On the influence of initial state on gyrokinetic simulations

Abstract

The influence of the initial state on the turbulence and transport is addressed in collisionless, global, and full- f gyrokinetic simulations solving both the equilibrium and the fluctuations. For two strongly differing initial states, it is found that the steady turbulent regime exhibits nearly identical statistical properties. This result is in marked contrast with the claim of different final states. In fact, a long transient with very different properties finally evolves towards the same turbulent regime for long simulation times. When the initial state is a local Maxwellian, i.e., constant on flux surfaces, a large-scale sheared electric potential develops on short time scales to compensate for the vertical curvature and grad-B drifts. We predict analytically (i) the temporal dynamics at short times of this electric potential, (ii) its poloidal structure, and (iii) its saturation time. All agree well with numerical simulations using the GYSELA code. The impact on the transport is twofold, as compared to the canonical initial state, where f only depends on the motion invariants: (i) the turbulence is delayed due to a weaker effective growth rate, (ii) the same transport level is obtained at long times and the turbulence exhibits nearly identical statistical characteristics. In agreement, the electric potential of these two cases has the same magnitude despite very different transients. © 2008 American Institute of Physics.