Consider this diagram of a focusing anode in a CRT: enter image description here

Question: Why does the electron beam not diverge again when it enters the focusing anode (or accelerating anode) while leaving the pre-accelerating anode (or focussing anode)?

I think the answer is that it does diverge but less than it converges at both entrances. And since it converges first, the resultant is relatively convergent. The following is how I show it, is the diagram below correct?

enter image description here


It does diverge it is really hard to get a bunch of electrons to stay in one place, an electron beam has an internal divergence due to self potential. If you stick two electrons close together, they will start to move away from each other, in a beam this is also the case.

It can kind of be seen in the diagram below (more in the voltage gradient shown than the electron lines), as electrons exit the cathode, they start to spread out.

enter image description here Source: https://www.quora.com/How-do-anodes-focus-and-accelerate-a-beam-of-electrons

Electrons are usually more useful when they are in a tight beam, so they need to be refocused. The beam focuses to a smaller point, however, if you did not refocus it it would diverge again.

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  • \$\begingroup\$ Apart from the self-potential, does the electron beam also diverge while entering the focussing anode? \$\endgroup\$ – Atom Jul 23 '19 at 18:55
  • \$\begingroup\$ Is my explanation correct? \$\endgroup\$ – Atom Jul 23 '19 at 18:55
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    \$\begingroup\$ I would say that is soon as it enters the focusing anode it starts to converge because the fields of the focusing anode start to force the electrons on a different trajectory. The electrons wouldn't start actually converging (mathematically speaking) until the center of the anode (or center of voltage gradient) in the diagram above. \$\endgroup\$ – Voltage Spike Jul 23 '19 at 19:04
  • \$\begingroup\$ If your talking rays, then they don't converge until they come together. It's a little unclear by the diagram when that happens. \$\endgroup\$ – Voltage Spike Jul 23 '19 at 19:32
  • \$\begingroup\$ What I'm saying is that if the beam doesn't get perfectly converged into one line by field of the pre accelerating anode, then the beginning field of focusing anode diverges it again. Correct? \$\endgroup\$ – Atom Jul 24 '19 at 2:22

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