S78 asked a simple question: what does MetroVolt give up to bring its peak magnet field onto the field the world has already demonstrated? The answer — almost nothing — was one of the design's best trades, and on 24 July 2026 the ratified Mode-D basis adopted it at the build limit: R_c = 1.50 m, B₀ = 6.37 T, 24.42 T peak.
Peak conductor field scales as B₀·R₀/R_c. Moving the winding-pack centre from R_c = 1.40 m to 1.44 m — four centimeters, absorbed by the build's 0.42 m spare — while re-rating to B₀ = 6.11 T holds the peak at 24.40 T: precisely the publicly demonstrated single-coil field, with fusion power unchanged and better Troyon (βN 4.41 → 4.33) and kink (q95 4.43 → 4.51) margins. Or spend the freed margin instead: density into the βN headroom buys +6% fusion power (2,409 → 2,557 MW at the hot-ion-era pricing), still at 24.40 T.
Both branches run through the full eight-constraint systems evaluation — every number survives the simultaneous check — and the robustness envelope (S79) confirms the lever stacks with the wall-stabilised upside rather than competing with it.
Financing risk prices the distance between a design point and a demonstrated anchor. S78 collapses that distance for the magnet — the single most scrutinized subsystem in compact fusion — for the cost of spare space the build already carried. It is the difference between 'trust our extrapolation' and 'here is the knob that lands us on the record book.'
| The move (as studied) | R_c 1.40 → 1.44 m + B₀ → 6.11 T |
| The move (as adopted, Mode D) | R_c → 1.50 m + B₀ → 6.37 T ⇒ 24.42 T |
| Spend branch | density into freed βN → +6% Pfus (2,557 MW) |
| Cost to plasma performance | none — margins improve |
| Status | design-space study S78, deposited, standalone |