What sets the residual transport in H-mode barriers?

One of the key questions for the success of the ITER project is the quality of the edge confinement. At present, there is not yet a self-consistent theoretical description of the L-H transition. However, gyrokinetic simulations have reached a level of maturity which allow them to address important open issues, thus contributing to a more complete understanding of this phenomenon. Employing the gyrokinetic turbulence code GENE (www.ipp.mpg.de/ fsj/gene), we have studied the behavior of the turbulence under H-mode edge conditions. Here, the physics model is quite comprehensive (e.g., magnetic trapping, magnetic field fluctuations, and collisions are all included), and the required geometric data is inferred directly from MHD equilibria. The relevant microinstabilities are identified, and their (unusual) linear as well as nonlinear properties are characterized. Our results are in line with the experimentally measured properties of the residual electron heat transport in the H-mode edge.