Some sci-fi questions

[Nik]

Handbook of Isotopes in the Cosmos: Hydrogen to Gallium By Donald D. Clayton

Around p55...

http://books.google.co.uk/books?id=22xo ... on&pg=PA55

Watch that mega-wrap...
My take on it is that Li/Be/B are under-represented because they're destroyed by few million-degrees, eg stars and some 'brown dwarf' interiors...

Um, I suspect that having polywell available allows H/D fusion a chance. Bit like ships have an auxiliary 'donkey engine' to start their big diesel...

[Brian H]

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

The Focus Fusion design is planning to use decaborane, B10H14. It is a solid at room temp, gas at 213°C, not too difficult to manage (except for toxicity). As far as availability, it can be refined from seawater at about $30/lb, I think, enough for about 1MWy or so. If the FF design flies, it would be able to handle 10X the current world power demand until approximately the time the sun goes red giant using local (terrestrial) supplies.

[taniwha]

Hello, I'm new here (though I've been reading for the last month or so). I'm a complete layman*, so anything I say is most likely to be swiss cheese. I too am interested in the polywell for scifi (and for real, too).

Anyway, regarding information on the CNO cycle, there's wikipedia (though I imagine this has already been referenced). Interestingly, the Japanese page has the information of the times (平均寿命: "average life span" (fast and loose translation)).

Quick translation:
12C + 1H → 13N + γ +1.95 MeV 1.3e7 years
13N → 13C + e+ + νe +1.37 MeV 7 minutes
13C + 1H → 14N + γ +7.54 MeV 2.7e6 years
14N + 1H → 15O + γ +7.35 MeV 3.2e8 years
15O → 15N + e+ + νe +1.86 MeV 82 seconds
15N + 1H → 12C + 4He +4.96 MeV 1.12e5 years

As I see it, the problem with the CNO cycle is the gamma rays. If direct conversion could be done on them, they wouldn't be so bad. I was wondering about the interactions between gamma rays and superconductors, but all I could find on the web was reports on the effect of gamma rays on the Tc (usually increasing it?), nothing about what happens the the energy in the gamma rays as they pass through the SC (it would be nice if the energy converts to voltage/current rather than heat).

I have to agree with chrismb that p+15N looks like a good choice as an alternative to p+11B.

* I studied Electrical and Electronic Engineering in university, but wound up as just a programmer, and I'm now teaching English in Japan.

[chrismb]

Hello, I'm new here (though I've been reading for the last month or so). I'm a complete layman*, so anything I say is most likely to be swiss cheese. I too am interested in the polywell for scifi (and for real, too).

Anyway, regarding information on the CNO cycle

The times included here are the 'half-life' times to that particular reaction. In other words, add them all up and that's how long a given set of reactants would take [viz. half-life] to go through the cycle.

The whole cycle is irrelevant for man-made fusion - it would take far too long.

Also, there is no interest in reactions that create gamma rays, such emissions would just go to waste. Gammas are generally very 'easy' to block [just stick more matter in the way], though there are some substances that will emit neutrons by irradiation by very hard gammas, so shielding can avoid those substances.

Lerner has spoken about direct conversion of x-rays to electricity. As far as I am aware, he is the only one and has not yet given any theoretical backing (nor, least still, pratical kit) to do this, so it remains in the imagination.

When deciding on suitable reactants, one needs to look at the fusion cross-sections and energy levels. Excepting for the p+15N, these reactions are all in the MeV range before they are slightly likely. The Sun, and other starts, get away with it by being so immensely dense by dozens of orders of magnitude, and that the reaction rate is the density squared! Just as well, as the Sun is a cool, smouldering tinder by comparisons with what we need to do here on earth to get anywhere near usable power densities, plus it uses the p+p reaction which is so unlikely it can barely be measured here on earth, but is the only thing that half-works at the cool temps of the Sun (which doesn't really 'do' much CNO).

[taniwha]

It's not the shielding of the gamma rays that bothers me (as you said, just add (appropriate) mass), but rather the waste (Carnot efficiency, radiator mass, etc).

p+15N being aneutronic, producing reasonable energy, and having the shortest "time" (and thus highest probability) is what made me agree.

I provided the information only because it seemed there might be interest in it.

Thanks for the confirmation that the times are the half-life. Do you have more information on the CNO cycle? Wikipedia is rather under-informative.

[chrismb]

I think CNO has been discussed before. The wiki article reads well enough to me, what aspects do you wish to know more about?

Mainly you need to understand more about cross-section and rate of reaction, then just look at the data.

The thing is, here on earth we'd not be trying to go for the whole cycle, we'd only pick the 'cream' reactions in any process. D+T is chosen for thermal plasmas due to the very large specific energy output and the biggest cross-section amongst light nucelii fusions at just about the lowest energy. Sure, it emits its energy as neutrons, but p+11B emits neutrons as well and you'd still need shielding, so why not make it easy on yourself?!

[KitemanSA]

12C + 1H → 13N + γ +1.95 MeV 1.3e7 years

These half-lives seem odd. Not being a nuclear physisist, I had the layman's notion that gamma decay typically happened VERY quickly as a result of the excited nucleus needing to loose energy. Seems odd that an excited nucleus can stick around for thousands of years before loosing the energy.

Where this is leading is, shouldn't the values listed be in Xe-Y years rather than XeY years? If it is as written, I have a concern expecting a viable fusion energy system with p15N given that it would take 112000 years to get half the energy out.

Please illuminate!

[MSimon]

I think CNO has been discussed before. The wiki article reads well enough to me, what aspects do you wish to know more about?

Mainly you need to understand more about cross-section and rate of reaction, then just look at the data.

The thing is, here on earth we'd not be trying to go for the whole cycle, we'd only pick the 'cream' reactions in any process. D+T is chosen for thermal plasmas due to the very large specific energy output and the biggest cross-section amongst light nucelii fusions at just about the lowest energy. Sure, it emits its energy as neutrons, but p+11B emits neutrons as well and you'd still need shielding, so why not make it easy on yourself?!

Easy fusion may no t be the best fusion. Plant cost enters in. And then you have to figure that a large steam TG plant takes about three years to build the turbines while a collector and associated electronics might take six months.

So more difficult would leads to faster roll out - if it can be done.

Which explains why you still need engineers even though the math is easier.

[chrismb]

These half-lives seem odd.

See my earlier reply. These are the 'dwell' times that these reactants need to dwell for, in the Sun at the Sun's core temperature, to lead to this reaction. This is a reaction rate, not a half-life of the fusion products.

[D Tibbets]

It's not the shielding of the gamma rays that bothers me (as you said, just add (appropriate) mass), but rather the waste (Carnot efficiency, radiator mass, etc).

p+15N being aneutronic, producing reasonable energy, and having the shortest "time" (and thus highest probability) is what made me agree.

I provided the information only because it seemed there might be interest in it.

Thanks for the confirmation that the times are the half-life. Do you have more information on the CNO cycle? Wikipedia is rather under-informative.

A model for comparing P-P and CNO fusion-
http://www.astrophysicsspectator.com/to ... NOSim.html

Concerning P-N15 fusion. I understand it is the quickest reaction in the CNO cycle. Any other CNO reactions that could follow from the C12 produced would be infrequent side reactions. How infrequent they would be I don't know. If the C12 produced has enough kinetic energy to escape (like the alpha particles in the P-B11 Polywell) there would be practically no side reactions.
I believe the fusion rate for P-N15 does not quite match that of P-B11, and presumably bremstrahlung radiation would be worse. If you can burn P-B11 realitively easily in an advanced Polywell, the increased handicaps for P-N15 might be tolorable if logistically it is easier to obtain the N15- such as in a spaceship harvesting fuel from ices obtained from small moons or comets.

Dan Tibbets

[chrismb]

The thing with p+15N is that it really is aneuontric whereas p+11B is a fake aneutronic seeing at it still puts out some 1% neutrons so that it needs some 50% of the shielding for DD. Why bother?

[kunkmiester]

Where are the neutrons from in PB11 again? I thought they were from some side reaction with the hydrogen. Would using Nitrogen really alleviate this?

[chrismb]

Where are the neutrons from in PB11 again? I thought they were from some side reaction with the hydrogen. Would using Nitrogen really alleviate this?

http://en.wikipedia.org/wiki/Aneutronic ... 1B_reactor

There are no other outcomes for p+15N excepting the a+12C (and possibly a very rare hv+16O outcome, easily shielded for).

[KitemanSA]

What would an alpha + 15N do? That is the analogous reaction to the main neutron producing reaction of the pB11 cycle.

[taniwha]

Ah, nice. I was wondering where the neutrons and gamma rays would come from. Even better, I've got some numbers to work with.

However, one important thing I noticed: that article is for thermal plasmas (tokamak etc?). I imagine a polywell would be quite different (though how is another matter). That would explain the comments I've seen about gamma power being about 0.1% and neutrons being 1e-3 to 1e-9 (for my shield calculations, I used 0.1% for both).