Formation of the high density front in the inner far SOL at ASDEX Upgrade and JET

Abstract

This article presents the development of a region of high electron density, being one order of magnitude larger than the separatrix density, in the inner divertor of ASDEX Upgrade (AUG) and JET. The high-field side high density (HFSHD) occurs about 10×λq away from the separatrix and about 20cm above the X-point. To trigger the HFSHD, the inner divertor has to be detached while the outer one remains attached and the heating power has to be sufficiently high. The HFSHD, determined independently from spectroscopic and interferometric measurements, is clearly correlated with neutral fluxes in the far scrape-off layer; both increase with heating power. Injection of N2 into the divertor private flux region suppresses the HFSHD in AUG and JET. The density distribution in the divertor with and without N2 seeding is consistent with the total radiation and the Dγ distribution.