In my travels in the tube world, I have been particularly fascinated by the effects of retarding fields. A retarding field is formed when a negative potential slows down the path of electrons. In the context of a tube, a retarding field exists when electrons pass near an electrode that is biased at a lower potential than the previous electrode, or when electrons approach a pre-existing space charge, as might have been accumulated by the beam plates of a beam tetrode, or right at the cathode, when thermionic emission is thwarted by the accumulated space charge of electrons around the cathode.
The effects of retarding fields on tube operation are quite varied: There is the beam plate action in a beam tetrode, there is the effect of plate potential when it is lower than the electrode adjacent to it, the third and last grid in a pentode which is usually designed with a sparse pitch to produce little retardation in electrons passing it, may have a more pronounced effect if the potential is sufficiently negative. Even an incandescent light bulb with a hard vacuum and a few hundred volts driving the filament may exhibit spontaneous VHF oscillations as electrons are accelerated and whip-lashed back as they travel from the negative side of the filament to the positive side of the filament.
I have explored some of this phenomena from the point of view of an antique radio hobbyist at the following links.
http://www.radiomuseum.org/forum/dual_c ... todes.html
This link deals with the effect that the third grid (suppressor) in a pentode has on plate and second grid (screen) currents. Follow some of the links within this page for a further treatment of the topic.
http://www.radiomuseum.org/forum/rustik ... ments.html
This link deals with VHF oscillations in an incandescent light bulb, and may have an incidentally closer connection to space charge behaviour in a fusor.
I would love to hear about any insights that come from the perspective other than that of of an antique radio-hobbyist.
Regards,
-Joe