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1. Thermonuclear fusion

“Thermonuclear fusion” is the nuclear reaction that takes place in the sun and in other stars. During the reaction an enormous amount of energy is released. How does this happen?
In a fusion reaction, the lightest nuclei – hydrogen – fuse at extremely high temperatures and pressures and form heavier nuclei – helium.
There are three isotopes of hydrogen: hydrogen (H) itself; deuterium (D) and tritium (T). The nucleus of each of these isotopes contains a single proton, which characterises the isotopes as a hydrogen element. The nucleus of deuterium also has one neutron, whereas that of tritium has two. In each isotope the neutral atom has an electron outside the nucleus to compensate for the charge of the single proton.
The most likely reaction is the one that occurs between a deuterium and a tritium nucleus, generating a helium nucleus (alpha particle) and a neutron. In this reaction the total mass of the products is lower than that of the interacting particles, and energy is released according to the mass-energy equivalence principle. The liberated energy gets distributed between the alpha particle and the neutron with an inverse ratio to their respective masses.

deuterium-tritium fusion reaction

The two nuclei interact only at short distances, equivalent to their size (10-13cm-3); in this case the nuclear forces are stronger than the electrostatic repulsion due to the positive charge of the nuclei (as the nuclei approach each other the repulsion increases inversely to the square of the distance).
To get the two nuclei close together within a sufficiently short distance, the velocity at which they collide has to be extremely high, that is, their kinetic energy (hence the temperature) has to be extremely high. If we want to obtain fusion reactions in the laboratory, for example, we have to heat a deuterium-tritium mixture to ultra-high temperatures (100 million degrees) and confine the mixture for a sufficiently long time for the nuclei to have as many collisions as possible, thereby increasing the probability of generating fusion reactions.

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