The effect of impurities on detachment, pedestal performance and global confinement

Livia Casali

General Atomics

Friday, May 17, 2019

3:00pm

NW17-218

PSFC Seminars

Abstract: Impurities in tokamaks, either originating from the wall or injected into the plasma with the aim of reducing power loads on divertor components, affect the pedestal and global confinement. As such, understanding the physics of such effects is vital for the operation of future devices.  At ASDEX Upgrade a reduction of the energy confinement has been observed with the introduction of the W metal wall attributed to the absence of a low-Z edge radiator. The confinement in AUG can be recovered with nitrogen seeding. Transport analysis of N₂ seeded discharges shows that the confinement improvement is primarily a pedestal effect propagating to the core via profile stiffness. Detailed modeling of the simultaneous evolution of N and W in the presence of ELMs in the pedestal show the importance of non-coronal effects and ELM-induced transport for low-Z impurities at the plasma edge. High ELM frequencies are required to avoid a radiation collapse by W accumulation in agreement with the experimental observations. AUG experiments combining low and medium-Z radiators such as N and Kr showed that a radiative layer just inside the separatrix can even improve confinement as long as Kr concentration is low enough to not trigger core impurity accumulation. Impurity seeding also promotes plasma detachment keeping good plasma performance as shown by recent experiments with nitrogen and neon seeding in the new Small-Angle-Slot (SAS) divertor at DIII-D. In these experiments, for matched conditions, while neon readily enters the confined region with consequent effects on the pedestal, nitrogen remains compressed in the divertor consistent with the higher/lower Ne/N ionization potential. The discussion of both AUG and DIII-D data aims to assess the impurity dynamics and their effects in high vs low Z wall machines.

Bio: Dr Livia Casali is a Research Scientist at General Atomics working on the DIII-D tokamak in San Diego. Her work is focussed on ‘core-edge’ integrated solutions with special emphasis on the role of radiative divertor and impurity behavior to achieve high performance operating scenarios with mitigated heat power loads which are essential for future reactors. She leads divertor experiments and scrape of layer simulations focusing on the role of neutrals and impurity dynamics in the newly installed divertor at DIII-D. Livia studied nuclear and subnuclear physics at the University of Rome Tor Vergata, Italy and at the Albert Ludwig University of Freiburg, Germany. With a fellowship awarded by the German Academy Exchange, she performed her master thesis at the Max Planck Institute for Plasma Physics in Garching studying the effect of ICRH on radiated power in the ASDEX Upgrade tokamak. She then stayed at the Max Planck Institute in Garching for her PhD thesis"Experimental studies and modelling of high radiation and high density plasmas in the ASDEX Upgrade tokamak" and received her doctorate from the Ludwig-Maximilian University of Munich with magna cum laude. She analyzed experiments at ASDEX Upgrade with a variety of impurities studying their transport properties and showing that the confinement depends on the injected impurity species and its effect on plasma pedestal. Her work includes a detail analysis of the radiation losses under non coronal conditions and a time dependent analysis of the impact of the Edge Localized Mode (ELMs) instabilities on the edge impurity concentration demonstrating the importance of the radial transport effects on the atomic processes.  For this work she was awarded the EPS/PPCF poster prize at the European Physics Conference in Berlin 2014. From the Italian Physics Society she received the award for highly proficient scientific activity entitled to “Pietro Blaserna”, the founder of the Italian Physics Society.