High resolution edge density measurements in ASDEX upgrade H-mode discharges with broadband reflectometry

Abstract

In high-confinement (H-mode) plasmas with edge localized modes (ELMs), the energy stored at the edge pedestal region directly affects the global plasma confinement. The measurement of the edge pedestal evolution and the study of the collapse mechanism of the pedestal structure due to ELMs is, therefore, of great importance to understand plasma confinement. In Axially Symmetric Divertor Experiment (ASDEX) Upgrade, information about the edge density profile has been obtained in H-mode plasmas both with Thomson scattering and by combining lithium beam and interferometry measurements. However, information around the edge pedestal region is not routinely available. Here we use microwave reflectometry to obtain high resolution measurements of the edge transport barrier and pedestal region, namely the pedestal position and density, the pedestal width, and the edge density gradient. The evolution of the pedestal density is obtained automatically with an algorithm applied directly to the group delay. The other parameters are derived from the corresponding density profiles. Measurements performed in the presence of type I ELMs reveal that the edge density profile modifications occur in the same time scale of the Dα radiation at the divertor. The collapse and recovery of the pedestal structure during each ELM is resolved. The values of the pedestal width (around 4–5 cm) measured in-between ELMs are similar to those obtained previously in similar ASDEX Upgrade discharges. Also, the evolution of the pedestal density given by reflectometry is found to be in good agreement with the changes of the line integrated density measured by the interferometer.