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Plasmas and magnetics!

  1. Oct 21, 2004 #1
    Questions garanteed to have you doing right handed impressions of rap artists by the end! :smile: :tongue:

    Now I am much happier with electronics, I decided to start reading about magnetics. However, I'd appreciate any help you could provide with a few things I'm thinking about.

    First of all - coils.

    If I wrap a piece of wire round and round into a helical solenoid shape, I get magnetic poles at the 'open' ends - the top and the bottom if your coil is vertically inclined.




    The flux here is trying to move between points on the same plane in the same axis.

    Now imagine if I was to take the coil an begin wrapping a new coil, identical to the first, with the original - forming a coil from coiled wire. From what I can tell, the coil should now have two dimensions of 'activity' if you like - as the original coil will be emitting a field, but the field forming coil is, it's self, in a coil formation. Where would the magnetic poles appear with the new coil?

    One step tricker... how about winding this coil into another new one. To give the coil three dimensional activity?

    Begin rapping! :eek:

    Another thing that was confusing me was plasma reactions.

    The Joint European Torus (JET) here in England is experiementing with fusion reactions.

    As it's name suggests, the JET reactor is a toroid - A donut shape. It's a jamless toroid.

    The reactions occur at hundreds of millions of degrees, so they can't touch any part of the reactor, otherwise it'll melt. So the reaction is plasma based and magentic confinement is used to lift the plasma off the walls of the reactor.

    Okay... the easy part first.

    Imagine you're looking at the reactor from above it or underneath it, so it looks like a donut on a table. If you have flux flowing clockwise or anticlockwise around inside the donut, you have something very similar to the core of a toroidal transformer - which is great at concentrating flux inside it's core and not emitting it as noise, since it's a sealed magentic circuit.

    Now imagine you slice into the donut and look into it so that you are looking at a circular shape - looking at what's happening inside it.

    If the field is coming towards you, or moving through you and into the donut cross section infront of you, the arrangement is very similar to a particle deflection chamber.

    If a charged particle was place into the field, it would try to spin or deflect around the circumfrence of your donuts circular cross section. Because the particles in the plasma are positive nuclei and negative shell electrons, one set will spin clockwise, and the other anticlockwise.

    As both have a component of force trying to push them around inside the toroid, they will form a helical shape with flutes going clockwise and anticlockwise.

    This is ignoring the fact that the flux it's self is flowing in a helical pattern inside the toroid's core - but I'm not sure if that makes a difference since it's polarity is not changing.

    This kind of containment would be useless on it's own. All a flux like this is doing is making the plasma spin inside the core. If anything, that would cause a centrifugal effect that would push the plasma out towards the walls of the reactor right?

    So, I started thinking, perhaps they have some other kind of coil arrangement to push the plasma back off the walls instead?


    But, how could it possibly push both halves of the plasma away from the walls at the same time?

    If the plasma is a usual thermal or cold plasma, it will contain positive and negative charges.

    Keeping things as simple as possible, if the magnetic containment is of a stationary polarity, it will contain one charge whilst simulatenously attracting the other.

    I know that these reactors currently have problems with plasma leaking out of the field. That can't be due to the methodology of the containment though since fifty percent of a neutral plasma would be leaking out. Which would probably melt the reactor in a few seconds. So it must be due to the real world practical limits.

    Imagine a single hydrogen atom. Split the electron away from the proton. In an alternating magnetic field, taking only their charge into account, there should be no net change between them. One will move one way, field changes polarity, it moves the other way to the same extent.

    The only difference would be their mass, and so, how rapidly they would respond to the polarity changes. I don't like that idea though because it seems far to rough.

    Now I'm left wondering how they manage the effect.

    Would it have something to do with the poloidal field that is also in use?

    I hope you enjoy thinking about these!

    Best wishes!
    Last edited: Oct 21, 2004
  2. jcsd
  3. Oct 21, 2004 #2


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    I must be dense this morning. I see nothing here that has anything to do with "Quantum Physics". Classical E&M, yes. But QM?

  4. Oct 21, 2004 #3
    The Quantum forum's description includes Field Theory.

    My questions are on magnetic fields.
  5. Oct 21, 2004 #4


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    Except yours is a CLASSICAL field theory (there's certainly nothing here that requires the rigors of QFT!). Ignoring the fact that you simply didn't not include the full magnetohydrodynamics effects, I think it might be a good start if you review your basic Maxwell equations first before jumping into the "quantum" aspect part. Plasma physics, especially all the self-interactions, is not this trivial.

  6. Oct 22, 2004 #5
    I'm not looking for an in depth review of plasma physics.

    Essentially, if I put a magnet near a plasma, what will happen to it?

    Is the entire plasma going to bend as if it's a single conducting wire, or are the separate charges in the plasma going to separate?

    If they separate, how can they be confined?

    If QFT is so much more advanced than classical physics, these questions should be easy for you to answer right?
  7. Oct 22, 2004 #6
    eeka chu,...

    Zz ain't no QFT-guy.

    Plasma physics is mainly described by classical physics and at best in a semi-classical way, just like laser-physics...

    Plasma's can be confined by surrounding toroidal magnets.

    Last edited: Oct 22, 2004
  8. Oct 22, 2004 #7
    Suppose you have a plasma in some torus with long axis R and small axis a. The poloidal coordinate p is used to describe the surface of the torus and the toroidal coordinate t is defined perpendicular to the great axis.

    To clarify : suppose the great circle with radius R is in the XY-plane and the torus is created by putting a little circle of radius a perpendicular to the XY-plane.The centre of this little circle is placed onto the great circle's circumference and just move the little circle 360° around the z-axis.

    Now t (in the XY plane) is the angle between the x-axis and some point on the great circle. p is the angle between the XY-plane and some point on the torus-surface.

    The torus will have a circular current on the surface called the poloidal current. The magnetic field will be toroidal. This magnetic field will cause the plasma to move outwards the torus (because of the Hall-plasma-effect) and in order to compensate this effect an extra magnetic field component is introduced : the poloidal component. Basically this component causes the magnetic field to have a torsion just like the fibres in a rope. These helical field lines are caracterised by the so called safety factor q. This factor expresses the MHD-stability of a plasma by stating that the helical field lines must be closed "on each other". This means that they must connect each other along the entire torus-surface so there are no openings for the plasma to escape...


    please, do feel free to ask more clarification if needed...

  9. Oct 22, 2004 #8


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    1.But your queston DOES require a review of plasma physics.

    2. You are NOT just putting a magnet near a plasma. Reread what you originally posted. You want a time-varying magnetic fields. This is where I pointed out the fact that you have ignored the self-interactions that occurs within a plasma. The plasma ITSELF generates E and B fields that it interacts with! These effects can be catastrophic in a constant magnetic field. So think of how NASTIER it can get in a time-varying magnetic fields! This is what mentioned as one of the things you simply did not include in your toy model.

    3. QFT? More "advanced" than classical physics? Where did THAT come from? None of the stuff we have talked so far have anything to do with quantum mechanics - which was my original question in this thread, no?

  10. Oct 23, 2004 #9
    Thanks Marlon,

    I am taking reference from JET's diagram here -
    http://www.jet.efda.org/pages/content/content-images/jg951132c.png [Broken]

    This helical pattern within the plasma will be due to the toroidal coils causing the charge of the plasma to deflect. If the plasma is electrically neutral, it will contain both positive and negative charges. So, does that mean that the pattern will be a double helix? One set of charges flowing clockwise in the helix and the others flowing anticlockwise?

    The poloidal coils in JET's diagram are shown as the inner and outer most coils - the inner most coil being used as the primary of a virtual 'transformer' and the plasma it's self forming the secondary. As I increase the inner poloidal coil's power, I pump more energy into the plasma and increase it's density.

    The JET diagram shows the out poloidal coils as encircling the torus's outer circumfrence. How would these coils, and the inner poloidal coils, actually be wound? I'm guessing as rings - like a cylinder around the torus's perimeters?

    The JET piccy refers to the poloidal coils as pumping and shaping coils.

    If I was to increase the power to the toroidal coils placed around the toroid it's self, would this increase the constriction of the plasma, it's 'safety factor', or would it simply increase the helical pattern in the plasma?

    To perhaps simplifying the geometry, I remember hearing that a lot of the earlier experiments used something that looked a lot like an straight electromagnetic coil without a core - the wire wrapped around a nail kind. Would this arrangement have 'squeezed' the plasma off the coil by just increasing the coil's power, or would they have needed more complex magnetics?

    Best wishes,
    Last edited by a moderator: May 1, 2017
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