Design for preferential cusp field weakening, guiding Alphas

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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EricF
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Post by EricF »

MSimon wrote:
Aero wrote:
EricF wrote:Would the net power gain from a direct conversion device be drastically affected by increasing the radius for larger polywells ...
The short answer is "No," as MSimon has already said. The alphas are basically traveling through vacuum and encounter no resistance once they escape the polywell, via the cusps.
Their is another question here that has yet to be discussed though. Dr. Nebel posted that the alphas cycle through the polywell about 1000 times before exiting the cusps. I interpret this to say that the alphas turn around (reverse direction) about 1000 times before they find a cusp through which to exit. My question is, if my understanding is correct, then what is the physics that cause the high energy aplha particles to reverse direction?
The magnetic field.
heh, I pulled a Homer Simpson while mulling it over last night before posting my first question, and just realized that the alphas would still be ionized wouldnt they? They wouldnt spontaneosly snag up electrons to become neutral particles because of the energy difference in the electrons and the alpha particle? But then what prevents the negative well from just sucking the alpha particle back in like it does the pB11 fuel that is flying in and out of the fusor?

KitemanSA
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Post by KitemanSA »

EricF wrote: heh, I pulled a Homer Simpson while mulling it over last night before posting my first question, and just realized that the alphas would still be ionized wouldnt they? They wouldnt spontaneosly snag up electrons to become neutral particles because of the energy difference in the electrons and the alpha particle? But then what prevents the negative well from just sucking the alpha particle back in like it does the pB11 fuel that is flying in and out of the fusor?
The fuel ions do not fly "in and out" of the MaGrid, they stay inside or in hopefully negligable quantities fly out and impact the chamber walls. They (theoretically) don't have the energy to climb all the way out of the well, so the potential well holds the fuel ions in. The potential well is created by the electrons. The electrons fly in and out though even here the Wiffleball keeps the electrons preferentially in.

The fusion products (alphas) have plenty of energy to escape the potential well. If they are not reflected by the magnets (the wiffleball) they will rapidly escape the MaGrid too. Dr.N has stated that they anticipate that a full size machine will have strong enought magnets to reflect the fusion product sufficiently so that they will escape only thru the cusps.

EricF
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Post by EricF »

Thanks! Clears up something I thought I had clearly understood, I didn't realize that the ion fuel was being contained within the magrid, just the electrons. Actually, that makes a lot more sense now that I think about it.

Robthebob
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Post by Robthebob »

Aero wrote:
EricF wrote:Would the net power gain from a direct conversion device be drastically affected by increasing the radius for larger polywells ...
The short answer is "No," as MSimon has already said. The alphas are basically traveling through vacuum and encounter no resistance once they escape the polywell, via the cusps.
Their is another question here that has yet to be discussed though. Dr. Nebel posted that the alphas cycle through the polywell about 1000 times before exiting the cusps. I interpret this to say that the alphas turn around (reverse direction) about 1000 times before they find a cusp through which to exit. My question is, if my understanding is correct, then what is the physics that cause the high energy aplha particles to reverse direction?
Oh really now? 1000 times? What's the level of interaction between the He and the electrons in the WB and the fuel ions? Maybe 1000 rounds is short enough at the atom's scale that not much interaction would take place, which I thought there would be a problem with thermolization of the alpha particles.
Throwing my life away for this whole Fusion mess.

Aero
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Post by Aero »

Robthebob Maybe. That is another of the questions to discuss.
Aero

D Tibbets
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Post by D Tibbets »

Aero wrote:Yes Simon, it can only be the magnetic field.

Knowing that gives very little insight into the actual process. Do the aplhas oscillate radially? Do they develop an orbit within the Polywell? Do they randomly walk about? Do 100% of the alphas escape through the cusps? If so, why and if not, what percentage does not?

I'm sure that Dr. Nebel made enough test runs on WB-7 to have high confidence that most alphas exit via the cusps. But WB-7 is a low power machine generating relatively few alpha particles. Can we really say with confidence that WB-100 will eject alpha particles only via the cusps? I think that some of the particles will find another route to ground. It all depends on the interacting electric-magnet mechanism turning the alphas away from the magrid.

What was the fuel, hence the energy of the alpha particles in WB-7? Is the resulting combination of magnetic field strength and alpha particle energy the only combination that works? Or can we increase the magnetic field by a factor without effect on the exit path of the alpha particles?

There are a lot of questions to be looked at here and either answered or dismissed. But saying, "Mikie did it," isn't very helpful.

Actually WB7 presumably only used D-D fusion when they were trying to do fusion at all (instead of studing heluim plasmas), so the charged fusion (high energy) products would have been tritium, protons, and He3. The numbers of particles would have been very small so I don't know if anything could have been measured. Neutrons are relatively easy, the detector can be outside the vacuum vessel. Detectors for charged particles would need to be exposed to the harsh enviornment inside the machine. Theoretically, you could measure local heating of the vessel wall opposite a cusp, but rember that total fusion power was probably in the ballpark of a few milliwatts. Seperating this heating effect from the less energetic but presumably much more prevelent ions and elecetrons escaping throught the same cusps would be like finding a needle in a field of hay stacks. In WB 8 with a few watts of possible fusion heating, usefull measurements might be possible. Secondary x-rays, etc might also be measurable at those levels. Dr Nebels estimates presumably comes from models.

Dan Tibbets
To error is human... and I'm very human.

TallDave
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Post by TallDave »

But then what prevents the negative well from just sucking the alpha particle back in like it does the pB11 fuel that is flying in and out of the fusor?
The thing to keep in mind is the alphas have WAY more energy than the fuel ions. Alphas come out in MeVs, the well will be tens of KeV.

I was surprised, as I think a lot of people were, when Nebel told us the B fields at reactor strengths could corral alphas. I had assumed they would just fly right on out. We were actually pretty worried about what they would hit and break on their way out (see "alpha sputtering").
Oh really now? 1000 times? What's the level of interaction between the He and the electrons in the WB and the fuel ions? Maybe 1000 rounds is short enough at the atom's scale that not much interaction would take place, which I thought there would be a problem with thermolization of the alpha particles
Rick doesn't seem to think so. He said they exit with essentially full energy.

I'm guessing they don't hit the fuel ions because the collisionality is low at their rarefied speed. I'm assuming this is true because I know this is how annealing is supposed to work (low-speed ions outside the core are more likely to hit each other) but maybe someone more enlightened than I can cite a source.

EricF
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Post by EricF »

The thing to keep in mind is the alphas have WAY more energy than the fuel ions. Alphas come out in MeVs, the well will be tens of KeV.

I was surprised, as I think a lot of people were, when Nebel told us the B fields at reactor strengths could corral alphas. I had assumed they would just fly right on out. We were actually pretty worried about what they would hit and break on their way out (see "alpha sputtering").
Thanks Dave.

More questions, sorry folks :D

What are the chances that the alphas themselves are involved in further fusion collisions before exiting the MaGrid. They obviously (to me) have enough energy to achieve fusion. Is the energy differential of the alphas from the pB11 particles enough to prevent it from fusing with the primary fuel, leaving only 4He+4He fusion from time to time? (what would that even create? a quick google search did not turn up possible products of an 4He+4He fusion.)

zimdlg
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Post by zimdlg »

Just a thought,

Would the +ve charge on the alphas leaving via the cusps interfere with/repel the +ve ions that are entering the polywell because of their attraction to the –ve wiffleball ?
Probably not, but can anyone give me a definite yes/no ?
the future is near.

EricF
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Post by EricF »

I don't think any will be entering the wiffleball. They pop into play from the p+B11 reaction inside, make their way out via intertia, charge up the collection grid and then become free floating particles to be extracted. Not only that but I don't THINK they would have the energy left over to squeeze back through the cusps even if they did wander over in that direction.

zimdlg
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Post by zimdlg »

high velocity +ve alphas are created in the wiffleball from the PB11 fusion reaction, are contained briefly by the magnetic fields around the polywell/magrid, then find a cusp hole in the magnetic field and exit the polywell/magrid towards the vaccum chamber wall or direct energy conversion device.

The P+ and B11+ ions have to enter the polywell/magrid via the cusp holes in the magnetic field as they are attracted towards the -ve wiffleball.

My concern was that having a lot of +ve alphas exiting through the cusps would interfere/repel the +ve ions trying to get into the polywell/magrid.
the future is near.

KitemanSA
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Post by KitemanSA »

I presume you two are talking -wiffleball as the contained plasma? Remember, the wiffleball is surrounded by a +MaGrid. Outside the Magrid, the system looks +.

It is not yet set on HOW the p+ and B11+ enter the plasma. That seems to be one of the items being investigated in this set of contracts. In the WB6, it was ionize from neutral gas while inside the MaGrid.

zimdlg
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Post by zimdlg »

As far as I understand the wiffleball/contained plasma must be net -ve so that the P+ and B11+ ions are attracted to it and oscillate about the centre of the reactor until they collide and fuse.

As far as I know the objective is to fire the +ve ions are fired from ion guns outside the magrid. They are given enough velocity to overcome the repulsion from the magrid and so get past the magrid.

Once inside the magrid the +ve ions are no longer affected by the +ve magrid. They are only attracted to the -ve wiffleball and the magnetic field around the magrid also helps to contain them in the centre.
the future is near.

KitemanSA
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Post by KitemanSA »

zimdlg wrote: As far as I understand the wiffleball/contained plasma must be net -ve so that the P+ and B11+ ions are attracted to it and oscillate about the centre of the reactor until they collide and fuse.
True, but the Polywell outside the MaGrid is + to attract the electrons into the center to create the wiffle ball in the first place.
zimdlg also wrote:As far as I know the objective is to fire the +ve ions are fired from ion guns outside the magrid. They are given enough velocity to overcome the repulsion from the magrid and so get past the magrid. .
The main problem is that if you "fire" the ions from outside, they arrive at the edge of the well with some residual velocity. This in theory will allow them to fall down the well, climb up the other side and still have that residual velocity. If they have it and are pointed at cusp, they will exit the MaGrid with increasing velocity and impact the vacuum chamber with great energy, ALL of which is lost.
then zimdlg wrote:Once inside the magrid the +ve ions are no longer affected by the +ve magrid. They are only attracted to the -ve wiffleball and the magnetic field around the magrid also helps to contain them in the centre.
That is the hope. So far I don't know if they have made it work like that.

Any info on that? anyone?

zimdlg
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Post by zimdlg »

True, but the Polywell outside the MaGrid is + to attract the electrons into the center to create the wiffle ball in the first place.
I agree, an electron outside the magrid must see the magrid as +ve overall so that it gets pulled towards the magrid. Then the magnetic fields will channel them into the interior of the magrid.
The main problem is that if you "fire" the ions from outside, they arrive at the edge of the well with some residual velocity. This in theory will allow them to fall down the well, climb up the other side and still have that residual velocity. If they have it and are pointed at cusp, they will exit the MaGrid with increasing velocity and impact the vacuum chamber with great energy, ALL of which is lost.
Can't this problem be solved by giving the ions just enough energy and no more to get past the magrid then the -ve charge on the wiffleball will hold the ions inside the magrid?

Once the +ve ions are inside the +ve magrid they feel equal repulsive forces from all directions so the forces cancel each other out and the ions feel no net repulsion from the magrid, not so?
the future is near.

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