Influence of Cross-Field Transport in a Divertor on Seeded Impurity Radiation and Divertor Plasma Detachment

Abstract

The dissipative divertor for the power and particle exhaust in a tokamak relies heavily on strong impurity radiation inside the divertor volume. In the present paper, the influence of the cross-field transport inside the divertor volume on the radiation loss by nitrogen and neon impurity seeding is studied in the SOLPS4.3 2D transport code. The study is carried out in a simplified tokamak geometry with long divertor legs, aimed at amplifying the effect of impurity radiation. It is shown that despite the simple analytical models predicting the enhancement of the low-Z impurity radiation loss with increasing cross-field heat conduction inside the divertor volume, the detailed 2D simulations demonstrate such a trend only if the electron density at the separatrix is used as the principal parameter quantifying the density of the edge plasma. However, if the total amount of the hydrogen isotope particles, which is a more suitable parameter for the edge plasma characterization, is considered, then the cross-field transport shows practically no impact on the impurity radiation in the divertor. It is found also that for a fixed ratio of the impurity to the main plasma content in the edge, two stable solutions can exist in a rather wide interval of the densities of the edge plasma. These solutions are attributed to the different distributions of the seeded impurity between the outer and inner divertors. Jumps between these two stable branches may result in bifurcation-like transitions between the attached and detached divertor regimes.