ARC engineering beyond the physics basis is a pair of 2026 papers that carry the 📝ARC design past plasma physics and into material limits: what accumulates beneath a 📝divertor surface held in detachment, and how structural metals fatigue at 4 kelvin.
Ane Lasa of the 📝University of Tennessee Department of Nuclear Engineering led "Subsurface Gas Dynamics in the ARC Divertor Under Prolonged Exposure to Detached Plasma Conditions" in Fusion Science and Technology, published online 13 March 2026 (10.1080/15361055.2026.2635253), with Sophie Blondel and Brian Wirth of Tennessee, Sicong He and Jaime Marian of UCLA, Michael Wigram of the 📝MIT Plasma Science and Fusion Center, and Lauren Garrison and 📝Jon Hillesheim of Commonwealth Fusion Systems. The problem it addresses is the one 📝divertor detachment creates rather than solves: detachment lowers the heat flux reaching the target, but it does so by delivering a dense, low-energy particle flux, and hydrogen and helium implanted at low energy do not sputter the surface so much as migrate, trap and collect in bubbles below it. Over a long pulse that becomes a question about 📝tritium inventory and the mechanical integrity of 📝plasma-facing components, not about erosion. The same UCLA–CFS pairing produced the 📝INFUSE final report on ARC divertor materials (10.2172/2589555), which reports in simulation that hydrogen buildup in 📝tungsten tracks particle flux more closely than temperature.
The second paper is about the cold end of the machine. Michael D. Marotta and R. Keith Kersey of Advanced Fracture Mechanics Associates, with Agnieszka Wusatowska-Sarnek, Taylor Pratt, Andreas Kulovits and Claire Saunders of CFS, published a first approximation for unified fatigue models for 316 stainless steel and IN718 at 4 K, 77 K and 293 K, derived from monotonic material properties (IOP Conference Series: Materials Science and Engineering 1345, 012001, 2026; 10.1088/1757-899x/1345/1/012001). The motivation is structural: a 📝toroidal field coil sees large 📝Lorentz force loads on every startup and shutdown, those loads are cyclic, and they act at 📝cryogenics temperatures where low-cycle fatigue data is scarce. Without a fatigue model at 4 K there is no defensible operational life for the coil structure. These two alloys sit in the magnet and structural stack; the neutron-facing structures are a separate materials problem, addressed by classes such as 📝RAFM steel.
Together they mark where 📝The ARC Physics Basis (2026) stops. The physics basis fixes a design point; these two papers ask whether the materials at either temperature extreme survive it for the life of the plant.
ARC's design point is settled on paper. What is not settled is how tungsten behaves after months of detached operation and how our coil structures fatigue at 4 kelvin — which is why we are publishing that work with Tennessee, UCLA and Advanced Fracture Mechanics rather than deferring it.