Making Electricity with the p-B Polywell

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

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93143
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Postby 93143 » Sat Jul 05, 2008 5:15 pm

TheRadicalModerate wrote:Just to be clear, you're back to discussing a trap grid where all the energy is extracted by a negatively charged grid as the alphas fly away from it, rather than classicpenny's positively charged collector.

This one always makes my head hurt. I can't make the charges balance to do useful work. As the alpha passes the grid, it pulls more electrons towards the grid, but they flow back out of the grid as the alpha gets further away. Where's the net current? As usual, I'm sure I'm missing something...


The trap grid is static. The net current is between the magrid and the collector. The electron uptake at the magrid has two components: the high-energy stuff that has to be minimized for efficient operation, and the low-energy stuff from ionization of the fuel. This latter is at basically the same potential as the magrid, so that the fuel is low-energy at the top of the well. This yields a ~2MV difference between the fuel electrons at the magrid and the alpha neutralization electrons at the collector, and by conservation of charge the two currents are equal and opposite. Hook them together and you have the ultimate nuclear battery.

I think the fuel electrons build up around the magrid until a loss rate equilibrium is reached. This shouldn't ruin the operability of the system.

Have I got it this time, do you think?

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Postby TheRadicalModerate » Sat Jul 05, 2008 6:11 pm

93143 wrote:The trap grid is static. The net current is between the magrid and the collector. The electron uptake at the magrid has two components: the high-energy stuff that has to be minimized for efficient operation, and the low-energy stuff from ionization of the fuel. This latter is at basically the same potential as the magrid, so that the fuel is low-energy at the top of the well. This yields a ~2MV difference between the fuel electrons at the magrid and the alpha neutralization electrons at the collector, and by conservation of charge the two currents are equal and opposite. Hook them together and you have the ultimate nuclear battery.

I think the fuel electrons build up around the magrid until a loss rate equilibrium is reached. This shouldn't ruin the operability of the system.

Have I got it this time, do you think?


Nope, still confused. To my mind, the magrid + wiffle ball is effectively neutral, with the ionized electrons from the fuel forming some fraction of the wiffleball. So then you've got a static trap that's strongly negative, and a collector that's--weakly negative? (Kinda has to be to provide electrons for recombination.)

I think my problem is that all of these schemes rely on a fairly small negative current flowing into the collector across a very high electrostatically-induced voltage. The current's fixed: 2 electrons per alpha * number of impinging alphas. So all the power is coming from the induced voltage, which in turn comes from the kinetic energy of the alphas.

So I'm missing how you get electrons to flow from the magrid to the collector.

I also may be confused by the terms "electron uptake at the magrid" and "fuel electrons."

By the former, I assume that you mean wiffle electrons that accidentally ground themselves on the magrid due to imperfect magnetic insulation, but I didn't understand the "low energy stuff from ionization of the fuel." To my mind, the ionized electrons get pumped into the e-guns and fired into the wiffleball, which keeps things neutral. The whole magrid+wiffleball will develop a gradually negative charge as alphas leave the system, but that charge can be drained off through a (small, low-power) current to ground (no doubt through a load).

And I assume "fuel electrons" are just electrons destined to be fired into the wiffleball?

So, ultimately, you've got three different electrodes, all with negative charge: the magrid, the trap grid, and the collector. Then you've got a powerful positive current radiating out through the alphas. But I don't see how you get any more current to flow than a low-energy recombination current.

93143
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Postby 93143 » Sat Jul 05, 2008 6:55 pm

TheRadicalModerate wrote:So I'm missing how you get electrons to flow from the magrid to the collector.


Hook them both to ground. Whichever one is at ground potential (pick one for now, rather than trying to strike a balance) will just dump current. Whichever one is at a couple of megavolts relative to ground will have to be hooked up through a load (ie: the power grid).

I also may be confused by the terms "electron uptake at the magrid" and "fuel electrons."


Electron uptake at the magrid is electrons that hit the grid and are sucked out. Fuel electrons are electrons from the ionized fuel. Since the fuel ions have to be at zero energy just inside the magrid, I assumed their electrons were too. There are probably multiple ways to swing this.

To my mind, the ionized electrons get pumped into the e-guns and fired into the wiffleball, which keeps things neutral.


That's why I tried to separate the electrons from fuel ionization ("fuel electrons") and the electrons used to form the wiffleball. They aren't the same; there must be more electrons in the reactor than you could have gotten from just ionizing the fuel, or else there wouldn't be a potential well.

On the other hand, with most of the electrons having matching ions (quasi-neutral), it becomes a bit funny to try to form a wiffleball, which requires the electrons to be at high energy. Maybe my picture of two electron populations at different energies was inaccurate. Perhaps we need to keep the electrons at high energies and just pump the ions in somehow... Or the wiffleball could just require a massive current to form, and then ECR of neutrals inside the magrid could produce that low-energy secondary population of electrons that equilibrates with the fusion rate...

The whole magrid+wiffleball will develop a gradually negative charge as alphas leave the system, but that charge can be drained off through a (small, low-power) current to ground (no doubt through a load).


The magrid+wiffleball is positive. That's how electron recirculation works. Perhaps beta=1+delta could be how electron losses equilibrate with fusion rate - keeping the well right on the edge of blowing out.

EDIT: Sorry, I see what you're saying. Yes, that's right - just replace "gradually negative" with "gradually less positive".

So, ultimately, you've got three different electrodes, all with negative charge: the magrid, the trap grid, and the collector. Then you've got a powerful positive current radiating out through the alphas. But I don't see how you get any more current to flow than a low-energy recombination current.


Okay, I think I see. You've fundamentally misunderstood the relationship of charge and potential. (Okay, that sounded harsher than it needed to...) The space within the trap grid is at a roughly constant potential of ~2MV with respect to the collectors, due to the trap grid charge. Anything that enters or leaves that space (like the magrid equalization current) is going to have to traverse that potential difference. The magrid perturbs the potential distribution, but not substantially (couple of %). So either you ground the magrid and have the trap grid be at a slightly lower potential and the collector at a massively higher potential (the magrid equalization current has to pass this too, net result ~0 and it's shielded from the actual gradients by the conductor it's flowing in), or you ground the collector and have the trap grid be at a massively lower potential and the magrid at a slightly higher potential than the trap grid.

Either way, one current or the other is going to be multi-megavolt, which is what we're looking for.

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Postby MSimon » Sun Jul 06, 2008 2:50 pm

So I'm missing how you get electrons to flow from the magrid to the collector.


They don't have to. Electrostatic induction. The current in the power collectors is electrostatically induced.

The current in the MaGrid is just that required to support acceleration.

The current in the low voltage alpha collectors is just that required to neutralize impinging alphas.

It all balances. Has to. The balance mechanism in these deals is often called earth or ground.
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93143
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Postby 93143 » Mon Jul 07, 2008 12:40 am

MSimon wrote:The current in the power collectors is electrostatically induced.


I'm pretty sure you've misunderstood this. If the concentrations of charges are constant everywhere, as in steady-state operation, the way to maintain a static potential is through a static charge.

If the deceleration grid were gaining or losing electrons, that would be one thing. But it isn't (ideally). So where is the current going? More importantly, where is it coming from?

I think my explanation above is probably the correct one.

I also think you'll find that the neutralization current multiplied by the magrid-to-collector potential difference equals the output power.

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Postby TallDave » Mon Jul 07, 2008 3:50 am

So where is the current going? More importantly, where is it coming from


I assumed it was coming from the deceleration and going to power my future home.

If it isn't, where does all that energy go? They'll be moving around 15,000 kps when they come out. My understanding was when they got to the collector they had no energy, only charge.

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Postby MSimon » Mon Jul 07, 2008 10:00 am

93143 wrote:
MSimon wrote:The current in the power collectors is electrostatically induced.


I'm pretty sure you've misunderstood this. If the concentrations of charges are constant everywhere, as in steady-state operation, the way to maintain a static potential is through a static charge.

If the deceleration grid were gaining or losing electrons, that would be one thing. But it isn't (ideally). So where is the current going? More importantly, where is it coming from?

I think my explanation above is probably the correct one.

I also think you'll find that the neutralization current multiplied by the magrid-to-collector potential difference equals the output power.


If what you said was true we could build particle accelerators that deliver kilo amps of beam current with micro amps (corona losses) of acceleration current. A perpetual motion machine. We are wasting our time with BFRs. :-)
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Postby charliem » Mon Jul 07, 2008 2:43 pm

When an alpha particle leaves the magrid its Kinetic energy is maximum and its Potential energy near cero, when it gets to the collector structure its Kinetic energy is very low and its Potencial energy maximum.

There is conservation of energy, the intermediate grids only job is to transform Kinetic into Potential, they dont need to give nor take any, so no current to or from them (ideally).

When that alpha impacts the collector it gives its Potential energy to it, energy that creates the external current [of electrons].

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Postby TallDave » Mon Jul 07, 2008 4:13 pm

Hrm. I think that kind of makes sense.

When the alpha leaves the core, it has ~3MeV of kinetic energy, but it uses all that energy to climb up the electrostatic hill to the ~3MeV collector. Exhausted, it wipes its brow, picks up its electrons, and begins its new life as a helium atom by being unceremoniously sucked out of the vacuum chamber.

Those electrons get passed at the top of the ~3MeV hill, so their departure creates an energetic current.

Is that right?

93143
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Postby 93143 » Mon Jul 07, 2008 6:38 pm

MSimon wrote:If what you said was true we could build particle accelerators that deliver kilo amps of beam current with micro amps (corona losses) of acceleration current. A perpetual motion machine. We are wasting our time with BFRs. :-)


Remember that if we didn't need to maintain the wiffleball, we wouldn't need the decelerator grid - the magrid would work fine to decelerate the alphas.

I don't see how there can possibly be a current in the decelerator grid. It's basically just there to prevent the potential difference between the magrid and the collectors from kicking in right away and destroying recirculation.

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Postby tombo » Mon Jul 07, 2008 6:52 pm

Perhaps a better analogy would be an electron microscope with its electrostatic lenses etc.
They have a fairly mature technology to mine for useful techniques.
I'm thinking broadly that what we want will be a cross between an electron microscope and a mass spectrometer to focus, separate and harvest the various energies.
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Postby MSimon » Mon Jul 07, 2008 7:53 pm

There is so much misconception in this thread I don't know where to begin.

Current flow depends on potential and load. It has zero to do with charge canceling.

Zero.

Collector grid potential in this machine is a quasi capacitive effect.

We are capturing the energy of the alphas with an electric field. Just as we would be using potential energy (requiring a current to maintain grid voltage) if we used the device as an alpha accelerator.

The alphas all have to get neutralized. The "power" grids neutralize nothing (ideally). It just slows the alphas down. You leak off enough voltage (by drawing current from the "power grids") so that the alphas have enough residual energy to hit the neutralizing plates.

The MaGrids are practically useless as electrostatic collectors. At even 200 KV (high energy density lower Q machines) they will collect about 15% of the available energy. Provided the collector plates are right at the surface of the magnetic bottle. A big no no as that ruins electron oscillation.

Really. The elegance of the current design still amazes me. It is going to be very hard to beat a guy who

1. Lived this stuff for at least 30 years.
2. Is a genius.
3. Knows the physics of plasmas and fields.

Where should you start for an education? If you have enough physics vacuum tube design is a very good place to start. Beam Power Tubes. Klystrons. Magnetrons. Cyclotrons.
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tombo
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Postby tombo » Mon Jul 07, 2008 8:23 pm

Would you please place a pointer to a drawing of that "current design" configuration?
To where in the device does the current from the load return to complete the circuit?
-Tom Boydston-

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Postby MSimon » Mon Jul 07, 2008 8:30 pm

tombo wrote:Would you please place a pointer to a drawing of that "current design" configuration?
To where in the device does the current from the load return to complete the circuit?


Ground. i.e. The outer wall. Or the "screen". I already said that. You know like 2 MV decelerator potential is between the decelerator grid and ground.

We are charging up a capacitor with the alpha energy and bleeding off some of the charge.
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Postby TallDave » Mon Jul 07, 2008 9:58 pm

Current flow depends on potential and load. It has zero to do with charge canceling.


That sounds more right.

All that deceleration has to be paid for somewhere. Otherwise you could run this in reverse as a free energy machine by putting alphas on the edge and accelerating them in.


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