Nils Leuthold is a 🏷️#physicist and associate research scientist at Columbia University, part of the 📝Columbia Fusion Research Center, and lead author of "ARC physics basis–magnetohydrodynamics" (2026) — the analysis of whether 📝ARC's plasma will hold its shape.

📝Magnetohydrodynamics is the study of how a conducting fluid and a magnetic field push on each other, and in a 📝tokamak it is the source of the instabilities that end discharges. Leuthold's paper works through them in sequence: 📝vertical displacement events, which he finds ARC's 📝poloidal field coils can control without dedicated in-vessel coils; ideal 📝kink instability analysis, which places the baseline scenario "deeply in the stable region"; 📝tearing mode calculations showing linear stability at the 2/1 and 3/2 surfaces; and 📝neoclassical tearing mode studies putting marginally stable 📝magnetic island widths below 0.1 percent of normalized poloidal flux. His error-field scaling places ARC's tolerance between 📝SPARC's and 📝ITER's, and the paper closes by specifying correction coils that would also serve to suppress 📝edge localized modes. "ARC was designed to be stable to these problems, and the modeling supports that," he has said — modeling being the operative word for a machine that does not exist.