Gyrokinetic modelling of light to heavy impurity transport in tokamaks

Abstract

Impurity transport is numerically investigated for different types of impurity, such as helium (He), argon (Ar), and tungsten (W). Both turbulent and neoclassical transports are treated self-consistently using the full-f gyrokinetic software GYSELA. For a light impurity (He), the transport is mainly controlled by turbulence, while neoclassical transport is found to be dominant in the case of a heavy impurity (W). The impact of a poloidal asymmetry of the impurity density is also studied in detail and it is found to be strong in case of a high charge impurity, due to its Boltzmann-type response. Such strong asymmetry might lead to a core accumulation of heavy impurities by reducing the thermal screening factor of neoclassical transport. The two main contributions to neoclassical transport—Pfirsch–Schlüter (PS) flux and banana–plateau (BP) flux—are also studied. Depending on their mass (A) and charge (Z), the magnitudes of each flux are determined accordingly. Tungsten shows a strong PS flux compared to the other impurities, while BP flux is dominant in the case of argon. An analytical model including the effect of poloidal asymmetry is compared with the numerical simulation and a good agreement is found between them.