Why is there no electric field in the hollow space of the dees of a cyclotron though it is open at one face?
Is electrostatic shielding applicable for space(hollow space) inside open conductors(like dees) also? 
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asked Aug 10, 2020 at 14:35
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3$\begingroup$ What is a "dees"? $\endgroup$– ACuriousMind ♦Commented Aug 10, 2020 at 14:41
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$\begingroup$ Those semi circular hollow disc like containers open at one face in the cyclotron..Sry I had not inserted the diagram.. $\endgroup$– Thirsty for conceptsCommented Aug 10, 2020 at 14:42
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$\begingroup$ The question has already been asked, but received no answers: physics.stackexchange.com/questions/238572/… $\endgroup$– ProfRobCommented Aug 10, 2020 at 17:39
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1 Answer
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Who says there is no electric field in the dees? The early designs by Lawrence included various grid or slit designs to close off the dees and keep the electric field out of the dees. But, in April 1931 Livingston (Lawrence's grad student, see APS.org) tried the cyclotron without those devices in an attempt to increase the beam current. To quote from "From Nuclear Transmutation to Nuclear Fission, 1932-1939" by Per F Dahl (Institute of Physics, 2002), p. 75-76:
In the absence of a grid or similar metallic shield across the dees of a cyclotron ... there is a slight penetration of the accelerating electric field near the entrance to the dee. Fortuitously, the penetrating field lines, or 'lines of force,' will act like an electric lens ...
So, yes, the field penetrates slightly into the dees, and helps keep the first few turns of the spiraling ions focused, increasing the current on the output. At higher energies, magnetic focusing is more important. Playing while the boss is away can be a good thing for a grad student.
answered Aug 10, 2020 at 15:33
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1$\begingroup$ Thank you🙏🏼.....so will the electric field also exist in that hollow space....and why does electric field penetrates just slightly and not more.....I mean why doesn't the curved surface of the dees just act like capacitors and the field should exist from the curved end of one Dee to the curved end of the other.....but instead of that electric field exists only in that narrow space between these dees........I am afraid that I may have got the concept wrong....so pls rectify🙏🏼 $\endgroup$ Commented Aug 10, 2020 at 15:47
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1$\begingroup$ @Thirstyforconcepts - no electric field exists in a volume completely enclosed by a conductor. So, there is a small fringing field that penetrates into the volume of the dees, but the height of the dees is small compared with the radius so the field doesn't penetrate very far. $\endgroup$ Commented Aug 10, 2020 at 16:01
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1$\begingroup$ Thank you.......But the volume of a dee is not "completely" enclosed by a conductor since one face is open...then how?........are u saying that as the height is very small as compared to radius we can consider that to be almost closed? $\endgroup$ Commented Aug 10, 2020 at 16:13
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1$\begingroup$ @Thirstyforconcepts - well, yes. In the same way that two plates acting as a capacitor have fringing fields around the edges. $\endgroup$ Commented Aug 10, 2020 at 16:15
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1$\begingroup$ Thank you so much @Jon Custer...very grateful to u.. $\endgroup$ Commented Aug 10, 2020 at 16:52