Overview of the SPARC tokamak is the 2020 Journal of Plasma Physics paper in which 📝Alex Creely and 45 co-authors published the design case for 📝SPARC — the anchor article of the seven-paper SPARC Physics Basis collection.
The machine it describes is defined by four numbers: an on-axis field of 12.2 📝tesla, a major radius of 1.85 meters, a minor radius of 0.57 meters, and a mission of 📝Q > 2 — which would make it the first magnetically confined plasma to return more fusion energy than was put into it. High field is what makes the compactness possible; SPARC is the continuation of the 📝Alcator C-Mod lineage of high-field 📝tokamaks, now with 📝high-temperature superconducting magnets in place of copper.
The projections are stated with their assumptions attached, which is the paper's most useful feature. Under conservative 📝confinement scaling — an H-factor of 0.7 — the Q > 2 goal still holds. Under the nominal assumption of H = 1, SPARC is projected to reach Q ≈ 11 and about 140 megawatts of fusion power, in a 📝burning plasma at a density near 3 × 10²⁰ per cubic meter and an 📝electron temperature near 7 📝keV, with a power density around 7 megawatts per cubic meter.
Every one of those figures is a projection. SPARC was under design when the paper was written and under construction in Devens, Massachusetts afterward; it has not yet produced a plasma. The distinction the paper preserves — a conservative floor of Q > 2 alongside a nominal Q ≈ 11 — is the honest form of a performance claim made in advance. Both figures were revisited afterward: 📝The 2022 SPARC Physics Basis Overview holds them while changing the wall material and the projected 📝divertor heat flux, and 📝Revising the SPARC Core Performance Prediction follows the physics-based gain projection from Q ≈ 9 to Q ≈ 8.
We published SPARC's design case, with its assumptions and its downside cases, five years before the machine could argue back. That is the standard we want the field held to, including us.