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 theimpurity fluxes are dominated by neoclassical transport.Equilibrium impurity profiles are evaluated for twotemperature profiles and compared with the resultsfor purely anomalous diffusion with a core diffusioncoefficient of Dan = 0.5 m2/s.The neoclassical fluxes are calculated taking collisions of all relevantplasma species into account.The diffusion coefficient in the core is below 0.1 m2/sand decreases with increasing Z of the impurity. The mean driftvelocities are always outwardly directed and cause an effective screeningfor high-Z elements.Due to the low diffusion coefficient, the He concentration profile in the core isstrongly rising. Helium concentrations on-axis are cHe ≈ 15-17%. In contrast to the calculation with purely anomalous transport andcHe ≈ 9% on-axis,the He density is not only determined by the recycling edge source butby 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%.