Overview of the SPARC physics basis towards the exploration of burning-plasma regimes in high-field, compact tokamaks is the Nuclear Fusion paper of 1 March 2022, led by 📝Pablo Rodriguez-Fernandez with 28 co-authors, that compresses and updates 📝The SPARC Physics Basis (2020) into ten pages.
The headline design did not move. 📝SPARC is still specified at 12.2 📝tesla on axis, 8.7 megaamps of 📝plasma current, 1.85 meters major radius and 0.57 minor, with 25 megawatts of 📝ion cyclotron resonance heating as its only external heating and a ten-second 📝flattop. The primary reference discharge still projects 140 megawatts of fusion power at 📝Q ≈ 11 from empirical power-balance analysis, and Q ≈ 9 from TRANSP coupled to the TGLF-SAT1 turbulence model — the same two numbers 📝Status of the SPARC Physics Basis introduced.
What moved is underneath. The paper records a decision to build the 📝plasma-facing components from 📝tungsten, which the 2020 collection had left open — carbon is easier to manufacture and survives thermal transients better, but its 📝tritium retention does not project to a power plant. The projected peak unmitigated parallel heat flux at the 📝divertor is stated as above 6 gigawatts per square meter, from a heat-flux width of about 0.3 millimeters mapped to the outer midplane and 29 megawatts crossing the last closed flux surface, where the 2020 divertor paper had projected above 10. A passive 📝runaway electron mitigation coil appears for the first time, absent from the 2020 collection entirely. And the advanced-divertor mission acquires a number: an 📝X-point target scenario capped at 5.7 megaamps that still projects Q > 2 at 37 megawatts of fusion power.
None of it is a measurement. The paper describes a machine then entering construction, and every figure in it is a projection for a 📝burning plasma that has not yet been produced.
Our physics basis is not a document we published once. This is where we recorded what two more years of design work did to it — the tungsten decision, a divertor heat flux revised downward, and a runaway mitigation coil that did not exist in 2020.