Comparing the bulk radiated power efficiency in carbon and ITER-like-wall environments in JET

We use a parameter β_r for all plasmas that allows detecting 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 and was used in previous works to characterize the efficiency of radiative mantles in Neon seeded discharges (Telesca G et al 2000 Nucl. Fusion 40 1845, Carraro L et al 2000 Nucl. Fusion 40 1983). 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 center of the discharge. In the carbon environment, the value of β_r is around 10^-40MWm⁶, 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 (Matthews G F et al 1997 J. Nucl. Mater. 241-243 450). In the ILW machine, the value of β_r is found to depend on the type of additional heating and confinement state of the plasma. We observe that neutral-beam injection introduces little W into the plasma, with a β_r between 2×10^-40 and 3×10^-40MWm⁶. Ion-cyclotron radio-frequency waves yield a β_r of order 5 in L-mode and 10×10^-40MWm⁶ in H-mode when no edge-localized modes (ELMs) are present. Conversely when ELMs are present, the parameter goes back to 5×10^-40MWm⁶, illustrating the positive effect they can have on the bulk pollution by tungsten.