Fusion Cross-Section & Reactivity

The cross-section measures how likely two nuclei are to fuse at a given energy. D–³He peaks at higher temperature than D–T — shaping the Kronos operating point.

Cross-section

Probability of fusion per encounter, vs energy

Reactivity

Cross-section averaged over the plasma temperature

D–T

Peaks near ~65 keV, easiest to ignite

D–³He

Peaks higher — needs hotter plasma

The fusion cross-section quantifies how likely two nuclei are to fuse when they collide at a given energy; averaged over a plasma's thermal distribution it becomes the reactivity, which sets the fusion reaction rate. Both rise steeply with temperature until they peak, so choosing a fuel is largely about where its reactivity peaks and how hot the plasma must run to get there.

Deuterium–tritium has the most accessible peak — one reason it ignites most easily. Deuterium–helium-3 peaks at higher temperature and lower absolute reactivity, which is precisely why Kronos MetroVolt must run hot (central ion temperatures of 65–70 keV) and lean on strong confinement. The reward for that harder plasma problem is a low-neutron, directly-convertible output.