Taming Tungsten in JET and ASDEX Upgrade
A short review of prior results on W-erosion, W-screening of the divertor and W-transport in the pedestal and the plasma core is given. The W-transport in the core plasma of JET is investigated experimentally by deriving the W-concentration profiles from modelling of the signals of the soft X-ray cameras. For the case of pure neutral beam heating W-accumulates in the core to W-concentrations of 10­3 in between the sawtooth crashes, which flatten the W-profile to a concentration of about 3¥10­5. When central ICRH heating is additionally applied the core Wconcentration decays in phases that exhibit a changed mode activity, while also the electron temperature increases and the density profile becomes less peaked. The immediate correlation between the change of MHD and the removal of W from the plasma core supports the theory that the change of the MHD activity is the underlying cause for the change of transport. Furthermore, two discharge scenarios from ASDEX Upgrade are investigated. In the first case the central W-radiation influences the core W-transport such that slowly increasing W-radiation leads to a steepening of the W-concentration profiles. In the second case two discharges are compared in which a small change of the heating trajectory leads to a change of the W-transport and a change of the sawtooth behaviour. It is speculated that the W-accumulation is the underlying reason for the irregular sawteeth, which in turn aids the W-accumulation.