A model for the turbulence in the scrape-off layer of tokamaks

The fluid theory of electrostatic perturbations in a scrape-off layer (SOL) plasma is analysed. The main difficulty is that the edge is theoretically found to be stable, while experimentally it is unstable. A possible explanation relies on the fact that the commonly used ballooning representation is not correct in the SOL. An alternative representation is proposed which reproduces the instability of the edge in several simple configurations and explains many experimental features.