Speaker
Mr
Rajesh Maingi
(USA)
Description
The understanding of regimes with 1) high pressure at the top of the H-mode pedestal, and 2) devoid of large ELMs is important for scenario optimization of ITER and future devices. Lithium wall coatings have been shown to both improve energy confinement and eliminate ELMs in NSTX. Here, we present analysis of variable pre-discharge lithium evaporation from multiple experiments, for more insight into the pedestal expansion and ELM suppression physics. First, a nearly continuous improvement of a number of discharge characteristics, e.g. reduced recycling, ELM frequency, and edge electron transport, with increasing pre-discharge lithium evaporation has been identified. These correlations ran contrary to initial expectations that the beneficial effects would saturate at much lower evaporation amounts than used in experiments. Profile and stability analysis clarified the mechanism responsible for ELM avoidance and the role of lithium: lithium coatings reduce recycling and core fueling; thus the density and its gradient near the separatrix are reduced. The temperature gradient near the separatrix is unaffected; hence the pressure gradient and bootstrap current near the separatrix are reduced, leading to stabilization of kink/peeling modes thought to be responsible for the NSTX ELMs. Thus, the enhanced edge stability with lithium coatings is correlated with the reduction of the pressure and its gradient near the separatrix. The key ingredient for ELM avoidance is control of the particle channel independent of the thermal channel at the edge: the density profile is continuously manipulated via the amount of lithium evaporation and resulting recycling control, leading to reduced neutral fueling. The surprising and beneficial facet of the NSTX data, however, is the continued growth of the edge transport barrier width in these circumstances, leading to 100% higher plasma pressure at the approximate top of the ne profile barrier with high pre-discharge evaporation. Analysis shows enhanced edge transport; coupled with the heating power reduction to stay below the global beta limit, the pressure gradient and associated bootstrap current are maintained below the edge stability limit, thus avoiding ELMs. This allows the H-mode edge transport barrier to expand further and in such a way that peeling stability improves as a result of the inward shift of the bootstrap current.
Country or International Organization of Primary Author
USA
Primary author
Mr
Rajesh Maingi
(USA)
Co-authors
Dr
Ahmed Diallo
(Princeton Plasma Physics Lab)
Dr
Benoit LeBlanc
(Princeton Plasma Physics Lab)
Dr
Charles Skinner
(Princeton Plasma Physics Lab)
Mr
Dennis Boyle
(Princeton University)
Dr
Dennis Mansfield
(Princeton Plasma Physics Lab)
Dr
Henry Kugel
(Princeton Plasma Physics Lab)
Dr
Janardhan Manickam
(Princeton Plasma Physics Lab)
Dr
John Canik
(Oak Ridge National Lab)
Dr
Jonathan Menard
(Princeton Plasma Physics Lab)
Dr
Lane Roquemore
(Princeton Plasma Physics Lab)
Dr
Mario Podesta
(Princeton Plasma Physics Lab)
Dr
Masayuki Ono
(Princeton Plasma Physics Lab)
Dr
Michael Bell
(Princeton Plasma Physics Lab)
Dr
Michael Jaworski
(Princeton Plasma Physics Lab)
Dr
Philip Snyder
(General Atomics)
Dr
Robert Kaita
(Princeton Plasma Physics Lab)
Dr
Roger Raman
(Princeton Plasma Physics Lab)
Dr
Ronald Bell
(Princeton Plasma Physics Lab)
Dr
Stanley Kaye
(Princeton Plasma Physics Lab)
Dr
Stefan Gerhardt
(Princeton Plasma Physics Lab)
Prof.
Steven Sabbagh
(Columbia University)
Dr
Tom Osborne
(General Atomics)
Dr
Travis Gray
(Oak Ridge National Lab)
Dr
Vlad Soukhanovskii
(Lawrence Livermore National Lab)
Dr
Walter Guttenfelder
(Princeton Plasma Physics Lab)
Dr
Yang Ren
(Princeton Plasma Physics Lab)
