Gyrokinetic simulations of tokamak microturbulence

We present direct numerical simulations of electromagnetic microturbulence in magnetic confinement experiments, especially describing progress achieved with the GS2 code. Particular emphasis is placed on the elucidation of the important physics of the nonlinear state, namely the roles of linear, secondary and tertiary instabilities and the resulting implications for the scaling of anomalous transport with, e.g., beta and collisionality. The parasitic instability model we present describes a wide range of new simulation results, and has the advantage of offering a straightforward route to predicting key turbulence simulation results. We will also review progress made in the area of comparing nonlinear turbulence simulations with experimental data. (author)