The burner's central-cell length is a degree of freedom, not a physics constraint. Fusion power, radiation, and axial loss all scale together with volume, so length sets the size of the machine — from a short reference cell to the longer production housings — while the engineering gain barely moves.
At fixed cell radius, every power channel scales with length, so their ratio — the engineering gain — is nearly independent of it. The reference solve confirms it: 15 m and 100 m give Q_E 0.615 and 0.632. Length can therefore be chosen for power output, wall loading, or siting without paying in closure.
This is what lets one physics point serve two products: the defense housing runs a shorter cell and the data-center housing a longer one, both on the same closure physics. The scaling is a design convenience, deposited and checkable; the specific production lengths and powers are held off the public record until the design cards are reconciled.
| Free knob | central-cell length |
| Scales with length | fusion power, radiation, axial loss |
| Nearly independent of length | engineering gain Q_E |
| Defense housing | shorter cell |
| Data-center housing | longer cell |