Scaling of the pedestal density in type-I ELMy H-mode discharges and the impact of upper and lower triangularity in JET and ASDEX Upgrade

Abstract

Experiments have been performed to disentangle the individual role of upper and lower triangularity on density buildup of lower single null, type-I ELMy H-mode discharges in JET. Comparison with corresponding data from ASDEX Upgrade allowed us to determine a dimensionless representation of the relation between the main chamber recycling and core density and to widen the triangularity variation in the data base. To incorporate the recycling flux density Γ in a dimensionless form, an effective scrape-off layer density ne,SOL∝Γ0.5 is introduced allowing us to parametrize and scale the core density by the density rise factor n̄e/ne,SOL. The scaling uses edge-specific definitions of normalized Larmor radius, collisionality and beta. Rewritten in dimensional form, a behaviour is found which is very similar to energy confinement scalings for the ELM-averaged conditions considered here: the density rise factor exhibits an almost linear dependence on plasma current, a weak negative toroidal field dependence as well as power degradation. While a pronounced positive dependence of density buildup on the upper triangularity is observed, no significant correlation with the lower triangularity is found. In particular the dependences on plasma current and upper triangularity emphasize the importance of transport physics for the density buildup.