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Questions Regarding Nuclear Fusion

  1. Sep 19, 2011 #1
    Hello there physicists! I am here to ask a few straightforward questions regarding nuclear fusion accomplished here on Earth.

    Question 1: How does a magnetic field contain the heat produced by nuclear fusion in the reactor? If we look at the example of Deuterium fusing with Tritium, a high energy nuerton is release with an ample amount of energy. I assume this energy is automatically transferred to heat. In order for the heat to be contained (and not incinerate absolutely everything that surrounds it), it would have to energize ions. Am I correct in saying that the heat is contained through fast moving ions that are held back with the magnetic field? If I am, then, what happens to the neutrons?(Yes, I know it was a series of questions).

    Question 2: Why is the fusion process not sustainable on it's own? What I mean here is, if we were to simply bring the temperature high enough to start the fusion process, why then, could it not sustain itself and continue indefinitely so long as we supply the fuel (just like the Sun)? This arises from the fact that the current process of fusion established in the Tokamak produces less energy than the energy required to keep the fusion process going.

    Question 3: Why is cold fusion considered "impractical?"

    Thank you very much to all those who answer these questions. I greatly appreciate it!
     
  2. jcsd
  3. Sep 19, 2011 #2
    Gosh...
     
  4. Sep 19, 2011 #3
    Sorry about that. I didn't word that correctly. What was meant to be said was "a high energy neutron is released ALONG with an ample amount of energy." In other words, a neutron is released and so is an x amount of energy. Sorry again.
     
  5. Sep 19, 2011 #4

    Drakkith

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    Heat is simply the motion of particles in a material. The charged particles that are being contained are moving very fast, and hence have alot of heat. They are kept that way via magnetic fields keeping them from impacting the walls of the reactor. The neutrons released are not charged and do not interact with the magnetic fields, so they pass right through and are caught by the shielding. When this happens they collide with the atoms in the shielding and transfer their kinetic energy into it, heating the material up.

    The magnetic confinement is not perfect. At such a high temperature and pressure combination many particles simply "leak" out of the containment fields and are lost. In addition, the plasma itself is usually used to generate magnetic fields to assist in confinement. The plasma runs into problems called instabilities that cause the plasma to be disrupted and unable to confine itself. So we can initially get the temperature up and the confinement up to where fusion is occuring, but very quickly the plasma becomes unstable and we lose part of the confinement.

    All of this is before we even look at efficiencies in converting the generated power from the reactions into electrical power.

    It is considered impossible, not impractical. Quite simply, all claims of cold fusion seem to violate the laws of nature. I will not go into details, as this subject is against PF rules.
     
  6. Sep 19, 2011 #5
    ^^For the above response...I hate the word impossible. I don't see why it can't be done. Maybe not with today's technology but who knows what the future will hold. Also, why is it against PF rules? Are they that high and mighty of themselves that they feel they can simply declare something as unfit to speak of? If that is the case of this forum, I do not wish to be a part of it. Many great ideas have been deemed silly and "impossible" at the time they were put forth, yet, like Quantum Mechanics, they are true no matter how unbelievable.
     
  7. Sep 19, 2011 #6

    Drakkith

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    Then you should leave. It is against PF rules because it does not agree with current mainstream science. As that is what this forum is for, anything else does not belong. If it should turn out that it does work then it will be allowed, however currently it does not. This isn't because we are "too high and mighty" it is to avoid countless arguments over a great many crackpot ideas that have been shown to violate the laws of nature and do not work.
     
  8. Sep 19, 2011 #7

    Astronuc

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    Cold fusion has not been demonstrated, and previous claims of cold fusion have been discredited. The topic generally attracts crackpots and nonsense, therefore we have banned it as a topic of discussion.

    As Drakkith indicated, the majority (~80) of the energy in d+t fusion is carried off by the neutron. It interacts with and heats the first wall. That thermal energy could conceivably be converted into electrical energy by various thermo-mechanical conversion cycles. However, the desire is to use direct conversion as much as possible in order to maximize efficiency.

    Besides the loss of energy from neutrons, the plasma radiates energy from cyclotron radiation, brehmstrahlung and recombination of ions with electrons.

    The current challenge for magnetic confinement is to improve stability and increase confinement times in order to produce more energy than consumed.
     
  9. Sep 19, 2011 #8
    Say that it becomes possible in the future. What would you say then? I am not suggesting that it is by any means a great area to spend money and resources on, but, suggesting that it is impossible limits the creativity of the mind. I personally hate the word. It clouds the minds of many whom try to solve the simplest problems in complex ways just because there is a barrier stopping them from thinking outside their own realm of knowledge.
     
  10. Sep 19, 2011 #9

    Astronuc

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    Then we would allow discussion of cold fusion.
    Anything can be imagined to be possible, especially in fiction. We do not permit discussion of overly speculative topics at PF. There are many other sites that do.
     
  11. Sep 19, 2011 #10

    jim hardy

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    ""There are many other sites that do. ""

    http://www.fusor.net/

    is a site for cold fusion hobbyists.

    you might find discussions there.

    old jim
     
  12. Sep 19, 2011 #11

    Morbius

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    Suppose we have D-T fusion. The energy of the reaction 17.6 MeV / reaction ends up as the kinetic energy of the product He4 ( 3.5 MeV ) and the product neutron ( 14.1 MeV ). The He4 is an ion and charged, so the magnetic field traps it. The neutron is not trapped by the magnetic field.
    In order to keep the reaction going, you have to trap the energy produced. You essentially have no hope of trapping that high energy neutron. At present, however, we can't even trap the He4. Inertial confinement fusion hasn't produced a big enough plasma to trap the He4 alpha particle. It is hoped that NIF will change that. In magnetic fusion, the He4 is trapped, but magnetic fusion plasmas have so many instabilities, that they fall apart.
    Because cold fusion doesn't exist!!! It was all a big mistake by a couple of chemists.

    Dr. Gregory Greenman
    Physicist
     
  13. Sep 20, 2011 #12
    Thank you very much for your response. Honestly, question 2 was the most important for me. I don't care too much for cold fusion as I know about the chemists you speak of. I just don't like it when people place so much certainty on a field we know so little about. We live in a wonderful world full of surprise and excitement. When I think of physics, it reminds of the stories I used to read as a child where there were extraordinary things, aliens, monster, etc. I loved it. Physics is my storybook at my current age. It is one of the few things that lets my imagination run wild. I don't wish to place a limit on my creativity, that is all. Anyone can say that it can't be done. It takes a man with courage and determination to suggest otherwise when all others laugh. Please don't take my response as ignorance. I know very little with respects to nuclear physics and I have no expert opinion in the subject, I just would like to suggest people keep an open mind. Thank you again!
     
  14. Sep 20, 2011 #13

    Drakkith

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    That is NOT cold fusion. It is a proven way of doing fusion on a tabletop scale. The fusor has commercial uses as a neutron source, but unless some kind of crazy breakthrough comes along it will never provide anywhere close to breakeven power.
     
  15. Sep 20, 2011 #14

    Drakkith

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    And you think that labeling things as impossible is close minded? It is necessary! It's not even a choice! The laws of nature require us to label many things as being impossible. Not because anyone is close minded, but because if they weren't labeled that then untold thousands of people would be working in vain on something that will absolutely never see the light of day. I suggest to you that you learn real science before suggesting that anyone keep an open mind on a subject that effectively was an error or mistake in the first place.
     
  16. Sep 20, 2011 #15

    jim hardy

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    ""That is NOT cold fusion. It is a proven way of doing fusion on a tabletop scale. The fusor has commercial uses as a neutron source, but unless some kind of crazy breakthrough comes along it will never provide anywhere close to breakeven power.""

    that's why i so carefully chose my words. I suppose same could be said of hot fusion, though.

    To people with perhaps an "excess of imagination" like myself and the OP it's a delightful site.
    Farnsworth's background as a practical vacuum tube designer made him naturally look to the electric field for confinement rather than magnetic. You dont need big wires to make a stout electric field.
    And if you read his patents you'll see Farnsworth was a master at the highfalutin' math for electric fields.
    If you ever pass through SE Idaho be sure to stop in Rigby, his hometown, and see the little Farnsworth museum . It's full of hand made vacuum tubes from the mid 20th century.

    no discussion of cold fusion here.

    old jim
     
  17. Sep 20, 2011 #16
    Course when the earth was flat and the sun went round it any suggestion that this might not be the true state of affairs would have been vigorously derided by the authorities of the day and the author of such fantastic untruths would have met a grizzly demiseo:)
     
  18. Sep 20, 2011 #17

    Drakkith

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    What do you mean?

    I don't see how you consider this an "excess of imagination". I myself am trying to build a fusor at home. It's just really hard for someone with no hands on experience with any of the concepts behind it.

    Really? If I'm ever up there I'll be sure to stop by, I'm sure I'd enjoy it alot.
     
  19. Sep 20, 2011 #18

    jim hardy

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    i meant - hot fusion isn't making any electricity either. And, well, i wanted to not sound like a commercial for the site.
    I didn't quite know how to take your statement about 'practical...tabletop fusion...' whether you were dismissing it or pointing that fact out.
    i am aspergers positive so you have to be blunt with me.

    Still it's a hobby site.
    Not to disrespect hobbyists for after all they are responsible for most innovation.
    They love what they are doing and will pour their souls into their avocation. Einstein, Wright Bros, Lindbergh and Steve Jobs all had day jobs..

    Get your National Parks map.. It's almost halfway between West Yellowstone and Craters of the Moon.
    Near Craters of the Moon is Arco Idaho and the expeimental breeder reactor that ran in 1950s. Another worthwhile visit.
    A local eatery features the "Atomic Burger" in honor of Arco's being first civilian use of nuclear generated electricity.


    Best of luck with your fusor.
    Mine is just a daydream and some parts in a box.
     
  20. Sep 20, 2011 #19

    Drakkith

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    I mean that a fusor will not be producing electricity in its current form. Either shielding the grids or removing them is required in order to reduce the losses incurred by the grid. You are correct in that magnetic confinement and inertial confinement have yet to produce power, however both of those have shown extensive work and improvement and could be said to be technically feasible. A standard fusor is not. Unless you just want a small device capable of producing neutrons.



    I have no idea what your getting at. Where did you get any of this from us saying cold fusion isn't possible?


    Nice! I'll have to remember to eat there one day. My fusor is some parts in my closet at the moment lol. Maybe one day...
     
  21. Sep 21, 2011 #20

    jim hardy

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    i guess i'd lumped cold fusion hobbyists and fusor hobbyists together because they're both outside the mainstream research community......

    as you observed they are quite different processes.

    Direct collection of charge from a fusor could be that crazy breakthrough.


    old jim
     
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