Wehnelt cylinder

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A Wehnelt cylinder is an electrode in the electron gun assembly of some thermionic devices, used for focusing and control of the electron beam. It is named after Arthur Rudolph Berthold Wehnelt, a German physicist, who invented it during the years 1902 and 1903. A Wehnelt cylinder is also known as a grid cap, and plays the role of a control grid. It is held biased to slight negative voltage relative to the cathode (usually a hot cathode). It has a shape of a hollow barrel with no top side (oriented towards the cathode) and a hole in the center of its bottom side (through which the focused electron beam escapes towards the anode).

Wehnelt cylinders are found in the electron guns of cathode ray tubes and electron microscopes, and in other applications where a thin, well-focused electron beam is required.