Response of Beryllium to Severe Thermal Shocks -simulation of Disruption and Vertical Displacement Events in Future Thermonuclear Devices

摘要

Beryllium will play an important role for plasma facing components in next step thermonuclear fusion devices such as ITER. In particular for the first wall beryllium will be used with an armor thickness of several millimeters. During normal plasma operation these components are subjected to quasi-stationary, cyclic heat loads; however, during plasma instabilities they also will experience severe thermal shocks. Here plasma disruptions with deposited energy densities of several ten MJm~-2 are the most essential damaging mechanism. However, a signifant fraction of the incident energy will be absorbed by a dense cloud of ablation vapor, hence reducing the effective energy density at the beryllium surface to values in the order of 10 MJm~-2. Even under these conditions which occur on a millisecond time scale severe melting of the plasma facing material to a depth of several hundreds of microns occurs. Splashing of liquid Be-droplets and the complete removal of the melt layer due to electromagnetic forces during the plasma disruption will result in an enhanced erosion. Beside material losses, the formation of cracks i nthe recrystallized zone and in the base material will have strong impact on the integrity of the beryllium tiles, in particular during the cool down phase. Plasma instabilities on a much longer time scale, i.e. with a duration of 0.3 to 1.0 s have to be anticipated. These so-called vertical displacement events (VDE) result in deposited energy densitites of approx. 60 MJm~-2. In contrast to the rather short disruption events beside surface effects also material damage to the joining zone has to be considered.