The advanced stellarator that proved modular coils could work and paved the way for Wendelstein 7-X — built at IPP Garching, it ran for over a decade and demonstrated key stellarator optimization principles.
Wendelstein 7-AS was a medium-sized stellarator that operated at the Max Planck Institute for Plasma Physics (IPP) in Garching, Germany, from 1988 to 2002. It was the direct predecessor to Wendelstein 7-X, the world’s largest and most advanced stellarator now operating in Greifswald, and it played a decisive role in demonstrating that the physics principles and engineering concepts behind optimized stellarators actually worked in practice. Without 7-AS, the multibillion-euro investment in 7-X would never have been justified.[1]
Wendelstein 7-AS was the first stellarator to use a system of modular (non-planar) coils as the primary field-shaping elements. Earlier stellarators — including IPP’s own Wendelstein 7-A — used helical windings wound around the vacuum vessel to create the rotational transform needed for plasma confinement. These continuous helical coils were effective but extremely difficult to manufacture, maintain, and modify. The 7-AS coil concept replaced them with a set of 45 discrete, non-planar modular coils, supplemented by 10 planar toroidal field coils for additional shaping flexibility. This modular approach proved that complex three-dimensional magnetic fields could be produced by manufacturable, separable coil units — a principle that became the foundation of the entire 7-X design.[2]
Wendelstein 7-AS had a major radius of 2.0 m and an average minor radius of about 0.18 m. The magnetic field on axis reached up to 2.5 T. The device featured five field periods (fivefold symmetry) and operated with plasma heated by neutral beam injection (NBI) and electron cyclotron resonance heating (ECRH). Pulse lengths were limited to roughly one second by the copper (non-superconducting) coil system, but this was sufficient for detailed transport and stability studies. The flexible coil set allowed the magnetic configuration — rotational transform, shear, and mirror ratio — to be varied over a wide range from shot to shot.[3]
Over its 14-year operational life, Wendelstein 7-AS produced a string of results that validated the optimized-stellarator concept and informed the design choices for 7-X.
The device demonstrated that confinement in an optimized stellarator could approach or match the H-mode energy confinement times achieved in tokamaks of comparable size. Experiments showed confinement scaling consistent with the ISS95 and later ISS04 stellarator scaling laws, with evidence that the optimization of the magnetic configuration genuinely reduced neoclassical transport compared with unoptimized stellarators.[4]
7-AS achieved high-beta plasmas — with volume-averaged beta values exceeding 3% — without encountering the hard beta limits that MHD theory predicted for some configurations. This result was important because it suggested that optimized stellarators might sustain higher plasma pressure than conservative estimates indicated, improving the power-plant economics of the concept.
Perhaps 7-AS’s most forward-looking contribution was the development and first experimental test of the island divertor concept. In this scheme, magnetic islands at the plasma edge — which arise naturally from the low-order rational surfaces in the stellarator’s rotational transform profile — are exploited to channel exhaust particles and heat to discrete divertor target plates. The concept was tested on 7-AS with a set of 10 divertor modules and demonstrated effective particle and impurity control. This island divertor concept was subsequently adopted as the primary divertor design for Wendelstein 7-X, where it is now being tested at much higher power and pulse length.[5]
Wendelstein 7-AS was decommissioned in 2002 after its scientific program was completed. By then, it had fulfilled its purpose: demonstrating that modular coils, optimized magnetic configurations, and island divertors were sound concepts worthy of a major next-step investment. Wendelstein 7-X, which achieved first plasma in 2015, is the direct result of the confidence built on 7-AS’s experimental record.