Mechanisms of XPR formation from SOLPS-ITER modeling

The early stages of X-point radiator (XPR) regime formation in a medium-size metal-wall tokamak with a single-null configuration and vertical divertor targets are discussed in detail, using the ASDEX Upgrade tokamak as an example. Based on the SOLPS-ITER modeling of ASDEX Upgrade with nitrogen (N) seeding, it is shown that the mechanisms associated with convective mixing of ionized particles play a key role in the initial plasma cooling in the X-point vicinity. Mechanism of XPR formation includes ion temperature drop and formation of density and potential hill above the X-point. E→×B→ drift vortex is formed around X-point, which amplifies cold plasma convection. The comparative contributions of the anomalous diffusion, the E→×B→ drift vortex, and the plasma expansion along the magnetic field to energy balance in the XPR vicinity are analyzed.