Nuclear Fusion
Nuclear fusion is the process in which two light atomic nuclei combine to form a single heavier nucleus, releasing energy because the resulting nucleus has greater binding energy per nucleon than its lighter constituents — the process that powers the Sun and all other stars, and a long-sought but not yet commercially achieved source of terrestrial power generation.
Curious
Just as splitting a heavy nucleus into medium-mass fragments releases energy (because the fragments are more tightly bound per nucleon), combining two very light nuclei — like two isotopes of hydrogen — into a slightly heavier nucleus, like helium, also releases energy, for exactly the same underlying reason: the resulting helium nucleus sits higher on the binding-energy-per-nucleon curve than the light hydrogen isotopes it was made from.
Fusion is harder to achieve than fission, however, because two positively charged nuclei strongly repel each other electrically, and this repulsion must be overcome — typically by extremely high temperatures and pressures, like those found in a star's core — before the nuclei can get close enough for the short-range strong force to take over and bind them together.
Exploring
| Fusion approach | Method | Status |
|---|---|---|
| Magnetic confinement (tokamak) | Strong magnetic fields confine hot plasma | ITER (under construction) aims for net energy gain |
| Inertial confinement | Powerful lasers compress a fuel pellet | NIF achieved net energy gain from the fusion reaction itself in 2022 |
Deep Dive
Hans Bethe's 1939 paper worked out the detailed nuclear physics of stellar energy production, identifying both the proton-proton chain (dominant in stars around the Sun's mass or smaller) and the CNO cycle (dominant in more massive, hotter stars, in which carbon, nitrogen, and oxygen nuclei act as catalysts for a net conversion of hydrogen to helium) — work that earned Bethe the 1967 Nobel Prize in Physics and finally resolved the long-standing puzzle of what powers the Sun, a question that had defied satisfactory explanation using only classical physics and known chemical energy sources.
The Coulomb barrier and quantum tunnelling: Even at the Sun's core temperature of about 15 million kelvin, the average thermal energy of protons is still far below what would classically be needed to overcome their mutual electrostatic (Coulomb) repulsion at the close range needed for fusion — stellar fusion proceeds at the rate it does only because of quantum tunnelling, allowing a small fraction of proton collisions to fuse despite lacking, classically, sufficient energy to do so. This is directly analogous to the tunnelling mechanism underlying alpha decay, applied in the opposite direction (particles tunnelling together rather than apart).
Terrestrial fusion research has pursued two main approaches for decades: magnetic confinement (using powerful magnetic fields to contain superheated plasma, as in tokamak reactor designs like ITER, currently under international construction in France) and inertial confinement (using powerful lasers to rapidly compress and heat a small fuel pellet, as pursued at the National Ignition Facility in the United States, which achieved a genuine fusion energy gain milestone in December 2022 — though considerable engineering work remains before fusion becomes a practical, grid-scale power source).
Concept Relationship Map
Requires: nuclear binding energy, quantum tunnelling
Enables: stellar nucleosynthesis, fusion power research
Commonly confused with: nuclear fission (fusion combines light nuclei; fission splits heavy nuclei — opposite processes, same underlying energy source)
Sources
- Bethe, H.A. (1939). Physical Review, 55(5), 434–456.
- National Ignition Facility (2022). "Lawson Criterion Achievement Announcement." Lawrence Livermore National Laboratory.
- Krane, K.S. (1987). Introductory Nuclear Physics. Wiley, Ch. 14.