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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Aero
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Design for preferential cusp field weakening, guiding Alphas

Post by Aero »

Would it be helpful, or even possible to modify the number of "turns" in the Magrid coils near one of the cusps so that Alpha particles would preferentially exit the polywell at that cusp? I'm thinking that two wires (multiple wires) could be deliberately shorted together in the vicinity of the selected cusp, in all coils bordering the selected cusp. Reducing the number of "turns" should reduce the strength of the magnetic field in that area. The reduced field strength should open the cusp allowing Alpha particles to preferentially escape through that cusp. Of course the field must remain strong enough for containment. My thought is to strengthen the B field overall beyond what is necessary for good containment, then weaken the field at the selected cusp to the level needed for good containment.

Why? I am thinking that direct conversion would be much easier if the major conversion system could be localized to a single cusp with only a simple (single energy) system installed at the other cusps.

Then again, it might do just as well to increase the coil separation at the selected cusp for the same effect.

What?
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D Tibbets
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Post by D Tibbets »

I can see your reasoning from an ion stand point. The fuel ions are contained well by the electrostatic field, so hopefully do not see the 'cusp holes'. The high speed fusion product ions (alpha particles) are not electrostatically contained so they would have a significanly greater chance to escape through a larger cusp hole. The problem would seem to be that the electrons are similar to the fusion ions. They are magnetically contained, not electrostatically contained. They also would see the larger hole and electron containment time would suffer. It might also change the shape of the semi spherical Wiffleball. I have no idea if it is possible that some combination of local electrostatic fields/ grids combined with the larger cusp hole would work.


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Post by MSimon »

POPS tuned to He.
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Post by Aero »

MSimon wrote:POPS tuned to He.
I already said it, but I'll say it again for you.

"What?"
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Post by MSimon »

Aero wrote:
MSimon wrote:POPS tuned to He.
I already said it, but I'll say it again for you.

"What?"
Do I have to explain everything? Sheesh! :-)

POPS - on the sidebar at IEC Fusion if you are not conversant - can be used to preferentially impart a radial velocity to He4 particles making the cusps leakier for He. In a pB11 system. In a D-D system with He and D frequencies so close together you have to figure out some other way. Dr. N. in one of his sporadic appearances here mentioned it. Being a mere engineer I defer to the physicists re: the physics. And you know - there appears to be some disagreement.

Engineering is the art of making what you want from what you can get at a profit.
Engineering is the art of making what you want from what you can get at a profit.

KitemanSA
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Post by KitemanSA »

D Tibbets wrote:I can see your reasoning from an ion stand point. The fuel ions are contained well by the electrostatic field, so hopefully do not see the 'cusp holes'. The high speed fusion product ions (alpha particles) are not electrostatically contained so they would have a significanly greater chance to escape through a larger cusp hole. The problem would seem to be that the electrons are similar to the fusion ions. They are magnetically contained, not electrostatically contained. They also would see the larger hole and electron containment time would suffer. It might also change the shape of the semi spherical Wiffleball. I have no idea if it is possible that some combination of local electrostatic fields/ grids combined with the larger cusp hole would work.
But the electrons, once they leave the cusps ARE electrostatically contained.

Could we include a longish solenoid like magnet outside one or more cusps to open the hole a bit but channel the electrons and ions in a constrained path so the electrons will turn around and re-enter the Polywell without roaming and the ions can be guided where they are wanted?

Aero
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Post by Aero »

Could we include a longish solenoid like magnet outside one or more cusps to open the hole a bit but channel the electrons and ions in a constrained path so the electrons will turn around and re-enter the Polywell without roaming and the ions can be guided where they are wanted?
Could we? I don't know but I do agree with Simon that we need several experiments going in parallel in order to find answers to the plethora of questions we're coming up with. Still, maybe the Navy's approach is to demonstrate net power, or at least it's real live potential, before spending funds to find answers that can be categorized as refinement. If so, well, that's not so bright when the current economy makes for relatively easy money from the government. Funding will soon become a lot harder to come by.
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D Tibbets
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Post by D Tibbets »

KitemanSA wrote:
D Tibbets wrote:I can see your reasoning from an ion stand point. The fuel ions are contained well by the electrostatic field, so hopefully do not see the 'cusp holes'. The high speed fusion product ions (alpha particles) are not electrostatically contained so they would have a significanly greater chance to escape through a larger cusp hole. The problem would seem to be that the electrons are similar to the fusion ions. They are magnetically contained, not electrostatically contained. They also would see the larger hole and electron containment time would suffer. It might also change the shape of the semi spherical Wiffleball. I have no idea if it is possible that some combination of local electrostatic fields/ grids combined with the larger cusp hole would work.
But the electrons, once they leave the cusps ARE electrostatically contained.

Could we include a longish solenoid like magnet outside one or more cusps to open the hole a bit but channel the electrons and ions in a constrained path so the electrons will turn around and re-enter the Polywell without roaming and the ions can be guided where they are wanted?
I don't know, except WB5 was an effort to improve electron confinement by electrostatically plugging the cusps and it apparently failed miserably. The electrons would presumably have a significantly greater chance to recirculate than hot alphas, but electron containement time based on both the small cusps and recirculation seems to be the critical facter in the machine. Handicaping it could cripple the machine.
Alternately, if the machine is so efficient that the heat loads, etc are a problem doing something like this might provide a compromise between power density, and directional alpha extraction.

My understanding of POPS is that it is a process where variable microwave energy at resonant frequencies can selectively heat/ accelerate ions resulting in interesting bunchings of ions that can possibly signigicantly enhance the fusion efficiency by a facter of 2 to 10 billion (I have no idea) :wink:
It apparently can also help to extract selective ions from the system. Dr. Nebel is an expert on this as he was involved in researching this at LANL befor he became involved with the Polywell.

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Post by KitemanSA »

D Tibbets wrote: I don't know, except WB5 was an effort to improve electron confinement by electrostatically plugging the cusps and it apparently failed miserably.
True, but that attempt tried to prevent the electrons from ENTERing the cusps in the first place. My suggestion is to coral them once they EXIT the cusps outside the MaGrid and shepherd them back thru the cusps into the machine, HOPEFULLY without running into neutrals outside the machine which I am assured is bad.

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Post by EricF »

Layman quesion here fellas

Concerning direct conversion of alpha particles and the r^7 scaling.....

Would the net power gain from a direct conversion device be drastically affected by increasing the radius for larger polywells, due to the increased distance the alphas have to cover while travelling from the well out towards the collection grid, considering it would be sheer inertia kicking them out of the well and not a magnetic force repelling them?

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Post by MSimon »

EricF wrote:Layman quesion here fellas

Concerning direct conversion of alpha particles and the r^7 scaling.....

Would the net power gain from a direct conversion device be drastically affected by increasing the radius for larger polywells, due to the increased distance the alphas have to cover while travelling from the well out towards the collection grid, considering it would be sheer inertia kicking them out of the well and not a magnetic force repelling them?
No.
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Post by Aero »

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?
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Post by MSimon »

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.
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Post by Aero »

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.
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Post by MSimon »

Can we really say with confidence that WB-100 will eject alpha particles only via the cusps?
I did some calculations here a while back and found that in a 10T field the gyroradius was on the order of .1 m for 6 MeV particles (IIRC). That says their odds of hitting the MaGrids is low. (assuming a grid diameter of 2 to 4 m in a production machine).

The preferential exit is at the point where the field/field gradient WRT particle velocity is lowest. Cusps.

Then the question becomes which cusps do you want them to leave from and how do you tune the machine geometry/magnetic fields to make that happen?
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