Impurity transport and its modification by MHD in tokamak plasmas, with application to Tungsten in the WEST tokamak

The neoclassical transport of impurities is investigated analytically and numerically in this work, and the effect of a magnetic island for a WEST plasma is described. It is shown that the poloidal asymmetry of the impurity density can be described with a simple analytical model, and numerical simulations with a non linear fluid code confirm the analytical findings. The poloidal asymmetry tends naturally to cancel as the impurity profile evolves towards its steady state. When a magnetic island is present, temperature screening is lost locally above a critical island size. Tungsten transport is however dominated by turbulence on WEST. Simulations with a turbulent transport model confirm that the Tungsten profile is flat, as observed, but that the poloidal asymmetry remains close to its neoclassical value. The transport of impurities in tokamak plasmas is a key issue in fusion research, since both plasma facing components and fusion reactions introduce undesirable ions in the fusion mixture that radiate plasma energy and dilute the fusion reactants. This transport can be separated into a collisional (neoclassical) and a turbulent component. The neoclassical impurity transport is extremely sensitive to the poloidal anisotropy of the impurity density [1]. We show that this poloidal asymmetry can be determined analytically from equilibrium profiles and that impurity fluxes are strongly modified even in the absence of toroidal rotation. The asymmetry is parametrized by a horizontal (d) and vertical (?) components: n_a/hn_ai = 1 + d cos ? + ? sin ? with n_a the impurity density and ? the poloidal angle.