Scaling of Prad with Density and Zeff in the Bulk of JET

We revisit the multi-machine scaling developed by G. Matthews in the new ITER-Like Wall configuration (ILW) of JET. The original scaling law establishes a relationship between total radiated power, average line density and Zeff for multi-machine comparisons. In this work, this scaling is used to elaborate the differences of the radiation behavior in the ILW and to monitor the different states of the plasma in series of discharges or even during one discharge when a large change occurs. We have restricted the analysis to the bulk of the plasma which is a region relatively simpler than the divertor where recycling, erosion and ionization take place. In the ILW, the two main impurities in the discharge are tungsten, eroded from the divertor plates and beryllium, eroded from the main chamber. Different to the JET with Carbon Plasma-Facing Components (C PFCs), the radiation is now mostly found in the bulk of the discharge and is associated to the presence of tungsten in this region. The radiation in the divertor is small and becomes less than the bulk one once some moderate level of additional heating is applied (~1MW). We find that the parametric form: Pradbulk/(Zeff-b)= ± ne2 (1) is always verified in the bulk of the plasma regardless of the level of the radiation, type and level of additional heating, confinement properties of the plasma, Limiter or divertor operation. The parameter b is found to be close to 1. It subtracts from the Zeff the contribution of deuterium. The parameter a characterizes the radiated power per effective charge. This parameter is very sensitive to the distribution of impurities present in the bulk region and describes the link between Zeff and Prad trough line radiation but certainly also with the small contribution of the Bremsstrahlung radiation. The scaling is valid for JET with C PFCs and ILW, but due to the different radiation characteristics of the main impurity in the two environments, the parameter a is two times larger in ILW than in C PFCs. In ILW, ICRH and NBI heating create different levels of impurities in the bulk and this can be immediately detected on the value of a. When impurities such as nitrogen are injected in the divertor for cooling purpose, the contamination of the bulk by nitrogen creates during the same discharge an additional scaling with a different value of a. We find that for a fixed additional heating configuration and during divertor operation, a is an invariant of the discharge. In particular a does not change when the discharge goes from L to H mode. In the case of ICRH heating, Prad is found to scale linearly with the total applied power and Zeff can be written as a function of the total applied power and the density. We find that the ICRH case is peculiar because no similar scaling could be derived for the other types of additional heating. The scatter obtained with expression (1) will also be compared with the original multi-machine scaling which was dominated by data from carbon walled machines.
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