Nuclear fusion—the process powering the Sun—fuses light nuclei (typically deuterium and tritium) into helium, releasing enormous energy from the mass difference (E=mc²). The D-T reaction (²H + ³H → ⁴He + n) releases 17.6 MeV per reaction, roughly 4 million times more energy per kilogram than coal combustion. Achieving net energy gain requires heating plasma to 100–200 million kelvin—hotter than the Sun's core—so that nuclei overcome their electrostatic repulsion and collide at the quantum-tunneling-assisted fusion rate.
The primary approach to confining this superheated plasma is the tokamak, a donut-shaped (toroidal) device using powerful superconducting electromagnets to contain the plasma with a combination of toroidal and poloidal magnetic fields that form nested helical flux surfaces. The Lawson criterion—nτT > 3×10²¹ keV·s/m³—specifies the minimum combination of plasma density n, energy confinement time τ, and temperature T needed for ignition. ITER, under construction in France, aims to achieve Q = 10 (10× more fusion power than heating power input).
This simulator models plasma confinement, heating, and fusion rate as functions of plasma parameters. You can adjust magnetic field strength, plasma density, and temperature to find operating points satisfying the Lawson criterion, visualize plasma pressure profiles, and observe how instabilities (kink modes, disruptions) limit achievable parameters. It illustrates why fusion has been "30 years away" for decades—and what recent advances in high-temperature superconductors and inertial confinement might change.
Why does fusion require such extreme temperatures?
Positively charged nuclei repel each other through the electromagnetic (Coulomb) force. To fuse, two nuclei must approach within ~1 femtometer (10⁻¹⁵ m) where the strong nuclear force takes over. Classically this requires kinetic energies of ~1 MeV, corresponding to 10 billion K. Quantum tunneling allows fusion to occur at lower energies (~100 keV, or ~1 billion K), but even this is 6–7 times the Sun's core temperature. The Sun compensates with enormous mass and confinement time; laboratory fusion must reach higher temperatures to achieve adequate reaction rates in much smaller devices.
What is the Lawson criterion?
The Lawson criterion, derived by John Lawson in 1955, states that the product of plasma density, energy confinement time, and temperature must exceed a threshold for fusion power to exceed the energy invested in heating: nτT > 3×10²¹ keV·s/m³ (for D-T fusion). Increasing any of the three factors can compensate for deficiencies in the others. Tokamaks typically operate at moderate density (n ~ 10²⁰ m⁻³) with long confinement times (τ ~ 1 s), while inertial confinement targets use extremely high densities for very short confinement times (nanoseconds).
How does a tokamak confine plasma magnetically?
A tokamak uses two magnetic field components: a strong toroidal field (the long way around the torus) generated by superconducting coils wrapped around the vessel, and a poloidal field (the short way around) generated partly by a current driven through the plasma itself. These combine into helical field lines that wind around the torus on nested toroidal surfaces. Charged particles spiral along field lines, and if the field lines close on themselves after winding around the torus, particles are confined. The safety factor q (toroidal/poloidal winding ratio) must exceed 1 everywhere to avoid kink instabilities.
ITER is a scientific experiment designed to demonstrate Q = 10 (output 500 MW of fusion power from 50 MW of heating) and to study plasma behavior at scale. It will not generate electricity. DEMO (a proposed demonstration plant) would follow ITER and demonstrate electricity generation. A commercial fusion plant would need to achieve Q > 30 to be economically competitive, continuously breed tritium from the lithium blanket (since tritium is rare), manage 14 MeV neutron activation of structural materials, and be maintainable remotely due to induced radioactivity—engineering challenges as daunting as the plasma physics.
Fusion has several potential advantages over fission: the primary fuels (deuterium from seawater and lithium for breeding tritium) are effectively unlimited, whereas uranium is a finite resource. Fusion produces no long-lived radioactive waste—the main activation product from D-T fusion is helium, and neutron-activated structural materials have half-lives of decades rather than millennia. Fusion cannot chain-react; removing heating power stops the reaction in seconds, with no possibility of runaway meltdown. The major remaining challenge is achieving net energy gain reliably and economically—a milestone that first appears within reach with ITER and private fusion ventures.