Revising the SPARC core performance prediction is the four-paper chain published between 2020 and 2024 in which the projected fusion gain of 📝SPARC's reference discharge moved from 📝Q ≈ 9 to Q ≈ 8 as reduced turbulence models were replaced by nonlinear 📝gyrokinetics — and then turned to a different question entirely.
The first number is 📝Pablo Rodriguez-Fernandez's 2020 Journal of Plasma Physics paper inside 📝The SPARC Physics Basis (2020), which chained TRANSP, the TGLF gyro-fluid model, EPED and full-wave heating codes into one 📝integrated modeling prediction and found Q ≈ 9 in standard 📝H-mode, against Q ≈ 11 from empirical power balance. TGLF is a reduced model: fast enough to run a whole profile, and calibrated against first-principles simulation rather than derived from it. Everything after 2020 is an attempt to remove that dependence.
📝Nathan Howard's 2021 Physics of Plasmas paper is the first step and produces no gain figure at all. Running the CGYRO code on the reference discharge, it establishes what the reduced model had assumed — ion temperature gradient turbulence dominates across most of the profile, electron-scale contributions are minimal, and the 📝ion temperature profile will sit pinned just above the ITG critical gradient. Rodriguez-Fernandez, Howard and Jeff Candy then closed the loop in 2022, wrapping Gaussian-process surrogates around nonlinear CGYRO with six gyrokinetic species so that a converged multi-channel solution could be reached without an impossible number of simulations. That paper's answer is Q ≈ 8: the first projection for SPARC's 📝burning plasma computed from nonlinear gyrokinetics rather than a model fitted to it.
The 2024 paper changes the question. Applying the same PORTALS-CGYRO machinery to SPARC's first campaign — 📝L-mode, before H-mode operation is commissioned — it finds breakeven reachable when edge pressure reaches roughly 35 percent of the EPED-predicted H-mode value, with Q ≈ 1.2 or Q ≈ 0.8 depending on input power, and reports the unexpected result that higher impurity content raises fusion power. A fifth paper, led by A. Di Siena in 2023, sits alongside rather than within the chain: it finds that supra-thermal ions from off-axis heating — the same population that drives 📝Alfvén eigenmodes — could raise the gain of a reduced-field scenario by up to 80 percent.
Nothing in any of these papers is a measurement. SPARC had not produced a plasma when the last of them was written, so the chain records exactly one thing: a prediction that was revised twice, in public, before the instrument capable of testing it existed.
Q ≈ 9 in 2020. Q ≈ 8 in 2022. In 2024, a different question — what breakeven actually takes in a first campaign. We publish the revisions because a projection nobody can show us moving is not one we would trust either.