In the core of the Sun, where temperatures are at 10-15 million degrees Kelvin, hydrogen atoms are fused into helium atoms to generate an immense amount of energy. Scientists can mimic this fusion process by fusing deuterium and tritium, which are different isotopes of hydrogen. This process is slightly different from the fusion that occurs in the Sun between pure hydrogen nuclei.
Deuterium occurs naturally as deuterium oxide or "heavy water" in proportions of about 1/5000. Tritium, a radioactive isotope, does not occur in nature and must be produced artificially through this reaction.
When deuterium and tritium combine, the mass of the products is slightly less than those of the reactants. Like in nuclear fission, the missing mass is expended as energy according to Einstein's famous equation E=MC2. The energy released in this reaction is greater than the energy released in ordinary chemical reactions because the energy that holds a nucleus together is much larger than the energy that holds electrons in orbit around a nucleus.
In the basic fusion reaction, an atom of deuterium (hydrogen with one neutron) combines with an atom of tritium (hydrogen with two neutrons) to form an atom of helium (with two neutrons), a neutron, and a vast amount of free energy.
Since the deuterium and tritium nuclei have similar positive charges, they repel each other and are reluctant to fuse except under extremely high temperatures of 100 million Kelvin (on Earth). The high temperature results in high kinetic energy of the individual nuclei, which can overcome the charge repulsion.
"This reaction that you see-deuterium plus tritium-is the easiest reaction to get energy multiplication out of... the first thing you have to do is to trip the electrons off of them to make a plasma...once you get it above 10 or 11 million degrees kelvin you end up stripping the electrons anyway...there's a natural repulsive force between the two which is actually quite strong..." - listen
"...This helium nucleus has about 3 million electron volts and [the neutron] has about 15 million electron volts... It only takes a fraction of the percent of the mass converted to energy under Einstein's relation to yield so much energy...the energy multiplication can be as high as 450 to 1..." - listen
Dr. Rajesh Maingi, Senior Research Scientist, Princeton Plasma Physics Laboratory