Experimental validation of the intensity refractometry principle for density measurements at the edge of a tokamak

Experimental validation is presented for a new type of microwave diagnostic, first introduced in the theoretical study in M. Usoltceva et al., Rev. Sci. Instrum. 93, 013502 (2022). A new term is adopted for this technique to highlight its difference from interferometry: intensity refractometry. The diagnostic allows measuring electron density, and in this work, it is applied at the edge of a tokamak. The implementation of this technique at ASDEX Upgrade, called Microwave Intensity refractometer in the Limiter Shadow (MILS), provides the first experimental proof of the diagnostic concept. Densities predicted by MILS are compared to several other diagnostics. The agreement and discrepancy in various radial regions of the density profile are analyzed and possible reasons are discussed. A wide density coverage is shown in the example discharges with densities from 2*10^17 m^-3 to 2*10^19 m^-3 at the limiter position. In these experiments, the radial location of the measurements varied from 5 cm in front of the limiter (up to 1 cm inside the separatrix was measured) to 3 cm in the limiter shadow. Experimental challenges of MILS operation and data processing are presented.