A possible mechanism responsible for generating impurity outward flow under radio frequency heating

The effect of poloidal asymmetry of impurities on impurity transport driven by electrostatic turbulence in tokamak plasmas is analyzed. It is found that in the presence of in–out asymmetric impurity populations the zero-flux impurity density gradient (the so-called peaking factor) is significantly reduced. A sign change in the impurity flux may occur if the asymmetry is sufficiently large. This may be a contributing reason for the observed outward convection of impurities in the presence of radio frequency heating. This paper extends a previous work (Fülöp and Moradi 2011 Phys. Plasmas 18 030703), by including the effect of ion parallel compressibility on the peaking factor, which is found to have a significant contribution in the presence of poloidal asymmetry. It is shown here that in the ion temperature gradient mode dominated plasmas the presence of an in–out poloidal asymmetry can lead to a negative impurity peaking factor, and it becomes more negative in regions with larger ion temperature gradients. In the trapped electron mode dominated plasmas an in–out poloidal asymmetry results in a strong reduction of the peaking factor; however, it remains positive for typical experimental parameters. Furthermore, it is shown that an up–down asymmetry reduces the peaking factor while an out–in asymmetry increases it