Turbulent generation of poloidal asymmetries of the electric potential in a tokamak


Poloidal asymmetries of the E × B plasma flow are known to play a role in neoclassical transport. One obvious reason is that an asymmetrical potential can produce a flux across the magnetic field. Also the associated distribution function may correlate with the magnetic drift velocity to enhance the neoclassical flux. Finally, poloidal variations of the electric potential can produce poloidal asymmetries of an impurity density, which in turn may modify the neoclassical transport coefficients. According to conventional neoclassical theory, the level of poloidal asymmetry of the electric potential is expected to be very small. Poloidal flow asymmetries can be driven by small scale turbulence via nonlinear coupling, and therefore change this result. In the present work, a general framework for the generation of axisymmetric structures of potential by turbulence is presented. Zonal flows, geodesic acoustic modes and convective cells are described by a single model. This is done by solving the gyrokinetic equation coupled to the quasi-neutrality equation. This calculation provides a predictive calculation of the frequency spectrum of flows given a specified forcing due to turbulence. It also shows that the dominant mechanism comes from zonal flow compression at intermediate frequencies, while ballooning of the turbulence Reynolds stress appears to be the main drive at low frequency.

Plasma Phys. Control. Fusion