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Thread: Tokamak physics

  1. #1 Tokamak physics 
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    I've got an understanding problem. It's about the behaviour of charged particles in plasma inside of a tokamak.

    It says that a particle in the non-homogeneous magnetic field (toroidal field), moving on the spiral trajectory all along the magnetic field lines, tends to drift downward (for ions) or upward (for electrons), together with the axis of the Larmor-spiral, which makes plasma highly unstable and damages the chamber walls. That's the reason why the second field (poloidal field) is necessary, which is provided by plasma current itself.

    Which force is it, that causes the particles to drift (toroidal drift) downward/upward?


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  3. #2  
    Universal Mind John Galt's Avatar
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    About the only thing I understand about fusion reactors is how to spell Tokamak, however, you have not stated in your post what you don't understand. That makes providing an answer difficult.

    Ugh! I'll swear your last line wasn't there when I read the post the first time. Surely, the force is the magnetic field. Electrons, being negative, are accelerated in one direction in the field, while ions (presumably positive) are accelerated in the other direction. No?


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  4. #3  
    Forum Radioactive Isotope cosmictraveler's Avatar
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    Experimental arrangement for controlled nuclear fusion. In a Tokamak, two superimposed magnetic fields enclose the plasma: this is the toroidal field generated by external coils on the one hand and the field of a flow in the plasma on the other hand. In the combined field, the field lines run helicoidally around the torus centre. In this way, the necessary twisting of the field lines and the structure of the magnetic areas are achieved. Apart from the toroidal field generated by the external field coils and the field generated by the flow in the plasma, the Tokamak requires a third vertical field (poloidal field), fixing the position of the flow in the plasma container. The flow in the plasma is mainly used to generate the enclosing magnetic field. In addition, it provides effective initial heating of the plasma. The flow in the plasma is normally induced by a transformer coil. Owing to the transformer, the Tokamak does not work continuously, but in pulse mode. Since, however, a power plant should not be operated in pulse mode for technical reasons, methods are examined to generate a continuous flow - for example by high-frequency waves. The fusion research plant JET is built according to the Tokamak principle. The fusion reactor ITER is also planned according to this principle.

    http://www.euronuclear.org/info/ency.../t/tokamak.htm
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  5. #4  
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    Surely, the force is the magnetic field. Electrons, being negative, are accelerated in one direction in the field, while ions (presumably positive) are accelerated in the other direction. No?
    Ok, thanks first guys for your replies.

    The thing I don't understand is why they speak only about the cross drift upward/downward? But why do the particles move upward/downward and not for example to the right or to the left? There is a gradient in these directions too!...because the strenght of magnetic field in a toroid depends on/is antiproportional to its radius from the big axis.

    Look at this, my question should be clear from this picture:
    http://www-fusion-magnetique.cea.fr/...ue/physiq2.gif

    you can read more on the same site:
    http://www-fusion-magnetique.cea.fr/...rajectoire.htm
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  6. #5  
    Forum Professor wallaby's Avatar
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    last i checked a Tokamak is in a donut shape, might this be the reason?

    might the source of the magnetic feild be the reason?
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