Some sci-fi questions

[kunkmiester]

Polywells look like they're much easier to "hand-wave" than other fusion systems, and I was looking to integrate them into my universe for what it's worth. There's a few questions that I've yet to find clear answers for though.

First, what's the actual size? I understand a gigawatt reactor isn't that much bigger than a 100 megawatt reactor, so a ballpark figure for a machine wouldn't be that big of a deal. I'm having trouble figuring out if the 10-12 foot figure is the positive grid, the magnets, or the inside of the vacuum chamber total.

Second, the miracle of the polywell is that due to the electrical versus thermal operation, it can gain the potentials needed for much higher power levels. If a civilization were to come to have a shortage of boron for some reason, and decided to go with heavier elements, would there be viable reactions for direct power production(makes charged particles that can be harvested directly, rather than having to be thermalized)? I'm thinking like finding the ruins of a city and a 100 foot chamber than burned carbon or something like that.

[Roger]

3 meter magnets for PB-11 100Mw, so 1000MW might be 3.1 meter magnets. But think of a 30-35 ft concrete building, or an engine room of a star ship.

IIRC David Weber had it close with his fusion bottles.

[kunkmiester]

Sounds good. How's the part about burning much heavier fuels sound?

[KitemanSA]

Sounds good. How's the part about burning much heavier fuels sound?

The Boron is not what actually provides the energy. Its binding energy for B11 is just almost equal per nucleon to the binding energy for Helium. The proton that combines with the Boron is where the energy comes from.

I suppose it might be possible to fuse the carbons and oxygens of the universe, but why bother? If you want more energy, then LIGHTER elements are typically the way to go.

[MSimon]

Sounds good. How's the part about burning much heavier fuels sound?

The Boron is not what actually provides the energy. Its binding energy for B11 is just almost equal per nucleon to the binding energy for Helium. The proton that combines with the Boron is where the energy comes from.

I suppose it might be possible to fuse the carbons and oxygens of the universe, but why bother? If you want more energy, then LIGHTER elements are typically the way to go.

A look at the binding energy curve can be helpful if you want to get exotic.

[IntLibber]

This may belong elsewhere, but as I recall, Bussard proposed catalysing fusion with CNO in his ramjet concept. Did he look at doing this with polywell also and would this make fusion any easier?

[kunkmiester]

A look at the binding energy curve can be helpful if you want to get exotic.

Most enlightening. I knew that stars will fuse up to iron or so, so the simple question is if you don't have a whole lot of boron, and you don't want to burn drinking water or some such(and with sufficient advancement, a large society would be burning enough to think, "this is only going to last so long), you'd go to another element that's easy to get to. Either that or I could go with an experimental reactor. I'd imagine that some researchers would be interested in building a very large reactor that can do reactions that couldn't otherwise be done.

According to the chart here:
http://hyperphysics.phy-astr.gsu.edu/hb ... ucbin.html
there's a steep curve up to ten, then a drop before going up again. Looks like you get diminishing returns after fifteen or twenty. Research might get up near iron, but twenty or so would be the limit if you wanted power.

[D Tibbets]

Sounds good. How's the part about burning much heavier fuels sound?

I had tried to get a handle on how the CNO cycle compared to deuterium fuel. The information was sparse, the most I found was that peak eV efficiencies CNO fusion was ~ 1/200th that of D-D. The acuracy of that comparison is iffy due to how the numbers may have been derived (may be comparing apples to oranges). If representative and the Polywell has a power gain to linier size scale of E+5; then if a two meter D-D burning Polywell produced 100 MW, a CNO Polywell producing 100 MW would be ~6 meters (ignores energy yield and conversion?).

CNO fusion is the dominate fusion path in massive stars, where p-p fusion dominates in smaller stars like the Sun. Both have extreamly small rates, but CNO fusion rates increase very rapidly with incresaing temperature. CNO fusion starts out with a proton (hydrogen) fusing with a carbon 12, subsequent reactions produce several isotopes of nitrogen and oxgyen, untill the original carbon 12 is regenerated while 4 protons end up as one helium 4. In a reactor, I would not be concerned with going through all the steps, just enough to get some energetic particles that could be converted into net power. I'm sure it is a pipe dream, but it would be cool to have a nuclear reacter that ran on methane (or perhaps booze- you could literally drink up your fuel ).


Dan Tibbets

[chrismb]

CNO, as a 'reaction' cycle, is extremely slow and unlikely.

However there are a couple of reactions within that cycle that aren't half bad.

I have always fancied the idea of using p+15N. It's only *a little* bit more difficult that p+11B, not really that much, even.

I have made such comments on http://www.fusor.net/board/view.php?bn= ... 1237731460

If you note the behaviour of the reactants p+3He, p+7Li, p+11B, p+15N. In each case, the proton is making up for 'the missing' part of a 4He (which each give off, as per a reaction mediated by the strong force). The next one would surely be p+19F but that's a rubbish reaction.

So p+15N is *my* preferred target. 15N is as free as...err.... the air. Well, as free as 0.35% of the air!!!

In fact, I reckon that with a fast MeV proton beam in air, the p+15N reaction is likely the most lively nuclear reaction. Funnily enough - we have such an example..

http://www.fusor.net/board/view.php?bn= ... 1202178966

and my modest attempts to quantify....

http://www.fusor.net/board/view.php?bn= ... 1213380714

[D Tibbets]

Well, amonia as the fuel then, if N15 is the best reaction in the CNO group. Not as easy to get as you need to seperate the isotope, but again the feed stock would be inexaustable, and aviable in a lot of places off the Earth (amonia ices on comets, frigid moons, etc.) and no need to wrestle with the boron gas/solid challenges. Can't drink it though...


Dan Tibbets

[KitemanSA]

According to the chart here:
http://hyperphysics.phy-astr.gsu.edu/hb ... ucbin.html
there's a steep curve up to ten, then a drop before going up again.

That is a pretty small graph. This one is a bit clearer.

http://en.wikipedia.org/wiki/File:Bindi ... otopes.svg

[chrismb]

4He and multiples of it are very stable and have high energy per nucleon. Hence, my suggestions above for the multiples of {N x 4He - 1 proton} + 1 proton. [As we know (but not necessarily why) this is excepting the 8Be which is unstable, though the p+7Li reaction still works as the 8Be splits into 2 x 4He's. Unfortunately, p+7Li is dominated by -> n+7Be.]

[D Tibbets]

After some further searches triggered by CB's comment about N15, I found some references ( and my possibly reasonable assumptions ) that C12 + P at 400KeV was ~ 0.3% of the rate of D-D at 100KeV and that the N15 - P reaction might be as high as ~ 100 times as great as the C12 - P reaction (unknown eV relationships). This means that at appropiate drive voltages the N15 - P reaction may approach that of D-D (1/3rd the rate). This ignores things like losses, and unknown N15 - P crossections at the relavent drive voltages. I'm extrapolating from reported relative rates under different conditions and assuming the N15 graph parellels the C12 graph as the eV's increase.
[EDIT] I'm also ignoring the energy yields.

http://fds.oup.com/www.oup.co.uk/pdf/0-19-856264-0.pdf
page 12

http://www.tim-thompson.com/fusion.html#cnoI


Dan Tibbets

[chrismb]

The cross-section of p+15N is bigger than D+D between ~300 and 400keV and then again between ~800 and 1100keV projectile energy (beam-target). (Bear in mind that the collision energy is a half if D+D is a beam-target system.) For the 'all-up' reactivity, the p+X is a bigger outcome as the p has a higher velocity than D for a given projectile energy, so you can say that p+15N is a more likely reaction than D+D for 'at least' 300-400keV, etc...

p+15N cross-section beats p+11B between ~800keV and 1MeV, and then again with a resonant peak ~1.2MeV. (collision energy is about the same as the proton projectile energy when the projectile mass is small compared with the target, as p is to 11B and 15N)

[djolds1]

Second, the miracle of the polywell is that due to the electrical versus thermal operation, it can gain the potentials needed for much higher power levels. If a civilization were to come to have a shortage of boron for some reason, and decided to go with heavier elements, would there be viable reactions for direct power production(makes charged particles that can be harvested directly, rather than having to be thermalized)? I'm thinking like finding the ruins of a city and a 100 foot chamber than burned carbon or something like that.

By the time boron supplies run down we will infest our solar system like locusts. Mine He3 from the gas giants. IIRC they have fuel supplies of He3 good for several million years. Use the He3He3 or DHe3 cycles, both of which are aneutronic/charged particle producers like the pB11 cycle.