Numerical study of the role of higher order flows in neoclassical transport of high-Z impurities

Obrejan, K.; Idomura, Yasuhiro   ; Honda, Mitsuru*

The use of tungsten to coat divertors or other plasma facing components, inevitably leads to the pollution of the fusion plasma by heavy and highly charged (high-Z) impurities. This kind of neoclassical and turbulent transport of high-Z impurities is not yet fully understood. To study high-Z impurity transport, the gyrokinetic full-f Eulerian simulation code GT5D was upgraded with an implicit collision solver based on a multi-species linear Fokker-Plank collision operator. The commonly employed low order approximation of the standard neoclassical theory showed good agreement for low-Z impurities, but large differences were observed for high-Z impurities in the presence of temperature gradients. We tested an improved form of the neoclassical transport theory, that takes into account higher order flows to more accurately recover the friction coefficients in a high collisionality regime, showing good agreement with GT5D. An important consequence of this improved method is the weakening of thermal screening effect of bulk ions on impurity transport.