Typical fusion reactions consist of Hydrogen and Helium isotopes. Figure 4.1 shows a plot of the reaction average cross section (reaction rate) as a function of plasma temperature and shows that DT reactions will ignite at the lowest temperature, followed by DD and D3He. DT ignites at the lowest temperatures because it has the smallest effective Coulomb barrier and the largest Maxwellian-averaged reactivity at 5–30 keV.