  SOLPS-ITER simulation is performed to reproduce the X-point radiator recently
observed in nitrogen-seeded TCV experiments, which is a scenario that may be
favorable to solve the power exhaust problems in future fusion devices. The
simulations reveal the transition from the detached regime without XPR to the
XPR regime, when increasing the nitrogen seeding rate. A cold X-point core
surrounded by ionizing and radiative mentals is formed inside the separatrix
and slightly above the X-point, where more than 90% of the total input power is
dissipated. The cold X-point core exhibits a temperature of approximately 1eV
and features high recombination rate to host the convective fluxes from the
ionizing mental. Increasing nitrogen seeding rate also moves the nitrogen
ionization front away from the target faster than the nitrogen stagnation
point, which enhances the divertor nitrogen leakage to the main chamber and
benefits the XPR region cooling. Carbon radiation decreases as the nitrogen
seeding increases, and carbon radiation contributes to above 5% of the core
impurity radiation before entering the XPR, which decreases to 2.8% when
reaching the XPR. Both baffled and unbaffled divertor geometries are simulated
and compared, showing that baffles facilitate the access to XPR by increasing
the X-point neutral density, but requires higher seeding rate to enter the XPR
regime.
