Fusionsrate

Fusionsrate is a term used to describe the rate at which nuclear fusion reactions occur within a plasma, expressed per unit volume and per unit time. It is a central quantity in both laboratory fusion experiments and in astrophysical contexts, where understanding how often fusion events take place helps characterize energy production and nucleosynthesis.

In a plasma containing two reacting species with number densities n1 and n2, the fusion rate per unit volume is given by Fusionsrate = n1 n2 ⟨σv⟩, where ⟨σv⟩ is the reactivity, the average product of the fusion cross section σ(v) and relative velocity v over the particle velocity distribution. For identical reacting species, the rate is R = (1/2) n1^2 ⟨σv⟩. The resulting units are reactions per cubic meter per second.

The reactivity ⟨σv⟩ depends on the plasma temperature and composition and is determined by nuclear physics data combined with the velocity distribution of particles. In practical terms, higher fusionsrate is achieved by increasing particle densities, extending confinement times, and maximizing the energy window where the fusion cross section is sizable. This interplay is encapsulated in the Lawson criterion, which relates density, confinement time, and reactivity to assess whether a reactor can achieve net energy production.

Fusionsrate can be inferred experimentally from measurements of fusion products such as neutrons or charged particles, after accounting for detector efficiency and geometric factors. Knowledge of the fusionsrate informs reactor design, heating requirements, and overall energy balance, while in astrophysics it governs stellar energy generation and the pathways of element formation.