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A Is Positron-Catalyzed Fusion Possible?

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  1. Dec 6, 2016 #1
    I was wondering if fusion based on positron-electron annihilation would be possible; my intuition tells me it is, but I'm not completely sure and wanted to consult this community.

    Essentially, I was wondering if a stream of positrons (ignoring the source and difficulty of production for a moment) could annihilate some of the "free" electrons in a plasma, releasing energy and causing fusion ignition. I would imagine this is possible, and it seems to me that companies like Positron Dynamics are pursuing this. Interestingly, they are smashing a target of non-ionized fusion fuel with the positrons; what would be the advantages/disadvantages of doing this instead of ionizing your fuel first and then effecting annihilation?

    Furthermore, I'm probably making an extremely elementary mistake here, but here it goes. I'm sorry if it's a dumb question, but it's really bugging me. Wikipedia states that the energy required for proton-boron fusion is 600 KeV, while D-T requires 66 KeV; meanwhile, the collision of an electron and positron produces at least 1022 KeV overall. This would make it seem as if you only need a single positron (perhaps a few more if some of the gamma rays don't hit the nuclei completely) to cause fusion ignition, which seems absolutely ridiculous and almost too easy. Where did I go wrong? Sorry if I'm missing something easy, but I'm curious.

    Thanks!
     
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  3. Dec 6, 2016 #2

    mathman

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    Positron+electron gives 2 511 kev photons, going in opposite directions. For the D-T reaction the nuclei have to collide. There is no obvious way that the 511 kev photon will force the collision.
     
  4. Dec 6, 2016 #3
    They supposedly energize the nuclei and coax them to eventually collide; the energy released pretty much energizes the ions from what I've heard. I asked an astrophysics graduate student who said it should work (he wasn't completely sure about the specifics) but I wanted to clarify.
     
  5. Dec 6, 2016 #4
    Sorry if my previous comments were essentially just speculation, here is a more concrete explanation and source. People have been thinking about using gamma rays to compress fusion fuel and effect fusion reactions. They've been thinking about using it to build a pure fusion weapon, but I'm sure the same concept could be applied to energy. This is essentially what I propose; we would use anti-electrons to produce gamma rays that are very high energy, highly compressing our fusion fuel and possibly igniting it.

    What do you think? Thanks!
     
  6. Dec 6, 2016 #5
    From https://en.m.wikipedia.org/wiki/Pure_fusion_weapon
    It seems to me these people thought about it a great deal and only found a dead end.
     
  7. Dec 7, 2016 #6

    Vanadium 50

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    Your Wikipedia source doesn't even mention positrons.

    You got the correct answer in #2. Not liking the answer is not a good reason to dismiss it.
     
    Last edited by a moderator: Dec 7, 2016
  8. Dec 7, 2016 #7
    I'm sorry if it seems like I'm dismissing mathman's answer, I'm not, but I'm still interested in this idea. I know that the Wikipedia source doesn't mention positrons, but one of the reasons I was thinking positron catalyzed fusion would work is that it releases gamma rays. The Wikipedia source is centered around the use of gamma rays to catalyze fusion. It is possible that the government did not know about/was not interested in working with positrons (they were very expensive and still are, I guess).

    I'm asking this question precisely because the wikipedia article did not mention positrons. It's possible they did not fully investigate the use of gamma rays released from positrons to catalyze fusion. That's why I'm asking this question: since positron-electron collision releases high-energy gamma-rays, couldn't those gamma rays theoretically compress plasma in a confinement chamber, resulting in fusion reactions, or even ignition?
     
  9. Dec 7, 2016 #8

    mfb

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    You underestimate the amount of effort put into those considerations by orders of magnitude. Those concepts are not "someone once thought about it". They all have computer simulations of various possible designs, with long discussions which designs to test and how to improve them.

    The Wikipedia article does not mention positrons because they are so much worse than the other approaches discussed there.

    Positron annihilation is a horribly inefficient way to produce gamma rays.
     
  10. Dec 7, 2016 #9

    ChrisVer

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    I was wondering, can 511keV photons be considered highly energetic to give enough energy [for fusion] to nuclei [too heavy]?
     
  11. Dec 7, 2016 #10

    mfb

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    The reaction ##\gamma + N \to \gamma' + N## (which is very unlikely on its own) leads to a tiny nucleus energy for 511 keV photons. Up to 1 eV for protons, lower for all other nuclei.
     
  12. Dec 7, 2016 #11

    ChrisVer

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    So I guess you agree with my point then (given by a questionmark)...
     
  13. Dec 7, 2016 #12

    mfb

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    They are not high-energetic enough to give nuclei sufficient energy. They can be used to heat and compress material - that is actually done with inertial confinement fusion. But not with positrons, because that would be way too inefficient.
     
  14. Dec 7, 2016 #13
    Hold on -- are you saying that if you had enough/enough energy gamma rays , you could theoretically compress plasma and cause fusion? Let's just suppose we had access to a bunch of positrons and didn't care about the efficiency for a moment (as a hypothetical), -- how many gamma rays/what energy rays would you need, and how many positrons would be necessary to produce this amount and ignite fusion? Just wondering, sorry if is too vague of a question. I'm interested specifically in the proton proton, deuterium tritium, and proton boron and reactions.

    I'm a bit confused by this claim; I'm inexperienced, so I'm probably going wrong somewhere, but wikipedia states that d-t reactions require energy of 66 kev and p-b11 requires energies of 600 kev.

    Thanks in advance!
     
    Last edited: Dec 7, 2016
  15. Dec 7, 2016 #14

    mfb

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    This is called inertial confinement fusion: use a high-energetic laser to heat a hollow target that emits x-rays. Those x-rays hit the outer layers of the fusion target, blowing them up and sending a shockwave to the interior, compressing the fusion fuel inside. Fusion is initiated by the compression and heat due to the exploding outer layers.

    For research, the energy per shot can be low, if you want to get more fusion energy out than laser energy in, you probably need megajoules of laser power (no one achieved this goal so far, so this is just a lower limit).
    Gamma rays are worse than x-rays (they are not absorbed that easily), so you probably need more like 100 MJ, or 5*1021 positrons. In ~5 picoseconds, the typical NIF pulse duration, this would correspond to a current of 2*1013 A. As comparison: Particle accelerators might deliver something like 1 A if they are optimized for highest beam currents, or 500 A in pulsed operation. 1-500 A of electrons which are easy to get. 11 orders of magnitude higher beam currents, and with positrons? No way.

    This is for DT fusion. All other fuels would need even more absurd numbers.

    Compare this to the 1 eV value and you see that you cannot heat up nuclei with gamma rays directly. That was my point. The gamma rays can interact with electrons, heating the material that way. But there is no point in using x-rays for that.
     
  16. Dec 7, 2016 #15
    I don't understand. Where did you get the 1ev value? Wikipedia states that 66 kev is necessary for dt fusion, and the gamma rays produced by annihilation have at least 511 kev of energy. I don't fully understand why these rays can't be used to heat up nuclei.


    A second, separate question. What if you shot the positrons at a non-ionized target instead? Wouldn't that rip the electrons out and also release gamma rays at the nuclei creating energized ions that are likely to fuse?


    Also I never proposed using X-rays, so I was confused by your last sentence...

    Also I was under the impression that while gamma rays are difficult to absorb for whole atoms, they are more easily absorbed by nuclei.
     
    Last edited: Dec 7, 2016
  17. Dec 8, 2016 #16
    The actual probability of two ions fusing when they collide is very small. Most of the time when the ions collide they bounce off of each, exchange a little energy, and do not fuse. This simple fact necessities the need for confinement. If you want to produce energy, it's not enough to heat a plasma up to several tens of KeV. You have to confine the plasma for many collision times. Heating the plasma is easy. Confining the plasma is hard.

    It sounds like you've been focusing on heating the plasma, and you haven't really thought about how your proposed concept can be used to confine the plasma. As a naive answer you might assume that standard inertial confinement techniques could be adapted to your idea. The problem is that the energy of the 500 keV photons is too high. The vast majority of the photons will pass through a small capsule without interacting. And the few that do interact will uniformly heat the capsule. To efficiently compress the capsule you want most of the photon energy to be deposited in its edge.
     
  18. Dec 8, 2016 #17

    mfb

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    Fusion is not something that magically happens if you just have some amount of energy somewhere. Drop a pen on a table. The impact releases something like 1017 eV. Will it start fusion? Of course not. You need hydrogen nuclei with a kinetic energy of tens of keV. A bunch of gamma ray photons doesn't lead to high-energetic hydrogen nuclei: even if they hit a nucleus (unlikely), the just "bounce off" - like a table tennis ball hitting a car: it won't speed up the car in any relevant way.
    That was my assumption already.
    Well, x-rays are better. Heating the nuclei only works via heating the electrons.
     
  19. Dec 8, 2016 #18
    Apparently at high velocities positrons can be useful for generating muon pairs. Could muon catalyzed fusion using high energy positrons be a viable alternative?

    Also, you said one must heat up the electrons to heat up the nuclei. Why is this? Thanks!

    Why doesn't the tennis ball analogy apply to X-rays too?
     
  20. Dec 8, 2016 #19

    mfb

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    Not much more useful than electrons at the same energy, worse if you take the effort to produce them into account, and way less efficient than proton beams.
    Muon-catalyzed fusion doesn't even work with the most efficient muon sources we have. And you want to use an even less efficient muon source?
    Electrons are lighter, they pick up more energy if electromagnetic radiation hits them.
    It does, but the heating mechanism is via the electrons, not via the nuclei, as mentioned.

    Where does that fixed idea "I must use positrons for something fusion-related!" come from?
     
  21. Dec 8, 2016 #20
    [QUOTE="mfb, post: 5639059, member: 405866] Where does that fixed idea "I must use positrons for something fusion-related!" come from?[/QUOTE]

    Not sure, it just seemed to me like the tremendous amount of energy released by antimatter annihilation could have uses in fusion catalyzation; most people have proposed antiproton use but they're even more difficult to make than positrons, so there's that. I read about Positron Dynamics, who claim that positron annihilation can effect fusion in deuterium targets; they seem like a fairly legitimate company and are backed by Peter Thiel.

    Even if electrons pick up radiation more easily, what effect does that have on fusion? It's the nuclei and their energies that matter, not the electrons, right?

    Further, since gamma rays are ionizing and high energy, isn't it also fair to say they'd transfer some energy to the ionized material itself? I would think the same applies to icf with X-rays.

    Lastly, why do you suggest that positrons are a terrible way to generate gamma rays? They generate fairly high energy rays on impact with electrons, and nearly all discussion of gamma ray lasers (which have been proposed as efficient ways to catalyze fusion) centers around their use.
     
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