Nuclear Fusion: How Stars Burn & Why It's Hard to Copy

The reaction: deuterium plus tritium

The most practical near-term fusion reaction fuses deuterium (²H) and tritium (³H): ²H + ³H → ⁴He (3.5 MeV) + n (14.1 MeV), releasing 17.6 MeV in total — about 10 million times more energy per unit mass than burning coal. The helium nucleus stays inside the plasma and helps keep it hot; the high-energy neutron escapes the magnetic confinement entirely and deposits its energy as heat in a surrounding blanket, which drives conventional steam turbines. Deuterium is abundant and stable (30 g per cubic metre of seawater); tritium is radioactive and scarce, so power plants are designed to breed their own from lithium-6 in that same neutron blanket.

The Coulomb barrier and quantum tunnelling

Both nuclei are positively charged, so they must overcome — or tunnel through — a repulsive electrostatic barrier of roughly 1.4 MeV at nuclear range. At 100 million °C (about 10 keV), thermal D-T ions have far less energy than that. Fusion happens anyway through quantum tunnelling: each nucleus's wavefunction extends slightly through the barrier, giving a small but nonzero probability of spontaneous penetration on every close approach. D-T has the highest fusion cross-section at the lowest energy of any practical reaction, peaking around 100 keV — which is exactly why it is the fuel of choice for first-generation reactors.

The Lawson criterion: three ways to reach breakeven

For a fusion plasma to produce net energy, the product of density n, temperature T and confinement time τ_E must clear a threshold:

n × T × τ_E ≥ 3×10²¹ m⁻³ · keV · s   (D-T, at 15–20 keV)

There are three routes to that threshold: extremely high density held together only briefly (inertial confinement, compressing fuel to 1,000× liquid density for ~10 nanoseconds), a moderate-density plasma held for seconds (magnetic confinement), or extremely high temperature, which raises the reaction cross-section directly. Real reactors mix strategies, but the two dominant approaches — tokamaks and laser-driven inertial confinement — sit at opposite ends of this trade-off.

Tokamaks and the 2022 ignition milestone

A tokamak confines plasma at 100-200 million °C inside a toroidal vessel using powerful superconducting magnets, with combined toroidal and poloidal fields creating helical field lines that keep the plasma from ever touching the walls. ITER, under construction in France since 2010, targets Q = 10 — ten times more fusion power out than heating power in — with D-T experiments expected around 2035. On the inertial-confinement side, the National Ignition Facility made history in December 2022: 192 laser beams delivering 2.15 MJ compressed a millimetre-scale capsule to 100 billion atmospheres in under 10 nanoseconds, yielding 3.15 MJ of fusion energy — the first time fusion output exceeded the laser energy driving it.

Frequently asked questions

Try it live

Everything above runs in your browser — open Fusion Reactor, set plasma temperature, density and confinement time, and test the Lawson criterion to see whether your reactor reaches breakeven or full ignition.