Formation of the High Density Front in the Inner far SOL at ASDEX Upgrade and JET

This article presents the development of a high electron density in the inner divertor region well above the X-point in ASDEX Upgrade and JET. In order for this high-field side high density (HFSHD) to be formed, the inner divertor has to be detached while the outer one remains attached and the heating power has to be sufficiently high. Spectroscopic and interferometric measurements of the density are consistent with total and D/gamma distributions and neutral flux measurements. It will be further shown that there is a clear correlation between the HFSHD and neutral fluxes in the far SOL and that both increase with the heating power. Injection of N2 leads to a suppression of the HFSHD, which is verified in ASDEX Upgrade and JET.
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