Coil Former and Electron Gun

6 01 2010

It’s looking feasible to run the Sydney experiment. The pieces I need beyond my current setup include:

  1. teflon coil formers
  2. electron gun
  3. coil drive electronics
  4. Langmuir probe

The coil formers will be turned on a lathe. I’m going to refer to these as coil formers, because unlike a magrid they do not carry a positive high voltage potential. The 2.5″ teflon rod arrived yesterday:

The electron gun is pretty straightforward. A metal cylinder placed at -10 kV in a few millitorr of hydrogen gas will produce the beam that you see in the Sydney experiment. I’m about to braise this copper tube to a lead to make the electron gun:

The coil drive electronics: Provided I can get the schematic, assembling the coil drive electrons should be straightforward.

The Langmuir probe: Still waiting to get quotes. Although the Langmuir probe looks pretty complicated, hopefully a commercial product will allow me to approach it as a black box… just a tool for taking plasma measurements. This exploration will force me to delve deeper in to plasma physics.


Joe Khachan has the following to add:

You are probably better off tapping a hole in the cylinder for the electron gun and screwing the stalk into it. This will get hot and brazing might melt. Another note, commercial langmuir probes might be a little limiting since they are made for the plasma processing market and will scan between -100 and 100 V. The polywell will be dealing with greater voltages than that. We make ours, which is a simple construction. Just a very thin wire (the width of a human hair) inside a ceramic sleeve, about 10 mm of the wire is outside that sleeve at the center of the polywell. You can find details on the web since many people make their own. For floating potential you can use a high voltage probe connected to the langmuir probe wire and the output of the high voltage probe goes to the oscilliscope, which is what we were using. These are the time varying potential that you see in our powerpoint presentation. After you’ve finished, you will find interesting behaviour in the well depth as you change the current in your coils.

Another note. The electron gun relies on the natural break-down of the gas in your chamber like gridded IEC. The breakdown voltage and/or pressure will depend on the size of your chamber (if you don’t have another ionizing source). You may have to increase the pressure until you get breakdown – then you can reduce it after that. You want as low a pressure as possible since the electrons are trapped for a long enough time to collide significantly with the background gas and therefore lose energy and therefore potential. If you have to go above 10 mTorr then so be it. You should still see interesting behaviour.

The langmuir probe should be the simplest of all that has to be done. Just ignore the wikipedia page. The maths makes it look bad. It is just a wire connected to a voltmeter (for the floating potential). You just need to shield most of the wire with a ceramic sleeve and only have 5mm to 10mm exposed in the plasma – that’s it. You will need electrical feedthroughs on your vacuum chamber so that you can connect the langmuir probe wire to the outside world.

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