Final steps to an all tungsten divertor tokamak

Currently 85% of the plasma facing components of ASDEX Upgrade are tungsten coated. Carbon influx from W PFCs is still observed but a reduction of the C content is found in plasma discharges and a lower C fraction is measured in deposited layers in agreement with modelling. W sputtering from the low field side guard and ICRF limiters is mainly due to fast particles from NBI as well as from ions accelerated in the rectified sheath during ICRF operation. The increase of the W source area is reflected in increased W concentrations. For medium to high density discharges the techniques developed so far, namely central heating and ELM pace-making, allow keeping the W concentration in the range of 10−5. Boronisation strongly reduces the W influxes and similarly the W content especially during ICRF operation, but this reduction is only temporary and equilibrium is reached already after about 100 discharges.