Lens inside Cathode?

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longstreet
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Lens inside Cathode?

Post by longstreet »

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
Todd Massure
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Re: Lens inside Cathode?

Post by Todd Massure »

What do you mean by a lens?
longstreet
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Re: Lens inside Cathode?

Post by longstreet »

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 Todd Massure »

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 »

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 »

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.

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Richard Hull
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Re: Lens inside Cathode?

Post by Richard Hull »

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
longstreet
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Re: Lens inside Cathode?

Post by longstreet »

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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longstreet
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Re: Lens inside Cathode?

Post by longstreet »

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 »

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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DaveC
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Re: Lens Musings

Post by DaveC »

Carter -

The field lines and potential contours tell some of the tale, but not all. It is a rule of thumb that electrons above about 10keV increasingly do not follow the field lines set up by electrodes. They are moving too fast to immediately redirect their paths as the fields change direction. They ARE influenced by the fields, however. Ions being far more massive, behave the same at much lower velocities. So one must do some careful modeling to have an idea how to shape the fields near the inner grid so that the D2 ions will miss the wires. There is, however, another issue that does not get discussed much here. And that is, that the glow around the inner grid at the higher pressures, is mostly the cathode fall glow of a simple gas discharge. This is a fairly low voltage, (low energy ) but often high current (lots of ions involved) activity. It is where most of the fusor enregy dissipation takes place. Evidence for this is that the grids melt, are sputtered onto the walls and viewports and insulators, within. Clean this up and the fusor power consumption for doing practically nothing useful, goes to nil.

But if the speculations are correct that the fusions occur out in the volume of the fusor and not at the center, then it would seem to say that trying to have a nice well focused "poissor" or star, may be entirely irrelevant to having high Neutrons/sec numbers.

If the outer regions are where the bulk of fusion occurs, then we do not want collisions in the center, UNLESS they are essentially elastic collisions.

It has always seemed to me that having a low density plasma everywhere except in the dead center, was the way to get low losses. But maybe the law of averages works more strongly in our favor than the concentrating effect of a central focus.

We probably need some more definitive experiments in this area.

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Re: Lens Musings

Post by longstreet »

I read the UoW paper on reaction location. They're conclusion was that this is because ions become neutral coming out of the core retaining their high velocity. I don't think this is good because a neutral definatly cannot recirculate. If 2/3 of the fusion is from these neutrals then I think that speaks to just how much energy is lost to neutrals flying out to heat the walls. Imagine how many fast neutral's didn't undergo fusion for every neutral fusion event, and then imagine if every one of those had stayed ionized?

I don't think it's usefull to try and increase the effect, but I also have no idea how to stop it. This is one of the reasons I would like to stop electron emmisions from the inner grid (be dielectrical insulation), to perhaps decrese the recombination effect. But there is still electron exchange with the surrounding gas. I suppose there's not much you can do with that short of trying to operate in a lower vacuum, or ionizing 100% of the gas in the chamber.

However, managing to focus the ion collisions better, I would hope anyway, works to increase fusion between ions before they become neutral, which is really the whole design of the fusor. If you wanted to increase the neutral fusion you would probably abandon the spherical design and just accelerate stuff linearly.

Thanks,
Carter
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Richard Hull
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Re: Lens Musings

Post by Richard Hull »

That we can certainly work for and actually achieve improved operation of the fusor is a virtual given (once the inertia of doing nothing is over come). A ten fold reduction in input losses from a 300 watt heating fusor would in theory imply that only 30 watts is in the crapper and 270 of our watts are no longer lost or heating anything. Are those watts fusing??!! Even at just break even, I hope not or the operator is dead!


Of course not!. Those watts still are not fusing they are still and will always be heating watts as misses are demanded by cross sectional limitations and these misses ARE HEAT, whether deuterons or neutrals or electrons or anything. Focusing and fusing 270 watts worth of deuterons on a kitchen table would really be undesirable from a longevity standpoint. If you actually did this, the first fusion power producing reactor using your principles would certainly bear your name. (posthumously, of course.)

I think what we ARE actually capable of doing is going from .00000001% efficiency to maybe even a 100 or even 1000 fold improvement per unit watt expended to maybe .000001% efficiency. This is the doable process that we might apply ourselves to.

The idea of turning even 10 watts of the 300 watts input into fusion energy is rather ludicrous.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
longstreet
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Re: Lens Musings

Post by longstreet »

If I stated somewhere that this is some kind of breakthrough I stand corrected. But I am fairly sure I never said such a thing. I think your opinion on the matter is fairly documented Mr. Hull, and your continuous pessimism is getting on the nerves.

Carter
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Re: Lens Musings

Post by DaveC »

I stand with Richard on this as I do on most things. We should at this juncture, be looking for increases in yield efficiency, not breakeven schemes. And huge levels of output. That will make for obituaries. Besides, most scientific progress is in the form of small steps, once the direction is shown.

As for me, I am not sure I have a clue about that "direction", although I have a few ideas about what I might try, next. But just getting an electrostatic lens from the arm waving concept to an actual piece of hardware, that works as planned, is a fairly involved task. And to do "focus" on a sub nanometer scale...!!! This is not trivial stuff. For example, how could you tell if focus is achieved. Testing for more output, of course is one way, but that actually only tests for.... more output, not better focus. So the experiments are increasingly sophisticated, and the prep time gets longer, and the instrumentation might get a bit more demanding...etc.

But not wanting to discourage anyone, systematic evaluation of what methods work - consistently and predictably, with amps, volts, neutron counts, and all relevant data... is the way to find the trail.

But I don't think there are any "breadcrumbs" on this trail, yet, because no one has been on it. So it could be a strange an wonderful direction. But it will definitely involve principles already known, mostly, and possibly something new, and as yet unforseen.

Dave Cooper
longstreet
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Re: Lens Musings

Post by longstreet »

I really don't understand you people's fixation on breakeven. WHO is talking about breakeven? Electrostatic lensing in IEC is the basis of it's operation, and I haven't even proposed any new scheme's; I mearly observed why Hirsch's outer grid increased transparency. Why do you have to warn me ten times in a posting about how it's such a long and hard road to breakeven?
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Re: Lens Musings

Post by Q »

carter,
the breakeven goal seems to be one of the main objectives in a lot of theoretical discussions. perhaps that's why breakeven even came up.
now, i'm not one of the brightest on this board, so perhaps i missed it, but i didn't really understand why you proposed the lensing. i realize you want to finely focus the ions, but is this to try and increase the reaction rate?

Q
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Re: Lens Musings

Post by longstreet »

You can use lensing, as Hirsch did, to decrease collisions with the grids. If for nothing else increase the lifetime of operation. However, getting the ions closer to the center of the inner grid spacings mitigates the defocusing effect near the grid wires (which I showed in the origional thread). This would increase the number of ions you actually have travelling to your target, which is the center of the fusor. If more ions are passing through the same region of space, ideally this would increase the number of high energy collisions.

Thanks,
Carter
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Re: Lens inside Cathode?

Post by LeePalmer »

Hi Carter-

Farnsworth had a similar thought, with regard to a 3D version of an Einzel lens, see his older fusor patent number 3258402, embodiment number two, and the drawings on page five and six: http://www.google.com/patents?id=6VBkAA ... +discharge

The idea of creating a potential well using a 3D analog of an Einzel lens is a good one, I think, because it opens up the possibility of getting rid of grids all together. I wonder how deep of a potential well it is possible to create this way?

Farnsworth's original idea was similar to Bussards current idea, just confine the electrons, and use this to create a virtual cathode that will attract positive ions, I think. He gave up on it because of excessive electron losses to the grids, I think, maybe.

What software are you using for your plots, by the way?
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Re: Lens inside Cathode?

Post by longstreet »

I think Bussard is on the right track about magnetic electron confinement. Using only electric grids is a dead end if you're trying to make power. Magnets maybe too but it has promise.
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