NSTX-U is the National Spherical Torus Experiment-Upgrade, the 📝spherical tokamak at the Princeton Plasma Physics Laboratory in New Jersey — a Department of Energy user facility that has been offline since 2016 and is being rebuilt toward experiments in 2027.
The upgrade, completed in 2015, replaced the centre stack of the original NSTX and added a second 📝neutral beam line. PPPL's design targets roughly doubled the machine on every axis that matters: a 📝toroidal field of 1 📝tesla at a major radius of 0.93 metres, a 📝plasma current of 2 megaamperes, and pulses lengthened from about 1 second to 5. The physics motive was collisionality: at low 📝aspect ratio a hotter, longer 📝plasma reaches conditions closer to a fusion nuclear science facility, where transport and stability behave differently than in the collisional regimes earlier spherical tokamaks could reach.
The machine ran for about ten weeks in 2016 before the upper inner poloidal field coil, PF-1a, failed. Forensic analysis traced the fault to a coolant blockage in one conductor path preceded by an undetected gradual deterioration; the project then chose to replace all six inner 📝poloidal field coils with a redesigned and remanufactured set. The recovery has taken a decade. A 23,000-pound centre magnet bundle built by Elytt Energy in Bilbao arrived at PPPL in June 2026 as the last major component before commissioning. NSTX-U's near-neighbour and long-standing collaborative partner is 📝MAST Upgrade, the other large spherical tokamak in the programme.