Power and Particle Exhaust Control in All W ASDEX Upgrade

Independent of the plasma facing materials in future fusion devices impurity seeding will become an inevitable element of the operation to protect the divertor from excessive heat loads. A very beneficial behaviour in terms of reduced power loads, moderate impurity concentrations and increased confinement has been found in N2 seeded discharges in ASDEX Upgrade. Radiative cooling has been applied in a large variety of plasmas ranging from improved H-Modes at intermediate density and heating power to discharges with very high heating power (Paux ≈ 20 MW) or high density and radiation fraction exploring the type-III ELM regime. Generally, the radiated power from the X-point and divertor region increased in N2 seeded discharges by more than a factor of two, but the core radiation is almost unchanged. Similarly, also the radiation during type-I ELMs is increased. In unseeded discharges with a pure tungsten wall, about 20% of the ELM energy is radiated, which is clearly less than in earlier discharges with mixed carbon and tungsten PFCs. For nitrogen seeded discharges, however, the ELM energy is generally smaller, and up to 40% of the ELM energy is radiated. This value is comparable to that found in former campaigns with mixed C/W PFCs. Consequently,the power load to the divertor targets during and in between type-I ELMs drops significantly with nitrogen seeding. Even for discharges at highest heating power, good energy confinement could be obtained simultaneously with a low impurity content of the core plasma and efficient power control in the divertor. In seeded type-III ELMy discharges a strong increase of the confinement with plasma pressure was observed. In parallel, a very moderate power flux in the outer divertor and a strong suppression of the W influx could be achieved. Investigations using a mixture N2/Ar as seeding gas revealed a change in central particle transport compared to discharges with N2 seeding only.