Thermionic device with gate electrostatic accelerator

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ohiovr
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Thermionic device with gate electrostatic accelerator

Post by ohiovr »

I was reading about Mannhart's thermionic generator:

http://phys.org/news/2013-12-highly-eff ... ronic.html

http://physicsworld.com/cws/article/new ... -from-heat

The innovation here is an electronic method of mitigating negative space charge. Negative space charge is the effect of electrons, on their way to an anode, being negatively charged and filling up a space, retards other electrons that have been thermionically emitted from freely moving to the anode.

The current limited by space charge can be increased by reducing the space between the electrodes. The relationship follows the Child's law, aka child langmuir law, aka, Three Halves Power law:

Child's Law states that the space-charge limited current (SCLC) in a plane-parallel diode varies directly as the three-halves power of the anode voltage Va and inversely as the square of the distance d separating the cathode and the anode.

http://ohiovr.com/documents/child's law space charge limited current.ods

Manhart's method of mitigating space charge is to put a gate (a grid) about 50 µm away from the cathode, which is positively charged to about 4 volts.

What I don't understand is, ignoring the electrons that hit the grid, wouldn't there be a power loss from accelerating the electrons? If so, how do I calculate what that is? I'm mean there is a cost with everything.. What does this cost in terms of power loss?

hanelyp
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Re: Thermionic device with gate electrostatic accelerator

Post by hanelyp »

Neglecting electrons that hit the grid, the energy applied to accelerate electrons is gotten back as the electrons cross the gap between grid and collector. The total potential between source and collector isn't changed by the grid. The potential profile of the path is changed.
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ohiovr
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Re: Thermionic device with gate electrostatic accelerator

Post by ohiovr »

Thanks for your help!

D Tibbets
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Re: Thermionic device with gate electrostatic accelerator

Post by D Tibbets »

After some head scratching, I have to agree, at least under certain conditions. Electrons accelerated to an anode hit that anode with all of their accelerated energy. When the electrons are accelerated past an anode (the grid). They are accelerated as they approach the grid, and equally decelerated as they pass the grid and pull away.

I think the exception to this, at least for a time, is Gauss Law. Electrons attracted into a gridded sphere are accelerated as they approach, but once inside / past the grid they are not effected at all (there are some qualifiers) by the charge on the spherical grid. The electrons further interact with the grid when they hit it or pass outside it on the other side. This of course is a principle utilized in the Polywell. Things become more complicated when you add in the grid collisions, space charge and electron- electron Coulomb collisions, ions, and finally the complex magnetic effects applied and inherent to the non neutral plasma.

Dan Tibbets
To error is human... and I'm very human.

ohiovr
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Re: Thermionic device with gate electrostatic accelerator

Post by ohiovr »

hanelyp wrote:Neglecting electrons that hit the grid, the energy applied to accelerate electrons is gotten back as the electrons cross the gap between grid and collector. The total potential between source and collector isn't changed by the grid. The potential profile of the path is changed.
Would there be an energy loss if the electrons were not decelerated before they hit the anode?

hanelyp
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Re: Thermionic device with gate electrostatic accelerator

Post by hanelyp »

The potential difference between grid and anode decelerates the electrons. This potential should be made greater by the potential between cathode and grid.

Electrons leave the heated cathode with thermal energy. Between the cathode and grid they pick up energy. Between grid and anode they lose energy.
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ohiovr
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Re: Thermionic device with gate electrostatic accelerator

Post by ohiovr »

hanelyp wrote:The potential difference between grid and anode decelerates the electrons. This potential should be made greater by the potential between cathode and grid.

Electrons leave the heated cathode with thermal energy. Between the cathode and grid they pick up energy. Between grid and anode they lose energy.
I read somewhere that a coulomb trans versing 1 volt requires 1 joule. Does it matter how far they travel?

I suppose if deceleration was not important, you could put your "gate" behind the collector and it would work basically in the same way.

D Tibbets
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Re: Thermionic device with gate electrostatic accelerator

Post by D Tibbets »

Yes and no- distance does/ does not matter. How to get around this +/- effect of a mobile charge and a fixed transparent electrode is a problem for the designer. Distance (inverse square law always applies provided the electrron/ mobile charge carrier can "see" the electrode. The electrode can be hidden by two ways that immediatly comes to mind. First is Gauss law considerations. The second is timing. If the electron is approaching the electrode, when it is turned on, it will accelerate (if opposite charge). If the electrode is turned off once the electron passes, then there will be no decelleration. I think this pulsed method is used in multiple electrode and/or multiple pass particle accelerators like cyclotrons. A chare on an emmiting electrode like a hot cathode only will accelerate in one direction. What happens inside the wire is different topic, and it is not. Ideas like space charge, debye length, etc applies to solids just like plasmas. Theexception is that electrons are the only mobile charge carrier possible. But, that is not always the case, at least for conceptual purposes, holes are sometimes involked for explaining some behaviors.
Also, the inverse square law doesn't always fully describe the motion of an electron relative to a fixed charge. Another aspect of Gauss law applies. Geometry matters.

Electrical engineering is of course dependent on physics, and the complexities fully justify the specialist electrical engineer. And, Bussard had lamented the neglect in the training of young physicists since the vacuum tube technology has been mostly supplanted by solid state electronics. Many of the interactions are glossed over, or expressed in such a way that some insight is lost. At one point a physicist (A. Carlson) here promoted Polywell criticisms without an understanding that the answers were already evident in power amplifier vacuum tube technology. Or, at least that was the counter argument, which was never resolved.

Dan Tibbets
To error is human... and I'm very human.

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