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 (Ti) is almost equal to the electron temperature (Te). As this ratio increases (attached state), Ti is larger than Te at the midplane. However at the target, Te is found to be slightly larger than Ti. 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.