Defining an equilibrium state in global full-f gyrokinetic models

This paper tackles the delicate choice of the initial distribution function in full-f gyrokinetic codes such as Gysela 5d, aiming at predicting the turbulent transport level in low collisional tokamak plasmas. It is found, both analytically and numerically, that a Maxwellian distribution function with constant profiles on magnetic flux surfaces leads to the fast generation of a large scale electric field. Such a field opposes the up-down charge separation governed by the inhomogeneity of the equilibrium magnetic field. If large enough, the shearing rate induced by the resulting poloidal E×B velocity could efficiently reduce the plasma micro-instabilities which account for the development of the turbulence. Starting in the ab initio code Gysela 5d from an equilibrium distribution function depending on motion invariants only is shown to cure such a problem. In this case, charge separation is counter-balanced by parallel flow, and the standard fluid force balance is recovered.