Direction of Impurity Pinch and Auxiliary Heating in Tokamak Plasmas

A mechanism of particle pinch for trace impurities in tokamak plasmas, arising from the effect of parallel velocity fluctuations in the presence of a turbulent electrostatic potential, is identified analytically by means of a reduced fluid model and verified numerically with a gyrokinetic code for the first time. The direction of such a pinch reverses as a function of the direction of rotation of the turbulence in agreement with the impurity pinch reversal observed in some experiments when moving from dominant auxiliary ion heating to dominant auxiliary electron heating.

Figure 1

Diffusion coefficient $D_p$ and pinch $R{V}_p$ for a trace impurity as a function of the charge number, in the case of $R/L_{Ti}$ drive (squares), $R/L_{Te}$ drive (circles) and $R/L_n$ drive (diamonds). In the inset, example of linear relationship between the quasilinear particle flux and the density gradient of a trace impurity, as computed by the code GS2, used to determine $D_p$ and $V_p$ for the case $Z_p=8$, $R/L_{Ti}=9$.

Figure 2

Thermodiffusive part of the particle pinch as a function of the charge of a trace impurity (symbol codes like in Fig. 1).

Figure 3

Particle pinch of a trace impurity as a function of the charge number with fixed impurity mass (symbol codes like in Fig. 1).

Figure 4

Pinch as a function of the impurity mass in the case of fixed impurity charge (symbol codes like in Fig. 1).