S78 asks a simple question: what does MetroVolt give up to bring its peak magnet field down onto the field the world has already demonstrated? The answer — almost nothing — is one of the design's best trades.
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), 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 | R_c 1.40 → 1.44 m + B₀ → 6.11 T (in 0.42 m spare) |
| Peak field after | 24.40 T = demonstrated single-coil field |
| 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 |