Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A What is wrong with particle acceleration based fusion?

  1. Mar 10, 2017 #1
    Recently, I was thinking about fusion and this thought struck my mind.

    • In tokamaks, the plasma is heated to extremely high temperatures in order to supply enough energy to the ions for them to fuse. But since, the plasma follows a boltzmann maxwell distribution curve,only a few ions have have enough energy to fuse.

    • But, while this is happening, there also is a lot of energy loss due to Bremsstrahlung radiation.
    Now, what I was thinking about is: If we somehow (rather mysteriously) confined a low temp plasma comprised of deuterium ions and an equal amount of electrons (quasi-neutral and approx 60000K or lower) magnetically, and accelerate tritium ions (like a dense particle beam) and make them bombard the plasma. Could we gain net energy?

    The reasons I think this would produce a Net gain of energy:

    • Well apparently, since the accelerator beam isn't in thermal equilibrium (wasn't given enough time), almost all collisions would result in fusion.

    • Previously, many people told me that the reason this can't produce net energy is because the coulomb interaction cross-section is much higher than the nuclear cross section, But I suppose that given enough density of the confined plasma, even when the nuclear cross section is smaller than the coulomb cross section to produce more energy than is lost. This is also the case in tokamaks.

    • The low plasma temperature can be helpful in producing high number densities in favor of the above point.

    • Yes, the energy of the alpha particles resulting from fusion heats the gas, but we can find ways to cool the plasma to maintain confinement.

    • I don't think that the cross section of coulomb interactions or Bremsstrahlung radiation would be considerably different from tokamaks, since we are dealing with the same energy levels (This statement is probably wrong, so it would be great if some one can help me on this)

    • Now, compared to tokamks, in which electrons have substantial amounts of energy and extremely small mass and emit a lot more Bremsstrahlung radiation than the particle beam I was talking about or ions in the plasma, radiation losses in our model might actually be smaller.
    And for the to compensate for the losses, thee particle beams energy per particle must be slightly higher, for example 30 Kev instead of 25 Kev

    With all this in mind, I think that the model we were talking about can produce net energy. So, where am I going wrong in my thinking? Where am I making wrong assumptions?

    Any help would be appreciated

    EDIT: Forgot to mention that we would actually add an equal amount electrons to the plasma as the number of ions penetrating the plasma to maintain quasi neutrality.

    EDIT: I just realized that we could use a helical magnetic field to make the particles spiral. So, the particles would spiral around the field lines and along the field lines. The lower the pitch of the helix, the more the particles interact with the plasma. So this would increase collision cross sections drastically wouldn't it? Are there any disadvantages in doing this?
  2. jcsd
  3. Mar 10, 2017 #2


    User Avatar
    2016 Award

    Staff: Mentor

    This is wrong, and I gave you the numbers for that comparison in a previous thread already. Only a tiny fraction of all tritium nuclei would fuse, most would just scatter from deuterium nuclei and electrons. This is independent of the densities of plasma and beam. High plasma density just means the whole process happens in a smaller volume.

    Adding a magnetic field wouldn't change anything. You still have particles entering a plasma.
  4. Mar 10, 2017 #3
    But why is it so? What does cross section depend upon. I was thinking that the more density you have, the smaller the spaces in between the particles. And hence, the collision would increase compared to Colomb interactions. If the particle didn't go head on into the ion, it would get scattered, and the probability is extremely high compared to the probability of bombarding an ion. But a 30 Kev tritium nucleus has about 5 Kev to lose and still fuse. The only way a particle could lose energyis through Bremsstrahlung radiation. Otherwise, all collisions would result in fusion. So, how would a particle lose more than 5 Kev of energy just through Bremsstrahlung radiation? And if a particle really loses that much energy that fast in a Quasineutral plasma, Then wouldn't the same loss apply to tokamaks? How are they not losing their energy almost instantly? Thermalization occurs almost instantly though, I understand. But if the ions have enough energy to fuse when they get close, would an elastic collision even be possible without fusing? Then how do they even attain thermal equilibrium (the plasma) and how would the particle lose energy apart from Bremsstrahlung radiation? This is what I am extremely confused about. Would elastic collisions even be possible without the ions fusing, and hence, how will the plasma ever attain thermal equilibrium?
    Last edited: Mar 10, 2017
  5. Mar 10, 2017 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    First, your avatar. You're no Einstein. Sorry, but that's what it is.

    Second, asking the same question over and over again is not going to work. Mfb answered you before.

    Third, spreading things out over multiple threads makes it impossible to help you.
  6. Mar 10, 2017 #5
    They were actually supposed to have different themes but on the same topic which I kinda messed up somehow. But the 2 questions turned out to be kind of similar. Sorry. Anyway..... you aren't vanadium, are you? Just kidding.

    Yeah, I will be posting questions in different threads next time If they have a completely different topics. Got it.
  7. Mar 10, 2017 #6


    User Avatar
    2016 Award

    Staff: Mentor

    The Coulomb scattering increases in exactly the same way as fusion (if you consider it as probability per length).

    A tritium nucleus scattering with a deuterium nucleus can easily lose more than 50% of its energy with the first scattering process. Scattering with electrons will lead to smaller losses per collision, but the cross section is much larger, so this process is important as well.
    This is wrong, and it gets boring to repeat this over and over again. Elastic scattering will dominate the nucleus-nucleus interactions. Even at the LHC energy of 6.5 TeV per proton elastic scattering is still a common process.
    If all particles are hot and thermal, elastic scattering is no energy loss, and emitted radiation can be re-absorbed by the plasma.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: What is wrong with particle acceleration based fusion?