We have structure with an cathode and filament at the center. A cylindrical shaped anode with cavities surrounds the cathode (fig 1). When the cathode get heated the electrons are emitted from cathode and move towards the anode. This is the thermionic emission. Here my question is, do the electrons emitted in any specific pattern or they are emitted uniformly around the cathode (fig 2)?
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Prerna
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what on earth makes you even entertain the idea that they might be emitted in patterns? Unless of course these mentioned but not illustrated cavities have anything to do with modulating the otherwise uniform and cylindrically symmetric electric field between cathode and anode. – Neil_UK Jan 21 '20 at 09:03
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i do not know the actual answer but logic tells me that it is neither of those ... nothing in nature is absolutely uniform ... so the pattern is probably something like a `plasma globe` – jsotola Jan 21 '20 at 09:03
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@Neil_UK of course they're emitted in patterns, as when we adjust the focus voltages on a CRT, so the emission patterns from the hot cathode surface are imaged on the phosphor screen. (It's a crude "electron microscope" used as a demonstration by physics teachers.) Unless the filament is perfectly polished, any small imperfections on the cathode, plus nonlinear threshold effects, will produce very nonuniform electron emission patterns. (I've never tried the trick myself. I should look up how it's done. Those $10 three-color Russian pixel-tubes on Ebay might make a good demo device.) – wbeaty Jan 21 '20 at 18:21
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Thermionic emission says to me that electrons "boil" off the cathode. If anode was not at +ve potential, a "cloud" of electrons would exist near the cathode (as @jsotola suggests) while the cathode is hot.
A pattern of electron emission is achieved by cold cathodes, where no electrons are emitted until a large anode potential is created. Electrons are ripped away, in a pattern determined by cathode shape (where electric field is high). Cold cathodes might have a sharp point, where electrons are emitted.
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Exactly right, and empirical testing shows that the same "sharp points" effect is present on hot filaments too. (Hrm, or maybe it's only there in the tests on oxide-coated cathodes? Might not be the same for non-oxide tungsten filaments?) – wbeaty Jan 21 '20 at 18:25
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The actual patterns become visible if the anode is coated with phosphor. GE had an old demo-tube for this. (Also, the grid-shadow was visible, and expanded when the grid was given negative charge.) Or, use a CRT tube, and focus the cathode surface pattern (that's a known "physics demo" trick...)
Imaging the nonuniform cathode emission pattern, crude "electron microscope" https://www.youtube.com/watch?v=l99e0KPtpqQ&t=1m51s
Rather than an expensive CRT, I bet this would work with those Russian stadium-sized flatscreen, hv pixel-tubes, the ILD3, for around ten bucks on eBay.
See: eye-tube electron microscope on sci.electronics.design
wbeaty
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