Spectroscopic measurements of tungsten erosion in the ASDEX Upgrade divertor

Tungsten erosion in the ASDEX Upgrade divertor has been measured by observing the WI emission at 400.9 nm. The sputtering yield is in the region of atoms/ion for typical divertor plasma conditions. By comparison of the measured sputtering yield with modelled sputtering yields one can conclude that tungsten erosion is dominated by plasma impurities. Adding impurities without cooling the boundary plasma increases the sputtering yield due to the additional impact energy gained by the particle in the sheath potential caused by its higher net charge. Cooling the boundary plasma by adding more impurities or increasing the plasma density leads to a strong decrease in the tungsten erosion. By comparison of the tungsten influx with the measured net erosion the prompt redeposition was determined. The redeposited fraction of tungsten increases significantly with a higher divertor electron density as expected.