Parameter dependencies of the experimental nitrogen concentration required for detachment on ASDEX Upgrade and JET

While current tokamak experiments are beginning to use real-time feedback control systems to manage the plasma exhaust, future tokamaks still require validation of theoretical models used to predict the threshold impurity concentration required to sufficiently reduce the power and particle fluxes to the divertor. This work exploits new spectroscopic measurements of the divertor nitrogen concentration, c_N, in partially detached N₂-seeded H-mode plasmas on ASDEX Upgrade (AUG) and JET with the ITER-Like Wall (JET-ILW) to test the parameter dependencies of the power flowing to the outer divertor, P_div,outer, and the separatrix electron density, n_e,sep. A least-squares regression of the AUG measurements demonstrates that the threshold c_N required for detachment scales as c_N∝P_div,outer^1.19±0.32n_e,sep^-2.77±0.36. This scaling of n_e,sep is also consistent with the measurements from JET which, at constant P_div,outer, show c_N∝n_e,sep^-2.43±0.27. The dependencies of P_div,outer and n_e,sep is demonstrated over at least a factor of two change in both parameters and indicates a stronger dependence on n_e,sep in comparison to the Lengyel model, which could be due to the assumption in this model that the heat flux channel width is independent of density. This first assessment of detachment scaling with impurity seeding highlights the need for further analysis of the systematic uncertainties of the measurement and more consistent scenarios from more tokamaks to investigate the machine size scaling.