Physics of pedestal density profile formation and its impact on H-mode density limit in burning plasmas

Based on semianalytic modeling of experimental H-mode density profiles, a pedestal density limit is computed for a prototypical next generation burning plasma. For a range of input assumptions, the projected limit lies near or above Greenwald density limit [M. Greenwald, J. L. Terry, S. M. Wolfe, S. Ejima, M. G. Bell, S. M. Kaye, and G. H. Neilson, Nucl. Fusion 28, 2199 (1988)]. Two separate models are developed and utilized: the first model determines the edge density profile from consideration of cross-field transport in the steep gradient region and the distribution of fueling sources. It is shown that the observed shape of the density profile in H-mode can be explained with constant particle diffusivity within the separatrix. The second model determines the separatrix density above which the conduction limited scrape-off layer (SOL) transport is no longer sustainable. These results imply that further increases in pedestal density (while maintaining separatrix density below the power balance limit) are...