The world’s largest and most advanced stellarator, built to demonstrate that the stellarator concept can confine fusion-grade plasma as effectively as a tokamak while offering inherent steady-state capability.
Wendelstein 7-X (W7-X) is an experimental stellarator operated by the Max Planck Institute for Plasma Physics (IPP) in Greifswald, Germany. First plasma was achieved on 10 December 2015 with helium, followed by hydrogen plasma operation in February 2016. The device represents the culmination of decades of stellarator optimisation theory and is designed to demonstrate that the stellarator concept can achieve plasma confinement quality comparable to tokamaks while offering the advantage of inherently steady-state operation without the need for a large plasma current.[1]
The 50 non-planar superconducting coils, each a precisely shaped three-dimensional form weighing approximately 6 tonnes, create the complex twisted magnetic field geometry that defines the stellarator. Their shapes were computationally optimised to minimise neoclassical transport—the main historical weakness of stellarators—while simultaneously achieving good magnetohydrodynamic (MHD) equilibrium, small bootstrap current, and effective particle confinement. This “optimisation” is what distinguishes W7-X from earlier stellarators.[2]
Magnetic topology verification. In 2016, flux-surface measurements confirmed that the as-built magnetic field topology of W7-X matches its theoretical design to better than 1 part in 100,000—a verified result published in Nature Communications that demonstrated the feasibility of manufacturing such complex coil geometries to the required precision.[3]
Record stellarator confinement. During operational campaigns OP1.2 (2017–2018) and OP2 (2022–present), W7-X achieved energy confinement times and plasma parameters that set records for stellarator devices. In OP1.2, with an island divertor formed by intrinsic magnetic islands at the plasma edge, W7-X demonstrated detached divertor operation and core impurity screening—results that had been predicted by optimisation theory but never previously confirmed in a large stellarator.[1]
High-performance plasmas. W7-X has reached ion temperatures above 3.5 keV, electron densities exceeding 2 × 1020 m−3, and stored energies above 1 MJ in transient phases. Importantly, these results have been achieved with low plasma current (~10 kA net toroidal current), validating the stellarator’s promise of current-free, disruption-free operation.[4]
In 2021, a study published in Nature provided the most direct confirmation of W7-X’s optimisation strategy. Measurements showed that neoclassical energy transport in W7-X was reduced by a factor of roughly 5–7 compared to an equivalent un-optimised stellarator, consistent with theoretical predictions. This verified result established that computational stellarator optimisation works as intended and that the approach can, in principle, be extended to reactor-scale devices.[4]
W7-X is currently in its OP2 operational phase, which employs actively water-cooled carbon-fibre-composite divertor targets rated for steady-state heat exhaust at 10 MW/m². The physics programme is focused on extending pulse lengths toward 30 minutes, demonstrating high-performance plasmas at reactor-relevant densities, and further validating neoclassical optimisation. W7-X does not use tritium and is not designed to produce fusion power; its role is to establish the physics basis for a future stellarator power plant. If successful, W7-X could motivate a stellarator DEMO that avoids the disruption risk inherent to tokamaks and operates in true steady state.