Steven D. Scott is a 🏷️#physicist whose career was spent at the Princeton Plasma Physics Laboratory, and who published "Fast-ion physics in SPARC" (2020) under a 📝Commonwealth Fusion Systems affiliation. The paper asks whether the particles that make a fusion plasma self-heating stay put long enough to do it.
In a burning 📝D-T plasma, 📝alpha particles carry a fifth of the fusion energy and are supposed to deposit it in the fuel rather than in the wall — the mechanism called 📝alpha heating, and the thing that distinguishes a 📝burning plasma from a heated one. Because a tokamak's field is produced by a finite number of 📝toroidal field coils, it ripples slightly between them, and fast ions can drift out through that ripple. Scott's team computed the effect for 📝SPARC with the ASCOT and SPIRAL orbit codes and found that with well-aligned coils the edge ripple stays at 0.15–0.30 percent, alpha power loss near 0.25 percent, and the resulting surface heating acceptable — but that coil misalignment raises the loss substantially, which makes the result an assembly-tolerance requirement as much as a physics one. Small classical losses are what would let SPARC study alpha redistribution by 📝magnetohydrodynamics instead, once it runs.
Steve's calculation is the reason our magnet alignment tolerances read the way they do. The alphas have to stay in the plasma, and how well we position eighteen coils decides whether they do.