Impact of the neoclassical distribution function on turbulent impurity and momentum fluxes: fluid model and gyrokinetic simulations

Abstract

The impact of the neoclassical background on turbulent impurity transport is investigated by means of gyrokinetic simulations supported by fluid equations. The latter are derived, using a Laguerre polynomials expansion of the first order neoclassical distribution function, and analytical expressions of the turbulent momentum flux and impurity transport coefficients are assessed. Comparisons of gyrokinetic simulations including this neoclassical background (coupling between the codes GKW and NEO) and the fluid model are used to identify the main mechanisms behind the modification of the turbulent transport channels and benchmark the numerical implementation. These mechanisms include a modification of the parallel dynamics of the main ions and direct contributions stemming from the asymmetry in the parallel velocity space of the neoclassical distribution function. The latter which is found dominant for turbulent impurity transport, increases with increasing collisionality, R/LT i , R/Ln, impurity mass, safety factor and aspect ratio. These contributions to momentum and impurity fluxes are also found to depend on the directions of the toroidal magnetic field and plasma current.

Light impurity transport in JET ILW L-mode plasmas

“…Also, the effects of the background neoclassical distribution function on turbulent particle transport have not been considered. These effects can affect in particular the roto-diffusion term [40], especially in presence of high plasma rotation. Nevertheless, in our case, due to the low plasma rotation in our discharges, they are not expected to change significantly the picture described above.…”

Diffusive impurity transport driven by trapped particle turbulence in tokamak plasmas

“…The dependence of impurity transport on the impurity species have been studied as well [2,[4][5][6][7][8][9][10][11][12][13], but without decoupling the impacts of Z and A, or focusing on the peaking factor. In a previous paper [14], we have shown how a large concentration of tungsten can change the nature of the turbulence in the case of active species.…”

Impurity density gradient influence on trapped particle modes

“…Fluid models 2,9 or gyrokinetic codes such as GYRO, GENE, and GKW containing realistic tokamak geometry 10 , auxiliary heating 11 , collisionality 12 , rotational shear 13 , toroidal rotation 14 , poloidal asymmetries 15 , have been successfully employed in the studies. Also the impact of the neoclassical distribution function on turbulent impurity and momentum fluxes has been recently investigated using fluid and gyrokinetic simulations 16 .…”