The Wendelstein 7-AS stellarator was in operation at Garching from 1988 till 2002.
The Wendelstein 7-AS experiment demonstrated that the novel modular coils can generate the precise magnetic field required. It broke all stellarator records in its size category by attaining plasma temperatures of 70 million degrees, energy confinement times of up to 40 milliseconds and reactor-like plasma densities.
The optimisation criteria used were also verified: The undesirable shift of the plasma column in the vessel as the plasma pressure rises was substantially reduced in Wendelstein 7-AS in relation to a conventional stellarator. Its successor, the completely optimised Wendelstein 7-X now being built at IPP’s Greifswald branch, is to demonstrate the power plant suitability of the new stellarators.
Wendelstein 7-AS was the first stellarator to be fitted with a divertor. Whereas previous devices limited the plasma column to the outside with material diaphragms, this was done in Wendelstein 7-AS since 1999 by magnetic means without making contact with the vessel wall: The plasma edge splits into individual cursors in accordance with the symmetry of the magnetic field. Accordingly energy and particles pass to limited areas on the vessel wall, these being protected by special collector plates. It was then possible here to neutralise and pump off the impinging particles along with undesirable impurities. This greatly facilitated impurity and density control, the plasma power then being safely distributed to the collector plates by means of radiation. This afforded advance information for Wendelstein 7-X, which will also be equipped with a divertor.
Wendelstein 7-AS was shut down on 31 July 2002 to make personnel and resources available for its successor at Greifswald. Long after shutdown of the device, the data and results obtained were still being evaluated for the world-wide stellarator programme and, in particular, for successor Wendelstein 7-X. A summary with extensive references to the original publications is presented in: Hirsch et al., Major results from the stellarator Wendelstein 7-AS (Review Article), in: Plasma Phys. Control. Fusion 50 (2008) 053001.