Performance of the microwave reflectometry diagnostic for density profile measurements on ASDEX Upgrade

The broadband reflectometry system on ASDEX Upgrade is now routinely providing electron density profiles with a minimum temporal spacing of 30 μs. The plasma is probed from zero density to ne=6.64×1019 m−3 at the low field side with combined O and X modes, and from ne=0.318×1019 to ne=6.64×1019 m−3 at the high field side using O mode. Here we analyze the performance of the diagnostic for profile measurements taking into account its design options and the experimental results obtained in a wide range of plasma regimes. We address several questions, in particular the measuring range of the antennas and the range of density gradients. It is shown that it is possible to obtain a detailed evolution of the density profile and density gradient even in the presence of turbulence, providing that sufficient data can be extracted from the plasma. The radial movements of the plasma can be measured with great accuracy, which is especially important for the use of reflectometry for plasma position control, as is proposed for the International Thermonuclear Experimental Reactor. The experimental results demonstrate that the frequency modulation of a continuous wave reflectometry system on ASDEX Upgrade can provide reliable and accurate density profile measurements at both sides of the plasma with automatic analysis.