The Coupling Nobody Models: Better Confinement, More Neutrons

There is a coupling in the burner that constant-fraction models cannot represent: the same longer confinement that pushes the machine toward closure also raises tritium burnup and neutron production. The design has to hold both ends of that trade at once.

The science

Longer particle confinement time means deuterium ions linger, more D-D reactions occur, more tritium is bred and then burned in secondary D-T reactions — each of which is neutronic. So the plug density ratio that buys closure also, indirectly, raises the neutron fraction. Both effects come from the same confinement improvement.

Why it matters

Naming the coupling keeps the low-neutron claim honest at the operating point rather than at the reference alone. It also tells the materials programme to qualify against the neutron budget of the closing machine, not the more comfortable baseline.