Measurements of X-point radiator and MARFEs using the high-field side Thomson Scattering diagnostic at ASDEX Upgrade

To ensure the longevity of the plasma-facing components and the divertor in a fusion reactor, it is crucial to maintain the incident heat flux to those surfaces at tolerable levels. Therefore, a fully detached plasma state is preferable. In this state, an X-point radiator (XPR) can be observed at ASDEX Upgrade (AUG): a highly radiating, poloidally localized region within the separatrix near the geometrical X-point [U. Stroth et al. NF 62 2022]. Controlling this XPR provides access to a detached scenario that has many of the properties required for a reactor scenario in ITER or DEMO. However, if the XPR migrates too far into the confined plasma region, it can become unstable and mutate into a MARFE, which moves from the X-point region to the high-field side of the plasma, and ususally terminates in a plasma disruption. A deeper understanding of the XPR/MARFE physics is therefore critical for predictive control in future fusion devices. This requires local measurements of electron temperature T_e and density n_e on the high-field side, which we have performed at AUG for the first time in over 30 years using the Thomson scattering diagnostic [B. Kurzan et al. RSI 82 2011]. T_e and n_e are measured at 15 vertical positions, predominantly below the midplane (ρ_pol > 0.9). During MARFE phases in the confined region, electron temperatures below 10 eV and densities exceeding 10²⁰ m^-3 are observed. Additionally, we present a comparison with the Thomson scattering channels located in the divertor [B. Kurzan et al. JINST 16 2021] in order to analyze the behavior of the XPR/MARFE.