Thomson scattering is the laser diagnostic that measures a plasma's electron temperature and density point by point, by collecting the light scattered from its free electrons — the gold standard of plasma measurement.
The technique is named for J. J. Thomson, discoverer of the electron, who described how a free charged particle scatters electromagnetic radiation. In practice a high-power laser pulse is fired through the 📝plasma and viewing optics collect the few photons scattered by free electrons. The Doppler broadening of the scattered spectrum encodes how fast the electrons move — their temperature — while the total scattered intensity is proportional to their density. Because the signal comes only from the small volume where the laser beam and the viewing optics intersect, the measurement is genuinely local: sampled along the beam, it yields complete profiles of 📝electron temperature and density from core to edge.
That locality is what separates Thomson scattering from interferometry, its near-neighbor among density diagnostics: an interferometer returns density averaged along an entire beam path, while Thomson scattering resolves the value at each point. The price is signal — only a vanishingly small fraction of the launched photons is scattered, demanding energetic pulsed lasers, careful stray-light suppression, and sensitive spectrometers. The technique settled one of 📝fusion's founding controversies in 1969, when a British team carried its lasers to Moscow's T-3 📝Tokamak and confirmed Soviet claims of unprecedented electron temperatures — the measurement that convinced the world to build tokamaks.