Neoclassical impurity transport in the core of an ignited tokamak plasma

Impurity behaviour in the core of an ignited fusion reactor (ITER FDR) is investigated under the assumption that the impurity fluxes are dominated by neoclassical transport. Equilibrium impurity profiles are evaluated for two temperature profiles and compared with the results for purely anomalous diffusion with a core diffusion coefficient of Dan = 0.5 m2/s. The neoclassical fluxes are calculated taking collisions of all relevant plasma species into account. The diffusion coefficient in the core is below 0.1 m2/s and decreases with increasing Z of the impurity. The mean drift velocities are always outwardly directed and cause an effective screening for high-Z elements. Due to the low diffusion coefficient, the He concentration profile in the core is strongly rising. Helium concentrations on-axis are cHe » 15-17%. In contrast to the calculation with purely anomalous transport and cHe » 9% on-axis, the He density is not only determined by the recycling edge source but by the interplay of low core transport, increasing dilution and decreasing fusion source. The increased dilution causes a reduction of total fusion power by » 8-11%. With an additional anomalous core diffusion for He of DanHe = 0.2 m2/s the central He concentration decreases to cHe » 10%.