Defining an equilibrium state in global full-f gyrokinetic models

Abstract This paper tackles the delicate choice of the initial distribution function in full-f gyrokinetic codes such as G ysela 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 G ysela 5 d 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.