Comparing the Scaling of Bulk Radiated Power between Carbon and ITER-Like-Wall Environments in JET
We define a parameter br for all plasmas that can be used to detect the pollution of the plasma bulk by highly radiative impurities. This parameter is defined as the radiative loss of the mixture of impurities relative to their mean Z2. We show that this parameter, though global, is very sensitive to the presence of highly radiative impurities in the bulk of the discharge. We use it to compare JET plasmas in the carbon environment and in the ITER-like wall (ILW), where it is highly correlated to the level of a bundle of spectroscopic lines of tungsten passing through the centre of the discharge. In the carbon environment, the value of br is around 1, indicating the absence of highly radiative impurities in the plasma. No change or even a small decrease is observed when going from L-mode to H-mode, this robustness being in agreement with the multi-machine scaling. In the ILW machine, the value of br is found to depend on the type of additional heating and confinement state of the plasma. We observe that neutral-beam injection (NBI) introduces little W into the plasma, with a br between 2 and 3. Ion-cyclotron radio-frequency (ICRF) waves yield a br of order 5 in L-mode and 10 in H-mode when no edge-localized modes (ELMs) are present. Conversely when ELMs are present, the parameter goes back to 5, illustrating the positive effect they can have on the bulk pollution by tungsten.