Author: Jae Heon Ahn
Requested Type: Pre-Selected Invited
Submitted: 2016-02-08 04:14:24
Co-authors: X. Garbet, H. Lütjens, R. Guirlet
CEA - IRFM
CEA/DRF/IRFM, CEA Cadarache, B
Saint Paul-lez-Durance, 13108
Understanding the transport of heavy impurities is an active subject of research. The aim is to find means to prevent impurity accumulation in the plasma core. The neoclassical theory predicts that the ion temperature gradient drives an outward convection velocity of the impurity, and is thus responsible for a favorable screening effect [1,2]. In the core region, MHD activities such as sawtooth oscillations add complexity to the impurity behavior [3-5]. A model considering the impurity transport due to MHD, collision and turbulence is then necessary.
The appropriate set of fluid equations for heavy impurities has been derived to model collisional transport, in the relevant case of Pfirsch-Schlüter regime and main ion in the banana-plateau regime. This model has been implemented in the non-linear full 3D MHD code XTOR  in order to study combined effects of MHD, neoclassical and turbulent transport.
First, the expected fluxes of impurities, including the thermal screening effect, are reproduced. In the core region of a sawtoothing plasma, the impurity behavior differs from the case without sawteeth, mainly due to the flattening of the temperature profile at each collapse. Second, 2D structures of impurity density are observed during sawtooth crashes due to convective cells that evolve in time. Preliminary results show that the accumulation is prevented by off-axis localized ECRH/ECCD deposition, which modifies these convective cells. This result is reminiscent of the scenario preventing tungsten accumulation in AUG and JET.
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