Nuclear Fusion
D-T fusion overcoming Coulomb barrier — plasma T sets kinetic energy.
Key Notes
Nuclear fusion: light nuclei combine to form heavier ones, releasing energy.
Most famous: ²H + ³H → ⁴He + n + 17.6 MeV (D-T reaction, basis of H-bomb and tokamaks).
Energy comes from mass defect: products are slightly lighter than reactants.
Fusion requires extreme temperatures (~10⁸ K) to overcome electrostatic repulsion between positive nuclei (Coulomb barrier).
In stars: gravity provides confinement; the Sun fuses H → He via proton-proton chain at ~15 million K.
On Earth: magnetic confinement (tokamaks) or inertial confinement (laser implosion of fuel pellets).
Fusion releases MORE energy PER NUCLEON than fission (~3.5 MeV/nucleon vs ~1 MeV/nucleon).
Advantages over fission: abundant fuel (deuterium from seawater), no long-lived radioactive waste, no chain-reaction runaway risk.
Formulas
Deuterium-Tritium fusion
Most-studied terrestrial fusion reaction.
Proton-proton chain (Sun)
Net stellar fusion: 4 protons → α + 2 positrons + 2 neutrinos + photons.
Coulomb barrier (D-T)
Quantum tunnelling allows fusion below this barrier at lower temperatures.
Lawson criterion (ignition)
Triple product of density, confinement time, and temperature required for net energy gain.
Important Points
Fusion of light nuclei is what powers the Sun and all main-sequence stars.
Net energy gain on Earth requires extreme temperatures + sufficient confinement (Lawson criterion).
ITER (under construction in France) aims for Q = 10 (output / input energy). Expected first plasma ~2025.
Cold fusion (room-temperature fusion in metal lattices) is NOT established physics — claims have not been replicated.
Stellar nucleosynthesis builds elements up to iron via fusion; heavier elements form in supernovae.
Fusion bombs: trigger D-T fusion using a fission bomb to compress and heat the fuel — first tested 1952.
Nuclear Fusion notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.