Effect of local wall clearance on scrape-off layer electron density profiles in ASDEX Upgrade

The plasma electron density is measured with multiple diagnostics in the scrape-off layer of the ASDEX Upgrade tokamak. The recorded values are typically in good agreement in the case of exponentially decaying profiles at low densities, but they become inconsistent as soon as a density shoulder is formed, and up to a factor of 3 deviation is experienced at high densities. An idea is put forward; namely, that the local plasma-wall proximity has an influence on the local density and thus the measurement results depend on the local limiter-plasma and limiter-diagnostic distances at the same time. The effect of local wall clearance dependence is demonstrated by creating a plasma having an extremely high scrape-off layer density $\left(n_\mathrm{e} \gt 3\cdot10^{19}\,\textrm{m}^{-3}\right)$ after the H-L back transition and varying the position of the separatrix in different directions. As a possible physical process, neutral recycling at the plasma facing components is identified. Similar behaviour is also observed in the QCE regime, where the appearance of small ELMs barely impacts the scrape-off layer profiles. There are also indications that the local wall clearance does have an impact on the density profile not only in the scrape-off layer, but also in the confined region.