Ion gun fusor

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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clarkmcc
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Ion gun fusor

Post by clarkmcc »

I have been studying the construction of Andrew Seltman's fusor which uses Ion guns to ionize deuterium (from what I understand) but I also read somewhere that there are Ion gun-less fusors as well. Is this what all of you use? How do they work?
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Re: Ion gun fusor

Post by Tyler Christensen »

There are probably thousands of posts here discussing ion guns. In fact, you posted in a sub-forum practically dedicated to answering your question. I'd start there.
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Richard Hull
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Re: Ion gun fusor

Post by Richard Hull »

There are only a tiny, microscopic number of fusors that have ever used ion guns. Virtually all amateur fusors DO NOT use ion guns.

There was an interest and push to use them at one time, but much of that enthusiasm has waned as standard non-gunned fusors are simpler, cheaper and do just fine for most amateur purposes.

Ion guns are typically investigated by the amateur fusioneer well after attaining superior fusion in a normal, non-gunned fusor.

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
clarkmcc
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Re: Ion gun fusor

Post by clarkmcc »

Where can I go to learn about how these non ion gun fusors work?
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Re: Ion gun fusor

Post by George Schmermund »

Clark - Cunning setup. Thanks for the laugh!
Anything obvious in high vacuum is probably wrong.
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Re: Ion gun fusor

Post by Dan Tibbets »

Ion guns? Some have worked to produce ions and aim them into the Fusor. But, actually the gridded glow discharge fusor is essentially an ion gun. My limited understanding of ion guns are that they essentially produce ions by having an electron source (hot wire), which produces positive (mostly) ions through collisions of the resultant electrons in a neutral gas. The electrons are extracted through a pos. or neutral electrode, while the ions are focused and directed through ring electrodes and/ or magnetic fields. The normal glow discharge fusor is actually similar, except the ions are directed towards the center instead of some selected vector, while the electrons fly quickly to the grounded walls.
Additional efforts with microwaves, etc can refine this further.

Ionization is by far dominated by high speed collisions with energetic electrons. A hot wire emits coupons amounts of electrons (helps to have a high negative voltage potential on the hot wire). Ions are not produced well by this method because the mass difference compared to electrons, the inertia of deuterium is ~ 60 times that of an electron, and this is my understanding of why simple thermionic emission (hot electrode) works well for electrons, but not ions (directly).

The glow discharge fusors produce ions through this secondary ionization effect from collisions of high energy electrons. Once the ions are born, they are effected by the electric potential on the wire grid and accelerate to high energy. This leads to fusion. The problem is that only a small portion of the gas molecules are ionized. I have seen estimates that only one in ~ 70-100 deuterium are ionized at any one time. This means that most collisions -both Coulomb scattering collisions and fusion collisions are between ions and background neutral atoms/ molecules. It is mostly beam- target type fusion. Some even argue that most of the fusion is occurring between ions and neutral deuterium atoms/ molecules embedded in the walls of electrode wires. In attempts to increase the ion to neutral ratio dedicated ion guns may be employed in an effort to at least in theory, among other things, allow for beam- beam fusion collisions, while keeping the neutral gas to a minimum. Beam beam fusion benefits from the ions colliding at up to head on conditions, essentially resulting in collisions at twice the KE, thus moving up the fusion cross section slope. Even with ion guns it is difficult to maintain a high percentage of ions because of recombination reactions and neutrals produced from wall collisions/ groundings.

Ion gunned fusors may work significantly better, but the glow discharge approach is much easier and works well enough. The historically best fusor was an ion gun design by Hirsch back around 1970. Recent efforts to duplicate this at the Univ. of Wisconsin has given confusing results (they did not duplicate Hirsch's work, but designed their own version).

PS: Even with ion guns, the dominate limiting factor in a fusor is probably the grid transparency. At best an ion may make ~ 20 passes/ orbits before hitting the negative wire grid and losing it's KE. It may make multiple collisions with other ions and neutrals during this time frame resulting in a little fusion , but mostly scattering collisions which up scatters ions to the walls, or charge exchanges so that the now neutral atom reaches the walls (no longer sees the potential well. It is a very complex mix of interactions. Trying to reduce one problem tends to make others worse. The possibly best mix of compromises may be the Polywell.

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Chris Bradley
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Re: Ion gun fusor

Post by Chris Bradley »

The theory behind using an ion gun is that it improves efficiency and encourages beam-beam fusion because the fusor can be operated at lower pressures, so there should be proportionately fewer collisions, thus lower losses, with the background. To operate at a lower pressure than the 'glow discharge' regime requires another means to generate the ions and get them into the electric fields because, visibly, once below the glow discharge regime the number of ions available to participate in accelerations is highly diminished.

There is scant evidence that using ion guns significantly improves fusion reaction efficiency over a glow discharge fusor, and as it is so much easier to just build and run a discharge device then that's the way amateur fusors have gone, in general.



Dan DT wrote:
>actually the gridded glow discharge fusor is essentially an ion gun.
It is a 'discharge device'. Not sure you can qualify it as an 'ion gun' because any ions produced are held within the electric field that formed those ions in discharge. I don't think I'd call that an ion 'gun', but that's just a 'definition' thing, I guess.



>My limited understanding of ion guns are that they essentially produce ions by having an electron source (hot wire), which produces positive (mostly) ions through collisions of the resultant electrons in a neutral gas.
Discharge devices work by a cascade of ionised particles. Clearly, the grid is 'cold' before a discharge is initiated, so the conclusion is that it is not a 'hot-wire' effect. We've discussed different possibilities of the initiating ionisation here at length and I seem to recall the conclusion was 'doesn't matter what the ionisation source is, you'd never be able to prove it and clearly it happens!!'.

Also, if the ionisation was related to the physics around the grid then you might expect the beams to intersect the grid, whereas because the beams sit in between the grid apertures they strike the shell at a given point so the mechanism would appear to be liberation of sputtered electrons that would then cause ionisations local to the beam-shell intersections.



> This means that most collisions -both Coulomb scattering collisions and fusion collisions are between ions and background neutral atoms/ molecules. It is mostly beam- target type fusion. Some even argue that most of the fusion is occurring between ions and neutral deuterium atoms/ molecules embedded in the walls of electrode wires.
Or even fast neutral deuterium hitting neutral deuterium embedded in the shell - when fast ions meet slow neutrals, one of the common reactions is charge-exchange and an electron jumps off the slow molecule and attaches to the fast ion. This would then high-tail into the shell, free of electr field effects (but still carrying on the same trajectory) and more would follow it at the same point. (Personally speaking, I have come to believe this is the main fusion route, in a 'regular' DD fusor).
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Re: Ion gun fusor

Post by clarkmcc »

So if I am understanding right, there is a grid that is negatively biased and a deuterium supply input straight into the chamber and I don't need to build a couple of ion guns to produce fusion?
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Richard Hull
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Re: Ion gun fusor

Post by Richard Hull »

Over and over again we must say......NO ION GUN IS NEEDED.... to do fusion. It is a costly, time consuming and tough to adjust item that has shown no tendency to improve fusion over the easy and simple spherical, gas discharge fusor.

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
clarkmcc
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Re: Ion gun fusor

Post by clarkmcc »

Sorry! The ion gun forum section tipped me off and I thought they were required.
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Richard Hull
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Re: Ion gun fusor

Post by Richard Hull »

You might read the forum title explanation in its forum box. The forum exists only for ion gun discussions related to advanced designs and for work where folks wish to work at reduced fusor pressures in possible differentially pumped designs. This is an advanced fusor device. Thus far, no one has reported neutron numbers using any ion gun that tops the common simple fusor's best reported record.

Accomplished fusioneer researchers need a forum to discuss possible improvements and deuteron sources to advance upon what they have already mastered in the simple fusor device.

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
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Doug Coulter
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Re: Ion gun fusor

Post by Doug Coulter »

Well, I'm one of those microscopic fraction that DOES use ion guns, and I've designed a few with differing results on the actual fusion process. My ECR microwave ion source will take you to very low pressures (e-6 millibar!), but it's hard to get the ions out of at any real current. Pretty reliable as these things go, though.

What I do now is silly, but due to my strange setup, it works great and is even more reliable. I'm doing my main grid fusion in a 6" sidearm of a much larger tank (cylinder grid 1" dia, 2" length). I added another tiny fusor grid right out in the middle of the bigger (14" by 26") part, and it will "light off" at far lower pressures and voltages than the main grid. I can literally control/modulate the main grid current with it. If I set the pressure below what the main grid will "light off" at, I can tun its current on and off and modulate how much by varying what I put into the little one out in the middle of the tank quite nicely. AC also has interesting effects here, and it only takes 10-20kv on the little grid to light things off when 50kv on the main grid will not...since it has a better PxD for Paschen's law out there - a lot better. Watching what voltage the little power supply I have on grid #2 current limits at is a more precise and repeatable indication of conditions than my PKR-251 gas pressure gage...

I do find I can get higher Q and more neutrons both with an ion source, but it's not a huge multiplier (so far, I haven't tried everything I've thought of there yet).

No question it makes it easier to run the main fusor, as it gives you another knob to control the current with, fairly smoothly, so things aren't as touchy about precise gas pressure or other variables when this is in use.

But I can also run without it just fine too - mainly things are a lot pickier re getting all the other conditions just so.
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Re: Ion gun fusor

Post by ab0032 »

Doug Coulter, I find this an very interesting comment. I dont have a 6" sidearm, so this is not really applicable to me, but I wonder, why you make the sidearm grid your main grid, when the small grid at the center of the big chamber works so well and starts so easily.

Can you explain this a little more?
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Doug Coulter
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Re: Ion gun fusor

Post by Doug Coulter »

I kind of wish I *could* explain this better than I have, but that's why we get in the lab and "play".

In my case, it was just the natural thing - I had this big tank (BillF found it in a scrapyard for the weight of the stainless steel, where the workers had been using it as a urinal, talk about a cleaning job), and it had this sidearm that was 6" - the same size most of the people here were telling me to shoot for, so....it just worked out that way - my first work was just trying to duplicate what others here have done. Just basic science to make sure everyone is getting the same results, as if you're not, something is wrong somewhere.

At first, when I wasn't making many neutrons, the advantage of getting the detector in a lot closer was important, now it's not so much, and in fact, my big 3He tube now gets into pulse pileup issues and won't count correctly past about 1m neuts/second (and I've seen 6-10 million). I'm now using a fast-only neutron detector (Hornyak button and phototube) and an old B10-lined detector, mostly, since both are on the "numb" side and very stable, particularly the Hornyak, since it only sees fast neutrons and thus isn't shifted when a big moderator (me) moves around.

I keep talking about Paschen's law, which CarlW actually put me onto. Here's some decent reading on it: http://en.wikipedia.org/wiki/Paschen%27s_law

I can find some other links (on my site) if that's not enough. Some of them are huge downloads, though (whole books, scanned as images). At least one of those is all about designing gas tubes (thyratrons, indicators, and so forth).

The deal is, at higher pressures, an electron can't gain enough energy between collisions with atoms to ionize them, given a certain field, and there's a magic pressure times distance where things are "just so" at that dip in the chart at the link. We run fusors way over on the left hand side of that plot, so yes, this works quite well having an ion source that sees a long electrical path, and a main grid that sees a shorter one. This makes the P x D a lot bigger for the grid out in the middle of the big tank, since D is bigger. It's lighting off at 5-10kv when the main grid won't at 50!

This is in general, the issue with trying to make a good ion gun. You're usually thinking it's going to be smaller - but then it's hard to get it to "light off" without a fairly huge magnetic field, which makes D bigger by making the electrons spiral around the H field lines. And even then it's hard, and most sources need a rebuild inside 100 hours use - sputtering of the metal parts messes them up.

In fact, my homebrew feedthrough is quartz, shielded by a grounded copper pipe right up to the back of my grid, maybe 1/4" clearance, and this doesn't discharge, arc, or draw much current! This is only true while the pressure is such that we are at the left hand side of the Paschen curve, however, it wouldn't have worked for me at first as I dumbly tended to turn on power and then pump down. Now I get everything else just so before turning power on, and "walk" that up slowly to get into the good part of a run.

I wound up shielding the internal quartz as it's something that fast D ions can reduce into conductive silicon, and I like not to have to replace the glass so often.

This isn't to say I might not start using the grid in the big part of the tank as the main grid sometime.
But this isn't broken, so for now, I'm not fixing it. I think JonR gets more neutrons than I do, but is using a lot more volts to get there, so this meets my definition of "works really well". At least compared to other fusors, that is. I want gain!

Frankly, my own research is going in another direction anyway - trying to influence the ratio of the 3 possible DD reactions via controlling spin of the reactants, for which I'll be injecting some RF on the little grid or another antenna in there to try and accomplish, along with some magnets here and there....we'll see what I can find out with that. As far as I know, no one has even tried to alter the thermal DD reaction ratios by using non-thermal preparation of the reactants. It's not Nobel turf - conservation of spin in all nuclear reactions was figured out IIRC, in about the 1930's, but it's interesting that the entire fusion community has utterly ignored this very basic and known-true aspect of things (it also explains in a way why DT has a larger cross section - the odd number of nucleons makes conservation of net spin easier). So if/when it works, I expect snarky comments from physicists who have themselves missed this for most of a century.

You have to wonder - to the extent a fusor affects spin - it's in precisely the wrong way to get to fusion, or at least the DD->He reaction (spin 2 in if the D's are aligned, zero out for 4He - I want + and - one spin going in here, no neutrons, more gain). Now I've got to figure out how to even measure the ratios so I can tell if it works! I am thinking I might find out some things with a gamma spectrometer if I can get one going here...The 16 mev gamma from that one ought to sort of stand out...you'd think. And I'd bet that you can tell between the other two as well, since they don't put the same energy into the results. But that's getting into guessing, and the lab will tell all, when I get there.

The trouble there is most big gamma spec heads are NaI:Tl, which will activate under neutron flux, so no one here with one has been willing to look, so far. I have one I'm willing to put at risk, but no MCA at the moment - and they count many thousands of times faster on a fusor run than the PRA software/hardware can handle.

The last picture is what it looks like in the light of the quartz "stab in" bakeout heaters. When things are running, the poisser on the little grid doesn't look too great, but it has rays. The main grid has such sharp focus that things disappear if you get aligned right to them. Note, you're looking at this through 2 pieces of 3/8" thick borosilicate glass, followed by 1/2" of lead glass scrounged from a hospital rad room. Else you'd be fried by the X rays in use. I just like to use my mark 1 eyeballs, which have far better resolution than any camera (and which don't bloom on high contrast ratios).
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