While a very small number of fusion reactions occur naturally under existing terrestrial conditions, the most spectacular steady state fusion processes occur in stellar media. Indeed, the formation of elements and the associated nuclear energy releases are conceived of as occurring in the burning of hydrogen during the gravitational collapse of a stellar proton gas; the initiating fusion process is

p + p-^>d + fi+ + v + 1.2 MeV (1.28)

where p+ represents a positron and v a neutrino. Then, the deuteron thus formed may react with a background proton according to

p + d h + 5.5 MeV. (1.29)

Subsequently, this helium-3 reaction product could fuse with another helium-3
nucleus to yield an alpha particle and two protons:

h + h—>a + 2p + 12.9 MeV. (1.30)

The next heavier element is beryllium, produced by

h + aWBe + 1.6 MeV (1.31)

and is an example of a rare helium-4 fusion reaction. Also, lithium may appear by

1Be + p-^1Li +0.06MeV. (132)

A progression towards increasingly heavier nuclides is thus evident. This process is known as nucleosynthesis and provides a characterization for the initial stages of formation of all known nuclides.

Closed fusion cycles have also been identified of which the Carbon cycle is particularly important:

nC + p—»13A + 1.9 MeV
13 A—»13C + p+ + v + 1.5 MeV

13 C + PW4N+ 7.6 MeV

14 A + p^X50 + 7.3 MeV

75 CM>I5A + ^ + v + 1.8 MeV
15 A + p^>nC+a + 5.0 MeV.

This sequence of linked reactions is graphically depicted in Fig. 1.3 and may be collectively represented by

4p^>a + 2p+ + 2V + 25.1 MeV (1.34)

if all the reactions of Eq. (1.33) proceed at identical rates. This relation suggests that protonium bums due to the catalytic action of the isotopes 12C, 13C, 13N, 14N, 15N, and 150.

Fig. 1.3: Graphical depiction of the Carbon fusion cycle.