Physics of the electric field in the scrape-off layer in ASDEX Upgrade L-mode discharges and comparison to experiments

The electric field in the Scrape-Off Layer (SOL) of fusion plasmas is a key quantity affecting the local plasma transport and possibly also the overall plasma confinement. However, the physics determining the SOL electric field is experimentally not well investigated. In this study a systematic experimental analysis of the electric field in AUG L-mode discharges at various plasma currents, densities and heating powers is presented. In particular, the relation of the electric field to the divertor condition, as measured by Langmuir probes, is analyzed in detail by applying a simplified form of Ohm’s law. The analysis shows that the peak value of the radial electric field Er in the near SOL measured by Doppler reflectometry decreases from about 8kV/m at the lowest densities to -2kV/m at the highest densities, which is accompanied by a flattening of the outer divertor target electron temperature profiles. The electric field obtained by integrating Ohm’s law from the divertor target to the midplane agrees with Er in the near SOL within the uncertainties, evidencing the quantitative validity of the underlying model. Based on the findings about the relation between the electric field and the target conditions, a scaling to obtain the maximum of Er in the SOL in terms of upstream parameters is developed.