Polywell size and radiation

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

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cgray45
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Polywell size and radiation

Post by cgray45 »

This is two questions from a history, not science major, so bear with me.

1. From the discussion here, it seems that the smaller polywell designs cannot produce net energy. Is that a "hard limit" in that the science just doesn't work, or could it be a matter of design. In other words, presuming the concept works, is it possible that miniaturized polywell fusion plants could be created in time, or is it simply impossible by the laws of physics as we know them today?


2. From the discussion about shielding, I believe that the implication is that while the P+11B reaction produces far less in terms of radiation than a fission or conventional fusion reaction, it still produces enough to require fairly extensive shielding. IE, standing next to an unshielded reactor would be just as lethal for a polywell design as it would be for a conventional fusion or fission design.
But from a few other sites, some of which spoke of designing the magnetic fields to reduce the radiation output or convert it directly to electricity (which quickly went far beyond my math understanding), it seems that their might be ways to reduce this radiation output without recourse to heavy shielding. My question here is the same-- is that possible, even if not today, or are we talking about physical laws that will require a polywell reactor always have a blanket of heavy shielding?

I haven't been able the answers here-- at least not in the form I can easily understand. Blame the fact that I flunked my science classes before deciding that a history of the British Slave Trade called to me.

D Tibbets
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Post by D Tibbets »

The power goes up as the size and magnetic field strength goes up. Theoretically(?), a small machine with disproportionately strong magnetic fields could work. But the engineering challenges would be impossible. Thermal wall loading would be too great (the vacuum vessel walls would melt), The necessary superconductors would be impossible to cool, wall outgassing, vacuum concerns would be magnified, etc, etc. As it is, if the P-B11, or D-D reactions work as advertised, engineering constraints are already near to being to the limiting factors.

X-rays from the reactor burning P-B11 would presumably be more than D-D because the machine runs at higher temperature (also other reasons). These x-rays would be mostly stoped by a few inches of steel in the walls of the vacuum vessel and converted to heat which could then be captured through a steam cycle at ~ 25-30% efficiency. Direct conversion of the x-ray energy at ~ 80% efficiency is essential for small positive Q machines like DPF, but is of minor concern for a potential Polywell. In either case there will need to be cooling of the reactor.

The P-B11 reaction will produce very few neutrons, but one of the rare side reactions will produce enough gamma rays (~ 1/10,000 reactions?) that at power outputs of millions of watts, the radiation hazard is significant. Some thick dense material is needed to stop it (or a lot of distance). Gammas are like x-rays on steroids. They are more penetrating and do more damage. If a direct x-ray conversion scheme can be made to survive long enough to be practical in the harsh x-ray environment, the same used to convert gamma rays would be even more fragile. Fortunately, I believe the gamma ray flux will be considerably less than the x-ray flux, so designing the collector to collect most of the x-rays, while allowing most of the gammas to heat outer thick shielding is probably the best bet.

The gammas heat load would be easy to manage. It might only be 1/10,000 the total power output- so for a 100 MW reactor the gamma heating might only be ~ 10,000 watts. This would be easy to manage (probably even for a small DPF systems. But, it is the damage these high energy photons do to electronics that is the problem (along with killing any non protected people near by).

PS: If MANY complications were ignored, a D-D fusing Polywell at 30 cm magrid diameter might breakeven with a magnetic field strength of ~ 35 or more Tesla (?).

Dan Tibbets
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TallDave
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Post by TallDave »

What Dan said. If you could squeeze ~35T magnets into a 30cm design, you might get a device that makes power if we ignore all the other considerations like first wall heat load.

I don't think you will ever see that done, even if it someday becomes possible, because even ignoring radiation concerns, you would probably not want (for instance) to drive a car powered by ~35T magnets.

It seems very unlikely you will ever see Polywell fusion in small mobile applications. I would say the lower limit is probably something like a Navy destroyer (~100 tons).
n*kBolt*Te = B**2/(2*mu0) and B^.25 loss scaling? Or not so much? Hopefully we'll know soon...

D Tibbets
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Post by D Tibbets »

In an interview on the defunct site- American Anti gravity, Bussard did say that a Polywell small enough to power a semi truck might be possible with later generation machines and utilizing some "interesting physics".

[EDIT] actually found the new(?) site holding the interview here.
http://www.americanantigravity.com/arti ... rough.html

Dan Tibbets
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cgray45
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Post by cgray45 »

I didn't catch the "interesting physics" comment, but he said third generation and "well down the road." in the part that I heard, so it's likely not impossible but very difficult.

the intersteing thing about that is that is that if you could get them that small, you could have a network charging a road grid or battery driven cars to make IC engines pretty much obsolete for most civilian purposes.

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Post by WizWom »

TallDave wrote:It seems very unlikely you will ever see Polywell fusion in small mobile applications. I would say the lower limit is probably something like a Navy destroyer (~100 tons).
Not to quibble, but a Destroyer is about 3000-5000 tons. Some have even been as large as 7000.

100 Tons is a small patrol craft.

The energy used to provide power will be on the order of 2 to 3 megawatts; then the system ought to produce on the order of 5 to 10 gigawatts. That's more power than any ship will use. The current nuclear Aircraft carriers have two fission plants which they say could power "100,000 homes" - assuming a typical draw of 1 kW gives us 100 megawatts. It might even be as high as a Gigawatt. That would be on the low side for a fusion reactor. A fusion powered Aircraft carrier, though, might be set up as a tender for a battery-powered fleet.

A fusion plant is much more a "power a whole city with one or two" type of thing.
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ladajo
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Post by ladajo »

WizWom: You are way off in your numbers. There are plenty of old threads where we beat the ship applications to death. It is not the Tonnage that drives Polywell install, it is the size of available engineering spaces.

FFG7 (Frigate): 4100 Tons - conventional Polywell will fit.
DDG (Destroyer): Easy fit for one, maybe two.
Flight I - 8,300 tons
Flight II - 8,400 tons
Flight IIA - 9,200 tons
Flight III - 10,000 tons
DD/CG (Spruance/Tico): 8000tons/9600tons - easily fits a Polywell, maybe two
CVN (Nimitz+): 100,000+ tons - No where near a fit problem

This includes support systems, and is based around a direct conversion plant. Submarines have also been discussed, albiet using a retro for the steam plants.

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D Tibbets
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Post by D Tibbets »

WizWom wrote:The energy used to provide power will be on the order of 2 to 3 megawatts; then the system ought to produce on the order of 5 to 10 gigawatts. That's more power than any ship will use. The current nuclear Aircraft carriers have two fission plants which they say could power "100,000 homes" - assuming a typical draw of 1 kW gives us 100 megawatts. It might even be as high as a Gigawatt. That would be on the low side for a fusion reactor. A fusion powered Aircraft carrier, though, might be set up as a tender for a battery-powered fleet.

A fusion plant is much more a "power a whole city with one or two" type of thing.
I wonder if you are confusing your reactors. I believe a practical Tokamac reactor might be limited to a minimal sizein the range of 5-8 GW. A Polywell could be anywhere from a little under 100 MW to hundreds of MW. A multiple gigawatt Polywell is also possible, but I understand that size is less attractive for the electrical grid, and is way more than a ship needs, unless it is using ridiculously powerful beam weapons or multiple large rapid firing rail guns.

Dan Tibbets
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Betruger
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Post by Betruger »

D Tibbets wrote:A multiple gigawatt Polywell is also possible, but I understand that size is less attractive for the electrical grid, and is way more than a ship needs, unless it is using ridiculously powerful beam weapons or multiple large rapid firing rail guns
or all that, and made to fly.

WizWom
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Post by WizWom »

The A4W produces 104 MWe, just about what I said (nominally; actual power is much higher, closer to 250 MWe).
A D2G is 150 (from back in the 60s, when we had an all-nuclear task force).
Similar power levels are used in Submarines.

Now, as to the power level at which a Polywell breaks even...
< http://nextbigfuture.com/2010/03/iec-fu ... isons.html > suggests break-even at 1.3m core size and 50kV potential (V not A???) to the magnets.
In any case 100MWe is expected break even... a factor of 100 more than I thought. If that's the case, then scaling by the 5th power for the input power and the 7th for the output means 5x the power to the core should give 10x times the output. So, a 500MWe net D-T plant would be about 2.8 m in diameter (just because if you need to carry 5x the current, you need to have the square root of 5 increase in cross-section).

It seems, from the drawing, that that's assuming no increase in containment efficacy from a truncated dodecahedron.
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D Tibbets
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Post by D Tibbets »

WizWom wrote:The A4W produces 104 MWe, just about what I said (nominally; actual power is much higher, closer to 250 MWe).
A D2G is 150 (from back in the 60s, when we had an all-nuclear task force).
Similar power levels are used in Submarines.

Now, as to the power level at which a Polywell breaks even...
< http://nextbigfuture.com/2010/03/iec-fu ... isons.html >
suggests break-even at 1.3m core size and 50kV potential (V not A???) to the magnets.
In any case 100MWe is expected break even... a factor of 100 more than I thought. If that's the case, then scaling by the 5th power for the input power and the 7th for the output means 5x the power to the core should give 10x times the output. So, a 500MWe net D-T plant would be about 2.8 m in diameter (just because if you need to carry 5x the current, you need to have the square root of 5 increase in cross-section).

It seems, from the drawing, that that's assuming no increase in containment efficacy from a truncated dodecahedron.
A modern destroyer might have 90-100,000 horse power and ~750 watts = 1 HP. So ~ 70 million watts is needed. With losses and good conversion to electricity a P-B11 Polywell with ~ 100-120 MW fusion output would fill the bill. A D-D Polywell would need to be ~ 200-250 MW with a steam conversion plant.

The advertised demo Polywell reactor is hoped to produce ~ 100 MW of fusion power at a cost of ~ 10 MW (a Q of 10) if the fusion and losses scale as expected. This was Bussard's goal. A scaled back design at half or even a quarter of that performance would be cheaper, validate the physics, and generate a lot of interest; but it would not serve as well as a prototype and thus would add another step to the commercialization process.

The power is supposed to scale at the 7th power, and the losses scale at the 2nd power of size, with a net gain of the 5th power of the size. Presumably this is for D-D fusion. The losses for P-B11 will scale faster due to the higher drive voltages, and higher bremsstrulung x-ray losses. That is why P-B11 Polywells will need modestly larger sizes as the net gain scaling is lower, perhaps at ~ 4.5th power of size . Atually, I have not seen numbers, except claims that perhaps a 4-5 meter wide P-B11 Polywell is needed to match a 3 meter D-D Polywell in fusion power (also the maximum Q attainable is more limited). Once the increased efficiency of direct conversion to electricity is taken into account, the P-B11 magrid diameter between the machines may be similar for the final electrical output.

Dan Tibbets
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ladajo
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Post by ladajo »

WizWom wrote:The A4W produces 104 MWe, just about what I said (nominally; actual power is much higher, closer to 250 MWe).
A D2G is 150 (from back in the 60s, when we had an all-nuclear task force).
Similar power levels are used in Submarines.
Besides Wiki and FAS, where are you getting your numbers?
Do not forget your friend Carnot when reverse engineering numbers.
Shaft HP(steam)==> MW ==> Total Plant Power needs to account for thermodynamic cycle losses. Ie: 50,000SHP (Steam) makes for a 150MW Primary.

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Post by WizWom »

ladajo wrote:
WizWom wrote:The A4W produces 104 MWe, just about what I said (nominally; actual power is much higher, closer to 250 MWe).
A D2G is 150 (from back in the 60s, when we had an all-nuclear task force).
Similar power levels are used in Submarines.
Besides Wiki and FAS, where are you getting your numbers?
Do not forget your friend Carnot when reverse engineering numbers.
Shaft HP(steam)==> MW ==> Total Plant Power needs to account for thermodynamic cycle losses. Ie: 50,000SHP (Steam) makes for a 150MW Primary.
The Abraham Lincoln Website was the 104 MWe figure for the A4W; for the rest, I didn't bother much. Actual performance for Aircraft carrier is known to be ~2.5 times published (have a friend who was a mastr electrician on one).

I'm aware of the Carnot cycle, and it's theoretical implications. Not important, on a ship, the "cool side" is always ocean temp, through a simple heat exchange.
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D Tibbets
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Post by D Tibbets »

Actually the carnot cycle is more restrictive in a steam plant. My feeble knowledge includes the point that the efficiency is based in part on the temperature of the steam entering and leaving the turbines. I've heard M. Simon say that you are limited to ~ 550 degrees C on the high end, because stainless steel starts to creep. More exotic and expensive alloys allows you to push this some. I guess that the hard floor temperature is ~ 100 degrees C as that is the boiling point of water depending on the pressure remaining after passing through the high pressure and the low pressure turbines.

Further cooling is presumably due to enviornmental concerns. A ship would have a modest advantage by dumping the hot water into the ocean- of course they would then need to produce new distilled water for the next cycle.
In short I doubt that ship based plants would have much advantage over a ground based plant- especially if it has a convient large cooling pond/lake.

So having a nuclear reactor that produces 2-3 times the heat energy as the final shaft horsepower rating is expected. Even if the steam directly drives the propeller shafts the advantage would be modest.

Dan Tibbets
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ladajo
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Post by ladajo »

For practical napkin purposes you can place PWR efficiencies at about 25% driving a steam plant.

So that means Total SHP is 1/4 of Total Power. Not counting the SSTG's, but they are small when compared to the total mix...also for a CVN you must account for the Catapults. They eat steam.

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