The number of particles per unit volume in a fusion plasma — a critical parameter that must be high enough for fusion but below the Greenwald limit to avoid disruptions.
Plasma density in fusion refers to the number density of electrons (ne) or ions (ni) per cubic metre. Fusion-relevant densities range from about 1019 to 1021 m−3, depending on the confinement concept. For comparison, atmospheric air has a density of about 2.5×1025 m−3 — fusion plasma is a very good vacuum by everyday standards.[1]
In tokamaks, the maximum achievable density is empirically limited by the Greenwald density limit: nG = Ip/(πa²) (in units of 1020 m−3, with plasma current in MA and minor radius in metres). Operating above this limit typically triggers a disruption. Exceeding the Greenwald limit safely remains an active research goal.[2]
Plasma density is measured using interferometry (measuring the phase shift of a laser beam passing through the plasma), Thomson scattering (measuring scattered laser light), and reflectometry (detecting the reflection of microwave beams from critical-density layers).[3]