Britannica Sources used in this answer

How does helium-3 help form helium-4 in the proton-proton cycle?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

laser-activated fusion — nuclear fusion Interior of the U.S. Department of Energy's National Ignition Facility (NIF), located at Lawrence Livermore National Laboratory, Livermore, California. The NIF target chamber uses a high-energy laser to heat fusion fuel to temperatures sufficient for thermonuclear ignition. The facility is used for basic science, fusion energy research, and nuclear weapons testing.

In the proton-proton cycle, helium-3 ( $^{3} He$ ) serves as a critical intermediate product that facilitates the formation of helium-4 ( $^{4} He$ ).

After hydrogen nuclei fuse to form deuterium and subsequently capture another proton to create helium-3, the cycle reaches a stage where helium-3 nuclei react with one another. In the most direct continuation of this chain, two helium-3 nuclei collide and fuse to produce one helium-4 nucleus and two protons. This reaction is represented as:12

$^{3} He +^{3} He \to^{4} He + 2 (^{1} H)$

At equilibrium, helium-3 burns predominantly through this self-interaction because its reaction rate with hydrogen is low and the concentration of deuterium is negligible. This process is a fundamental step in the nuclear fusion reactions that power stars like the Sun, ultimately resulting in the synthesis of helium from hydrogen.12