Instrument 3 of 4 · S78

The field lever.

The REBCO story lives in one identity: B_peak = B₀R₀/R_c — the peak field the inboard conductor actually sees. The frozen point sits at 24.6 T, just above the demonstrated 24.4 T single-coil record. The deposited S78 study shows the lever: slide the winding-pack centre 4 cm outboard and raise B₀ to 6.11 T, and the peak lands exactly on the demonstrated value — with the freed margin available to bank (as stability headroom) or spend (+6 % fusion power). Try it.

starting …

Toroidal field B₀ 6.00 T

Winding-pack centre R_c 1.40 m

B_peak on conductor

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tesla

β_N (scaled)

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q₉₅ (scaled)

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The three chips are the deposited S78_field_lever.csv rows. “Spend” raises density into the freed β_N margin: 2,409 → 2,557 MW at the demonstrated 24.40 T.

Frozen means frozen The paper's design point does not move: the S78 lever is deposited as a constraint-checked upgrade path for the next iteration, gated on an inboard-neutronics re-run — not a revision of the frozen basis. β_N (Troyon) remains the binding constraint in all three cases. Source: the deposit, design_space/S78_field_lever.py.

The writeup

The plain-language paper behind this instrument: One Turn Outboard: The Field Lever That Buys Demonstrated Margin.

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