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I was reading this simple description of magnetron working principle.

enter image description here

Can you explain me why when electrons go near the cavities, they start to resonate at microwave frequency? I do not see the link between these two phenomena.

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Can you explain me why when electrons go near the cavities, they start to resonate at microwave frequency?

It's not the electron that resonates. Or the cavity.

It's the electric field produced by the moving electron, as constrained by the geometry of the cavity.

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Due to the combination of the magnetic field and the cathode-anode potential, the electron stream naturally wants to orbit the cathode. A magnetron with no resonating cavities at all will still generate energy at microwave frequencies (see the Wikipedia article or maybe search on "Hull Magnetron" or "Single-anode Magnetron").

I believe (but am not totally sure) that the cavities just amplify the effect of the interaction of the electrons with the magnetic field.

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  • \$\begingroup\$ Does it actually continuously orbit the cathode in such a perfect circle? Wouldn't the electrons still have a radial velocity component that would make it more of an outward spiral rather than a circle? \$\endgroup\$ – DKNguyen Nov 13 '19 at 23:24
  • \$\begingroup\$ @DKNguyen Yes. That is what wikipedia says. See my answer below. \$\endgroup\$ – Oldfart Nov 14 '19 at 3:51
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I have just read the Wikipedia article as I also know very little about cavity magnetrons. It is much better. What I read from the wiki page the following is wrong/missing from the text above:

  • The electrons do NOT travel in a circle but in a spiral finally hitting the anode. (Yes @DKNguyen you where right)

  • This causes local charge which has to be equalized an thus a current starts flowing in the anode. (The local charge also makes that the next electrons want to land elsewhere)

  • Due to the cavities that current has to go "around" the holes.

  • This causes microwave radiofrequency energy.

Please do not upvote this. I know nothing about magnetrons, I only repeated the wike data.

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  • \$\begingroup\$ That's pretty cool. It almost sounds like an electrostatic version of induction, and instead of producing a torque as a byproduct you produce microwaves. \$\endgroup\$ – DKNguyen Nov 14 '19 at 4:14
  • \$\begingroup\$ Magnetrons evolved. So there are a variety of types. But of them, the cavity magnetron is perhaps the most interesting for an electronics engineer. As the electrons sweep outward away from the cathode towards the surrounding anode, there is a very strong magnetic field that causes the electrons to curl (the force is always perpendicular to the magnetic field lines and the motion of the electron. So the electrons wind up spinning around in a circle. \$\endgroup\$ – jonk Nov 14 '19 at 5:44
  • \$\begingroup\$ The anode is cut much like a "six-shooter" with holes (which are effectively "inductors") and slots leading from the circling electron current in the vacuum to those holes (which are effectively "capacitors" -- parallel plates.) The upshot is an LC resonance is made from a physical construction of a slot and adjacent hole. (In fact, they used gun tooling for revolvers to make these units [for a time.]) The whirling electrons supply energy to the resonating cavities to sustain their oscillations and one or more are then fed to a waveguide for emission. The OP's diagram closely captures it. \$\endgroup\$ – jonk Nov 14 '19 at 5:47
  • \$\begingroup\$ It may help to look up "Hull field," "Hull cutoff", and the "Buneman-Hartree condition" (ironically, the "B-H condition.") \$\endgroup\$ – jonk Nov 14 '19 at 5:55
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    \$\begingroup\$ @Oldfart I also don't know anything about the community wiki. No clue what it means. The problem with my writing about magnetrons (as a high school student we got to play with klystrons and then magnetrons, building crystals out of styrofoam balls wrapped in tin foil and held together with wood rods and doing diffraction experiments, as my teacher had a master's in physics from Northwestern U) is that there is so much to them, both historically and with the cavity invention later. It's more interesting than the D cyclotron. If I get some time to write? Adjusting your words wouldn't be fun. \$\endgroup\$ – jonk Nov 14 '19 at 13:19

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