The tungsten-coated tiles of the ASDEX Upgrade outer divertor have been replaced with tiles made of solid tungsten. This change brings various benefits including the possibility of new types of material studies with regard to ITER. Operation will start in January 2014.

Tungsten is the material intended for the first reactor wall in ITER. With the exception of JET, which features an ITER-Like Wall made from tungsten and beryllium, ASDEX Upgrade is currently the only other divertor machine in Europe which is equipped with tiles of solid tungsten. And will remain so until the French WEST experiment starts operation in 2016. Besides investigating the behaviour of solid tungsten when compared to tungsten-coated tiles, the change opens up options for long-term studies. Material erodes under heat and particle fluxes from the plasma. This effect is at its worst at the divertor, where the plasma touches the vessel wall. The ten micrometre tungsten-coating of the previously used graphite tiles would vanish in part after a longer period of operation, so tiles needed to be reworked regularly. The 15 millimetre thick tungsten tiles, on the other hand, may be exposed to the plasma over longer operational periods. During plasma pulses in ASDEX Upgrade, which may last up to ten seconds long , the tiles will experience a temperature excursion from room temperature up to melting temperature (around 3400 °C). This simulates the conditions during ITER overloads which will occur, for instance, during plasma edge instabilities which thrust very high heat and particle loads at the vessel wall. After some time of operation, the surface of the tiles will show signs of erosion and cracks. ASDEX Upgrade will help us to gain experience about operation with tiles degraded in this way.

Engineering challenges
In 2007, ASDEX Upgrade was the first tokamak to have an inner vessel wall coated with tungsten. Now it takes a big step and implements solid tungsten on a large scale. The tungsten tiles are a lot heavier than the previous tungsten-coated graphite tiles. And, for this reason, the divertor and the cooling system had to be reconstructed. Manufacturing the tungsten tiles is also not a standard industrial process, so IPP used its test stand GLADIS to first test the prototype tiles and then test the samples from the serial production under conditions which simulate two to three years of ASDEX Upgrade operation.

Changing tiles between experimental days
Within these reconstructions, a divertor manipulator was designed and installed, which allows the replacement of two tiles without breaking the vacuum. This enables dedicated experiments with special targets. While many material and divertor concept tests can be conducted in test stands such as GLADIS or in linear plasma machines, some conditions can only be realised in tokamaks. These include, for instance, special magnetic field configurations, grazing plasma incidence, or the effect that gyrating ions have on the tiles. The new divertor manipulator has a water inlet and thus also enables the testing of actively cooled targets, as planned for ITER. The IPP task force for plasma edge processes is currently discussing with ITER what possibilities ASDEX Upgrade offers to address specific ITER questions, for example, testing of various tile geometries or tessellations.