Plasma boundary

Whereas previous devices restricted the outward extent of the plasma column with material limiters, in Wendelstein 7-X this is done contact-free by magnetic means with the aid of a divertor.

Different to a tokamak no additional magnet system is needed to divert the plasma boundary: In a stellarator the plasma boundary splits – in keeping with the symmetry of the magnetic field – into individual offshoots through which energy and particles move to limited areas of the vessel wall, just like the divertor plasma in a tokamak.

These areas of the wall are protected by special collector plates – ten such along the plasma column. Here the incident particles together with the undesirable impurities from the plasma can be neutralised and pumped off. This greatly facilitates impurity and density control; the plasma power can be sparingly distributed on the collector plates by radiation.

Divertor plates along the plasma column of Wendelstein 7-X. The water-cooled wall elements can remove heat loads of 10 megawatts per square metre. The plasma-facing surface is protected with carbon-fibre-reinforced carbon.

The Wendelstein 7-AS experiment (1988 - 2002) at IPP was the first stellarator to be equipped with a divertor, divertor experiments starting in 1999.

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