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Why do they use a huge tokamak?

  1. Oct 6, 2008 #1
    why do they use such a huge machine like the tokamak to try and sustain fusion? wouldn't it be easier to build a really small one? I'm guessing it would be a lot easier to recreate the conditions of the sun's core (extreme temp./pressure), assuming thats the blueprint for fusion, in a small area than something huge like the tokamak. if you had something a lot smaller, maybe you wouldn't have to use as much power to start it. plus it would be easier to create extreme pressure in a small area than a large area. and wouldn't creating a vacuum in the chamber create the opposite effect of the density needed for atoms to collide? does somebody want to help clarify this for me? thanks

    one more thing. in order to a plasma to remain a plasma in such an extreme pressure (150,000kg/m3) is it heat alone that sustains it?
    Last edited: Oct 6, 2008
  2. jcsd
  3. Oct 6, 2008 #2


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    Most of the tokamaks are prototypes aimed ultimately at power generation so it makes sense to build the prototypes at the same scale you will need for the power station.
    Creating fusion is a solved problem, it's sustaining it thats hard - and you can always compress the plasma into a small volume in the larger chamber,

    There is a totally different research path of inertial confined fusion which uses very small amounts of material heated very quickly by eg. lasers. These are much smaller vessels but it's hard to see how they would scale upto a power station.
  4. Oct 6, 2008 #3
    okay I understand now. thanks.

    do you think it would be practical to create a very small model just for experiments? maybe it would be easier to sustain it then, and get an idea of how to sustain it easier in the larger one. or just use it for other purposes.

    also, I'm probably lazy asking this (I could probably google it) but do we have the capabilities to create the pressure in the suns core? (150,000kg/m3) and/or temperature? 13 -15 million K

    for example instead of using a lot of power to adequately power a lot of huge electromagnets, why not use that same power to immensely power a lot of small (superconducting of course) electromagnets in a small area in order to more easily create high pressure. this may be totally off. I haven't read up on electromagnetic properties lately. please pardon my ignorance :P maybe it would be more realistic to use large and powerful magnets in a very small area. this wouldn't solve for a power plant, but at least you might be able to sustain it.

    also what if you injected the hydrogen in as a liquid under pressure so it starts out as dense already, then (if it is at all possible) after heating it into a plasma while the vessel is still at the pressure used when it was liquid, the electromagnetic field would have a head-start and only further compress it. but this might be a problem because I don't remember why they use a vacuum, and if its possible to keep something like hydrogen compressed into a liquid in a vacuum. I'm just brainstorming here.

    you say the sustaining part is whats the problem. is it heat or pressure or both? the reason the sun does it must be because the density is so great the atoms of no choice but to collide. (resulting in it being superheated) I would assume pressure is the problem and not heat as they already can easily make the plasma.

    Last edited: Oct 6, 2008
  5. Oct 6, 2008 #4
    I think the dimensions of the tokamak are also limited by the strength of the superconducting magnets as this in turn affects the radius of curvature of the beam.
  6. Oct 6, 2008 #5


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    There are lots of small fusion experiments, you just have to heat a little pinhead of liquid dueterium very quickly with lasers. You can also acheive the same thing with a very magnetic field, you can generate one by collapsing a magnetic material with explosives.
    The problem is that both of these tend to be a high energy bang and a lot of cleaning up - the aim of fusion is to create relaible power 24*7 .
    There is a minimum size you can make a tokamak simply because you have to thread the magnets through the middle as jimmy said.

    The aim of the plasma is to generate a high density, you can do this with either temperature or pressure. Tokamaks actualy run at higher temperatures than the core of the sun but at lower pressures, you can generate higher pressures than the sun but it's hard to sustain them - and very hard to introduce new fuel at the same time.

    It doesn't really matte rif you start with a liquid or a gas, comapred to the eventually temperatures you are going to reach anyway. The very high vacuum is necessary to stop heat escaping from the plasma.

    As I said, generating the plasma isn't hard. It's mostly now a problem of keeping it hot and confined for long enough to get power out, and a lot of engineering details about introducing new fuel and dealing with the effects of the radiation.
  7. Oct 6, 2008 #6
    I think I understand what you're saying. thanks! a lot of good information :)

    I'd like to experiment with it myself with smaller electromagnets. see how powerful and small I can get, trying to compress a small amount of plasma with the most power possible. just something for fun. I know you can instantly create a plasma inside a microwave oven by cooking a lit match or paper clip. people put glass cups over it to prevent it from escaping, but it explodes the glass cup within seconds. it might also fry the magnetron, not sure. but this would be plasma of normal air and not hydrogen.

    thanks. this makes me think that if you can attain the densities and temperatures even higher than the core, why not try them both at the levels they are said to be at the core. when I say small fusion experiments, I mean literally a miniature tokamak. again, I don't know if its practical or possible even for small experiments, but I'd like to try anyway just for fun.

    just hypothetically speaking, irrelevant to physical engineering of the tokamak, I imagine it would be easier to sustain fusion with a few atoms compared to a lot, since I assume less power is needed to heat/pressurize a very small amount of fuel. but thats just me thinking out loud again.

    just a matter of time before technology develops to sustain it all I guess.
    Last edited: Oct 6, 2008
  8. Oct 6, 2008 #7


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    It's harder to maintian fusion in a few atoms, since you are losing power over the surface area and generating it in the volume - the more volume to surface the better.

    The easiest way to sustain fusion is to put an awfull lot of it together - 1 solar mass works quite well, but it's difficult to find somewhere to mount the experiment!
    I don't think you are going to make a tokamak without superconducting magnets but you can see the effects of magnets on a plasm with one of those plasma globes from the gadget shop
  9. Oct 6, 2008 #8
    lol okay. thanks for all your help anyway. well maybe my next thing will be to figure out how to build a mini superconductor :P
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