In nuclear fusion reactions we know that temperatures can be as high as 10 million degrees Celsius. This is well beyond the melting and vaporization temperatrures of any known substance. How is it then that scientists are able to create controlled and contained nuclear fusion reactions?
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It is true that in the traditional nuclear fusion reaction that the heat required to bring the nuclei of the hydrogen atoms together and fuse into helium is extremely high--approaching, and in some cases exceeding, the temperature of our Sun.
This of course is not a problem when the nuclear fusion reaction is used as a bomb as the point of a bomb is to release as much heat and energy as possible. However, to use nuclear fusion to generate heat to create electricity requires that the materials involved be contained. This is a very big technological challenge.
The operating temperatures of a traditional nuclear fusion reaction are well above the melting point of any naturally occurring or man made material. Therefore the fusion reaction must not come into contact with (or even close to) any materials. If it did it would simply melt (and vaporize) the material releasing the heat and pressure required to maintain the reaction.
The technique which is being developed to contain nuclear fusion reactions is based upon the property of the hydrogen and helium in the reaction. At the operating temperatures of the reaction, the hydrogen atoms that are fused and the helium atoms which are produced are stripped of their electrons and are therefore an electrically charged, superheated gas called a plasma. The fact that they are electrically charged allows them to be manipulated by electric and magnetic fields.
The engineering solution is to cause the superheated plasma to move by exposing it to a strong electric field. The moving plasma produces a magnetic field (a la Ampere's Law). The magnetic field of the plasma can then be manipulated by an external magnetic field which can cause the plasma to flow in a circular path. This allows the reaction to occur and to be kept away from the physical materials which are necessary to building the reaction vessel. The plasma does not melt the material containing it because it is prevented from coming close enough to the containment walls by the strong electric and magnetic fields used to move and bend the plasma.
One engineered soluition is to create a containment vessel which is a hollow "donut" shaped tube called a toroid in which the plasma is forced to move in the center of the tube by the electric and magnetic containment fields.
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