Lens inside Cathode?

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
longstreet
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Lens inside Cathode?

Post by longstreet » Thu Aug 25, 2005 1:18 am

I didn't know if putting lenses inside the inner cathode (eg between grid wires) could even affect the ion trajectory, much less focus the collision area any. Could use input on either of these issues. This isn't necessarilly a gunned fusor, but it seems appropriate in this forum.

Carter

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Re: Lens inside Cathode?

Post by winterhaven » Thu Aug 25, 2005 1:52 am

What do you mean by a lens?

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Re: Lens inside Cathode?

Post by longstreet » Thu Aug 25, 2005 2:55 am

An electrostatic lens. It has varied voltage to create a shaped voltage gradient that curves the path of ions.

My thinking is it could be used to finely tune the collision zone. The design ratling in my brain would have a dodecahedron shaped cathode, and 10 of the spaces would have a variable lense. The other two used for voltage supply areas. However, I don't know if this would even be practical given the varied and high energy of the ions entering.

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Re: Lens inside Cathode?

Post by winterhaven » Thu Aug 25, 2005 4:45 am

My first thought on this is that, assuming you are talking about a Hirsch-Meeks type fusor with a negative cathode grid in the center of the sphere, in order to focus the positive ions you would have to use a positive charge on the lens. This seems like it would not be benificial to the acceleration of the ions, but I may not be understanding what you mean, maybe it would work with a with the original Farnsworth design where the anode is the central grid.
Actually, in a way, the cathode grid already acts like a lens.
Maybe you can describe your idea further.
-Todd

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Re: Lens inside Cathode?

Post by longstreet » Thu Aug 25, 2005 5:38 am

Well, I like pictures...

This is a equipotential crossection of the inner grid rotated about the vertical axis (left). The top and bottom are most accurate since they for closed loops.

Anyway, looking at this under my understanding of electrical lensing, the shape of the field around the cathode would tend to defocus the stream of ions. So, maybe there should be a more sofisticated lens here to focus the ion stream.
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DaveC
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Re: Lens inside Cathode?

Post by DaveC » Thu Aug 25, 2005 6:41 am

Carter - We have discussed these points quite some time ago... probably back on the old fusor site...

I have also done a few field plots for the inner grid which show similar contours to those you show.

In order to provide focus, which in my opinion is a worthy goal, the inner electrode structure, must guide ion trajectories, so that they do not end on the grid electrode structure itself. For most electrodes with some sort of aperture, this is possible to do over a limited range of entrance angles and positions.

The unsolved issue to date, is to make a lens structure that has a wide acceptance angle and tight focus. This would allow ions from all over the region near the shell of the fusor to move toward the inner electrode structure and converge into a few main beams that focus at the fusor's center.

Presumably, with a tight focus, the ion density would be enough higher that even with the high ion velocities, a greater number of "ion collisions" would occur.. enhancing fusion probabilities.

Dave Cooper

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Richard Hull
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Re: Lens inside Cathode?

Post by Richard Hull » Thu Aug 25, 2005 1:57 pm

I have seen and have in my set of Farnsworth images, I believe from George Bain, an attempt to create a big central bulk machined lens system in the decahedron form. I know for sure that I have no outcome on the lens system as no one alive can remember it being in a test situation. But, I have seen the system. I will have to cast about for the image. It may be nothing, but I will look. I am going to Rochester for the big weekend Teslathion there, so it might be a few days.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Lens inside Cathode?

Post by longstreet » Fri Aug 26, 2005 12:15 am

I don't suppose it would as "simple" as having several concentric inner cathodes to make something like a spherical einzel lens.

(I just have one hole in my cathode since that's the rotation axis)

edit:

I added a 3D model of what I'm talking about.
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Re: Lens inside Cathode?

Post by longstreet » Mon Aug 29, 2005 6:49 pm

I think I missed a very important piece of the puzzel. The ions collected in the center form another anode of positive potential. So, this causes even worse diffusion in the inner grid.

Here I take another plot with an anode representing the ion clump. Normally, I beleive, entering ions are highly deflected as soon as they pass the inner grid wires. This may be why the U of W findings suggest that most fusion doesn't happen in the very center, since most ions nevery make it that far (just speculation).

Now, if you have a lensing mechanism to cause the ions to "converge" somewhat further out of the center, then perhaps they will make it further into the core of the plasma.

Thoughts?

Thanks,
Carter
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Lens Musings

Post by longstreet » Sat Oct 15, 2005 6:37 am

I've been considering the issue of enhancing the focus and circulation of ions using pure electrostatic lensing. Hirsch did this by lining up an outer grid wires (of the same voltage as the shell) with the inner grid wires. I'll explain how this works.

The natural voltage drop inside from the inner grid follows the inverse law. This is slightly off since 0 voltage is not at infinity, but at the chamber wall. However, if the inner grid is much smaller than the chamber then you can make the approximation. If you place grids of voltage concentrically at exactly this drop (ie each concentric grid follows the inverse to determin it's voltage) the electric field looks exactly the same. However, if the voltage on these grids drops faster than the inverse, it will actually alter the voltage distribution between the wires such that the voltage in middle of each wire space is higher than the the wire (see attached voltage plot with grid voltages droping at 1/x^(1.1), which is faster than the natural 1/x). This means that any ions will accelerate to the center of each concenter grid spacing.

Why the Hirsch outer grid increases transparency is that it forces a voltage drop faster than the natural voltage drop (ie faster than inverse , or rather approx to the chamber wall). This forces any ions to the middle of the grid wire space so that as they drop into the core they miss the inner grid.
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