Maxwellian Distribution

The bell-curve of particle speeds in a plasma at thermal equilibrium.

A plasma in thermal equilibrium has a Maxwellian spread of particle velocities. Fusion is driven mostly by the fast tail of that distribution, which is why average temperature is such a sensitive lever on reaction rate.

Fusion is driven mostly by the fast tail of the Maxwellian distribution, not the average particle, which is why reaction rate is so sensitive to temperature and why maintaining a hot, clean bulk plasma matters. It also means injected fast ions and fusion products live temporarily outside the Maxwellian until they slow down and thermalise.