Summarizing Fusion

Fusion Summary

• Nuclear fusion is a reaction in which two nuclei are combined to form a larger nucleus. It releases energy when light nuclei are fused to form medium-mass nuclei.
• Fusion is the source of energy in stars, with the proton-proton cycle,

  \({}^{1}\text{H}+{}^{1}\text{H}\to {}^{2}\text{H}+{e}^{+}+{v}_{\text{e}}(0.42 MeV)\)

  \({}^{1}\text{H}+{}^{2}\text{H}\to {}^{3}\text{He}+\gamma (5.49 MeV)\)

  \({}^{3}\text{He}+{}^{3}\text{He}\to {}^{4}\text{He}+{}^{1}\text{H}+{}^{1}\text{H}(12.86 MeV)\)

  being the principal sequence of energy-producing reactions in our Sun.

• The overall effect of the proton-proton cycle is

  \(2{e}^{-}+4{}^{1}\text{H}\to {}^{4}\text{He}+{2v}_{\text{e}}+6\gamma (26.7 MeV),\)

  where the 26.7 MeV includes the energy of the positrons emitted and annihilated.

• Attempts to utilize controlled fusion as an energy source on Earth are related to deuterium and tritium, and the reactions play important roles.
• Ignition is the condition under which controlled fusion is self-sustaining; it has not yet been achieved. Break-even, in which the fusion energy output is as great as the external energy input, has nearly been achieved.
• Magnetic confinement and inertial confinement are the two methods being developed for heating fuel to sufficiently high temperatures, at sufficient density, and for sufficiently long times to achieve ignition. The first method uses magnetic fields and the second method uses the momentum of impinging laser beams for confinement.

Glossary

break-even

when fusion power produced equals the heating power input

ignition

when a fusion reaction produces enough energy to be self-sustaining after external energy input is cut off

inertial confinement

a technique that aims multiple lasers at tiny fuel pellets evaporating and crushing them to high density

magnetic confinement

a technique in which charged particles are trapped in a small region because of difficulty in crossing magnetic field lines

nuclear fusion

a reaction in which two nuclei are combined, or fused, to form a larger nucleus

proton-proton cycle

the combined reactions ¹H+¹H→²H+e⁺+v_e, ¹H+²H→³He+γ, and ³He+³He→⁴He+¹H+¹H