Detached regime with highly radiating X‐point: Physics and modelling

A possibility to operate with fully detached targets and highly radiating X-point was recently demonstrated on ASDEX Upgrade, JET, and DIII-D with intensive impurity seeding (N, Ne, Ar) and feedback control. Notable feature of such a regime is the radiated power fraction of up to 90% of discharge power inside the separatrix without a confinement degradation, or even with confinement improvement (in terms of H-factor), which leads to a full detachment of both targets. Therefore, this regime might be attractive for the next generation reactor scale tokamaks like DEMO or CFETR, where most of the power should be radiated. In the present report, the recent achievements in the understanding of such regime are reviewed. The effectiveness of divertor target shielding by different impurity gases is discussed, paying attention to its first ionization potential. SOLPS-ITER modelling results illustrating the impurity flow pattern are presented, and the importance of drift flows is demonstrated. The effect of the machine size on the effectiveness of the divertor shielding by impurity radiation is discussed. It is demonstrated that in bigger machines impurity radiation is better kept in the divertor and divertor asymmetry is less pronounced. As the seeding rate increases, both targets fully detach, and neutrals start to penetrate inside the separatrix above the X-point, leading to the plasma cooling there and to the formation of an observable localized radiation spot in the X-point vicinity. The cold zone above the X-point thus behaves as an energy sink like a divertor in conventional regime, so that the perpendicular decay length of perpendicular turbulent energy flow appears to be of the order of lambda q of conventional divertor. During the formation of the radiating X-point a potential hill/well (depending on the direction of Bx backward difference B drift) is formed on perturbed closed flux surfaces, and the corresponding ExB drift vortex fluxes appear to give major contribution to the particle balance. SOLPS-ITER simulations show that the radial electric field significantly deviates from neoclassical formula and may even change sign (become positive). Thus, the zone of maximal poloidal rotation shear may shift inwards, which might be responsible for the inward shift of the transport barrier and for the confinement improvement. This effect is different to the discussed extension of peeling-ballooning stability boundary by intensive impurity injection, which might explain experimentally observed ELMs suppression or their frequency reduction.