Basic Duoplasmatron Ion Gun

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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Jeremy Adams
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Basic Duoplasmatron Ion Gun

Post by Jeremy Adams » Wed Jul 29, 2015 8:13 pm

I've been planning a deuterium ion source that's fairly simple and just wanted some feedback on it. A diagram is posted below of the ion gun but I'll describe it too. Basically it uses a filament emitting electrons through thermionic emission. The filament will be negatively charged DC which should enhance the emission due to the schottchky effect. A plate with a strong negative bias placed behind the filament will push the electrons away as well. A small pipe will be charged with a negative bias and concentrate the electrons. This will change into a smaller pipe which concentrates the electrons into a more dense e beam. An inductor will be placed throughout this area and direct the electrons motion using magnetic field lines. The concentrated e beam will be shot into the second stage of the ion gun where deuterium gas leaks into the chamber. The gas, initially not ionized, will be bombarded with electrons from the e beam. The electrons will "knock off" valence electrons from their orbit of the deuterium gas, ionizing it. The deuterons will reach a pipe with a magnetic field created from an inductor and be organized into a specific direction. The ions will be concentrated using a smaller positive bias rod which will also terminate remaining electrons from the plasma. They will then reach my main vacuum chamber where the fusion happens!

It's not too hard of a design; however, I need to figure out the correct voltages for my filament (which will have a DC neg. bias) and for both pipes with negative and positive biases. If someone could suggest a range of voltages and currents for these it'd be great! I don't want to do anything with radio frequency yet because it's my first time building an ion gun and I've read about how complicated it can get- just out of curiosity though is the RF pulsed through an inductor creating a magnetic field?

As far as materials go for this design I'm probably going to use some parts from McMaster Carr. I want to use a thicker filament so it doesn't risk burning out like a regular tungsten lightbulb filament might. Also more surface area should increase emissions which is good. It's hard to find a thicker filament, any suggestions on what to do for this? It should probably be coated too because that will help emission rate. I was wondering if class 150 black steel flanges will create a strong enough seal for a low pressure ( ... /G1657162/)?

- Jeremy
Basic cross section schematic
Ion gun material design dimensions, cross sectional view

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Re: Basic Duoplasmatron Ion Gun

Post by prestonbarrows » Wed Jul 29, 2015 11:51 pm

I have never personally worked with a Duoplasmatron, but generally filament sources use biases in the range of 10's-100's of volts. You will need to float the power supply which is running current through the filament to heat it up also which can be tricky. Small filament sources are usually limited to a kilowatt or two; whenever you start pulling enough arc current to begin melting things. The electron current you can pull off depends on the thermionic emission laws; basically the filament temperature, area, work function, and local electric field.

Don't expect the filament to last terribly long before burning up. Negatively biased filaments repel electrons but they also attract big burly ions which sputter them away fairly quickly especially in the high pressures within an ion source.

The second accelerating voltage of course will be much higher, more on the order of kilovolts of bias and micro/milliamps of beam current.

I have only ever seen Duoplasmatrons which are linear, not involving the 90 degree bend you have drawn. The Handbook of Ion Sources is a good place to look if you haven't already.

Look for tungsten filaments for electron sources and the sputtering industry if you don't want to make your own. For example ... askets.htm
You can find fancy doped filaments which will boost the maximum electron current density you can pull out of them also.

The correct material from Mcmaster is fine, but stay away from plain steel for vacuum systems. Pretty much only stainless steel, aluminum, copper, and certain ceramics/glass should be used.

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Dennis P Brown
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Re: Basic Duoplasmatron Ion Gun

Post by Dennis P Brown » Thu Jul 30, 2015 11:16 am

A few points that I would like to add and may or may not help:
Use the search function here on ion guns. Many have built very good units and their posts are very helpful.
Not clear why you are accelerating electons to collide with neutral deuterium.
Don't put your inductor wiring within vacuum system near/in the plasma - it will tend to short out in a plasma depending on pressure and being insulated by the vacuum, tend to overheat.
Don't try and draw ions through a right angle - they are not a gas but a plasma and you will waste a great deal of power with ions hitting walls.
Look at a PIG (Penning Ion Gun) type system - very efficient and effective. It will give you a look at a great gun that is both practical and effective (and rather simple to build as many here have.)
Otherwise, everything Prestonbarrows said is the best advice.

Jeroen Vriesman
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Re: Basic Duoplasmatron Ion Gun

Post by Jeroen Vriesman » Fri Jul 31, 2015 9:11 am

This might be handy:

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