Erosion and migration of tungsten employed at the main chamber first wall of ASDEX Upgrade

In ASDEX Upgrade, tungsten is employed as plasma facing material at the central column of the plasma main chamber. The campaign averaged tungsten erosion flux was determined by measuring the difference of the W-layer thickness before and after experimental campaigns using ion beam analysis methods. The lateral variations of the observed tungsten erosion flux reflects the pattern of magnetic field lines intersecting the respective plasma facing surfaces. Long term migration and redeposition of eroded tungsten were investigated by ion beam analysis of deposited tungsten on wall samples from main chamber and divertor. The obtained results, as well as the spectroscopically observed low tungsten plasma penetration probability, indicate that a major fraction of the eroded tungsten migrates predominantly through direct transport channels in the outer plasma scrape-off layer without entering the confined plasma.