The Bussard Fusion Reactor for Dummies.

Discuss how polywell fusion works; share theoretical questions and answers.

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MSimon
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Post by MSimon »

Billy Catringer wrote:
chrismb wrote:Both experiment and calculation.

Let me know if you need any more than;

viewtopic.php?p=14072#14072

Thanks, Chris. I'll read it and think as best I can about it.
Take a look at my response to chrismb in the very next comment.
Engineering is the art of making what you want from what you can get at a profit.

Billy Catringer
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Post by Billy Catringer »

chrismb wrote:The evidence and discussions are scattered around the fusor.net forum. Basically, if you plot out the reactivity curve for DD on a linear scale, you'll see it pretty much asymptotes to zero below 10keV, collision energy. If you then plot out actual neutron emissions from a fusor, that same shape of curve repeats but comes off zero at 20kV drive voltage, which shows it's fast-neutral. If it were working fast-fast, then it would come off the line at 2.5kV drive voltage. The experimental results are plotted out for various experiments, and they come up with pretty much the same numbers.

You might argue that there are some fast-fast reactions in the data uncertainty of the results, but if you think about the transit times for deuterons and electrons, bearing in mind that this is a gas discharge device, i.e. like a neon light, the electrons just peel off the cathode central grid and head straight for the anode (unlike the ions that reciprocate across the centre). This takes 1/60th of the time it would take a deuteron, but at the same time there must always be as many electrons as there are ions (there is no polarisation in the device), so the consumed current of the device, which is typically a couple of 10's of mA, will be at least 120 times the deuteron current travelling into the centre of the device. Do the back-of-envelope sums on ion current beams of 100uA or so and you get a tiny reaction rate, units to 10's per second. Do the same on a beam passing through a neutral background and it pretty much adds up to the 100,000's of neutron's/sec, and this is what is actually seen.

This is a broad-brush treatment and interpretation of the data, but I feel these issues are generally agreed upon, notwithstanding a few remaining debatable caveats.

Okay, I think I am following this. It basically boils down to the voltages required to get fusion reactions inside a fusor. Were the reactions occurring where expected, in the center of the machine, we would see high reaction rates at 2.5kv. That is NOT what see in the fusor. We do not see high reaction rates until the voltage is somewhere between 10-20kv.

Tom Ligon reports that Doctor Bussard's team saw significant fusion rates at about 5kv. Clearly then, we are dealing with two very different machines. Also, we do not have any experimental evidence that tells us where inside the BFR these reactions are occurring. This thanks to the embargo imposed by the US Navy.

Let's say that the BFR breaks net power no matter where the fusion occurs. If the reactions are occurring between the walls of the vacuum chamber and the magrid instead of at the center of the magrid, we have some serious mechanical design issues to deal with. What I am trying to model right now would not be remotely workable. We need a "Plan B". We also need plans "C" through "D" and hope one of them works before we get to Plan F.

There is another worry that has been nagging at the back of my mind and that has to do with the Navy's embargo. No, I shan't discuss it here. Those with experience will know what that worry is, nuff said.

chrismb
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Post by chrismb »

Billy Catringer wrote:Tom Ligon reports that Doctor Bussard's team saw significant fusion rates at about 5kv.
I've not seen any sort of formalised material of that nature. I'm not sure there has ever been 'significant' fusion rates, though I suppose amongst the sweet few neutron counts that have been seen a single count would be 'significant'. Let me know what the data actually is at 5kV and we can visit that and discuss it some more.

You should also bear in mind that, just as you raised to start with, fast ions exiting the central region of the device may fuse with embedded materials in the chamber walls/structures. This is quite common in a fusor. It can confuse the data, particularly if you are presuming the reaction is at the centre and you multiply up your neutron count accordingly to a 1/r^2 relationship, yet the neutron counter is actually close to the chamber wall.

Billy Catringer
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Post by Billy Catringer »

chrismb wrote:I've not seen any sort of formalised material of that nature. I'm not sure there has ever been 'significant' fusion rates, though I suppose amongst the sweet few neutron counts that have been seen a single count would be 'significant'. Let me know what the data actually is at 5kV and we can visit that and discuss it some more.

A completely fair demand under the circumstances.
chrismb wrote:You should also bear in mind that, just as you raised to start with, fast ions exiting the central region of the device may fuse with embedded materials in the chamber walls/structures. This is quite common in a fusor. It can confuse the data, particularly if you are presuming the reaction is at the centre and you multiply up your neutron count accordingly to a 1/r^2 relationship, yet the neutron counter is actually close to the chamber wall.

Duly noted, but notice that I accept your word on the matter.

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Post by chrismb »

There is a paper on this from Uni Wisconsin that shows most DD reactions are outside the grid and accounts for fusion with embedded nucleii. I think they came to the conclusion that there *were some* fast-fast reactions, but I can't find the paper to remind myself of their exact conclusions.

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Post by D Tibbets »

chrismb wrote:You should also bear in mind that, just as you raised to start with, fast ions exiting the central region of the device may fuse with embedded materials in the chamber walls/structures. This is quite common in a fusor. It can confuse the data, particularly if you are presuming the reaction is at the centre and you multiply up your neutron count accordingly to a 1/r^2 relationship, yet the neutron counter is actually close to the chamber wall.
The 'fast ions' reaching the walls are hopefully rare. This would be above their initial kinetic energy. The ions are created by ionization within the chamber, are acellerated twoards the central cathode, and delcllerarted as they climb back up to their initial position. The only ions that could reach the walls are upscattered ions. There is probably a wide variation possible with different setups, but I'm guessing most ion- structure collisions are with the central cathode wires. This is why having a virtual cathode as in the polywell is supposed to be much more efficient. What I am uncertain about is ions created and injected by an ion gun, they have some kinetic energy at birth, so as the pass through the center and out the otherside, their 'start position' would be higher than the ion gun, thus possibly hitting the walls, or at least the ion gun- if they can get to it (Debye sheaths and all that).

As far as neutrons originating in the center or at the walls, the distance to the detector would be the same on average. One neutron may be created at the near wall, but another may be created on the opposite wall, so the distance would average out to be the same as if they were created at the center of the machine. Off the top of my head, I'm guessing that to differentiate them you would need two detectors; one looking at the center, the other looking at a lateral edge, with lots of shielding (tube) so it is not hit by centrally produced neutrons.


Dan Tibbets
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Billy Catringer
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Post by Billy Catringer »

chrismb wrote:There is a paper on this from Uni Wisconsin that shows most DD reactions are outside the grid and accounts for fusion with embedded nucleii. I think they came to the conclusion that there *were some* fast-fast reactions, but I can't find the paper to remind myself of their exact conclusions.

That's okay, I do NOT doubt your veracity on this issue. I know that the odds against this thing are fairly large, but I also see some real promise in it if it DOES work. As I see it, the BFR can be built at reasonable cost and poses far fewer operational problems than any other fusion program that i have been able to examine. This does not mean that I assume that it will work, only that I am HOPING it will work.

chrismb
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Post by chrismb »

D Tibbets wrote:One neutron may be created at the near wall, but another may be created on the opposite wall, so the distance would average out to be the same as if they were created at the center of the machine.
a) you can't do a linear average on a 1/n^2 function
b) you can't get a realistic average from 3 data points -especially if it a non-linear function!!
D Tibbets wrote:Off the top of my head, I'm guessing that to differentiate them you would need two detectors; one looking at the center, the other looking at a lateral edge, with lots of shielding (tube) so it is not hit by centrally produced neutrons.
Yes. Anything like that. Just readings from two or three poisitions at differential radii would do it and you could solve simultaneously for grouped regions of neutron emissions.

But the bloomin' big problem here is that no Polywell has ever put out a consistent stream of neutrons ever to be able to do such an objective set of measurements with. If it did, then it would being to become 'an experiment'.

D Tibbets
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Post by D Tibbets »

chrismb wrote:Both experiment and calculation.

Let me know if you need any more than;

viewtopic.php?p=14072#14072
At the risk of being anoying, I need to point out that a perfect foucus is only possibly theoretically. Actually, all that is needed is a reasonable convergence towards a central region, as per Dr Nebel. There has been a study in Japan that measured the fusion locations and most fusions were occuring in a zone both inside and outside the cathode in a gridded fusor, but definity orented towards the center. And, to my knowlege, there is no encompasing theory that explaines the expermental findings in a gridded fusor (neutron counts in the Hirsch Fusor), let alone a Polywell.


Dan Tibbets
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Post by KitemanSA »

D Tibbets wrote: The 'fast ions' reaching the walls are hopefully rare. This would be above their initial kinetic energy.
I should think they were fairly common. Wouldn't fast ions be akin to upscattered electrons, without the wiffleball there to keep them in?

chrismb
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Post by chrismb »

D Tibbets wrote:And, to my knowlege, there is no encompasing theory that explaines the expermental findings in a gridded fusor (neutron counts in the Hirsch Fusor)
An elementary calculation of fast-ions fusing with 'lab-stationary' nucleii matches the neutron-versus-drive voltage for all amateur fusors that I have seen. This fits a simple calculation on ion densities. It is not just spatial density, it is also the 'temporal density' as the ions don't all get to the centre at the same time. They have to get pretty damned close to fuse! Lining them up to pass through a narrow region of space just isn't good enough, they have to 'dwell' long enough in a given space to have any chance of fusing.

I think I know the paper you are referring to and it covers D+3He. Differing fuels appear to behave differently, some (like 3He) tend to prefer to adsorb onto the grid so that a larger fraction of the fusions are on the grid itself, whereas DD tends to mostly occur outside the grid (depending on how big the grid is, of course).

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Post by TallDave »

It still baffles me every time I see a comment here on why ITER won't work. Isn't this a Polywell site? Does it make Polywell more likely to work if ITER won't?
Yes, in a practical sense. If everyone admits ITER doesn't work as a commercial power source, that frees up vast amount of fusion research resources relative to what we need for Polywell.
I don't see Art saying at all that tests on the Polywell shouldn't happen.
Well, to be fair, he did sort of imply that at one point, saying we had "sweet talked" someone into funding WB-7. But he's entitled to his prejudices, just as we are re ITER. That's what makes free societies work. He's also a bright guy who's made some good arguments and helped us (well, me at least) extend our understanding.

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Post by D Tibbets »

chrismb wrote:
a) you can't do a linear average on a 1/n^2 function
b) you can't get a realistic average from 3 data points -especially if it a non-linear function!!
...
But the bloomin' big problem here is that no Polywell has ever put out a consistent stream of neutrons ever to be able to do such an objective set of measurements with. If it did, then it would being to become 'an experiment'.[/quote]

a) Oh well... Using a couple of example positions on a circle ( near pole, opposite pole and equatorial ), I got results suggestive that the counts, if originating on the surface would be ~twice the counts if originating from the center. A central zone of some smaller diameter than the circle would give intermediate results. I'm guessing if I could derive a formula it would be more acurate, or if I added more points on the near and far hemispheres the difference would increase some more, but not much. Sound reasonable?

b). I admit that 3 counts (confidence range 2-4 counts?) is very little to work with.

Wild speculation- I wonder if Dr Nebel's team managed (or plan) to obtain longer runs, and/or stronger B fields to flesh out the counts, and if so did they obtain differentiated neutron counts?

Dan Tibbets
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