Heat and Particle Transport in a Tokamak : Advances in Nonlinear Gyrokinetic Simulations

We present gyrokinetic simulations of plasma turbulence driven by ion temperature gradient (ITG) modes and trapped electron modes (TEMs). These studies have become possible through an upgrade of the nonlinear gyrokinetic Vlasov code GENE [1] which now includes magnetic particle trapping. The new version of GENE has been benchmarked successfully against semi analytic theory and other gyrokinetic codes.[2, 3] Several issues are addressed which are interesting both from a theoretical and from an experimental point of view. Finite ITG turbulence. The dependence of ITG turbulence on the plasma beta is studied. Here, beta is increased from zero all the way to the ballooning mode regime. Our results confirm earlier fluid simulations [4] qualitatively but not quantitatively. Magnetic flutter transport is usually subdominant. This finding can be reconciled with test particle calculations if cross phase relations are taken into account. Electron heat transport by TEM turbulence. The electron heat transport induced by TEM turbulence exhibits three key features which cannot be explained in terms of conventional (quasi- ) linear theory : (a) a strong dependence on the safety factor, (b) an effective threshold in the normalized electron temperature gradient, and (c) a decoupling of the particle and electron heat transport for sufficiently steep gradients. These three features will be discussed, and a simple transport model will be presented which is able to capture at least the first two of these effects. Co-existence of ITG modes and TEMs. Taking a step towards increased complexity, we then focus on the co-existence of ITG modes and TEMs. Varying the density gradient, a transition from down-gradient to up-gradient particle transport is observed as the dominant mode changes. The null point obtained in gyrokinetic simulations tends to correspond to the experimental working point. (Author)