Focus Fusion On Slashdot

[bcglorf]

I add fairly cold ions, at a distance from the wiffleball, which are then accelerated towards it...

This is like trying to get blood out of a turnip. When you learn to talk in more than a half-sentence at a time, when you have a coherent picture and are ready to explain it in detail, including how it relates to my analysis of collisional energy tranfer, then maybe we can talk. Until then, I've got more important things to do.

The physics is way over my head, but it seems to me that at least both you(Art) and Nebel understand it well enough to grasp Rider's objections. From my level, I understand from what I can follow of Rider's paper that he has laid a rather comprehensive proof that net power from pB11 is impossible under any assumptions he used for a Polywell like device.

My question relates to the apparent view of both Bussard and Nebel that as far as they understood, it could at least potentially produce net power, in at least so far as it was not yet a proven theoretical impossibility. Is it not possible for the Polywell to be functioning outside of the assumptions that Rider based his paper on, moving at least from the realm of impossible to nearly impossible?

I ask because I think the main thing is a question of if Polywell as a concept falls under the realm of 'proven' impossible, or very likely to turn out to be impossible. With pB11, the later really is kind of the best that is left anyways.

[Aero]

I add fairly cold ions, at a distance from the wiffleball, which are then accelerated towards it...

This is like trying to get blood out of a turnip. When you learn to talk in more than a half-sentence at a time, when you have a coherent picture and are ready to explain it in detail, including how it relates to my analysis of collisional energy tranfer, then maybe we can talk. Until then, I've got more important things to do.

The physics is way over my head, but it seems to me that at least both you(Art) and Nebel understand it well enough to grasp Rider's objections. From my level, I understand from what I can follow of Rider's paper that he has laid a rather comprehensive proof that net power from pB11 is impossible under any assumptions he used for a Polywell like device.

My question relates to the apparent view of both Bussard and Nebel that as far as they understood, it could at least potentially produce net power, in at least so far as it was not yet a proven theoretical impossibility. Is it not possible for the Polywell to be functioning outside of the assumptions that Rider based his paper on, moving at least from the realm of impossible to nearly impossible?

I ask because I think the main thing is a question of if Polywell as a concept falls under the realm of 'proven' impossible, or very likely to turn out to be impossible. With pB11, the later really is kind of the best that is left anyways.

The thing is, this is research. EMC2 is trying to find out what the truth is and that can't be done by looking at everything you already know, unless you already know the truth. Rider is a smart guy I'm sure, and so is Art, but they can not give absolute proof for the real world operation of a BFR.

Don't fall into the trap of believing the weather forecast for next month without checking with the weatherman again in a few weeks. He may learn something new.

Technical people frequently fail to take reality into consideration by believing that their simulations include everything that there is, when in fact the simulations only include what is known. If a thing is unknown, then we don't know what it is. Rider doesn't know, Art doesn't know, Rick doesn't know, no body knows. But EMC2 is doing the research and they will find out, hopefully.

It would be nice if they could tell us when they do find out but that is a different hope all together.

[Art Carlson]

I add fairly cold ions, at a distance from the wiffleball, which are then accelerated towards it...

This is like trying to get blood out of a turnip. When you learn to talk in more than a half-sentence at a time, when you have a coherent picture and are ready to explain it in detail, including how it relates to my analysis of collisional energy tranfer, then maybe we can talk. Until then, I've got more important things to do.

The physics is way over my head, but it seems to me that at least both you(Art) and Nebel understand it well enough to grasp Rider's objections. From my level, I understand from what I can follow of Rider's paper that he has laid a rather comprehensive proof that net power from pB11 is impossible under any assumptions he used for a Polywell like device.

My question relates to the apparent view of both Bussard and Nebel that as far as they understood, it could at least potentially produce net power, in at least so far as it was not yet a proven theoretical impossibility. Is it not possible for the Polywell to be functioning outside of the assumptions that Rider based his paper on, moving at least from the realm of impossible to nearly impossible?

I ask because I think the main thing is a question of if Polywell as a concept falls under the realm of 'proven' impossible, or very likely to turn out to be impossible. With pB11, the later really is kind of the best that is left anyways.

Rick Nebel apparently believes that (non-trivial) spatial non-uniformities can improve the result. He has never explained exactly how this is supposed to work, and for the life of me I can't figure out anyway it could work. Does that count as a loophole (impossible to plug, since we don't even know what it looks like) that makes the difference between impossible and nearly impossible?

[TallDave]

Art you are leaving out that while the particles are whizzing passed each other their "point" of maximum interaction is at "1/2 r" where the velocities are equal and opposite. Every where else (if it is a beam machine) the electrons are fast while the ions are slow and vice versa.

It's a heavy mantle to bear...

Here's what Rick said a while back:

Posted: Tue Apr 15, 2008 12:06 am

I had a long talk with Luis Chacon about the ion scattering a few weeks back. We concluded that for the Polywell, these issues were a red herring. The reason is that the densities are so high in the Polywell that you really don't need the ion focussing to be all that good. If you are running a gridded system where particles are lost every 20 passes or so, then it is an issue. For the Polywell, the electron recirculation fraction appears to be ~ 1e5 so the effective energy loss from the electrons is small. Consequently, you don't need huge focussing to get the density high.

This was interesting too:

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.

[bcglorf]

This is like trying to get blood out of a turnip. When you learn to talk in more than a half-sentence at a time, when you have a coherent picture and are ready to explain it in detail, including how it relates to my analysis of collisional energy tranfer, then maybe we can talk. Until then, I've got more important things to do.

The physics is way over my head, but it seems to me that at least both you(Art) and Nebel understand it well enough to grasp Rider's objections. From my level, I understand from what I can follow of Rider's paper that he has laid a rather comprehensive proof that net power from pB11 is impossible under any assumptions he used for a Polywell like device.

My question relates to the apparent view of both Bussard and Nebel that as far as they understood, it could at least potentially produce net power, in at least so far as it was not yet a proven theoretical impossibility. Is it not possible for the Polywell to be functioning outside of the assumptions that Rider based his paper on, moving at least from the realm of impossible to nearly impossible?

I ask because I think the main thing is a question of if Polywell as a concept falls under the realm of 'proven' impossible, or very likely to turn out to be impossible. With pB11, the later really is kind of the best that is left anyways.

Rick Nebel apparently believes that (non-trivial) spatial non-uniformities can improve the result. He has never explained exactly how this is supposed to work, and for the life of me I can't figure out anyway it could work. Does that count as a loophole (impossible to plug, since we don't even know what it looks like) that makes the difference between impossible and nearly impossible?

That's what I was trying to ask you.

Is there even an outside possibility Nebel's "(non-trivial) spatial non-uniformities" fall outside of the assumptions Rider used in his calculations that demonstrated net power pB11 was impossible?

I think all I'm looking for is the difference between having already been proven to be impossible versus an extreme long shot, I think that distinction is vital to the project's legitimacy.

[TallDave]

Is there even an outside possibility Nebel's "(non-trivial) spatial non-uniformities" fall outside of the assumptions Rider used in his calculations that demonstrated net power pB11 was impossible?

Well, obviously if you could get arbitrarily dense ion focusing at the core and an arbitrarily thin mantle region full of collisions you could do it.

If all of Rider's objections were correct, it's hard to see how any of the WB machines could have generated neutrons.

WB-8 may tell us whether Polywell reactors are unlikely, impossible or just around the corner.

[Art Carlson]

Is there even an outside possibility Nebel's "(non-trivial) spatial non-uniformities" fall outside of the assumptions Rider used in his calculations that demonstrated net power pB11 was impossible?

Well, obviously if you could get arbitrarily dense ion focusing at the core and an arbitrarily thin mantle region full of collisions you could do it.

What's obvious about that? Both fusion power density and bremsstrahlung power density are proportional to n^2, so any density profile effects cancel out (at least to lowest order).

If all of Rider's objections were correct, it's hard to see how any of the WB machines could have generated neutrons.

Why not? Rider calculates that a polywell will have a miniscule Q. The few neutrons produced also indicate a miniscule Q. It fits.

@bcglorf: Since you insist, I think that polywell research as a p-B11 project has no "legitimacy". It squeaks through as a legitimate D-T project, but just barely. I'm "very" confident that the cusp losses are too large to allow any kind of net power, but I'm "really, really, truly" confident that the bremsstrahlung calculation rules out p-B11.

[TallDave]

What's obvious about that? Both fusion power density and bremsstrahlung power density are proportional to n^2, so any density profile effects cancel out (at least to lowest order).

Not if the collisions are mostly in the mantle and we're talking about the density in the core.

Why not? Rider calculates that a polywell will have a miniscule Q. The few neutrons produced also indicate a miniscule Q. It fits.

Because the machines are also miniscule. There should be too much scattering and too much electron loss.

Will you hazard a prediction on WB-8/8.1? This is the first time anyone's actually tried to burn p-B11 in a machine of this kind.

[D Tibbets]

Is there even an outside possibility Nebel's "(non-trivial) spatial non-uniformities" fall outside of the assumptions Rider used in his calculations that demonstrated net power pB11 was impossible?

Well, obviously if you could get arbitrarily dense ion focusing at the core and an arbitrarily thin mantle region full of collisions you could do it.

What's obvious about that? Both fusion power density and bremsstrahlung power density are proportional to n^2, so any density profile effects cancel out (at least to lowest order).

If all of Rider's objections were correct, it's hard to see how any of the WB machines could have generated neutrons.

Why not? Rider calculates that a polywell will have a miniscule Q. The few neutrons produced also indicate a miniscule Q. It fits.

@bcglorf: Since you insist, I think that polywell research as a p-B11 project has no "legitimacy". It squeaks through as a legitimate D-T project, but just barely. I'm "very" confident that the cusp losses are too large to allow any kind of net power, but I'm "really, really, truly" confident that the bremsstrahlung calculation rules out p-B11.

I think that answers a question I was getting ready to ask- Which causes more bremsstrulung, a near encounter between a fast ion and a slow electron or a fast electron and a slow ion. Is it correct that the realitive velocities are the same so the bremstsrulung is the same?

But, this assumes a symetrical distribution of the realitive velocities and densities.
Because of the ions greater inertia they travel a little further out than most of the electrons and a little further inwards twoards the center (virtual anode), so there is some segragation of the two populations. I have no idea how much of an effect this would have(probably quite small). With a thermalized square potential well this would be the only contributor (?).

But, again, with an elliptical potential well resulting from a nonthermalized electron population (and ion population) the 1/2r region of maximal electron - ion cross velocities would be shifted and/ or brodened so that a smaller percentage of the electrons and ions would be colliding at their realitively maximal velocities to each other(?). Or, if my reasoning is reasonable the relative velocities of the ions and electrons are better represented by two logrithmic lines with opposite slopes intersecting rather than two linier lines with opposite slopes intersecting in the center. A distint difference between a non thermalized system and a thermalized system (maybe ).


Dan Tibbets

[alexjrgreen]

I can read your words, but I don't have any idea what you're talking about.

OK. All the electrons in a relatively thin shell. Then you add hot ions. They'll make an excursion outside by about lambda_D. The excursion on the inside will be determined by the mutual repulsion of the ions (since electrons are by assumption absent), which will also end up being lambda_D. Each ion will go back and forth from inside to out, and each time it passes through the electron layer it will make Coulomb collisions. If there is actually any physics behind what you are thinking, you will have to be less telegraphic in describing it.

I add fairly cold ions, at a distance from the wiffleball, which are then accelerated towards it...

This is like trying to get blood out of a turnip.

Art, despite the Halloween imagery I'm not your worst nightmare. I'm trying to help you.

When you learn to talk in more than a half-sentence at a time, when you have a coherent picture and are ready to explain it in detail, including how it relates to my analysis of collisional energy tranfer, then maybe we can talk. Until then, I've got more important things to do.

On the face of it, it seems quite reasonable to ask for more information, but it's disingenuous to pretend not to recognise a fusor...

In fact, it's quite consistent with the psychology of avoiding something you haven't thought through properly. You're completely comfortable with the scientific language here, and well used to students' questions, so when you resort to bluster you give yourself away.

[chrismb]

If all of Rider's objections were correct, it's hard to see how any of the WB machines could have generated neutrons.

Not really. You've got high voltages in there, so you'll be making fast neutrals of a sufficient rate to account for neutrons by irradiation of the interstitial deuterium in the walls of the chamber.

I have provided the calculation;

viewtopic.php?p=21955#21955

This is not to say that this is happening, as I emphasise there, but it has to be excluded before it is a well-founded 'demonstration' of fast-fast fusion.

If so, running p11B will mean you've just got yourself an expensive light bulb.

[TallDave]

Which causes more bremsstrulung, a near encounter between a fast ion and a slow electron or a fast electron and a slow ion.

http://en.wikipedia.org/wiki/Nuclear_fu ... ic_plasmas

You've got high voltages in there, so you'll be making fast neutrals of a sufficient rate to account for neutrons by irradiation of the interstitial deuterium in the walls of the chamber.

At a rate far higher than anything Farnsworth got at those drive levels, just as the device sweeps through beta=1? Very, very unlikely.

But WB-8 neutron counts should absolutely confirm or disprove fast-fast fusions, since the scaling is so different.

[Art Carlson]

At a rate far higher than anything Farnsworth got at those drive levels, just as the device sweeps through beta=1? Very, very unlikely.

"Far higher"? When we're talking about 3 neutrons, if Farnworth had just half the density, he would have measured a neutron rate of zero. If his detectors were not quite as good as Bussard's, he might even have been producing more neutrons.

Also, you're over-estimating the sophistication of Bussard's diagnostics. It was more like, I see a flash of radiation somewhere in my sweep . Aha! That must be the beta = 1 condition. I saw a whiffleball!

[TallDave]

Actually iirc we're talking about 4 sets of 3 neutrons, each occurring in the span of a quarter-millisecond. Coincidence is not likely.

There are also neutron counts from the other WB machines, and I'll have to check to be sure but I seem to recall there were other measurements to indicate beta=1 at the time the counts were recorded.

It's also reasonable to assume Rick's team got similar results, or the Navy wouldn't be funding WB-8/8.1 and explicitly requesting a Polywell reactor design that surpasses anything on the board for tokamaks.

People really need to read the available literature before shouting "only three neutrons!" I'm seeing this claim all over and it just isn't true.

I'm still curious what skeptics think we will see in WB-8/8.1. Beam-background scaling? Ions fleeing through the cusps?

[D Tibbets]

At a rate far higher than anything Farnsworth got at those drive levels, just as the device sweeps through beta=1? Very, very unlikely.

"Far higher"? When we're talking about 3 neutrons, if Farnworth had just half the density, he would have measured a neutron rate of zero. If his detectors were not quite as good as Bussard's, he might even have been producing more neutrons.

Also, you're over-estimating the sophistication of Bussard's diagnostics. It was more like, I see a flash of radiation somewhere in my sweep . Aha! That must be the beta = 1 condition. I saw a whiffleball!

While I understand much of Bussards efforts were starved for resources, I think assuming the paltry amount of information thus far reveiled, represents most of the data accumulated over ~ 20 years of effort is unreasonable. While semi-quantitative neutron measurements are reassureing, I suspect other types of data that might have been accumulated gives more information over what is going on.
Certainly the confidence intervals in WB 6 neutron counts is poor, though even one count per 0.25 milliseconds is thousands of times higher than gridded fusors. As a rough gestimate in a gridded glow discharge fusor, you might be lucky to get a few counts per second at similar drive voltages. The parameter that makes comparisons more difficult (for me) is the large difference in current conditions. . Also, the density and ion/neutral ratios are presumably different.
That is where the WB7 tests with the advantages of hindsight and at least some improvement in diagnostics presumably greatly improved on the confidence intervals and perhaps understanding of complex relationships. The closed peer review and subsequent continuing research suggests encouraging results, or foolishness, depending on viewpoint. The arguments of the viability of P-B11 fusion is in some ways premature. The successful burning and scaling of D-D would in itself be heroic.

An optimistic viewpoint in the WB 8.1 proposal for P-B11 testing would be that there is already enough confidence in the process that serious consideration of developing cleaner and direct converting p-B11 reactors for the first generation BFR's, as opposed to easier D-D reactors, is being considered.
A less optimistic viewpoint would be that they want to quickly rule out the possibility of P-B11 reactors , so that they (the Navy) can concentrate on and plan for D-D reactors. Or, that the research is driven by curiosity and not practical concerns. Or, the most pessimistic view (some would say the most realistic view) would be that they are all fools. In any case, however it turns out, the current arguments on this formum are interesting and educational.


Dan Tibbets