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Fast ignition fusion approach

  1. Apr 7, 2014 #1
  2. jcsd
  3. Apr 7, 2014 #2


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    I am not sure, but I think the answer to your question is "neither". I think it is still considered a promising approach which has not yet been experimentally verified because the necessary laser hardware isn't there yet. Anybody else know more?
  4. Apr 7, 2014 #3
    Is there any difficulty to construct a laser set they describe? It should be pretty easy to do?
  5. Apr 7, 2014 #4
    Fast ignition is an ongoing area of active research in the ICF community.

    In theory both laser beams and ion beams can be used to heat the hot spot and ignite the pellet. Fast ignition requires a lot of energy being deposited in a very small volume, at a very precise time. Building such a laser or accelerator is not easy. The physics that governs how these high energy dense beams interact is also not trivial.
  6. Apr 8, 2014 #5


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    Take a look at the scale of the lasers at the National Ignition Facility. Now you have to build a new laser with a tighter focus and a very short pulse width and integrate it with the other 192 lasers which are already in place. I don't think I would describe this as "pretty easy to do."
  7. May 24, 2014 #6
    Fast ignition is pretty much dead. We can't make the electrons do what we want them to do. There is little funding left for it.
  8. May 25, 2014 #7


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    I wouldn't say fast ignition "is pretty much dead", AT ALL!. As the_wolfman states, it is an ongoing area of research of the ICF research community. The research in fast ignition isn't at the experimental stage yet; but more in the theoretical and design phases as the details of the technique are worked out.

    For example, there's a LOT of computer simulation of fast ignition ongoing at Lawrence Livermore, including some of the largest single simulations ever done:


    As far as the coordination mentioned by others, the delivery of the ignition pulse laser beam would be similar to coordination of diagnostics already routinely performed on NIF. While NIF has 192 lasers, not all of them may be used to drive the fusion capsule. Some of the beams may be used to drive the diagnostics, like X-ray backlighting.

    Let's say thhe NIF scientists would like to get of measurement of the capsule geometry at a precise point in time during the capsule implosion to see how well the experiment matches their numerical simulations. One way to do that is to take a fast X-ray of the capsule at the time of interest.

    Rather than use all 192 beams to drive the capsule, NIF uses some of the beams to irradiate a target to produce X-rays at precisely the time of interest. The X-rays "backlight" the capsule and the X-ray image is recorded on detectors to see if the position of the various shells in the capsule are where the numerical simulation predicted they would be. That means that certain beams on NIF are fired in a way different from the main group that is driving the experiment. Those beams fire so that one gets a very short pulse of laser energy which is directed at the X-ray backlighter target at the appropriate time to give the short pulse of X-rays that will record the position of the capsule shells. LLNL lasers like NIF and its predecessor Nova, have had the ability to assign different missions to different beams for decades now.

    Well, if you think about it, the firing and coordination of the X-ray backlighter lasers is similar to what one needs to do when one if preparing an laser ignition pulse.

    Last edited: May 25, 2014
  9. May 25, 2014 #8

    What you are describing is an x-ray backlighting diagnostic. This is different than trying to ignite the compressed fuel with a beam of hot electrons.

    It was concluded in a National Academy of Sciences report last year that fast ignition was not the most promising path to fusion due to complications with controlling the fast electron beams. Many of the scientists who specialized in fast ignition are now changing research directions.
  10. May 26, 2014 #9


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    Evidently you have a reading comprehension problem because I stated explicitly in the above post that what I was describing was a backlighting diagnostic. Evidently, you didn't understand the last paragraph where I ANALOGIZED the coordination of the backlighting technique to the coordination needed for fast ignition.

    There is actually renewed interest in fast ignition since the "central hot spot" technique championed in the National Academy of Sciences report FAILED to achieve fusion ignition in recent trials at the National Ignition Facility. The "central hot spot" technique requires a high degree of symmetry which is proving to be rather difficult to achieve.

    As detailed in:


    the fast ignition technique relaxes some of the constraints of high degree of symmetry imposed by the "central hot spot" technique.

    There's lots of not well understood phenomena and much difficult physics to take into account, that it is really premature for anyone, even the National Academy to be eliminating or tossing aside any ICF approaches.

    When "central hot spot" or some other scheme achieves success in achieving fusion ignition; then we can eliminate other contenders. But it would really be irresponsible to eliminate contenders as of yet.

    The National Academy of Sciences has NOT done the irresponsible and called for the elimination of fast fusion; contrary to the ERRONEOUS contentions you have made earlier in the thread. Must be more of that poor reading comprehension thing.

  11. May 26, 2014 #10
    Why are my claims erroneous? The report specifically quotes " Conclusion 4-5: At this time, fast ignition appears to be a less promising approach for IFE than other ignition concepts." Complicated target design and problems with laser-target energy coupling were cited.

    You can argue all you want about how we have the technology to create the beams that we want, but it doesn't change the fact that we are unable to achieve good energy coupling between the beam and target. You are delusional if you think fast ignition is a "hot topic" in the ICF community.
  12. May 26, 2014 #11


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    Yes - during the past year, the approaches that the National Academy of Sciences recommended pursuing did NOT achieve ignition on NIF.

    Not achieving good energy coupling, and not achieving good symmetry can also be said for the schemes that were recommended by the National Academy of Science report.

    In fact, the best results that have been obtained on NIF have been using techniques pioneered by LLNL's WCI Directorate, and not the National Ignition Campaign.


    Evidently, YOU are the one that doesn't understand what work is being done when you characterize the "fast ignition" concept as being totally dead and the researchers moving on to other pursuits. As I showed previously in this thread, there is still a lot of ongoing work in pursuit of the fast ignition concept, including some of the most detailed supercomputer simulations done to date on LLNL's Sequoia supercomputer:


    The fact that these simulations are being researched doesn't seem to be compatible with your characterizationn of fast ignition being a "dead" concept.

  13. May 26, 2014 #12

    jim hardy

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    Fascinating !

    from first link:
    What discourages the alphas from going their own separate ways ?
  14. May 26, 2014 #13
    You are giving me a link of a fast ignition simulation dating more than a year ago. Can you tell me what useful results have been generated with since then?

    I'm also not saying that there is nobody in the US who is studying fast ignition, but I am seeing a trend of researchers moving away from it. If you really do not notice this, then I am guessing that you are not actively involved in the ICF community.

    If we cannot understand how normal cryo implosions are performing, then how can we possibly understand what is going on in a fast ignition scheme which has much more complicated targets?

    I also don't understand your point about the hight foot campaign. That has nothing to do with fast ignition. They increased the adiabat to stabilize their implosions.
  15. May 27, 2014 #14


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    I didn't say that the high foot work had anything to do with fast ignition. I was merely commenting that the most promising results of late have been coming from a different group, and not the ICF community. When I begin a new paragraph, then I'm beginning a new topic that not necessarily connected to the statements of the previous paragraph.

    Most of the problems aren't so much with understanding the implosion; but has more to do with the drive and laser / plasma interactions, and how small perturbations in the geometry can seed instabilities that destroy symmetry.

    No - I'm NOT actively involved in the "ICF community". I'm in the same field / lab directorate as the people who did the "high foot" work.

  16. May 27, 2014 #15


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    The alphas are charged; doubly so. Each alpha has a charge of +2; so it feels the Coulomb ( electrostatic ) force from all the ions and all the electrons. That's why alpha radiation is so relatively short ranged; because the interaction is so strong. Prior to NIF, the lasers weren't as powerful and could only implode really small capsules. Because the capsules of previous lasers were so small, they couldn't trap the alphas let alone the neutrons. ( Of the 17.6 MeV of energy that comes from a D-T fusion reaction, 14.1 MeV goes to the neutron and 3.5 MeV goes to the alpha. Small capsules have very little chance of trapping the neutron; the mean-free path of a 14.1 MeV neutron in most materials is measured in 10s of centimeters ) One needs to recover the energy of the alpha in order to sustain the fusion process; just like the heat released in a fire is needed to sustain the fire. Take away the heat, and the fire goes out; which is why dousing with water is a good way to put out a fire.

  17. May 27, 2014 #16


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    Inertia, for the time required, as in the name of this approach to fusion.
  18. May 27, 2014 #17


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    The goal of controlled fusion is to obtain sufficient energy from the alphas to 1) heat a magnetically confined plasma, or 2) release maximum amount of thermal energy.

    The disadvantage to d+t fusion is that so much energy (14.1 MeV of ~17.6 MeV) is carried away by the fast neutron. That has implications on the first wall and structures, as well as the thermal conversion system, e.g., thermal blanket, in which most of the neutron energy is deposited as heat, and then as activation of whatever structure or coolant there is.

    In magnetic confinement, the alpha particles would curve around the magnetic lines as well as travel along the lines, and in the process, collide with other nuclei and electrons, thus heating the plasma. They would also recombine with electrons.

    In the inertial confinement capsule, the alphas would travel in all directions, out of the capsule at the surface, and into the capsule. However, the capsule first compresses then expands. As the capsule expands, the alpha heating diminishes as does the fusion of d+t (or d+d, t+t)
  19. Nov 14, 2014 #18
    Initially they proposed to use a picosecond laser as the ignitor in fast fusion. What if instead we use femtosecond or even attosecond laser for this purpose? Could it change conditions of fast fusion drammatically or not too much?
  20. Nov 14, 2014 #19
    We can't build a femto or attosecond laser with sufficient intensity to "spark" the fusion reaction. In order to spark fast fusion you need to deliver sufficient energy to your target. As you go to shorter pulses, the necessary intensity of your laser increases.
    And lets not ignore all the physics of the laser-plama interactions. These tend to get more detrimental as you go to extreme laser intensities.
  21. Nov 15, 2014 #20
    As you go to shorter pulses, the necessary intensity of your laser increases.
    What do you mean as "intensity"? Lasers are characterized by power and energy. As we go to shorter pulses their power automatically increases, but energy requirements (for the fusion) diminish. The very point of fast ignition is to use picosecond laser which has higher power, but much smaller energy demands. I wonder what will happen if they will proceed to even much shorter pulses.
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