The SPARC Physics Basis is the special issue of the Journal of Plasma Physics published on 29 September 2020 in which the 📝SPARC team set out, in seven papers plus an introduction, the complete physics case for a machine that had not been built. Nothing in it is a measurement. It is a design argued in public, in the order the argument has to be made: here is the machine, here is why we think it will perform, here is why we think it will survive doing so. That order is how this pillar is arranged, and it is worth reading in that order rather than by topic.
The machine and the framing
📝Martin Greenwald's introduction, 📝Status of the SPARC Physics Basis, makes the argument about timing: decades of collectively built plasma understanding met a newly practical 📝superconductor, and SPARC is what that combination permits. 📝Overview of the SPARC Tokamak, led by 📝Alex Creely with 45 co-authors, is the anchor paper — 12.2 📝tesla on axis, 1.85 meters major radius, a mission of 📝Q > 2 and a nominal projection of Q ≈ 11 at roughly 140 megawatts. Between them they fix every number the other six papers work against.
Will it perform
Two papers carry the performance case, and they depend on each other. 📝Pablo Rodriguez-Fernandez led the core prediction, chaining TRANSP, the TGLF turbulence model, EPED and full-wave heating codes to find Q ≈ 9 attainable in standard 📝H-mode — deliberately below the empirical Q ≈ 11, because physics-based modeling is the more conservative instrument. 📝Jerry Hughes led the 📝pedestal projection that the core prediction sits on top of, finding a clear H-mode access window and a peeling-limited pedestal above 0.3 megapascals — and, more usefully, that even halving that pedestal leaves Q > 2 intact.
Will it survive
Heat has to get in, stay in, and get back out, and three papers handle those separately. 📝Yijun Lin designed the 📝ion cyclotron resonance heating system that is SPARC's only external heating — twelve four-strap antennas, no beams, no fallback. 📝Steve Scott computed whether 📝alpha particles stay confined long enough to heat the fuel, and turned the answer into a magnet-alignment tolerance. 📝Adam Kuang published the number that should worry a reader most — unmitigated 📝divertor heat flux above 10 gigawatts per square meter — together with the strike-point sweep that short pulses make possible. 📝Ryan Sweeney covered 📝magnetohydrodynamics and 📝disruptions, finding comfortable stability margins alongside disruption loads comparable to 📝ITER's, including halo current forces up to 50 meganewtons.
What the collection is not
It is not a results paper, and no figure in it has been confirmed by an experiment. It is also not quite the nine-paper set it is sometimes described as: the collection is seven research papers plus Greenwald's introduction, and Tünde Fülöp's January 2020 paper on plasma elongation and 📝runaway electrons — which CFS lists alongside them and which genuinely bears on the disruption case — was published separately. Nor is it the last word: 📝The 2022 SPARC Physics Basis Overview revises several of these numbers two years later, including the divertor heat flux, and adds a runaway mitigation coil the collection does not contain. What the collection is, is falsifiable. Every projection carries its assumptions, its codes and its conservative case, which is precisely what will make the comparison meaningful when the machine finally runs.
We published the whole physics case in one issue, five years before SPARC could answer back, because a prediction made in advance is the only kind that can be wrong in public.