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5. Magnetic confinement of plasmas

In magnetic confinement fusion the hot plasma is contained in a vacuum vessel, in which an appropriately configured external magnetic field and/or a field generated by a current induced in the plasma itself prevents the plasma from hitting the vessel walls.
Diverse magnetic configurations have been studied, for example, mirror configurations, in which the force lines of the magnetic field are open linearly, and toroidally symmetric configurations (e.g., stellarator and tokamak).
The concept that has given the best results so far in magnetic confinement fusion is the tokamak.

Nuclear binding energy vs. massTokamak as a "doughnut" & magnetic fields

 

The tokamak is a toroidally shaped device characterised by a hollow vessel or chamber, forming the “doughnut”, in which the plasma is confined by a magnetic field and bound to force field lines along a spiralling path.

This type of magnetic configuration is obtained by combining an intense toroidal magnetic field, produced by magnetic coils placed around the doughnut, with a poloidal magnetic field, obtained by externally inducing a current in the plasma. The poloidal current also helps to prevent the plasma particles from migrating towards the vessel walls.
The plasma particles spiral around the force field lines.

Another set of external magnetic coils is used to provide auxiliary magnetic fields that control the position of the plasma in the doughnut.

The tokamak configuration is particularly stable and allows the plasma to be confined for a long time.

Plasma Heating

As the plasma is an electric conductor it can be heated by inducing a current from the outside. The plasma in the doughnut forms the secondary circuit of a transformer whose primary circuit is external.
So, the induced current has two purposes: to generate the poloidal field and to heat the plasma to high temperatures ("4 current" in the figure below). This type of heating is called “Ohmic” or “resistive” and obeys Joule’s law. Actually, it is similar to the heating that occurs in an electric lamp or heater.

Nuclear binding energy vs. massTokamak as a "doughnut" & magnetic fields

 

However, the effect of Ohmic heating is limited ("4 current" in figure) because the resistance of the plasma decreases as the temperature increases, so the maximum temperature that can be obtained in the plasma is only a few million degrees. If we want to reach the temperatures necessary for thermonuclear fusion, we have to use additional heating methods:

  • absorption of electromagnetic waves, which are injected into the plasma by means of waveguides or antennas (1 in figure);
  • injection of neutral atoms at high kinetic energy, which cross the magnetic field, become ionised and transfer their energy to the plasma through collisions (2 in figure);
  • adiabatic compression of the plasma, obtained by moving the plasma to regions where the magnetic field is higher, which consequently increases the plasma temperature (3 in figure).

 

 

Have a look of some Plasma light video-clips
(duration about 2s - QuickTime plugin required)
viewed from a window of FTU tokamak:

Plasma light: 1sec movie15039.mov        Plasma light: 1sec movie15030.mov        Plasma light: 1sec movie13711.mpg

samples taken from "TV images of FTU plasmas" archive.

 

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