To Focus or not to focus - the next generation

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
DaveC
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Re: To Focus or not to focus - the next generation

Post by DaveC »

Greg -

Here's a little bit about how the electron optics works. You should probably think about getting hold of some text on electrostatics for a more thorough treatment.

Simply put, whenever charges are placed on an electrode, a voltage or potential is present. The potential is actually a measure of the energy a charge particle would either gain or lose as it leaves the electrode. At points away from the electrode, the particle would have different energy and thus a different potential. Potentials around the electrode can be plotted as lines of equal potential known as equi-potentials.

The rate at which the voltage changes ( gradient of potential) is expressed in volts per unit of length, and is what we call the "Electric Field". The term "electric field" is simply a way to describe the force experienced by a charged particle at that particular point in space.

So, when a voltage is applied to an electrode, it creates around the electrode an electric field which is capable of exerting a force on a charged particle.

Now depending on the shape of the electrodes we can get differently shaped electric fields, which can bend the paths of moving charged particles - electrons or ions.

The electrodes with voltages on them, act to the ions like a lens acts to light waves or photons. The two cylinder electrodes comprise what is called an "Einzel" lens, which works very much like a simple optical lens. If an ion is going through the cylinders, say from left to right, and if the first cylinder has a lower voltage than the second, the electric field in the gap between the cylinders, acts to bend the flight path of the ion towards the axis of the cylinders. If the first cylinder has a higher voltage than the second one, the fields bend the ion outward. This gives either a converging lens in the first case, or a diverging lens in the second case.

In the case of the two cylinders, lines of equal potential curve into the bore of the cylinder. making what actually closely resembles the outline of a convex or concave lens. Since electric field lines intersect the equipotential lines at right angles, and since at low speeds, ions and electrons more or less follow the electric field lines, we can actually draw the paths of the ions as they go through the lens.

The plots I posted above could be displayed with the equipotentials and the ion trajectories. I'll rerun one and post it for you.

That's the broad generalities of how bits of metal with voltages on them can bend the paths of moving ions. Hope the explanation was somewhat useful.

Dave Cooper
DaveC
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Re: To Focus or not to focus - the next generation

Post by DaveC »

Here's a much simpler version of the reflex ion gun. It can hardly be much simpler than this.

The ion source is on the left emitting positive ions of 2 AMU. It has a potential of +1V.

At center is a ring electrode at - 10kV.

On the right side is the reflector at same potential as the source, +1 V.

I put a grounded (0V ) metal can around everything along the beam axis.. and... dodged the issues of how to get the -10 kV to the ring in the center, for now.

Notice the beam path resembles a bow tie, somewhat. The return paths of some ions can clearly be seen. What is interesting is that this dense pattern is from only 30 ions!. The plotting took a number of seconds to complete, as each ion flies separately, until it collides with an electrode. Most of the ions made many passes before finally colliding with the reflector or source.

The second picture is an enlarged view of a portion of the source end, showing how the returning ions are reflected again and again. The squares are 1mm in both pictures.

Looks like the reflex ion gun is a pretty simply thing. We should note too, that this model does not account for residual gas molecules. So the pressure assumed is such that the mean free path is a few tens of meters, perhaps around 10^-6 Torr.

I have done some models of scattering in air at these pressures. There is not much. So the trajectories should be more or less representative of the real thing.

Keep in mind too, that the earth's magnetic field will have an effect if the tube is not enclosed in a magnetic shield.


Hope this helps stir the imagination and guide the construction a little.

Dave Cooper
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Simplified Reflex Ion Gun.jpg
DaveC
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Re: To Focus or not to focus - the next generation

Post by DaveC »

Here's the enlargement mentioned in the post above. Tried to send both together, but seemed to print one over the other.

Hope this is not too confusing..

Dave Cooper
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Enlarged View Reflex Ion Gun.jpg
hellblazer
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Re: To Focus or not to focus - the next generation

Post by hellblazer »

One thing I'm wondering about, though, is the scattering at the focus. The cross section of scattering is much larger than the cross section of fusing. So, what I'm thinking is that this will be similar to a magnetic mirror in that there will be a lot of losses in the equitorial plane at the focus. Also, those ions that aren't lost through scattering, most will still have their trajectories significantly altered through the scattering. This will change their energies, and thus their ultimate focus. It seems like this would have the effect of diffusing the fine point focus at the center.

Anyways, just some completely random thoughts. I still think this is a pretty cool idea, and I think it will be extremely interesting to see how it turns out.
pfostini
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Re: To Focus or not to focus - the next generation

Post by pfostini »

This thread has been very informative. I am going to try some of the ideas on an actual gun. I will also rig some type of target to prevent the scattering and such in order to truly see where the beam hits. On the last gun I will use a much smaller extractor and seal it in such a way as to have a higher pressure behind it and use the pinhole as an orfice for slowing the gas flow and have a higher pressure gradient.
DaveC
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Re: To Focus or not to focus - the next generation

Post by DaveC »

Concern about scattering is certainly in order here. Been posting early results to help everyone see what is going on.
I can model some degree of charge repulsion. Need to do that to see the magnitude of the effect. Stay tuned.

Dave Cooper.
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