Conditions for Fusion

Since nuclei carry positive charges, they normally repel one another. The higher the temperature, the faster the atoms or nuclei move. When they collide at these high speeds, they overcome the force of repulsion of the positive charges, and the nuclei fuse. In such collisions, energy is released. The difficulty in producing fusion energy has been to develop a device which can heat the deuterium-tritium fuel to a sufficiently high temperature and then confine it for a long enough time so that more energy is released through fusion reactions than is used for heating. Temperature In order to release energy at a level of practical use for production of electricity, the gaseous deuterium-tritium fuel must be heated to about 100 million degrees Celsius. This temperature is more than six times hotter than the interior of the sun, which is estimated to be 15 million degrees Celsius.

Confinement The required temperatures have been attained. The problem is how to confine the deuterium and tritium under such extreme conditions. A part of the solution to this problem lies in the fact that, at the high temperatures required, all the electrons of light atoms become separated from the nuclei. This process of separation is called ionization, and the positively charged nuclei are referred to as ions. The hot gas containing negatively charged free electrons and positively charged ions is known as a plasma.	Plasma Confinement
Because of the electric charges carried by electrons and ions, a plasma can be confined by a magnetic field. In the absence of a magnetic field, the charged particles in a plasma move in straight lines and random directions. Since nothing restricts their motion the charged particles can strike the walls of a containing vessel, thereby cooling the plasma and inhibiting fusion reactions. But in a magnetic field, the particles are forced to follow spiral paths about the field lines. Consequently, the charged particles in the high-temperature plasma are confined by the magnetic field and prevented from striking the vessel walls.
Many different plasma configurations have been studied. Presently, the advanced tokamak, the spherical torus, and the compact stellarator are of particular interest.

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Updated: 15 November 2004 Send questions or comments to: Anthony R. DeMeo at ademeo@pppl.gov