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Author Topic: Would this electron gun setup work?  (Read 3486 times)

Offline scientist1234

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Would this electron gun setup work?
« on: 06/05/2010 15:01:33 »
I have a design for an electron gun that incorporates a Faraday cage setup and I would like anyones opinion if this setup could actually work.
 
see image below message

Explanation of image
 
There is a Faraday cage which is connected to a negative high voltage source. A wire which is shielded from the electric field of the Faraday cage is going into the Faraday cage to supply power to the cathode (electron emitter).
 
Since the Faraday cage has the principles that when there is only an electric field on the outside there is no electric field on the inside but when there is an electric field on the inside there will be an equal an opposite charge on the inside of the Faraday cage surface.
 
So if the cathode has a voltage of -100 then the inside of the Faraday cage will be +100 volts even though the outside of the Faraday cage may be at negative 10 kilovolts.
 
By the way the area around the cathode and the outside tube on the Faraday cage are in a high vacuum
 
My question would be is this setup possible, is it possible to emit electrons inside a Faraday cage accelerate them to a positive voltage on the inside of the Faraday cage and exit through a hole in the Faraday cage and be then accelerated again by the negative voltage on the outside of the Faraday cage.
 
My idea is very similar to the setup of a van de graaff accelerator which has a penning ion source to generate positively or negatively charged ions and to accelerate them. I can not however find very much information on the design of penning ion sources other then extremely complicated designs.
 
Any ideas/suggestions are highly appreciated.
« Last Edit: 06/05/2010 18:26:32 by scientist1234 »


 

Offline Atomic-S

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Would this electron gun setup work?
« Reply #1 on: 29/05/2010 06:20:46 »
First, I think there is a bit of a problem here with understanding the concept of voltage. Voltage is always measured with respect to some point. A conductor always is everywhere at the same potential (voltage with respect to the chosen reference point) as long as the current in it is zero. Therefore, it is not possible for the faraday cage to be at both +100 volts and, say -10,000 volts -- except in the sense that each of those measurements is taken with respect to a different reference point.

Which, however, is possible. But in the case of your proposed apparatus, you need to consider where the electrons will go once they exit the faraday cage. They have to go somewhere, and in the case of a TV picture tube, where they go is the screen, which is yet a 3rd electrode of the system, and which carries a high positive charge. There they are captured by a microlayer of metal and bled back into the circuitry. That would be like metallizing the inside of your vacuum chamber and using it as a drain for electrons. Using that as the reference point, and calling it 0, your faraday cage would be at, say, -9900 volts, and your emitter inside would be at -10100 volts. The voltage difference btween the emitter and the cage would be -200 volts. Actually, this arrangment is not all that different that found in many vacuum tubes. However, a -200 volt difference will not cause emission from a cold cathode in an apparatus of reasonable construction. You would need to put the voltage up much higher and make the cathode very sharp, to induce field emission. I don't have firm figures for what the voltage would be, but let us for argument's sake say we would have the cathode at -10100 volts, the cage at -200 volts, and the drain at 0 volts. Here, the strong field at the tip of the cathode could induce cold emission, and the electrons would be propelled with great speed at the cage, and any that got out through the hole would continue on at high speed to the drain. Now, one problem is that the electrons would mostly not go out through the hole, but rather at the cage itself. 2 ways to deal with this problem: One: place the tip of the cathode much closer to the side of the cage having the hole, than to other sides. Two:  Place the cage in an electromagnetic coil whose axis passes from cathode through the center of the hole, which, when an auxiliary current flows in it, will whirl the emitted electrons into a vortex that will exit the hole.
« Last Edit: 29/05/2010 06:30:40 by Atomic-S »
 

Offline scientist1234

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Would this electron gun setup work?
« Reply #2 on: 08/06/2010 09:12:08 »
All voltages are taken with a reference to ground (earth, 0 volt).

I got some information of an electron accelerator engineer and he says that when the outside of a Faraday cage is charged there will be no or a very small electric field on the inside of the cage even when the cathode is placed inside the cage. If the cathode has a voltage of -100 then the inside of the Faraday cage will be basically 0 volt. The outside of the Faraday cage can be at -2000 volts for example but there will be no noticable electric field inside the Faraday cage (Faraday cage effect).

Lets say you can emit electrons with 100 volt potential difference (cathode and inside Faraday cage), would it then be possible to let the electrons exit the hole in the Faraday cage with or without the help of electromagnetic coils (magnetic lenses) and so let them be accelerated again by the -2000 volts on the outside of the Faraday cage to a target that is at ground (Earth, 0 volt) potential.
 

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Would this electron gun setup work?
« Reply #2 on: 08/06/2010 09:12:08 »

 

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