Hot Cathode Electron V2

29 04 2012

All photos.

Today I’m taking some small steps toward building hot cathode electron gun v2.

Here is a physical diagram and schematic:

This is the hot cathode intended for  an electron beam welder. It has 34 mΩ resistance:

The parts of the electron gun will be arranged like this:

This is more simple than most electron guns. I don’t need a carefully focused beam, I just want to shoot a crude beam of electrons into the Polywell.

The next step is to design and build a simple armature to hold the pieces together.



4 responses

2 05 2012

This sounds great. Even if it doesn’t work, troubleshooting will be much easier with a simplified design. If you haven’t stumbled across it already, this might be a useful resource for all things egun related: I imagine that the new guns are pricy, but there are also a whole bunch of odds and ends for sale which might help us.

25 05 2012


Not sure if anyone has covered this in emails/comments yet. A few points:

20kv is a huge acceleration potential. As far as I’m aware, you don’t necessarily want electrons that are this fast. There’s a lot of complicated theory involved in determining the optimal energy to reduce the electron loss rate through the device’s cusps, and (I think) cold electrons will give you longer confinement times.

You probably don’t need to apply the potential to the filament itself.

You will get better results with a positively biased extractor grid that is placed as close as possible to your filament. This has to do with the extractor potential changing the profile of the potential barrier that electrons must overcome within your filament material, thus lowering its effective work function.

Surface area is an important parameter. Your cathode probably won’t produce very high electron currents, and will need a large amount of power to get hot enough.

A much simpler design that will probably be more effective involves using a bulb’s filament. Bulbs filaments are typically wound coils within coils within coils. If you were to stretch one out, you’d be surprised to find that they’re close to a meter in length! They have a lot of surface area as a result, and will yield orders of magnitude more current than a single wire.

Crush a bulb in a vice using tape to control the glass shards and you’ve got yourself a filament. Add a positively biased grid as close as possible (without touching) the filament and you should get a good electron current. You probably only need about 50-100V of positive bias.

I would recommend against using a transformer for supplying your filament’s heating current like your diagram suggests. Shorting against the chamber could be catastrophic, as could arcing from any high voltage parts within the chamber to your filament, which will then be able to couple onto your mains supply. A car headlight powered by a car battery is a very safe and effective option.

25 05 2012
Mark Suppes


Excellent point on the safety issue caused by an arc from transformer to ground.

I actually started down the path of using an incandescent filament. I have all the parts on hand.

I wonder if I would be able to see the glowing phosphor with a bright incandescent filament behind it.

29 09 2012
Clark McCauley

From my research electrons going that fast and then colliding with a metal substance gives off x-rays so be careful!

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