetry for uniform focusing.