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