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Magnetic confinement questions

  1. Oct 31, 2014 #1
    Hello all,

    1) In tokamak, since torodial magnetic field is present everywhere, how come plasma is confined without touching the walls? Is the magnetic field stronger inside (I suppose it should be radially uniform) or is it the doing of position control coils/cusps?

    2) If there are two arc shaped coils close to each other creating magnetic fields in the same direction, and a large torodial coil containing them which also creates magnetic field in the same direction but in much higher magnitude, is the plasma confined in the torodial field or does it escape to weaker field between the two little coils? If yes, does increasing the magnitude of torodial field or decreasing those of smaller coils limit the amount of loss?

    Thank you.
     
  2. jcsd
  3. Oct 31, 2014 #2

    mfb

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    Charged particles spiral around magnetic field lines. If those field lines are closed like in a tokamak, they cannot escape. There are more details, but that is the most important thing that stops them from hitting walls.

    I don't understand the coil geometry you describe in (2).
     
  4. Oct 31, 2014 #3
    mfb,

    solenoid_lines.png
    As seen in the figure, magnetic field is not "focused" to the center of the coil that I suggest therefore plasma cannot be confined merely with torodial field coils but position control coils must also operate in order to keep plasma away from the walls (or wires in the picture).

    I will draw how the second system would look and post it on here tomorrow.
     
  5. Oct 31, 2014 #4

    mfb

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    This has nothing to do with the shape or field geometry of a tokamak.
     
  6. Oct 31, 2014 #5
    mfb, I think you don't understand. I am asking you why plasma does not touch the walls where magnetic field is as strong as the inside. Why is plasma confined only in the middle?
     
  7. Oct 31, 2014 #6

    Drakkith

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    The tokamak requires that the magnetic field lines be twisted along the toroidal axis, otherwise the plasma will simply leak out. This can be done by inducing a polodial plasma current, or by using another set of external magnets.
     
  8. Nov 1, 2014 #7
    By twisted along, do you mean it should be in a torus shape or do you mean a polodial field must also be applied, creating a helix? I though the polodial field or current was needed to prevent charge accumulation and sustain the confinement. What I am saying is not relevant to that. I will ask it in a simpler way: There is a selenoid and plasma confined inside it. Two ends are plugged with mirror coils. Can the plasma escape from the selenoid, not the two ends? Where magnetic field is there might be plasma, so why does the plasma not reach the coil itself, or the walls?

    Thank you.
     
  9. Nov 1, 2014 #8

    Drakkith

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    The latter.

    I believe the prevention of charge separation is part of it, but in a homogeneous magnetic field (like in your solenoid above) particles will drift in a direction that is perpendicular to both the magnetic field lines and another applied force (such as gravity or an electric field).

    I believe so. To my knowledge the particles can drift out of the center and collide with the solenoid walls.

    See the following links:

    http://en.wikipedia.org/wiki/Guiding_center
    http://en.wikipedia.org/wiki/Tokamak#Toroidal_design
     
  10. Nov 1, 2014 #9
    OK, I read more about the drift but I still cannot understand why magnetic confinement is even possible.

    Imagine a magnetic mirror and plasma confined within. How does plasma not touch the coils? There is magnetic field around the wires as well as the inner part.
     
  11. Nov 1, 2014 #10
    The basic idea behind magnetic confinement is that charged particles are bound to individual field lines like beads on a string. Most of the individual field lines in a tokamak do not intsect the wall, and therefore most of the particles in a plasma do not interact with the wall.

    That being said, the plasma does interact with the wall. The interaction is limited to a tiny fraction of the plasma volume. We call this region the "edge." What happens at the edge can have a huge effect on the core plasma performance. The physics of the edge plasma is significantly more complicated than the beads on a string picture. In the edge you have to worry about plasma/material interactions, sheath physics, atomic physics, etc. In truth, we don't fully understand the edge. And it is an active area of research.

    Also the magnetic field in a tokamak is not radially uniform. Crudely speaking the torodial fields scales as 1/R.
     
  12. Nov 1, 2014 #11
    Thanks the_wolfman. One more thing, if a little volume of plasma touches the walls, how is it different than no magnetic fields at all? If it is not different, then is tokamak meaningless without the polodial field?
     
  13. Nov 1, 2014 #12
    Great question. The aim of magnetic confinement is to achieve a high temperature high density core plasma. However, the edge plasma has to be cold and low density so it doesn't destroy the wall. These means that there is a pressure gradient from the edge in to the core. Magnetic confinement is really about building up that pressure gradient within the plasma.

    The popular science description that magnetic confinement prevents plasma from touching the walls isn't exact. But its still a pretty good description of what we do. In large scale tokamaks, the core temperature is usually hundreds to thousands of times hotter than the edge. Similarly the core is orders of magnitude more dense than the edge.

    Also all tokamaks have a poloidal field. The poloidal field provides stability and helps maintain force balance. It is an integral part of the tokamak concept. However, the poloidal field is usually small compared to the toroidal field. In simple models, the poloidal field is sometimes neglected because it is so small. This helps elucidate the underlining physical process. Such models are great for teaching purposes, but in reality we have to account for both fields. I should probably also mention that the poloidal field circles a minor axis (the magnetic axis) and does not cause the field to hit the wall.
     
  14. Nov 2, 2014 #13

    mfb

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    Without the magnetic field, particles could directly move outwards and hit the wall. You would lose the plasma in microseconds. The particles cannot do that with a magnetic field, as particle migration from the inner parts to the outer parts is significantly reduced by the path a charged particle takes in the magnetic field.
     
  15. Nov 2, 2014 #14
    Thanks mfb and the_wolfman, for your patience. Also I figured out the second question myself.
     
  16. Nov 9, 2014 #15
    Update: https://www.iter.org/mach/magnets

    In the "Polodial Field System" section, it states that:

    The poloidal field (PF) magnets pinch the plasma away from the walls and contribute in this way to maintaining the plasma's shape and stability. The poloidal field is induced both by the magnets and by the current drive in the plasma itself.
     
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