Author Topic: designing and building electron gun help!  (Read 105 times)

Offline Luca c.

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designing and building electron gun help!
« on: May 10, 2024, 09:38:05 PM »
Hello everyone, recently I recently started a project at my university in witch I will try to build an electron gun using some of their equipment, it needs to shoot electrons at a target material to study the emission of light using different machines like spectrophotometer ecc. the parameters for the build are: accelerating voltage variable between 0-30kV, it would be better to reach 50KV, but 30 are ok, high current is not all that important, so in the order of mA, deflection of the beam is not necessary because the target can be positioned where the electron goes, but it wouldn't hurt to have a way to steer the beam, more important is the collimation, that is necessary and important to achieve a more or less focused beam.
The first try I made was to take apart an old oscilloscope and extract the CRT tube from that:

and this is the complete system:

on the right the tube goes to a turbomolecular pump, that can reach 1*10^-5 mbar, my goal was to try and reuse the crt tube controls grid to create the beam and steer and collimate it using the already existing structure, but unfortunately, that is too complicated, also this kind of crt should normally operate at 1-2kV, so using this system at 7,10 or even 30 kV seems really hard, and also it uses an indirectly heated cathode, that from what I found can only operate at an anodic voltage of <5kV (https://www.r-type.org/articles/art-249.htm)
and also I don't know at which potential the collimating plate should be set, so I decided to change strategy:

I want to use this x-ray tube filament that came from a broken tube (the glass was broken in transit but the filament has continuity so should be ok) that should be made of tungsten, and I want to build a very primitive and as simple as possible collimating system, that should be electrostatic, if anyone has a clue on how to do magnetic collimation I'm open to that option too, but electrostatic collimation seems easier to me because I'm also building the high voltage power supply, it will be made of a zvs+ac flyback, going into a 4 stage + polarity CW multiplier, that will give me 5-40kV without many problems.
I tried to do some research on the topic but I have found only superficial instructions or extremely theoretical books, the only useful one is the following: https://sbe.org/handbook/fundamentals/Video/Video-Electron_Optics.pdf
I need specific construction/engineering information, like at which distance the electrode should be placed, at which potential, how many focusing plates and relative potential, how to calculate the focal spot ecc.
From what I understand, if the ratio of the voltage at which the different electrodes are placed with respect to each other is more or less constant the focal spot should remain the same, (in my case it is in the order of 20cm) so I was thinking of using different output from the voltage multiplier coming from different stages, to be able and achieve different voltages all at once, that can be changed by changing the input ac so that the ratio of the voltage is kept constant and so the focal spot, in this way I should be able to maintain the same focal spot varying the accelerating voltage, right?
so if anyone has any practical information on how to build such a focusing system I would be really happy to hear, and also if I made any mistake in the reasoning or principle please correct me;
Many thanks!

Offline Twospoons

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Re: designing and building electron gun help!
« Reply #1 on: May 11, 2024, 12:10:44 AM »
I think you might find this to be a useful resource https://reprap.org/wiki/MetalicaRap .
Its a project to create a metal sintering 3d-printer using electron beam melting. There's a lot of detail on the electron gun and its focusing system.

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Re: designing and building electron gun help!
« Reply #1 on: May 11, 2024, 12:10:44 AM »

 


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