National Spherical Torus Experiment – Upgrade (NSTX-U)

Inside the NSTX spherical torus at the Princeton Plasma Physics Laboratory.

The NSTX-U is a magnetic confinement fusion facility employing a spherical torus confinement configuration to explore the potential stability and confinement advantages of this compact tokamak concept.
Princeton, NJ Location
2016 Start of Operations
307 (FY 2022) Number of Users


The National Spherical Torus Experiment Upgrade, or NSTX-U, is the most powerful spherical tokamak in the world. With its nearly spherical plasma shape, it provides a test of the theory of toroidal magnetic confinement as the spherical limit is approached.  During experimental studies in the original NSTX, plasma with a spherical torus shape (viz. a shape resembling a cored apple) was found to be stable even when ratios of plasma-to-magnetic pressure and self-driven current fraction were large. Beyond stability, NSTX-U will test the impact of the spherical torus shape on plasma energy confinement properties. If present theoretical predictions are verified, it would indicate that spherical tori use applied magnetic fields more efficiently than most other magnetic confinement systems and could, therefore, be expected to lead to more cost-effective fusion power systems in the long term.  The NSTX-U construction was complete in 2015 and plasma operations began in 2016. This upgrade project included a new center stack and a second neutral beam injection system. Together this hardware upgrade has doubled the magnetic field, plasma current, and auxiliary heating power of the device, while extending the plasma pulse length by nearly a factor of five.