How many neutrons the burner makes is chosen at the fuel line. On the locked closing config the design mix is x=0.30 (f_n 5.44%); shifting to x=0.35 cleans it to 4.18% while staying net-positive. Neutron output is a knob, and the design turned it down.
Neutrons in the burner come from D-D side reactions and the secondary D-T burn. Fewer deuterium-deuterium encounters means fewer neutrons, so a helium-3-richer mix is a quieter mix. Across the closure window x_He3 ∈ [0.20, ~0.43] the neutron fraction falls from 9.53% to 2.77%; the design point sits at x=0.30 (5.44%), with a free clean-shift to x=0.35 (4.18%). Beyond ~0.43 the machine no longer closes.
A designer who can trade neutron output against fuel demand can place the machine wherever the mission needs it. The published trade curve makes that a transparent engineering choice, not a hidden assumption.
| Neutron knob | helium-3 fraction x_He3 (locked config) |
| Design point x=0.30 | f_n 5.44% |
| Free clean-shift x=0.35 | f_n 4.18% (net-positive) |
| Closure window | x in [0.20, ~0.43], f_n 9.53% -> 2.77% |
| Above ~0.43 | does not close |