Pure deuterium-deuterium fuel actually confines better in the burner's mirror than D-3He does — yet it cannot reach net electricity. The failure is not confinement; it is that at a high neutron fraction, too little charged power is left to fund the axial loss.
In the deposited arbiter, pure D-D reaches an engineering gain of only 0.4314, a net deficit (-8,255 MWe), despite a longer confinement time than D-3He. The reason is the charged-power budget: at the deposited point (ion temperature 90 keV), a 41.4% neutron fraction leaves just 173.7 MW of charged power against an 8,779 MW axial loss — a 51-fold shortfall. (A higher-temperature 250 keV scan reports 47.3% / 420 MW / 7,815 MW; those are scan values at a different temperature, not the deposited point.) The machine starves regardless of how well it confines.
This is the counterintuitive result that proves the mechanism. Better confinement does not rescue a fuel whose energy leaves as neutrons, because neutrons cannot pay end losses. It is the cleanest demonstration of why the burner must run helium-3-rich.
| D-D engineering gain Q_E | 0.4314 (net -8,255 MWe) |
| D-D confinement | longer than D-3He, still fails |
| Deposited point (Ti 90 keV) | f_n 41.4%, 173.7 MW charged vs 8,779 MW axial (51x) |
| Ti 250 keV scan (labeled) | 47.3% / 420 MW / 7,815 MW |