Experimental study on the role of the target electron temperature as a key parameter linking recycling to plasma performance in JET-ILW

Changes in global and edge plasma parameters (H_98(y,2), dimensionless collisionality ν^*, core density peaking, separatrix density n_e,sep) with variations in the D₂ fueling rate and divertor configuration are unified into a single trend when mapped to ⟨T_e,ot⟩, the spatially averaged spectroscopically derived outer target electron temperature. Dedicated JET with the ITER-like wall (JET-ILW) experiments in combination with an extended JET-ILW database of unseeded low-triangularity H-mode plasmas spanning a wide range of D₂ fueling rates, I_p, B_t and heating power have demonstrated the importance of ⟨T_e,ot⟩ as a key physics parameter linking the recycling particle source and detachment with plasma performance. The remarkably robust H_98(y,2) trend with ⟨T_e,ot⟩ is connected to a strong inverse correlation between ⟨T_e,ot⟩, n_e,sep and ν^*, thus directly linking changes in the divertor recycling moderated by ⟨T_e,ot⟩ with the previously established relationship between ν^*, core density peaking and core pressure resulting in a degradation in core plasma performance with decreasing ⟨T_e,ot⟩ (increasing ν^*). A strong inverse correlation between the separatrix to pedestal density ratio, n_e,sep/n_e,ped, and ⟨T_e,ot⟩ is also established, with the rise in n_e,sep/n_e,ped saturating at ⟨T_e,ot⟩ > 10 eV. A strong reduction in H_98(y,2) is observed as ⟨T_e,ot⟩ is driven from 30 to 10 eV via additional D₂ gas fueling, while the divertor remains attached. Consequently, the pronounced performance degradation in attached divertor conditions has implications for impurity seeding radiative divertor scenarios, in which H_98(y,2) is already low (∼0.7) before impurities are injected into the plasma since moderate gas fueling rates are required to promote high divertor neutral pressure. A favorable pedestal pressure, p_e,ped, dependence on I_p has also been observed, with an overall increase in p_e,ped at I_p = 3.4 MA as ⟨T_e,ot⟩ is driven down from attached to high-recycling divertor conditions. In contrast, p_e,ped is reduced with decreasing ⟨T_e,ot⟩ in the lower I_p branches. Further work is needed to (i) clarify the potential role of edge opacity on the observed favorable pedestal pressure I_p scaling; as well as to (ii) project the global and edge plasma performance trends with ⟨T_e,ot⟩ to reactor-scale devices to improve predictive capability of the coupling between recycling and confined plasma fueling in what are foreseen to be more opaque edge plasma conditions.