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PostPosted: Fri May 30, 2008 7:52 pm 
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I am trying to understand how the p-B polywell makes electricity. My present understanding is that it works in somewhat the same way as Henry Moseley’s 1913 Beta Cell. His apparatus was a glass globe, silvered on the inside, with a radium emitter mounted on the tip of a wire at the center. Betas emitted by the radium carried a negative charge to the silvered inside of the globe. The negative charges accumulated on the globe, and an electric field developed between the inside surface of the globe and the wire – in the same way that a charge accumulates on a capacitor.

My questions are these: First, is it accurate to say that the alphas emitted
(as a result of the p-B fusions in the potential well) are carrying a positive charge to the “power capture” grid (screen) that is placed between the magrid and the vacuum tank shell?

Second, is it accurate to say that the grid (screen) accumulates a positive charge as the ions remove its electrons and become neutral He Atoms?

Third, does an electric field develop between the spherical grid (screen) and the potential well – in somewhat the same way as a charge accumulates on a capacitor?

If these understandings are essentially correct, then other related questions come to mind:
1. How close do the alphas have to get to the grid (screen) in order to remove its electrons? (Probably important because it seems to me that the answer would determine the size of the grid mesh.)
2. Will there need to be any kind of an initial grid “bias” or will it just start to accumulate charge naturally, without any outside help?
3. Since the grid will charge like a giant capacitor, then it would seem that there must be provision to control the rate at which the charge bleeds off – some sort of current regulation? What would that look like?
4. It would also seem that the charged grid will be the “shielding” that protects the vacuum tank shell from the sputtering and heat effects that are such a problem for the magrid. If so, then it would further seem that even the earliest of the WB-100 series polywells will need to have a feature like this – which suggests the possibility that we might want to try to generate electricity in even the earliest of the 1.5 meter polywells.[/img]


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PostPosted: Sat May 31, 2008 2:57 am 
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That's an interesting extrapolation, but the number density of alphas coming from a fusion reactor is going to be a million times larger (at least) than a radioactive decay source. So the impacts will do a great deal of damage if that is what you relied on.

If you extract the mechanical energy from the alphas before they impact and neutralize, you can prevent radiation damage to your reactor walls. Fortunatly, the particles are charged, so putting up a repelling field will slow them down. It's the same thing as using water flowing down a hill to move a wheel - you can extract energy from the alphas as they slow down in a positive electric field. The field generator becomes your energy source, and the battery return is the place where the alphas neutralize. If you do it right, the alphas won't be moving when they turn into Helium and you'll have maximum power transfer.

Not easy to do, but it's the ideal.

I agree that as soon as we get reactions, we should start looking at extracting energy. Even if there is no net gain, the proof of principle is really important.


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PostPosted: Sat May 31, 2008 4:28 pm 
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Using a simple deccelerating E-field outside the magrid might have one drawback, it would atract the electrons and impair their recirculation. That would reduced confinement.

I think a configuration with multiple grids would solve it:

A grid charged to a small negative potential with respect to the coils and not far from them would prevent that effect without affecting the ions very much (G1).

To (almost) stop the ions, two highly positively charged grids outside G1, one slightly under 1.23 MV (G2) and the other slightly under 1.73 MV (G4) (for the two different energies expected).

And, at last, two collectors, beyond but near the deccelerating grids, at a little negative potential from them (the first one should be a grid (G3), the second could be solid).

Problems of this design: First the size of the whole contraption, I think it would be (much?) bigger than the magrid itself. And second the extremely high voltages we are talking about.

All the grids should be as transparent to the ions as possible and, most probably, all of them would need active cooling.

Another possibility is using magnetic fields but, given that the emission of alphas is expected to be isotropic in direction, I don't see how that could be achieved (yet :wink: ).

.... Alpha charge error corrected.


Last edited by charliem on Thu Jun 12, 2008 3:48 am, edited 2 times in total.

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PostPosted: Sat May 31, 2008 5:23 pm 
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charliem,

You have to divide the decelerator voltage by 2 to account for the +2 alpha charge.

In addition there will be a spread of energies depending on the angle of fusion particle collisions and other stuff. In other words the backstop may be required to handle 0 to 100 KV alpha impingement depending on energy distribution.

Other wise your idea of requiring a number of grids to get the correct voltage gradient is correct.

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PostPosted: Sat May 31, 2008 8:10 pm 
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You are right Simon, I forgot to account for the charge of the alphas. Thanks.

About the spread of energies a theoretical analysis, or even better an experimental determination would be in order, although I'd be surprised if the spectrum didn't show a strong anisotropy favoring those two values.

Anyhow, I think that a multigrid like the one I depicted (or even one with more pairs) could, at least, aminorate the degradation of the walls expected for such a heavy bombardment (and the subsequent contamination of the vacuum).


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PostPosted: Sun Jun 01, 2008 3:10 pm 
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Thinking it twice, instead of using a decelerator grid and a collector grid, just one grid at a potential a bit under each energy peak in the distribution would be enough.

If the spread on the spectrum energies is to high, like Simon suggested, two or more, separated lets say 25 kV, would catch them while getting sure that the alphas (at least most of them) dont hit the collectors with more than 50 keV.

..... Ouch, again the alpha charge mistake. Corrected. Sorry.


Last edited by charliem on Thu Jun 12, 2008 3:43 am, edited 1 time in total.

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PostPosted: Thu Jun 12, 2008 3:37 am 
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Thank you for your most helpful replies – I guess I’m obsessed with these collector grids – trying to understand how they would work:

I am picturing two spherical collector grids inside the vacuum chamber, charged to 1.2 MV and 1.7 MV, but far enough from the chamber walls to make arcing unlikely (and how far would that be?). When I do this, two questions come to mind:

1. Why would electrons want to travel from the emitter/electron gun toward a magrid charged to a measly 100 kV, when there are collector grids in the area charged with 12 and 17 times as much voltage? (Or would the electrons “ignore” the charged collector grids because the electrons are inside the collector grid sphere, and therefore to the electrons, the grid is just a Faraday cage?)

2. Would the newly minted alpha particles even “see” the collector grids – or would they also “ignore” the collector grid for the same reason that the electrons ignore it (Faraday cage)?


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PostPosted: Thu Jun 12, 2008 1:27 pm 
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Not directly related, but just to finish up the description of the process, once the Alphas have been decellerated to generate the electricity, then there has to be a removal process. Or soon you're going to have less and less of a vaccuum. Can they be removed while still charged? So as to give something to "grab onto?" Otherwise... a vaccum pump? (That helium has to end up in balloons for kids somehow... what... don't tell me that this isn't the envisioned end use of the waste product to finish the cycle?)

Somehow I envisioned the magrid being "open" more on one side than the others, so that the alphas would have an "escape rout." We could then create a beam of alphas, which would make energy and waste extraction simpler (I think). This might solve Classic's problems above, and might further have the benefit of making the generator directly into a drive that could propel a space vehicle. In fact, if I'm not mistaken, this is generally what they envision for fusion drives, no?

Basically it's the same principle, use the momentum of the alphas to either propell a vehicle, or hold the drive still, and use the momentum to create electricity if it's a generator. Either way, the design of the generator would be the same. Kill two birds with one stone.

Of course it also seems to me that any such "escape rout" method would have a problem with containment overall, making achieving the fusion more difficult to start with.

So it occurs to me that we already have, with the cubic polywell geometry currently in use, eight "escape routs" already, the cusps caused at the vertices. This might be problematic for a drive (you'd have to change the vector of the momentum to make it beneficial for most of the beams), but for a generator, might it be possible to take a "flaw" of the geometry and make an advantage of it, by simply making those the collection points? Just put your "backstops" at each of those locations?

Once again, layman here, probably tons of problems with these ideas. Just trying to spur the conversation on.

Mike


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PostPosted: Thu Jun 12, 2008 2:02 pm 
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If the electron guns are inside the grids, then yes, it is just a Faraday cage. If you give the electrons enough energy they can blast past the grids as well, it acts more like a TV scanner and you just need to aim the electron beam accordingly.

A shaped reaction chamber might be possible, but to start off with a spherical geometry makes the most sense. The alphas are going to come off spherically from the center and the external grids can absorb that energy by braking the ions. Once the ions slow down, getting them to neutralize so they can be sucked into balloons should not be too hard to do. That's just "engineering details".

If the p->B11 cross section is a function of energy and angle, then some interesting games can be played. If this works at all we'll be measuring that
in fine detail for sure. Shaped reactors for rocket engines will be most useful.
:D


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PostPosted: Thu Jun 12, 2008 4:49 pm 
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Mike Holmes wrote:

Somehow I envisioned the magrid being "open" more on one side than the others, so that the alphas would have an "escape rout." We could then create a beam of alphas, which would make energy and waste extraction simpler (I think). This might solve Classic's problems above, and might further have the benefit of making the generator directly into a drive that could propel a space vehicle. In fact, if I'm not mistaken, this is generally what they envision for fusion drives, no?

Mike


I had tried to start a discussion on the NasaSpaceflight website about using the alphas for propulsion, but at the time no one seemed interested. Tom Ligon eventually responded that Dr. Bussard had not suggested using the alphas directly, rather to use the electrical energy extracted from them to accelerate a propellant. I always felt that this was an unfortunate loss of efficiency, but it is obviously feasable.

The problem with the alphas is that they are about 8000 times heavier than an electron, and they only have twice the charge, so moving them with a magnetic field is a lot harder. They are also emitted in every direction from the core,(unless as Dr.Mike suggests there may be a way to make their direction controllable) some of which will unavoidably strike the Maggrid, etc.

I was thinking that by building a large enough engine (In the vacuum of space of course) a sufficient combination of magnetic and electric fields (outside the MagGrid) could possibly steer the emissions from the core into a specific direction, thereby using the alphas directly for propulsion.
A helium atom with +1-4MEV should produce a better reaction force than any other process of which i've ever heard.

Wasn't the drive on Deep Space 1 something like 1.3 kv ?

http://lasp.colorado.edu/~bagenal/DS1/ionthruster.jpg

Mike Holmes wrote:

Basically it's the same principle, use the momentum of the alphas to either propell a vehicle, or hold the drive still, and use the momentum to create electricity if it's a generator. Either way, the design of the generator would be the same. Kill two birds with one stone.

Of course it also seems to me that any such "escape rout" method would have a problem with containment overall, making achieving the fusion more difficult to start with.

So it occurs to me that we already have, with the cubic polywell geometry currently in use, eight "escape routs" already, the cusps caused at the vertices. This might be problematic for a drive (you'd have to change the vector of the momentum to make it beneficial for most of the beams), but for a generator, might it be possible to take a "flaw" of the geometry and make an advantage of it, by simply making those the collection points? Just put your "backstops" at each of those locations?

Once again, layman here, probably tons of problems with these ideas. Just trying to spur the conversation on.

Mike



I believe the "Cusps" are an escape route for electrons only. For He atoms with 1-4 MEV every direction that isn't blocked by solid matter is an "escape route".



David


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PostPosted: Thu Jun 12, 2008 6:11 pm 
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Something i've known about for several years, and I assume others have as well is "Lord Kelvin's Thunderstorm. " For those out there who aren't familiar with it, here are a few links.

http://amasci.com/emotor/kelvin.html

http://www.juliantrubin.com/encyclopedi ... opper.html

http://www.linux-host.org/energy/akelv1.html


It has always been in my mind the closest example of an existing system for extracting energy from the motion of charged particles, and it was conceived a hundred years ago !


In any case, this seems to be as good a time and place as any i've encountered up till now.


David


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PostPosted: Thu Jun 12, 2008 6:54 pm 
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Quote:
I had tried to start a discussion on the NasaSpaceflight website about using the alphas for propulsion, but at the time no one seemed interested. Tom Ligon eventually responded that Dr. Bussard had not suggested using the alphas directly, rather to use the electrical energy extracted from them to accelerate a propellant. I always felt that this was an unfortunate loss of efficiency, but it is obviously feasable.


That was a big part of Dr. B's genius. He did everything with feasible in mind. I think it had a lot to do with his raising of venture capital. Those guys won't listen unless you can show profitability. At least on paper.

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PostPosted: Thu Jun 12, 2008 6:58 pm 
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ravingdave wrote:
Something i've known about for several years, and I assume others have as well is "Lord Kelvin's Thunderstorm. " For those out there who aren't familiar with it, here are a few links.

http://amasci.com/emotor/kelvin.html

http://www.juliantrubin.com/encyclopedi ... opper.html

http://www.linux-host.org/energy/akelv1.html


It has always been in my mind the closest example of an existing system for extracting energy from the motion of charged particles, and it was conceived a hundred years ago !


In any case, this seems to be as good a time and place as any i've encountered up till now.


David


I built one of those when I was a kid. Fun. I used an NE-2 between the two cans to indicate the rate of charge build up.

The deal is: they are not very efficient. They do show how charge can be extracted from moving particles.

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PostPosted: Fri Jun 13, 2008 4:32 am 
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I'm sensing some confusion how the direct conversion would work in a polywell.

In the center is the plasma, ~-50kV potential in the center relative to the magrid.

Surrounding this is the magrid, containing the electrons in the plasma.

Outside the magrid we have electron injectors, ~-50kV or so, and an open grid of about the same electric potential. This grid prevents the injected electrons from seeing the collector shell and being pulled to it.

Finally we have the collector plates at a very high voltage.

Alpha particles generated by fusion shoot out of the plasma and, unless they hit something on the way, lose most of their energy on the way to the collector plate. Hitting the collector plate the alpha picks up a pair of electrons and becomes helium. The extraction of electrons from the high voltage plate allows the plate to accept more from the external voltage converter. The electrons left behind in the plasma, from the fuel that fused to become the alphas, eventually leaks to the magrid.


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PostPosted: Sat Jun 14, 2008 6:33 pm 
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I'm still confused. I tried to get an answer on this in a previous thread and I'm ashamed to say I didn't completely understand the answer, although looking back on it, I sort of asked the wrong question.

So here's my problem:

We've got a big anode charged up to some high positive voltage. We're firing 8.6 MeV alpha particles at that anode. Now, we know that there are now Coulomb forces between the alpha and the metallic atoms in the anode.

However, unlike electrons, you can't make the metal atoms go anywhere interesting. So, even though we're producing a huge E-field between the alpha and the anode (even bigger than the E-field of the anode all by itself), we can't generate any current. Instead, conservation of energy says that that 8.6 MeV has gotta go somewhere, so, as best I can tell, the metal atoms in the anode are going to move as far as they can in their lattice and then vibrate, i.e. they're going to heat up.

Now, you can obviously run a working fluid through the anode and extract that heat and make steam and turn a turbine, but I just don't see how you generate electrical energy directly using an electrostatically coupled anode. Clearly I was asleep during some crucial day in college physics, but I can't figure this out.


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