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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Mattski wrote ...
Proud Mary wrote ... Aren't these things simply properties of the interelectrode capacitance due to the vacuum dielectric, which I mentioned above?
Nope, what I hboy007 and I are talking about is that there are electrons flying between the electrodes when a diode is forward conducting. If the voltage is reversed quickly then those electrons are still there and since there is no recombination mechanism they must be swept in the reverse direction by the reverse voltage to be removed. During this time there should be a reverse current, and the time it takes to sweep those charges away would be the reverse recovery time. You can't treat the vacuum purely as a vacuum dielectric since there are in fact electrons in the dielectric, but capacitance will exist between the electrodes, and I think between the electrodes and space charge.
So do you think it is the self-same electrons as enters the space charge from the cathode as leave it at the anode, as if 'transit time' should be understood quite literally as a time-of-flight of individual electrons from cathode to anode?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
I've not posted in this thread yet because the more I think about it, the more my brain hurts....
As soon as the anode goes neutral (or negative) the electrons 'in flight' will no longer be attracted to it, so it's a 'capacitance thing'.
If the cathode goes positive, the electrons 'in flight' will be attracted to it.
The same electrons don't necessarily go from cathode to anode (it isn't a 'perfect vacuum')
electrons have momentum, they will continue in the same direction......my brain hurts.....
So far as I understand it, the picture is rather more complex than that.
Space charge comes into being around a heated element in a vacuum without there being any necessity for there to be an anode at all.
A thermionic diode will generate a very small but measurable current without the necessity for there being any supply voltage on the anode, because the energy distribution within the space charge allows the escape of a small number of very energetic electrons at the anode.
I have three or four books dealing with space charge theory from the thermionic heyday of the 1930s and 40s, mostly compendiums of peer-reviewed papers, and one has to read them with the care due to a foreign language, because both theory and terminology have changed a great deal in the decades since.
Is the velocity of electrons leaving the cathode acutally zero at the moment of departure, as many calculations suppose?
I've forgotten more than I ever knew about one dimensional collisionless Child-Langmuir law, and all the rest of it, so I'll put it all down to a bad hair day, and let the real experts get on with it without my chatter.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Proud Mary wrote ...
Mattski wrote ...
Proud Mary wrote ... Aren't these things simply properties of the interelectrode capacitance due to the vacuum dielectric, which I mentioned above?
Nope, what I hboy007 and I are talking about is that there are electrons flying between the electrodes when a diode is forward conducting. If the voltage is reversed quickly then those electrons are still there and since there is no recombination mechanism they must be swept in the reverse direction by the reverse voltage to be removed. During this time there should be a reverse current, and the time it takes to sweep those charges away would be the reverse recovery time. You can't treat the vacuum purely as a vacuum dielectric since there are in fact electrons in the dielectric, but capacitance will exist between the electrodes, and I think between the electrodes and space charge.
So do you think it is the self-same electrons as enters the space charge from the cathode as leave it at the anode, as if 'transit time' should be understood quite literally as a time-of-flight of individual electrons from cathode to anode?
Yes to all of that. Transit time is literally the time of flight of individual electrons from cathode to anode, transit time effects don't matter for a steady state current because the electron flux is the same at all cross sections between cathode and anode, but it certainly matters when the current changes. Perhaps the space charge stored in the space between electrodes could be modeled as a nonlinear capacitance similar to how gate charge storage on a MOSFET can be viewed as a nonlinear capacitance.
The way I'm picturing it is when the diode is operating as we know the charge density decreases with distance from the cathode in a shape at least superficially similar to an e^(-x) function (I know I have a book somewhere with the exact function) both because the electrons further from the cathode are traveling faster, and the high electron density near the cathode has a shielding effect on the electrons as they leave the cathode so the accelerating force also increases with distance from cathode. When the current slew rate di/dt is slow I imagine that most of the electrons far from the cathode make it to the anode based on their momentum, so you have mostly the denser but short region of space charge next to the cathode that needs to be depleted. But if it's a fast enough slew rate then the electrons won't change direction immediately, so it seems that some of them will make it to the anode using their momentum, but some will just almost make it to the anode and then stop right when the voltage reverses, and then they have to travel all the way back in the opposite direction.
wrote ... Perhaps someone can put up a copy of: F.B.Llewellyn "Electron-inertia effects, "1st ed., Cambridge University Press. London. 1941
If somebody has this reference that would be great, my discussion is based on my theoretical knowledge from semiconductor devices, so while I'm pretty sure my principles are correct I might be missing details.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
.
So far as I understand it, the picture is rather more complex than that.
Space charge comes into being around a heated element in a vacuum without there being any necessity for there to be an anode at all.
A thermionic diode will generate a very small but measurable current without the necessity for there being any supply voltage on the anode, because the energy distribution within the space charge allows the escape of a small number of very energetic electrons at the anode.
I have three or four books dealing with space charge theory from the thermionic heyday of the 1930s and 40s, mostly compendiums of peer-reviewed papers, and one has to read them with the care due to a foreign language, because both theory and terminology have changed a great deal in the decades since.
Is the velocity of electrons leaving the cathode acutally zero at the moment of departure, as many calculations suppose?
I've forgotten more than I ever knew about one dimensional collisionless Child-Langmuir law, and all the rest of it, so I'll put it all down to a bad hair day, and let the real experts get on with it without my chatter.
While your points regarding thermionic emission are valid as usual PM, I'm not sure how relevant they are as far as recovery time is concerned.
I'd also like to hear more from the 'experts', all we seem to have so far is manufacture's 'max fequency' figures, with no explanation of the principles involved.
I'm interested in the theory because I'm building a vacuum system that will utilise these principles, therefore I'd appreciate a better understanding of the subject.
(this theory also applies to capacitatively coupled plasmas and inductively coupled plasmas subjected to electrostatic forces.)
EDIT: Mattski posted while I was writing this, I appreciate his input.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Looks like you managed to get in a whole post before I got my last post submitted PM :) And Ash you weren't far behind ;)
Proud Mary wrote ...
Space charge comes into being around a heated element in a vacuum without there being any necessity for there to be an anode at all.
Correct, the thermal velocity the electrons have will eventually allow it to break free of the attraction of the atomic nuclei as temperature increases and the electrons move faster (well that's the particle way of viewing what is really more quantum mechanical).
wrote ... A thermionic diode will generate a very small but measurable current without the necessity for there being any supply voltage on the anode, because the energy distribution within the space charge allows the escape of a small number of very energetic electrons at the anode.
That seems likely yes. And to anyone who worries if this violates conservation of energy there is a temperature difference so power can be extracted from it. With two cathodes at the same temperature electron flux between them must be equal in steady state.
wrote ...
Is the velocity of electrons leaving the cathode acutally zero at the moment of departure, as many calculations suppose?
I've forgotten more than I ever knew about one dimensional collisionless Child-Langmuir law, and all the rest of it, so I'll put it all down to a bad hair day, and let the real experts get on with it without my chatter.
It's been a while since I've studied it at all too, but as I remember it the space charge density near the cathode has mostly to do with the temperature of the cathode where the rate of electrons leaving the metal is equal to the number re-entering it due to electron-electron repulsion, and the actual conduction current does not perturb this charge density much. The electrons are all actually moving quite fast in that region I imagine due to the temperature, but their average movement will be near zero. The current at any point in the diode is equal to the charge density times the average speed of the electrons and so near the anode the electrons are much less dense and traveling much faster while near the cathode they are very dense and so their average speed is quite slow. Average speed can be slow even while individual electrons travel quickly because two electrons travelling at the same high speed in opposite directions cancel each other out.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Proud Mary wrote ...
.
So far as I understand it, the picture is rather more complex than that.
Space charge comes into being around a heated element in a vacuum without there being any necessity for there to be an anode at all.
A thermionic diode will generate a very small but measurable current without the necessity for there being any supply voltage on the anode, because the energy distribution within the space charge allows the escape of a small number of very energetic electrons at the anode.
I have three or four books dealing with space charge theory from the thermionic heyday of the 1930s and 40s, mostly compendiums of peer-reviewed papers, and one has to read them with the care due to a foreign language, because both theory and terminology have changed a great deal in the decades since.
Is the velocity of electrons leaving the cathode acutally zero at the moment of departure, as many calculations suppose?
I've forgotten more than I ever knew about one dimensional collisionless Child-Langmuir law, and all the rest of it, so I'll put it all down to a bad hair day, and let the real experts get on with it without my chatter.
While your points regarding thermionic emission are valid as usual PM, I'm not sure how relevant they are as far as recovery time is concerned.
I'd also like to hear more from the 'experts', all we seem to have so far is manufacture's 'max fequency' figures, with no explanation of the principles involved.
I'm interested in the theory because I'm building a vacuum system that will utilise these principles, therefore I'd appreciate a better understanding of the subject.
(this theory also applies to capacitatively coupled plasmas and inductively coupled plasmas subjected to electrostatic forces.)
EDIT: Mattski posted while I was writing this, I appreciate his input.
I don't think I've ever come across the term 'reverse recovery time' - or an equivalent term of similar meaning - in any thermionic literature, though absence of evidence is not the same as evidence of absence.
One sees transit time mentioned in RF valve data sheets from time to time, but as Ash has pointed out, data sheets usually confine themselves to maximum frequency and inter-electrode capacitance, presumably because it was believed that these figures (together with all the general characteristics) were all that was necessary for successful circuit designs to be developed using these products
To cheer Nah up, here is the data sheet of the wonderful ML-6422 UHF Planar Triode, which can be made to oscillate at 5 GHz. Note the dark reference to 'transit-time effect' and its relationship to heater voltage and half a dozen other variables which may need the advice of the Machlett Engineering Department to sort out 'above 500 Mc.'
Registered Member #3567
Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260
oooo, fancy!
I'm asuming that this was used in pulsed low power radar, correct? The data sheet is heavy on pulsed aplications. As stated, the only way that tubes can be used at such high frequencys is physicaly making them small. Just look at the nuvistor. I belive even the most common can be used in the hundreds of megahetz or is that my brain going soggy?
Fun Fact- the nuvistors were made in a vacuum chamber using simple robotic arms.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
Most tubes can be operated well into the hundreds of MHz but that's not the same as reverse recovery time. In the case of operating freq, the electrons are always traveling the same direction and the signal is hindered only by the speed of light and interelectrode capacitance but in the case of reverse recovery, the plate and cathode switch relative potentials causing the electrons to switch directions and deplete the space between the elements.
I'm not sure if there is such a thing as reverse recovery time for vacuum tubes as I haven't seen anything on datasheets talking about anything related to it.
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vacuum diode
