Electron Gun Armature

21 05 2012

One of the components of the electron gun is an armature which will hold the hot cathode, accelerator anode, and phosphor screen all in the same line with the center of the reactor.

This is a challenge because the armature must be an excellent electrical insulator and have a  high heat tolerance. The ideal material is ceramic.

The problem with ceramic is that it we cannot machine it into the unique shapes required, but we can 3D print it! I’ve modeled the armature shape in OpenSCAD:

Here’s a link to the source code

The three curved “feet” have the same curvature as the inside of the reactor chamber, so it will fit nicely and sit still in the bottom of it.

the first column on the left holds the cathode, the middle column, the accelerator anode, and the last, the phosphor screen. the black line will be the path of the electrons to the center of the reactor. Everything here is pretty much how it’s going to be on the final armature, except the phosphor screen will have a different shape, and the distances between the columns will be different as well.

The MakerBot wouldn’t be able to print this all in one shot, so I printed it in sections, and glued them together to get a feel for the final one.

The accelerator cradle

Column for hot cathode

Base of the armature

Earlier version of the base

Assembled armature with cathode and accelerator

Hopefully when I send this file out to be printed in ceramic, they will be able to do it all in one piece. If not, I’l have to find some way of gluing pieces together

Domenick Bauer

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5 responses

22 05 2012
JoeP

You might insertion holes in the base and in the bottom of the cradle so that the standoff parts simply plug into it. That way you avoid introducing whatever glue you were thinking of using in the chamber. And maybe some smaller holes in the insertion holes too so they are vented, that way you don’t trap air molecules.

22 05 2012
dbauer7890

Great point. If I cant get it made in one piece, I’ll try the holes and pegs thing.

22 05 2012
hyakugei

What JoeP said. I was wondering about the high-heat and what kind of glue might work in that environment. Creating pegs and holes is (sounds like) a great solution.

22 05 2012
Remy Dyer

Maybe give each their own base, and have the inline base parts fit together with a vertically aligned “half lap scarf joint” (google image it). That way you can secure them together with small bulldog clips, and easily change the spacing if you want to move the screen closer to the accelerator.

Keep in mind that the electrons will be attracted back towards the accelerator as they leave it, but because the E strength should be higher on the gun side it shouldn’t make much difference. The E field strength will be volts difference / length difference, I suggest putting the accelerator dead center in the chamber, and calculating it from wall to filament to accelerator, then from accelerator to wall on the screen side. If the screen doesn’t accumulate a negative charge, then it will be at the voltage depending on it’s position along that last gradient. If it does collect charge, then it may become negative, repelling further electrons. If it’s at earth potential, then the beam voltage will end up just whatever the filament is below ground. Best bet would be to tie it to ground via a resistor – changing the resistance will change what voltage the screen ends up at, adjusting it or tying via a resistive divider to the same voltage it would have in the gradient without accumulating charge will eliminate any effect of charge saturation near the screen.

31 05 2012
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