Electron recirculation

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

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Indrek
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Post by Indrek »

Very good. I once tried to pick the same thing up on the iec fusion list but nobody took the bite:

http://tech.groups.yahoo.com/group/IEC_ ... ssage/1065

- Indrek

TheRadicalModerate
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Post by TheRadicalModerate »

Seems like the CUSPDEC thing is a simple (more-or-less infinite) plate-charge decelerator. That seems straigtforward. But we've been discussing two different things here:

1) A spherical postive charge decelerator. I think I understand how this works and stays electrically neutral: You're pulling electrons from ground and recombining them with alphas, and in turn generating a lot of power by how fast you pulling the electrons from ground. But, as has been discussed, there's no net force inside a spherical conductor, so we don't think this will work.

2) A spherical negative charge decelerator grid, which pulls energy from the alphas, leaving them to recombine with electrons at a positive grid. Since the spherical conductor is effectively inside the radius of deceleration, there's a net force, but I can't figure out how the currents balance out with this scheme. You've got to supply zero-energy electrons to the postive recombiner and they come from--where? And the kinetic energy from the alphas makes current flow from the decelerator grid to--where? Makes my head hurt.

So here's Yet Another Half-Baked Direct Conversion Scheme: If you can generate a series of fairly lazy cusp B-fields at some distance from all this nasty magrid business, you ought to be able to guide all of the alphas into one of the cusps and then do good ol' fashioned ExB induced current energy conversion. (Just think of it as a set of standard generators with alpha particles instead of armatures.)

tonybarry
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Post by tonybarry »

TheRadicalModerate wrote:But, as has been discussed, there's no net force inside a spherical conductor, so we don't think this will work.
I think this is not correct. There ought to be a net force able to exist inside a spherical conductor, where there is a dielectric separating the inside "centre" from the outside sphere.

You could run a thought experiment ... imagine a large sphere made of conductive material. Inside that sphere is a vacuum, and a battery (a voltaic pile) with the positive plate electrically connected to the sphere. According to your postulate, because the voltaic cell is inside a conducting sphere, there is no negative potential at the other end of the voltaic pile. This is obviously incorrect, as a meter connected across the voltaic pile will show a voltage regardless of the box it is contained in.

Let us imagine further. Let us imagine that the pile generates a million volts, and the negative pole by good fortune happens to reside in the centre of the sphere. Let us also suppose our meter has an infinite input impedance, so it does not consume current as it measures voltage. Something like an electrometer. We connect our meter positive terminal to the sphere inside, and have the negative terminal left as a probe. As we move our meter probe from the sphere inside surface towards the centre of the sphere, through the vacuum, what do we observe? A voltage gradient, I believe.

Now I do understand that a sphere with a conductive outside acts as a Faraday cage ... but ONLY to impinging EM fields that come from the outside. Not the inside. A radio transmitter can send signals out of a Faraday cage ... it just does not receive them.

Regards,
Tony Barry

MSimon
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Post by MSimon »

If you couldn't have potential differences in a faraday cage then there is no way you could test transmitters inside one.

BTW faraday cages are reciprocal. If nothing can get in nothing is going out.
Engineering is the art of making what you want from what you can get at a profit.

tonybarry
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Post by tonybarry »

Yes Simon you are correct, and a moment's thought should have alerted me to the falsity of my previous statement. A Faraday cage is reciprocal, else a microwave oven would radiate nicely.

However, a voltage gradient must still be able to exist within a cage, providing a boundary without making demands on the volume inside the cage. Again the example of a microwave oven comes to mind. There is a substantial voltage present across the magnetron element, although the device is fully enclosed in a cage (and in most cases the voltage generating part is outside the cage).

Regards,
Tony Barry

kcdodd
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Post by kcdodd »

It doesn't matter what you call them; pusher, puller, eggcracker, etc. What you're doing is putting an alpha particle into a piece of metal which has a +2e charge. Whether the alphas are going at like 5MeV or 2eV its still +2e charge getting put in the metal. The current is totally fixed you can't change that. What you can change is the voltage. If you put it at +5MV then you have a tiny freaking current at a huge freaking voltage = noticeable power output. +5MV to what? Ground, why not. We like to ground everything. The core of the reactor is at like -100kV or something from ground right? The alphas are made in the core so they will just make it to our collector right? All the current loops are to ground because we like the ground. Don't try to make it more complicated.
Carter

93143
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Post by 93143 »

I didn't say there could be no potential gradient inside a conducting closed surface. I said there could be no potential gradient inside an EMPTY conducting closed surface - ie: the charge on the conducting shell itself has no effect within that volume.

dch24
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Post by dch24 »

So I love physics, but I haven't had the chance to devote the time to master everything. So a lot of polywell physics still goes over my head.

But the direct conversion makes sense to me, so let me explain what I understand. Then shoot it full of holes :)

Code: Select all

          ______________ Outer wall
         /
        /
       /    ____________ Direct Conversion collector
      /    /
     /    /
    /    /    __________ Direct Conversion cage
   /    /    /
  /    /    /
 /    /    /    ________ MaGrid
/    /    /    /
|   |    /    /  Electron Gun
|   |   |    /     |
|   |   |   |      V
|   |   |   |      e-   Electrons form a "potential well" in the center
|   |   |   |    e-  e-
|   |   |   |      e-
|   |   |   |
o   o   o   o
|   |   |   \
|   |   |    \
|   |   |     \
|   |   |      \
|   |   |     ___ (+)
|   |   \_     _  "great big battery"
|   \__   \   ___ from Ground to MaGrid
|      \  |    _  (-)
|      |  |    |
Ground |  Ground
       |
       Power out (electrons in)
Outer wall of vacuum chamber
  • grounded (E = 0 eV)
  • few to no impacts from particles in the system - so it shouldn't heat up much
Direct Conversion collector
  • charged to E = +5 MeV
  • alpha particles (Helium, 2+ charge) pick up 2 e- here. If the collector wasn't charged all the way to 5 MeV (for example, during startup), alphas would impact it even more energetically, claim their 2 electrons, and the charge on the collector would thus increase. Likewise, if the collector wasn't able to move current, the charge would go above 5 MeV (I don't know that much higher is actually possible), until the voltage was sufficient to move charge. So efficiency of the collector (ratio of heat from alpha impacts / watts of electricity generated) is directly proportional to the charge on the collector.
  • alpha particles give up their velocity (kinetic energy) which is how power is generated
Direct Conversion cage
  • Could be grounded (E = 0 eV). Or could be at negative potential. Just so the voltage difference from the cage to the collector matches the energy of the alphas. (And yes, there are two different energies of alphas...)
  • hides the Direct Conversion collector charge to anything inside the cage - specifically, recirculating electrons
  • alpha particles escape the cage with their high velocity, but not before they are pushed outward a little: they get a small energy boost from the "great big battery"
MaGrid
  • charged to a moderate positive potential (E = +10 KV - converting that to eV requires physical dimensions of the BFR)
  • high intensity (B) magnetic field
  • for particles that have escaped confinement: positive potential attracts electrons, and repels alphas
  • magnetic field B confines electrons inside
  • magnetic field B + electric potential E both bring electrons back that escape
  • lots of alpha impacts produce heat and pick up 2 e-
Here's the answer I needed, the "ah hah" moment that cleared it up:

The MaGrid will lose positive charge, because the alphas formed leave behind their electrons, which despite the close-to-perfect confinement still build up so much that they can collide with the MaGrid and decrease its positive charge.

The electron gun is only needed for startup. After that, (at least, in my mind) the electrons stripped off the Boron (5+) and hydrogen (1+) join the potential well in the center and increase its negative charge.

But electrons hitting the MaGrid will flow into ground. The "great big battery" gives up a little energy when alphas are flying out to the Direct Conversion collector. It also does some work to maintain positive charge as electrons hit it and must be moved out to ground. I don't know how much power it will take to do this work.

Other electrons might collide with other parts of the polywell. They all end up in Ground, where they can flow into the Power out connection as needed, thus completing the circuit. I don't think it's possible right now to say what the balance will be among the different currents from ground. But the MaGrid current was the one I was missing.

tonybarry
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Post by tonybarry »

Hello dch24,
I liked your post. Good to see you grappling with the ideas.

A point. The direct conversion grid is charged to a bit less than +1.8MV, not +5MV. The alphas emerge from the fusion region with about 1.8MeV of kinetic energy, and the DCG provides a field to offer electric braking. When an individual alpha hits the grid, it collects two electrons to become neutral helium.

The equations may be simplified as:-
One hydrogen atom -> one proton + one electron
One boron atom -> one boron nucleus + 5 electrons
---> one proton + one B11 nucleus -> three alphas at 1.8MeV + a few other things.

So the electron movement path (current flow from -ve to +ve) may be seen as being from the ion guns (liberated by the ionisation of the hydrogen and boron), up to the DCG, into the alphas, which collect the electrons and enter a neutral state as helium. This current path provides considerable current at enormous voltage ... = power.

The current is proportional to how many electrons pass a given point in the circuit per second (one ampere = 6.25e18 electrons per second moving past a given point). The voltage is derived from nuclear physics, and represents the kinetic energy of each alpha as it emerges (in a random direction) from the fusion region. It is measured in MeV for convenience.

The system requires an earth point. It doesn't matter where, although for engineering reasons it might be the ion guns, or perhaps the MaGrid casing, or somewhere. Anywhere. Earth is just a point of reference in this case, a place to measure from. The "earth point" or "ground" for WB-6 was the electron emitters; the MaGrid was about +12kV with respect to this ground.

Yes you will lose electrons to the MaGrid casing. According to Dr. Bussard, this is a Bad Thing, and should be less than 1 electron per 1e6 fusions to permit net positive energy generation. That's about 160 nanoamps per ampere of DCG current assuming no loss of alphas to the MaGrid (recall that one fusion reaction produces three alphas, which require six electrons to neutralise; 1e6 fusions requires 6e6 electrons).

MSimon reckons there ought to be 20% loss of alphas to the MaGrid. That represents a substantial current too.

Regards,
Tony Barry

drmike
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Post by drmike »

I don't see why you need the "direct conversion cage". The center is negative, the Collector is max positive, and everything in between doesn't matter.

It's just a spherical electron tube. But it uses ions instead. :)

MSimon
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Post by MSimon »

Tony,

You forgot to account for the +2 Charge on the alphas.

A common mistake. I've made it myself.
Engineering is the art of making what you want from what you can get at a profit.

kcdodd
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Post by kcdodd »

Electron recirculation needs a reference outside the magrid at lower voltage then the magrid so the electrons will be pulled back in. If the alpha collector is at higher voltage then magrid then you need something in between.
Carter

TallDave
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Post by TallDave »

All these grids worry me. You're talking about a LOT of energy hitting that negative grid in a p-B11 reactor. I worry it's going to melt or need constant cooling.

If you had no intervening negative grid, how far back would the collector plates have to be before the electrons ignore them in favor of the Magrid which is right next to them? Given how close to the grounded outer wall WB-7 is operating and how the fields bend close to the Magrid, I've always assumed it wasn't that far but I've never tried to calculate it.

kcdodd
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Post by kcdodd »

If the magrid is at lower potential then the wall it doesn't matter how far away the wall is. The electric field will always point toward the magrid, repelling the electrons.
Carter

93143
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Post by 93143 »

dch24 wrote:[ASCII art]
Why is the direct conversion collector separate from the outer wall? Practical considerations?

Also, why is there current going through the direct conversion grid? The collector, not the decelerator, is what provides the electrons to neutralize the fusion products.
tonybarry wrote:Yes you will lose electrons to the MaGrid casing. According to Dr. Bussard, this is a Bad Thing, and should be less than 1 electron per 1e6 fusions to permit net positive energy generation. That's about 160 nanoamps per ampere of DCG current assuming no loss of alphas to the MaGrid (recall that one fusion reaction produces three alphas, which require six electrons to neutralise; 1e6 fusions requires 6e6 electrons).
Where did he say that? I recall the "1e6 electron transits before loss" thing, but the reduction in energy out from even 1 electron lost per fusion would be the electron energy times the fusion rate, which is nowhere near the fusion power.
TallDave wrote:All these grids worry me. You're talking about a LOT of energy hitting that negative grid in a p-B11 reactor. I worry it's going to melt or need constant cooling.
Hopefully putting them in the magrid shadow will be enough. If not, the intercept problem at that distance will not be nearly as bad, and cooling will be much simpler. On top of that, there are no superconducting magnets to worry about. It may still be hard, but if we can make the magrid work we can make the trap grid work.
If you had no intervening negative grid, how far back would the collector plates have to be before the electrons ignore them in favor of the Magrid which is right next to them?
It doesn't matter. There has to be a +1.8MV potential difference from the magrid to the outer wall in that scenario, which means the electric field is uniformly inwards. Electron recirculation - indeed, probably wiffleball formation - is impossible under those circumstances.

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