The deepest result in the burner's physics is not about confinement at all: an open system must fund its axial end losses out of the charged power it produces. That charged-power budget, not confinement quality, is the mechanism that decides whether a mirror closes.
In a mirror, particles leave along open field lines, carrying energy out the ends. That loss must be balanced by the fraction of fusion power that arrives as charged particles — the rest, in neutrons and radiation, cannot pay it. The frozen record makes it concrete at the deposited point (ion temperature 90 keV): pure D-D radiates 41.4% of its power as neutrons, leaving only 173.7 MW of charged power against an 8,779 MW axial loss — a 51-fold shortfall, so the machine cannot close. At the helium-3-rich design mix (x_He3 0.30) the neutron fraction is 5.44% and the charged-particle power covers the axial loss, so the budget closes at engineering gain Q_E 1.31 — provided the end-plug density requirement (n_p/n_c ~16) is met.
This is why the distinction between the burner and the breeder is load-bearing, not cosmetic: the same result that sinks pure D-D in a mirror does not transfer to Hyperion's closed tokamak, which has no open-end loss to fund. Naming the mechanism is what makes the fuel choice defensible.
| Governing mechanism | charged-power budget (not confinement) |
| Pure D-D (deposited pt, Ti 90 keV) | 173.7 MW charged vs 8,779 MW axial (51x), f_n 41.4% |
| Design D-3He mix (x 0.30) | f_n 5.44%, closes Q_E 1.31 (plug ~16 required) |
| Applies to | open (mirror) systems |
| Does not transfer to | Hyperion (closed tokamak) |