Approaching the radiating X-point in SOLPS-ITER modeling of ASDEX Upgrade H-mode discharges

Abstract

In the present paper the ASDEX Upgrade (AUG) experimental trend of reaching the radiating X-point with nitrogen seeding is reproduced by SOLPS-ITER code modeling. It is demonstrated that for big prescribed seeding rate SOLPS-ITER simulations result in that the confined plasma goes into the radiation collapse as a certain threshold in seeding rate is exceeded. This threshold value increases with increasing discharge power. No stable regimes with the T_e below 5 eV in the confinement zone are achieved in the modeling if the seeding rate is large enough, in contrast to experiment. However, such a regime may be achieved if the fueling, seeding and pumping rates are changing in time. Since the SOLPS-ITER code can simulate only steady state, another modeling strategy is chosen. The fueling and seeding rates are artificially reduced by 3 orders of magnitude and the impurity content are set to so that the number of electrons originating from fuel atoms is about number of ones coming from impurity. It is suggested that the radial width of the cooled region in the confinement zone is of the order of the SOL width $\lambda_q$, since it is driven by the same physics leading the energy flux to go from mostly radial to mostly parallel. In extreme regimes, the width of the cold region inside the separatrix may exceed $\lambda_q$, and up to 90\% of discharge power can be radiated from the confined region. An estimate of the poloidal length of the radiating spot is suggested as well. The formation of an electric potential peak above the X-point is observed in the simulations, so that $E\times B$ drift flux gives the largest contribution to the main ion and impurity fluxes. This drift flux together with the large ionization source change the parallel velocity with respect to its neoclassical profile.