ASDEX Upgrade Gefäß, Photo: Volker Steger
​ASDEX Upgrade Gefäß.
Photo: Volker Steger

The pedestal at ASDEX Upgrade

​Seminar by Elisabeth Wolfrum, Max-Planck Institute for Plasma Physics, Garching, Germany

​Abstract of talk

The tokamak ASDEX Upgrade situated in IPP Garching, Germany, is equipped with an excellent set of edge diagnostics, which is used to improve our understanding of the complex mechanisms determining the transport and the limits of the edge transport barrier, also called pedestal. This talk will give an overview of our current picture of the development of the pedestal from the low confinement mode (L-mode) to the high confinement mode (H-mode), the temporal evolution of edge profiles (ne, Te, Ti, Er) in between type-I ELMs and the effect of impurities on the pedestal stability.

Fast measurements of the radial electric field, Er, show that the transition from L- to H-mode is determined by the shear in the ExB drift velocity. The measured Er is consistent with the neoclassically calculated Er, and therefore both, the ion heat flux as well as the density gradient length at the separatrix, are decisive quantities for the L-H transition. The development of the density pedestal following the L-H transition is independent of the temperature and can be modelled with a diffusive term only. The inter-ELM pedestal evolution occurs in consecutive phases with the density recovering earlier than the temperature, accompanied by quasi-coherent modes, which can cause significant transport.

In plasmas with high gas puff and sufficient heating power a region of very high density ( > 1021 m-3) develops in scrape-off layer at the high field side at the height of the x-point, fuelling the plasma via diffusion due to the locally inverted ne profile. The seeding of radiating impurities, such as N, reduces the density in this region and the subsequent change in the density profile location strongly impacts the ideal MHD stability of the plasma edge.
Category Seminar
Location: Room F-N6115, Origo building north 6th floor, Fysikgården 1
Starts: 18 August, 2016, 10:00
Ends: 18 August, 2016, 11:00

Published: Tue 02 Aug 2016. Modified: Wed 03 Aug 2016