The most powerful spherical tokamak ever built, at Princeton Plasma Physics Laboratory, which achieved record-high toroidal beta but has been offline since 2016 due to a magnet failure — with restart projected for 2027.
The National Spherical Torus Experiment Upgrade (NSTX-U) is a spherical tokamak at the Princeton Plasma Physics Laboratory (PPPL), Princeton, New Jersey. The original NSTX achieved first plasma on 12 February 1999 and operated until 2012, when it was shut down for a $94 million upgrade.[1]
The original NSTX (1999–2012) achieved record toroidal beta (βT up to ~35%), demonstrating the high-beta advantage of the spherical tokamak configuration. It reached 1.4 MA plasma current (exceeding the 1 MA design) and pioneered lithium wall conditioning techniques.[2]
NSTX-U achieved first plasma on 10 August 2015 and operated for approximately 10 weeks in early-to-mid 2016, reaching H-mode at ~1 MA. On 22 July 2016, operations were suspended due to failure of the PF1A Upper inner poloidal field coil. Forensic analysis found the root cause was inadequate vacuum pressure impregnation during subcontracted manufacturing.[3]
All six inner poloidal field coils were scrapped and rebuilt. A new 23,000-pound centre stack magnet bundle was fabricated and delivered in approximately June 2026. The recovery project is ~93% complete as of mid-2026, with restart projected for 2027.[4]
NSTX-U explores the physics basis for compact spherical tokamak fusion pilot plants, complementing the conventional-aspect-ratio research at ITER.