The problem with ion convergence

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

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rnebel
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Post by rnebel »

Art:

OK. I'm on the same page with you. Actually, I believe that Tom Dolan brought this up some time ago. I don't have access to Sci-Search but I believe that Tom wrote a short comment about this topic in response to the Fusion Technology paper (and I think that either Bussard or Krall might have in turn written a response) in the early 90s or late 80s. If someone has access to Sci-Search, perhaps they can look it up.

My response to this is two-fold. If you find that you need more focussing, then you can reduce the non-spherocity by going to a higher order system such as a dodecahedron. Secondly, the the problem arises because you are forming the ions in a region with significant magnetic field. If that proves to be a problem, then don't do that.

drmike
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Post by drmike »

The idea of forced radial currents comes from wakefield acceleration. Forcing radial waves in the system will force radial currents. I would hope that it's an efficiency enhancement because it won't solve a zeroth or first order instability problem.

Art Carlson
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Post by Art Carlson »

rnebel wrote:OK. I'm on the same page with you. Actually, I believe that Tom Dolan brought this up some time ago. I don't have access to Sci-Search but I believe that Tom wrote a short comment about this topic in response to the Fusion Technology paper (and I think that either Bussard or Krall might have in turn written a response) in the early 90s or late 80s. If someone has access to Sci-Search, perhaps they can look it up.
Thanks for your patience. I realize that some, perhaps all of my objections will have been discussed somewhere at some time, and that I have not done any serious literature search. I'm looking forward to seeing these comments.
My response to this is two-fold. If you find that you need more focussing, then you can reduce the non-spherocity by going to a higher order system such as a dodecahedron. Secondly, the the problem arises because you are forming the ions in a region with significant magnetic field. If that proves to be a problem, then don't do that.
(1) I would expect that you would have to quadrupol the number of faces in order to halve the average angle relative to a spherical surface. To get significant convergence, the average angle should be less than the inverse of the number of transits before fusion. Since (I believe - correct me if I'm wrong) something like 10,000 transits are required before a fusion is expected, you will never get to that angle by increasing the order of the polyhedron. It's not enough to make the potential surfaces a little a bit more spherical. You need to make them orders of magnitude smoother.
(2) Not sure what you're getting at here. The magnetic field determines the shape of the equipotential surfaces by its effect on the electrons. The ions see these lumpy equipotential surfaces no matter where they are born.

Roger
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Post by Roger »

Art Carlson wrote:I realize that some, perhaps all of my objections will have been discussed somewhere at some time,
Art, I for one appreciate your posts, your descriptive wording has enabled me to be better able to "Visualize" some concepts previously discussed with less descriptive eloquence.

Art Carlson wrote: (1) If the electrostatic well is not spherically symmetrical, the initial in-fall will not be radial, and the convergence on the center will get worse with every bounce,
Considering my lackings, I was actually following this. LOL.

If the surface of the well, the equipotential surfaces are not spherical, does this mean that the well is non spherical when looking at each equipotential surface ? To the core of the well ?

I'm not seeing an onion's core being exactly reflective of the outer layers. Would the well core equipotential surface average out the "lumpiness" of the surface equipotential layer ?

Thanks to all in advance.
I like the p-B11 resonance peak at 50 KV acceleration. In2 years we'll know.

Solo
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Post by Solo »

Two thoughts: the electrostatic potential will probably diverge some from the magnetic field lines. I think the driving force for this is the kinetic energy stored in the radial (machine radius, along a cusp axis) motion of the electrons. Assuming that the saddle point in the e-field is about 80% of the magrid potential, the electrons will have several dozen keV energy passing through the cusp (coming into the machine from the e-gun). As they move into the potential well, they loose energy until they have very little (10% of magrid potential or less, I think). The electron KE + PE(from the well) will be the same along each field line, but the PE (in other words, the potential) can vary along the field line.

Second thought: suppose the equipotential surface is a 'spikey sphere' like the b-field. The gradient is perpendicular to the field lines, so that an ion not on the spike will experience a force tilted toward the axis of the spike. This seems like it would concentrate the ion motion into beams rather than cause them to scatter randomly (electrostatic lens?). But maybe the angular momentum from that would cause problems, I don't know.

Roger
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Post by Roger »

Solo wrote: This seems like it would concentrate the ion motion into beams rather than cause them to scatter randomly (electrostatic lens?). But maybe the angular momentum from that would cause problems, I don't know.
What do ions "see best" ? The core of the potential well... no ? Are the spikes areas of lesser effect or acceleration, compared to the well core ? I think yes.
I like the p-B11 resonance peak at 50 KV acceleration. In2 years we'll know.

TallDave
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Post by TallDave »

A lot depends on the Debye length and how exactly the electrons are arranged. As Nebel suggested, that's probably a fairly complex and dynamic picture.

I'm very skeptical that any amount of theorizing or simulation can be very useful, absent a lot of experimental data to base them on.

For what these cost and may deliver, it's probably worth spending the money to find out experimentally, even if there is a lot of uncertainty about whether they'll be useful in the end.

drmike
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Post by drmike »

You need experiments to guide models. It's too complex to be purely empirical. If we can get a fluid model that's close enough for scaling in a regime of factors of 10 it will enable engineering to proceed.

A fluid picture is different than a particle distribution model. In a fluid I don't care about where the particles go, I care about the whole system. If I can describe the fluid as a radial motion with little or no transverse motion then the whole thing should be stable. That doesn't mean ions don't move transversely - it just means on average I can ignore that motion.

It will be interesting to see what the data says. If it's on average radial, then fusion is possible. If the ion current spins around and doesn't move radially, the system won't have much fusion reactions. In any case, a model that explains the data to some level will help with understanding if such problems are fixable or not. With luck there's no fixing - it will just work!

Tom Ligon
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Post by Tom Ligon »

I've never felt anything but experiment was going to be satisfactory on these machines. I watched Dr. Bussard do enough calculations on blackboards and whiteboards to know that there were some generalizations, approximations, and assumptions going on. It was patently obvious no whiteboard has the processing power needed to fully model these things. The EIXL code ran a better model, but even that was only about 1.5 dimensions. Running earlier theories, I saw the results of one 2D supercomputer study: looked great but was it correct?

While Langmuir probes and microwave beams and lasers and other neat toys are certainly invaluable in figuring out what they are really doing, the only diagnostic I ultimately trust regarding the ability of these devices to produce fusion is fusion. The few coughs of fusion produced by WB6 were, for me, sufficient cause to become excited. The improvment in performance caused by the geometry changes made absolute sense. Knowing how crude the fuel metering was on that machine, and suspecting gross fuel dilution and non-optimal birth locations for the ions, I thought it was a miracle they found the sweet spot at all. That it could work with such a crude setup is, to me, extremely promising.

I'm following the theoretical arguments with some amusement, because I know physics will not be fooled by our poor understanding of what is going on in magrid machines. Dr. Park, Mike Skillicorn, and the brothers Wray are my heros ... they've built WB-7, have it "running like a top", and will uncover the truth, to the extent such a small machine can produce.

Art Carlson
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Post by Art Carlson »

You guys are amazing. The theory of this machine is really weak, but the experimental data is non-existent. The only positive data you have is a number of neutrons that I can count on my fingers. A reactor will have to produce around 10^30 neutrons. ZETA produced millions of neutrons per pulse in the 50's. People got excited about this, but it turned out they were irrelevant because they were not coming from the plasma. I haven't even seen a calculation telling me that 2 neutrons per pulse from a polywell is a lot. Maybe we should be expecting thousands, but the experiments are a dismal failure.

But we digress. The purpose of this thread was to discuss whether there is any reason at all to expect ion convergence. Or have you decided if theory is against you you'll just circle up your eight neutrons and try to wait out the Indians? Some parts of polywell theory are complex, perhaps hopelessly so. This isn't one of them.

Does anyone see any reason to expect the equi-potential surfaces to have irregularities in the radius of less than the order of 10^-1 to 10^-2? Does anyone see any reason to expect significant ion convergence if the irregularities are greater than 10-4 to 10^-3?

scareduck
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Post by scareduck »

Art Carlson wrote:You guys are amazing. The theory of this machine is really weak, but the experimental data is non-existent. The only positive data you have is a number of neutrons that I can count on my fingers. A reactor will have to produce around 10^30 neutrons. ZETA produced millions of neutrons per pulse in the 50's. People got excited about this, but it turned out they were irrelevant because they were not coming from the plasma. I haven't even seen a calculation telling me that 2 neutrons per pulse from a polywell is a lot. Maybe we should be expecting thousands, but the experiments are a dismal failure.
ZETA's false neutron readings were due to plasma instability. Are you saying that's the case here? And if so, on what grounds?

Art Carlson
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Post by Art Carlson »

scareduck wrote:
Art Carlson wrote:You guys are amazing. The theory of this machine is really weak, but the experimental data is non-existent. The only positive data you have is a number of neutrons that I can count on my fingers. A reactor will have to produce around 10^30 neutrons. ZETA produced millions of neutrons per pulse in the 50's. People got excited about this, but it turned out they were irrelevant because they were not coming from the plasma. I haven't even seen a calculation telling me that 2 neutrons per pulse from a polywell is a lot. Maybe we should be expecting thousands, but the experiments are a dismal failure.
ZETA's false neutron readings were due to plasma instability. Are you saying that's the case here? And if so, on what grounds?
I'm saying when you have tens of kV hanging around, there are a lot of ways to produce neutrons besides thermonuclear fusion. A plasma instability seems unlikely in this case, but what happens to a deuterium ion that happens to form near the outer wall? It will be accelerated to 10 kV or more and slam into the maggrid, which is likely to be saturated with deuterium. Bang! You get a fusion neutron with no plasma at all. Since deuterium always comes with a small fraction of tritium, you could also get D-T fusion from this mechanism (requiring less voltage). I don't know if this is actually happening. Has it been ruled out? Did anybody even try to rule it out?

TallDave
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Post by TallDave »

It will be accelerated to 10 kV or more and slam into the maggrid, which is likely to be saturated with deuterium.
That seems unlikely, with the Magrid having a large positive charge that would decelerate an approaching ion, in addition to the well pointing the other way. Also, very few ions should get out this far anyway. It seems overwhelmingly unlikely that the neutron counts could have come from this. Besides, its not like neutron counts from fusors are a new phenomenon.

Ligon has gone on in a couple posts about the various measures employed to rule out false positives.
The only positive data you have is a number of neutrons that I can count on my fingers.
Heh, that seems a little unfair. Also, we don't know what WB-7 has produced.
Maybe we should be expecting thousands, but the experiments are a dismal failure.
Well in some sense, all fusion experiments to date are dismal failures; despite the billions spent, not one has produced any meaningful net power at a reasonable cost, or even a plausible path to such. Polywell has the advantage of failing at orders of magnitude less cost than the mainstream failures.

rnebel
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Post by rnebel »

Let me see if I can summarize this. The basic claim is that the electric potentials will take on the shape of the magnetic field structure. This will give electrostatic potentials that are defocusing for the ions and they add angular momentum to the ions with each pass (i.e. we have an ion optics problem). Is that a fair characterization? If this is a problem, I’m suggesting the following mitigation strategies:
1. Look at higher order, more spherical systems like dodecahedrons which will move the aberrations to the edge. Regularity requires that aberrations go like r**m where m is related to the order of the polywell (it’s 3 for a truncated cube, and higher for other systems). This may not cure the problem, but it should help.
2. Ion collisionality will mitigate some of the accumulation of angular momentum. Ions spend most of their time near their turning points and this is where they are the slowest and the most collisional. These collisions will take angular momentum out of the ions. This should help too
3. Introduce the ions you want to focus at a radius that’s a little inside the nonspherical region. Pellets are one possibility. This will require you to apply a higher potential to the coils to get the same ion energy, but it should improve the focusing.
4. As I pointed out in another thread, the notion that flux surfaces will become equipotential surfaces is incorrect for inertial electrostatic confinement. That only happens if you allow the plasma to thermalize, which we don’t intend to do. These are driven systems and the finite electron inertia will allow you to impose potential gradients along field lines.
Now let’s suppose that mitigation factors 1-3 don’t sufficiently improve the focusing. How do you take advantage of number 4? The answer is probably in the electron optics. Although this hasn’t been explored in Polywells (at least to my knowledge) there is an analogous problem that crops up in gridded systems. Grid wires tend to add angular momentum to ions or electrons which also leads to defocusing. We had this problem when we were doing the POPS studies (see previous references). We were trying to make harmonic oscillator potentials and we kept getting potential wells with the electrons clustered near the edge. What we learned was that we could reduce the angular momentum and improve the focusing by using a two-grid system where the inner grid had a retarded potential compared to the outer grid. Ron Moses had studied this effect 10 years earlier (Ron did his thesis work on electron optics) and had demonstrated that a properly aligned two grid system with a retarded inner grid could produce stable electron orbits and vastly improve the effective grid transparency. I believe that the MIT people are using similar techniques for their ion-based IECs. While this technique was used to modify radial profiles, similar things could be used to reduce aberrations in the virtual cathode.

Tom Ligon
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Post by Tom Ligon »

Art,

I hear you ... all we've seen from the earlier work is a wiff of fusion. I'm content, at the moment, to await peer-reviewed results to see if the scent is now stronger, and if the dogs know which way to track.

The point has already been answered that a magrid charged to attract electrons will not attract the sort of ions we expect to form. I allow the possibility of negative ions, but doubt they're much of a factor in such a high-energy environment. Driving ions to the outer wall is certainly a possibility, although in that case I would expect reactions during the big discharges that terminated the WB6 runs, rather than immediately preceeding the discharge and while the well was deep. What reactions would you get in stainless steel at 10 keV? Would the energy of the neutrons be distinguishable from DD fusion? I've recommended methods other than electronic thermalized neutron counters be employed. Bicron 720 comes to mind (fast-neutron specific, and with some neutron energy resolution capability), with a backup of bubble dosimeters (require fast neutrons to make bubbles).

In my test runs, I deliberately tried to saturate the machines with deuterium, but the RGA said I was not having much success ... the gas that blows off in the bright glows remains mostly hydrogen, although you do start to see some HD peaks with continued use. But we've noticed the present EMC2 website photo shows a helium plasma ... I have to wonder if they're deliberately trying to avoid deuterium loading.

I would add that the extensive runs of WB-4, essentially WB-6 with a bad geometry, should have made the same apparent high fusion results if the effect were ions going to the walls. The reports say it produced fusion, but at a rate about three orders of magnitude down from WB-6. The evidence for either is relatively thin, but it does suggest WB-6 was doing something unique and worth looking in to.

I set up the counters to not trigger on electric arcs (I shielded the entire setup strongly and super-filtered the power lines). The result was a background count rate of something like 3 counts per minute. They would not respond to a 20 kV cap-discharge arc right beside the counter. I understand they've now taken this a step or two further. I do understand any concerns about the possibility of false counts due to this cause ... Farnsworth reportedly faked results for one early demonstration before Dr. Hirsch showed an easier way to build fusors that actually worked like a charm.

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