On the locked closing config the burner closes across a helium-3 fraction window of about 0.20 to 0.43. Below 0.20 the charged-power budget cannot fund the end losses; above ~0.43 the fuel no longer reacts hard enough. The design point sits inside it at x = 0.30. The window is a computed result, stated as one.
Sweeping the helium-3 fraction at the closing plug ratio (n_p/n_c=16), the machine is net-positive from about x=0.20 to x=0.43, with the design point at x=0.30 giving Q_E 1.31. Across the window the neutron fraction falls from 9.53% to 2.77%. Above ~0.43 the reaction rate drops too far and closure is lost — a redesigned machine, not a fuel change, would be needed for near-aneutronic operation.
A published closure window tells a fuel planner exactly how helium-3-rich the mix must be, and tells a physicist exactly which mixes to test. It is the fuel-side companion to the plug-density requirement on the confinement side.
| Closure window | x_He3 in [0.20, ~0.43] |
| Design point | x_He3 0.30 (Q_E 1.31) |
| Neutron fraction across window | 9.53% -> 2.77% |
| Above ~0.43 | closure lost (redesign needed) |
| Also required | plug density ratio ~16 (not demonstrated) |