Enhanced modification of tungsten surface by nanostructure formationduring high flux deuterium plasma exposure

The morphology changes induced by low energy deuterium ions on tungsten surfaces is studied for particle fluxes similar to those expected in the ITER divertor, as high as 1024 m2 s1. A new type of surface modifications by formation of nanostructures is observed to occur under those conditions. The nanostructures formation is critically dependent on the particle flux and ion energy, and affected by surface temperature and particle fluence. It correlates with the formation of nano-cavities at a depth of about 40 nm below the surface. In addition, the positron annihilation Doppler broadening (PADB) measurements reveal a significant increase of defect concentration (mostly vacancy-type) after plasma exposure, and the evolution of defects is in line with the surface morphology changes with fluence and surface temperature. The possible mechanism of cyclic trap mutation and dislocation loop punching due to high flux D exposure is discussed.