p + 15N

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

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chrismb
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p + 15N

Post by chrismb »

{This is a continuation thread from; viewtopic.php?p=54353#54353 }

Helius wrote:Where can I briefly see/read more about P+N15
You can get reaction data off of http://www.nndc.bnl.gov/, but as for a discussion on its use as a fusion fuel then as far as I am aware, only what I have written about it exists.

It just won't work as a thermonuclear fuel due to a lowish energy output and what would, ultimately, be a crippling amount of brems.

For IEC methods, I think the general conclusion is p+11B is hard enough and p+15N just isn't on most people's map yet. I crunched the numbers because it was relevant for me to do so with regards my device.
Helius wrote: I'm surprised that the reaction would sputter carbon 12; You'd think there'd be too much energy for the carbon to hold together, and instead come apart into Be and an He
'Sputter' isn't the correct term. An excited 20Ne momentarily exists at 5MeV above ground state. That is enough to shove off a 4MeV alpha, leaving a 12C recoiling with 1MeV, which isn't enough energy to split the 12C further.

Helius wrote:does it spill its energy in a Gamma, or in the energy of the first alpha to prevent it?

What's the scoop? Has there been anyone whacking N12 with protons in a collider or IEC device somewhere?
The p+15N is a strong-mediated reaction. In fact, there is only this one strong-mediated outcome, which makes it truly aneutronic [rather than p+11B for which there are neutron-producing reactions, so neutron shielding is still required for p+11B].

There are weak mediated reactions, where a proton shrugs off a positron to form a neutron (e.g. p+p->D), which are very very very low in terms of probability (i.e. have a very small fusion cross-section). There are electromagnetically-mediated reactions that shove off a photon and leave the nucleon count the same in one nucleus (e.g. the D+D->4He + hv), and these are far far more common that weak ones (by 10's of oom), but are ~4 oom less likely than strong mediated reactions.

Strong-mediated reactions are where the energy goes into recoiling parts of the nucleii (without any photons emitted - because it is not mediated by the electromagnetic force!). What we want to aim for, here on earth, is only the strong mediated reactions because recoiling nucleons are heat, and as such we can do something with that to get energy from it.

The p+15N is the most common strong-mediated fusion reaction in the solar system, because it is the final step of the CNO cycle and it is the only strong mediated reaction in CNO. Each of the other 3 fusion steps are EM mediated [therefore take a longer time to actually happen, within the Sun]. This final step, which finally joins all the 4 protons together to liberate a 4He, is the quickest CNO fusion, due to it being strong-mediated (notwithstanding the 13N and 15O positron decays, which are much quicker than any of the fusions).

12C + p -> 13N [takes average 1e6 years in our Sun]
13N -> 13C + positron [takes av. 14 minutes]
13C + p -> 14N [takes av. 3e5 years]
14N + p ->15O [takes av. 3e8 years]
15O -> 15N + positron [takes av. 82 seconds]
15N + p -> 12C + 4He [takes av. 1e4 years]

(this time data as given on http://www.tim-thompson.com/fusion.html)
Last edited by chrismb on Sun Jan 09, 2011 12:56 pm, edited 5 times in total.

chrismb
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Post by chrismb »

p+11B in green. p+15N in blue.

p+15N is 10% reaction rate of p+11B at ~200keV, then the gap closes and both are 0.1 barn at ~300keV.


Image


{For my device, my simulations suggest it would run more efficiently the higher the fast ion energy is, so p+15N at 1MeV may actually be preferable to p+11B at its peak.}

Stoney3K
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Post by Stoney3K »

Food for thought here: If we could make p-15N work, we'd essentally solve the world's energy problem once and for all.

You could simply drop one of these things in the middle of nowhere and bootstrap it with some energy storage, after which it would literally run on thin air.

0,37% of the air we breathe is 15N, which would also make it a very good candidate for vehicle power sources, since you don't need to carry any fuel. The H2 can be pulled from the atmosphere or the water vapor in the air sucked in.

The 3He production is also an interesting prospect.
Because we can.

chrismb
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Post by chrismb »

Indeed, it is a component of air.

The 15N present is as a result of cosmic radiation bombarding the upper atmosphere. Without cosmic radiation, there would be no 15N, as far as I have read.

There is enough 15N in the atmosphere, mind, to generate the same energy as around 100 million years of current power output. I think that was the figure I previously calculated. Mind you, it'd depend on how efficiently you could extract this, as to how long that'd last you.. I do not know if the amount of cosmic rays would keep up with the consumption, mind you. Would 100 M years be long enough to replenish all the 15N?

But it is an interesting point to note; that if p+15N were possible then the 15N would be acting as an 'energy vector' by which we are extracting energy from cosmic radiation!

icarus
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Post by icarus »

Intriguing topic.

chrismb:
{For my device, my simulations suggest it would run more efficiently the higher the fast ion energy is, so p+15N at 1MeV may actually be preferable to p+11B at its peak.}
Care to elaborate on this point?

Guess: the fast ion being N15 or B11, so similar charge:mass ratio of N15 vs B11 gives similar orbital radius, (frequency) for the same E and B fields ....
but
KE = 9/2*(m)*(E^2/B^2) .... so higher energies for same E and B due to higher mass ... and how does this affect efficiency?

happyjack27
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Post by happyjack27 »

icarus wrote:Intriguing topic.

chrismb:
{For my device, my simulations suggest it would run more efficiently the higher the fast ion energy is, so p+15N at 1MeV may actually be preferable to p+11B at its peak.}
Care to elaborate on this point?

Guess: the fast ion being N15 or B11, so similar charge:mass ratio of N15 vs B11 gives similar orbital radius, (frequency) for the same E and B fields ....
but
KE = 9/2*(m)*(E^2/B^2) .... so higher energies for same E and B due to higher mass ... and how does this affect efficiency?
you really need to consider the rate of energy dissipation (loss) rather than the amount of energy stored in the system (conserved). but presumably the former would be proportional to the latter, so i suppose the point still stands.

though i have to say i think the "energy from thin air, literally" thing is pretty frickin' cool!

icarus
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Post by icarus »

happyjack:
you really need to consider the rate of energy dissipation (loss) rather than the amount of energy stored in the system (conserved).
Since you choose to interject why don't you just tell us the answer you are alluding to know .... ?

Cryptic half-statements just smell like BS .... surely not.

Stoney3K
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Post by Stoney3K »

chrismb wrote:Indeed, it is a component of air.

There is enough 15N in the atmosphere, mind, to generate the same energy as around 100 million years of current power output.
Wouldn't that mean that, once we get even *a* viable fusion reaction out of p-N15, the whole energy economics would collapse like a house of cards?

There is no reason for it to be efficient anyway -- since we're essentially bathed in and walking through potential fusion fuel on a daily basis, a terrible reactor just needs to be big enough to give enough output. The abundance of N15 would completely overshadow the economic challenges of making the whole darn thing efficient.

Once there is such a thing as p-N15 fusion, it means plenty of power anywhere, anyplace, anytime you never dared ask. Irrigation of a desert? Consider it done...

If the 3He byproduct is actually ejected from the reactor as such, it could in itself serve as fusion fuel for spacecraft or capital ships. Or, if necessary, as a bootstrap agent to get a reaction started on p-N15. It would also solve the problem of 3He mining on a commercial scale, as you could produce 3He fuel on any planet with a Terran-like atmosphere.

There is also the question to what extent the 15N gets replenished by cosmic radiation. If the irradiance is enough, we could be looking at a sustainable power source - indefinitely.
Because we can.

chrismb
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Post by chrismb »

I think the issue is power and power density. Yes, a viable p+15N would mean unlimited energy, but no scheme can supply unlimited power. If a p+15N reactor were x1000 the size of a fission reactor, which well it might be because the reaction doesn't give much per-reaction, then clearly there would be other power sources that would remain part of the mix, according to the requirement for specific power density as may be demanded for a given application.

chrismb
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Post by chrismb »

Stoney3K wrote:If the 3He byproduct is actually ejected from the reactor as such, it could in itself serve as fusion fuel for spacecraft or capital ships.
I'm not sure where 3He has been mentioned. Have I made a typo somewhere? The reaction is; p+ 15N -> 4He + 12C + 5MeV

Stoney3K
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Post by Stoney3K »

chrismb wrote:
Stoney3K wrote:If the 3He byproduct is actually ejected from the reactor as such, it could in itself serve as fusion fuel for spacecraft or capital ships.
I'm not sure where 3He has been mentioned. Have I made a typo somewhere? The reaction is; p+ 15N -> 4He + 12C + 5MeV
http://en.wikipedia.org/wiki/Aneutronic_fusion reads:

p + 15N → 12C +3He + 5.0 MeV

(Ref. 1: Harms, A. A., K. F. Schoepf, G. H. Miley, and D. R. Kingdon. (2000.). PRINCIPLES OF FUSION ENERGY An Introduction to Fusion Energy for Students of Science and Engineering. World Scientific Publishing Company. pp. 8–11.)

It might also have been a typo on one of the WP editors' part. 12C + 3He doesn't really add up to being aneutronic ;)

I don't have the book referenced so there's little I can say about its correctness.
Because we can.

kunkmiester
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Post by kunkmiester »

The brem losses would be an issue I suppose. My though was that with nitrogen, you don't need neutron shielding, therefore, a portable polywell would be much lighter, though larger in volume due to the deeper well needed. This might be preferred for making an SSTO vehicle or other such related systems.

If cosmic radiation generates the nitrogen isotope needed, then it's a renewable resource. :lol: This might beat using boron anyway, since boron is somewhat limited, and has a variety of uses besides fusion fuel.
Evil is evil, no matter how small

Stoney3K
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Post by Stoney3K »

kunkmiester wrote:This might be preferred for making an SSTO vehicle or other such related systems.
It would even be a perfect candidate for SSTO, since the reason it carries an enormous load of fuel is because it needs to produce a truckload of thrust to counteract air resistance along with gravity.

Hey, wait, what was this thing using as fuel again? ;)

I can already imagine the coolness of having launch vehicles with massive jet intakes. Given the fact that you need to carry only a tiny bit of fuel (if any at all), this would make spacecraft almost featherlight. The only thing that would add weight is payload (and some fuel supply that you'd use when you're clear of atmo). The word 'holy grail' comes to mind.
chrismb wrote:I think the issue is power and power density. Yes, a viable p+15N would mean unlimited energy, but no scheme can supply unlimited power. If a p+15N reactor were x1000 the size of a fission reactor, which well it might be because the reaction doesn't give much per-reaction, then clearly there would be other power sources that would remain part of the mix, according to the requirement for specific power density as may be demanded for a given application.
True, but if the power density of a pN15 reactor proves to be insufficent or impractical for the application needed, the limitless supply means you can just run a land-based reactor forever to produce any fuel you want. Hydrogen? Gasoline? Just determine what's practical enough and fill your vehicle of choice up with that.

However, the produced fuel would only be a mode of energy transport, not requiring any of our planet's resources to make it. Other than air and water, which are readily available.
Because we can.

chrismb
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Post by chrismb »

Stoney3K wrote:http://en.wikipedia.org/wiki/Aneutronic_fusion reads:

p + 15N → 12C +3He + 5.0 MeV

(Ref. 1: Harms
It's just a typo. Count up the nucleons on the left, 16, and on the right, 15....

chrismb
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Post by chrismb »

Stoney3K wrote:True, but if the power density of a pN15 reactor proves to be insufficent or impractical for the application needed, the limitless supply means you can just run a land-based reactor forever to produce any fuel you want.
It still depends on the final power density of such a reactor. There is no guarantee one could be built, so our whole ignorance of the technology might simply mean we're as yet unaware that such a reactor cannot produce more than 1W per m^2... in which case some solar panels would be better!

So there is still a lot riding on what power outputs are actually possible from fusion reactors. If a country requires a PW of power, but the reactors to generate a PW take up more space than that country...well.... y'see, it is still all to play for. But this thread is about a wishful, though very slightly plausible, hope that one day we can fuse thin air and get lots of power yet no neutrons whatsoever.... we can but hope and dream... (and maybe some of us can even design and build what we hope might do it!).

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