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Negative Temps and Fusion

  1. Dec 7, 2014 #1
    I was reading about negative temperature here http://www.nature.com/news/quantum-gas-goes-below-absolute-zero-1.12146 and how the state is "hotter than hot" and thought of fusion.


    Since you need very high temperatures for fusion does the discovery on negative temperatures bring fusion as a power source closer to reality? I know my thinking is simplistic but I thought I'd give it a shot!



    Regards,
    JDM
     
  2. jcsd
  3. Dec 8, 2014 #2

    Drakkith

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    No, negative temperature is a result of a system that has a limit on the number of high-energy states available. These are very specialized circumstances and the temperature becomes negative because of the relationship between energy and entropy. It doesn't have any bearing on fusion.
     
  4. Dec 8, 2014 #3
    Fusion isn't economical because "Currently, here on Earth the amount of energy you'd need to put in to produce that kind of heat or pressure is much, much higher than what you get out in usable energy" [ http://www.popularmechanics.com/sci...ration/why-dont-we-have-fusion-power-15480435 ] but "a negative temperature is not colder than absolute zero, but rather it is hotter than infinite temperature" [ http://en.wikipedia.org/wiki/Negative_temperature ] hotter than infinity seems like it should be enough to achieve fusion economically!
     
  5. Dec 8, 2014 #4

    Drakkith

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    Negative temperatures don't apply to fusion plasma since there are a near-infinite number of states available for the plasma. In other words, we can keep inputting energy into the plasma and the entropy keeps increasing. To reach negative temperatures, the entropy would need to decrease while the energy increases.
     
  6. Dec 8, 2014 #5
    Normally fusion occurs at high temperatures but "You need a lower bound for the energy in order to get positive temperatures and an upper bound in order to get negative temperatures" [ http://www.quantum-munich.de/research/negative-absolute-temperature/ ]. We know a typical system with a lower bound fuses at high temperature and high entropy might a system with upper bound fuse at negative temperatures and low entropy?
     
  7. Dec 8, 2014 #6

    Drakkith

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    No, fusion occurs because the particles slam into each other at high speeds. This high-energy thermal motion cannot have an upper bound put on it, so it can't be made into a negative temperature system.
     
  8. Dec 8, 2014 #7

    Vanadium 50

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    John, you have asked the same question three times. Are you going to keep asking it until someone tells you "yes"? Drakkith has explained why it is not three times. If you would like him/us to elaborate on the answer, please explain what parts are confusing you. But don't just start from the top again.
     
  9. Dec 8, 2014 #8
    I feel like I'm not explaining myself; couldn't there be a point of such high energy and so little entropy that the atoms have no place else to go but will smush together?
     
  10. Dec 8, 2014 #9

    Vanadium 50

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    And the answer is still no.
     
  11. Dec 8, 2014 #10
    lol; you're funny
     
  12. Dec 8, 2014 #11

    collinsmark

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    Perhaps this might serve as an introduction.

     
  13. Dec 8, 2014 #12

    Drakkith

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    No, because in order to "smush" together and fuse, they must be traveling at a high velocity. Velocity is a type of state that has no upper limit. You can just keep inputting energy all you want and you'll never hit a boundary. The states that negative temps apply to are things like the spin of atoms, which have only up or down. If we look at a system of 100 atoms in an electric/magnetic field, we could excite the atoms in this system to an up spin to increase their energy. But at a certain point the number of atoms in a high energy state (spin up) becomes equal to the number of atoms in a low energy state. If we continue to add energy we begin to run out of high energy states and the entropy decreases instead of increasing. This is the point where negative temperatures come into play.

    Note that systems where negative temperatures can be used are typically very close to absolute zero in a "traditional" view of the word temperature. We have to cool them off first and then set up this very sensitive system of up/down states. In the example above, the thermal motion of the atoms even at room temperature would wreak havoc on our system, flipping spins randomly and exciting atoms and their electrons to other states that we didn't have before.
     
  14. Dec 9, 2014 #13
    I'm working in fusion. We have no problem reaching fusion temperatures (~150MK) in the lab. The problem is keeping the power balance favorable. Negative temperatures won't help with this. In fact, if you get too hot, the fusion rate starts to decrease.
     
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