Net movement of plasma in tokamak

  1. Hi, I've got conflicting impressions on the motion of particles in tokamak reactors. Wikipedia says that they generally follow the poloidal field lines due to the Lorentz force, whilst another site claimed that the promising results were due to the random motion of the particles within the plasma, travelling in many different directions, suggesting that overall the plasma is almost static with zero net momentum.

    Is the reality that they are all generally travelling fast in the same general direction of the field lines, either clockwise or counterclockwise, but that relativistically they appear to have random motion, just as on earth we are all travelling east at hundreds of miles an hour due to the rotation of the earth, but relativistically we appear to all be moving in different directions?

    Thanks, I hope my question makes sense :smile:
  2. jcsd
  3. Astronuc

    Staff: Mentor

    There is random motion and net currents in a magnetically confined plasma.

    Neutrals (neutral atoms) simply leave the plasma.

    Electrons and nuclei/ions move randomly, as well as drift in a toroidal direction, and those near the periphery follow the magnetic field lines, which are a vector sum of the various fields. Electrons (-) and nuclei/ions (+) travel/drift in opposite directions, e.g., in the toroidal current.

    Plasma exclude (acutally attenuate) externally apply magnetic fields, so the magentic field strength falls off from the surface inward to the plasma.
  4. Drakkith

    Staff: Mentor

    Astronuc, since the plasma is composed of both + and - ions, how are both confined at the same time? Something to do with the applied current in the plasma? My knowledge of magnetic fields and the structure of the reactor is limited, so I apologize if it is because of something obvious.
  5. Astronuc

    Staff: Mentor

  6. Drakkith

    Staff: Mentor

    Thanks Astronuc! I'll look into those links.
  7. If we talk about TOKAMAKs there are three types of magnetic fields:
    -poloidal created by induced toroidal current
    -toroidal created by winding around vacuum chamber
    -vertical mag field excluding "baloon effect"
    Namely poloidal mag field is responsible on confinement.
    Superposition of poloidal and toroidal creates so called "rotary transformation" by which power lines rotate around torus with some step. That step is different at different distance from torus axis and this configuration called "shir".
    Such a configuration interferes with development the most types of instabilities.

    The balloon effect caused by that the external torus area тора exceeds internal and as plasma kinetic pressure is the same, torus tries to increase its diameter.
    Also the vertical magnetic field is created aspiring to reduce diameter. By this way the equilibrium can be achieved.
  8. Drakkith

    Staff: Mentor

    Thanks Joseph. I understand how something like a Fusor or Polywell works, but when it comes to multiple types of magnetic fields my understanding is sorely lacking. I know that the 2 charges experience opposite directions in force, so are the magnetic fields set up so that one confines one type and the other confines the other?
  9. Wow :D Thanks for everyone's contributions, I understand a lot more now and those links were great.

    I've got another question now, in a cyclotron, is the majority of the particle energy supplied by the surrounding magnetic field or the wave function between the Dees? Are there any equations linking the accelerated particle energies and the input power, or is it as simple as say a 15kV results in a 15 keV energy per elementary charge?
  10. Drakkith

    Staff: Mentor

    Interesting, I just read about the magnetic mirror effect. I did not realize that both + and - charges both tend to bounce away from a higher strength magnetic field.
  11. Drakkith

    Staff: Mentor

    I think the acceleration is solely due to the electric field.
  12. If we talk about TOKAMAK, first of all there is induced toroidal (cyclic) electric field with intensity sufficient for discharge in gas. Discharge and ionization.
    When ionized electrons in that electric field move at one direction and ions to the opposite. So, very high current is created - about 10-20 millions amperes in latest TOKAMAKs. But due to much lower mass firstly the main carrier of that current are electrons.
    That current creates poloidal mag field in which plasma compressed in radial direction (pinch) increasing number density of particles and so collisions intensity.
    Then due to multiple collisions kinetic energy of arranged motion of particles transferred to random (chaotic) motion - in the other words growth of temperature. This is so called "Ohmic heating". From the beginning the temperature of electron gas is much higher than ion's. But then those temperatures go to alignment.
    But only Ohmic heating can not increase plasma temperature till 10-15keV required for ignition of self-sustained process.
    Because by increasing of temperature also increases the conductivity of plasma. The second factor is growing braking radiation (Bremstahlung). And at certain temperature energy input into plasma through induced electric field will become equal to energy losses mainly via Bremstahlung. And so, Ohmic heating allows to reach only about 1 or few keVs.

    For reaching higher temperature two other ways of heating are used: RF heating utilizing cyclotron resonance and injection into plasma of beam of neutrals (mostly deuterium). Neutrals because it is difficult to inject charged particles through mag field’s lines from externally.
    Last edited: Aug 11, 2011
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