Where the Burner's Neutrons Come From
The burner's 5.44% neutron output comes from D-D side reactions and the D-T that D-D breeding feeds, not from the main D-³He channel.
In a deuterium–helium-3 plasma the intended reaction is D + ³He → ⁴He (3.6 MeV) + p (14.7 MeV). Both products are charged, so direct energy conversion can recover them and no neutron is made. The neutrons that do appear come from unavoidable competing reactions among the deuterium ions.
The two side channels
- D + D → ³He + n (2.45 MeV): one of the two roughly equal D–D branches emits a neutron directly.
- D + D → T + p: the other branch makes tritium, which then fuses with deuterium as D + T → ⁴He + n (14.1 MeV), adding a high-energy neutron.
The relative rates depend on ion temperature and the deuterium fraction. The design deliberately runs helium-3-rich and hot to suppress the D–D rate, but it cannot reach zero. The net of both channels is the 5.44% neutron energy fraction quoted for the design point.
Design levers
The tritium bred in situ by the D–D path is a subtle point worth stating plainly: the burner makes a little of its own tritium, which then burns and adds 14.1 MeV neutrons. This internal breeding is small but is part of why the neutron fraction cannot be driven to zero in any real deuterium plasma.
Two knobs reduce the neutron fraction: raising ion temperature (which favors D–³He over D–D) and lowering the deuterium-to-helium-3 ratio. Both are constrained by the plug and confinement physics, and the burner's operating regime is far beyond any existing device, so the 5.44% figure is a simulation target, not a demonstrated value. Reducing it further trades against fuel supply and confinement, which is why the honest number is stated rather than an aspirational zero.