Research news from the division Plasma Edge and Wall
Impact of poloidal impurity asymmetries on edge current and pedestal stability
The improvement and maintenance of plasma performance on a steady-state and economical basis is a key issue for the development of future fusion power plants. At present, the highest attainable pressure is achieved in the high-confinement regime (H-mode), which is characterized by the formation of an edge transport barrier. The stability of this barrier is thought to be driven by a complex interplay between the edge current j and the pressure gradient ∇p. In order to test current pedestal stability theories, the profile shape and the magnitude of both j and ∇p are required at a high level of accuracy with a temporal (spatial) resolution on the ms (mm) range. However, accurate measurements of j are challenging due to the complicated nature of the measurement.
Combined with the unique edge diagnostic suite available at AUG, the research project aims at the accurate measurement of j and ∇p to gain a solid understanding of the dynamics between the current density and pressure gradient in the edge transport barrier. In addition, the impact of impurities on the edge current and stability of the H-mode pedestal will be studied by means of an integrated approach based on experimental measurements combined with theoretical tools.
Influence of Different Defect Types on Hydrogen Isotope Transport and Retention in Tungsten
Hydrogen can barely be dissolved in a perfect tungsten crystal, but hydrogen atoms that actually make it into the material can diffuse through it quickly. Defects such as dislocations, grain boundaries and vacancies can strongly alter the behaviour of hydrogen in tungsten. They can trap hydrogen atoms, thus increasing retention and slowing down the effective diffusion. But there is also indication that, e.g., grain boundaries may serve as diffusion highways. In this project, the creation of defects by exposure of tungsten to hydrogen plasmas will be investigated by electron microscopy and ion beam analysis. In addition, diffusion of hydrogen in tungsten will be measured with an electrochemical double cell. This newly constructed set-up allows highly sensitive measurements near room temperature, where up to now only very few data are available.