Status of the SPARC physics basis is 📝Martin Greenwald's single-authored 2020 introduction to the Journal of Plasma Physics special issue on 📝SPARC — the shortest paper in the collection and the one that explains why the other seven exist.
Its framing argument is about timing rather than physics. Decades of collective work across the world's fusion programs produced an empirical and theoretical understanding of magnetically confined plasmas good enough to design against; 📝high-temperature superconducting tape then arrived as a practical engineering material. SPARC is what happens when the second is applied to the first. Greenwald states the design plainly: 12.2 📝tesla 📝toroidal field, 8.7 megaamps of 📝plasma current, a major radius of 1.85 meters and a minor radius of 0.57, with up to 25 megawatts of 📝ion cyclotron resonance heating, targeting a minimum 📝Q of 2 and more than 50 megawatts of fusion power.
The paper also names SPARC's two missions in the order that matters: produce net fusion power, and retire technological risk on the path to a plant. A machine can accomplish the second while falling short on the first, and a reader who takes only one thing from this collection should take that both goals were projections stated in advance of construction, and in advance of any magnet test or plasma measurement that could check them. The collection this introduces did not stay fixed either: 📝The 2022 SPARC Physics Basis Overview compresses and updates it two years later, and 📝Revising the SPARC Core Performance Prediction traces what happened to the gain projection underneath.
Marty wrote the paragraph that puts SPARC in the field's history rather than in our marketing. We inherited an understanding of plasmas we did not build, and the honest version of our claim says so.