Bright spots on the divertor plates of ASDEX-Upgrade during stationary phases

The appearance of bright spots at several parts in the tokamak vessel during disruptions is a well known phenomenon. In ASDEX-Upgrade an enormous number of bright spots is observed at the lower target plate and at the diaphragm ring of the divertor from a density limit disruption. The investigation of carbon tiles after an operation period of the tokamak shows, that arcs are the reason for the bright spots. In ASDEX-Upgrade, however, bright spots at the divertor plates are observed also during the stationary phases of the tokamak operation. These spots seem to arise from uneven power deposition on the target plates. The energy of the bulk plasma enters the scrape-off layer crossing the separatrix so far it is not radiated from the bulk and the boundary of the plasma. Along the SOL the energy is transferred to the divertor plates. During the stationary state ASDEX-Upgrade is heated with a power up to 10 MW. Power loads of about 10 MW/m2 at the target plates are measured by thermography supposing uniform deposition. However due the tile geometry and other irregularities on the surface of the tiles a local power overload will occur unavoidably. Then the sublimation temperature on the carbon tiles may be exceeded. The increased impurity production can produce carbon blooms and following plasma disruptions. Also damage of divertor tiles is possible. (author) 3 refs., 6 figs