Talk:Wehnelt cylinder

Problem with image

The image for the article gives the impression that the electron cloud first diverges as it passes through the hole in the Wehnelt cylinder and then magically changes direction and converges to a choke point after it has left the device. A more accurate image would indicate that the cloud is caused to converge toward the choke point as it passes through the hole - for example this image would be less confusing (if it weren't for the text) — Preceding unsigned comment added by Norlesh (talk • contribs) 19:38, 13 July 2015 (UTC)

In fact, the beam actually does diverge (the emitter is an imperfect point source and the aperture is not a collimator). However, there is no magic involved in changing the electron paths: The curvature of the equipotential surfaces around the aperture acts as a converging electrostatic lens, which does indeed change the paths of electrons and thereby focus them to a crossover as they accelerate toward the anode. The crossover occurs at a focal plane located some distance away from the Wehnelt, in a fashion similar to light focused by an optical convex lens. Have a look at similar diagrams (e.g., Egerton, Fultz & Howe) and you will see similar divergence as the beam exits the Wehnelt, and subsequent focusing by the electrostatic lens. Lambtron (talk) 21:38, 13 July 2015 (UTC)

I fixed the svg image. --FDominec (talk) 09:43, 3 February 2017 (UTC)