One-dimensional model of detached plasmas in the scrape-off layer of a divertor tokamak

Abstract

Numerical and analytical study of a detached divertor equilibrium is presented. The model uses one-dimensional equations for continuity, momentum and energy balance with radiation, ionization, charge-exchange, and recombination processes. A reasonably simple neutral model is also employed. Analytical calculation, using a simple five-region model for a case with negligible convective heat flux and constant sources/sinks, captures the essence of detailed numerical calculation for the same case. More general cases are handled numerically. The detachment is studied as a function of the ratio Q_⊥/S_⊥ [the ratio of power and particle volume source, coming from the core to the scrape-off layer (SOL) region]. For low values of Q_⊥/S_⊥ (detached state), at the midplane and at the target, the ion temperature (T_i) is almost equal to the electron temperature (T_e). As this ratio increases (attached state), T_i is larger than T_e at the midplane. However at the target, T_e is found to be slightly larger than T_i. It is also observed that as Q_⊥/S_⊥ increases, the region of most intense radiation shifts progressively from closer to the X-point towards the target plate.