Reactivity

The temperature-averaged fusion reaction rate for a fuel.

Reactivity averages the fusion cross-section over the plasma's velocity distribution, giving the reactions-per-second per pair of nuclei at a given temperature. It rises steeply with temperature and is what makes D–T practical at 15 keV while D–³He needs far more.

Reactivity rises steeply with temperature and then saturates, which sets each fuel's operating point: D–T peaks near 15 keV, while D–³He requires the burner's much hotter 90 keV. Kronos does not choose these temperatures arbitrarily — each is dictated by where its fuel's reactivity pays off against the losses.