Microwave reflectometry diagnostic for density profile and fluctuation measurements on ASDEX Upgrade

Abstract

The broadband microwave reflectometry system on ASDEX Upgrade has been recently upgraded with new channels. It has now 12 channels that probe simultaneously the high and low fields plasma, with ultrafast sweeping (20–100 μs). X mode (33–75 GHz) is used for the scrape off layer, and the O mode (16–110 GHz) is reflected from densities between 0.3×1019 and 1.5×1019 m−3. A fast sweeping heterodyne system was developed for the highest frequency channels (50–110 GHz) to cope with higher losses and lower incident power. Fixed frequency operation is employed for fluctuation measurements. A dedicated channel (33–55 GHz) operating in fixed frequency provides a signal to continuously monitor the level of density fluctuations (e.g., L–H transition), during the whole discharge. The diagnostic is fully operated by remote control. The acquisition system is based on specially developed VME boards with up to 1 Gsamp/s sampling rates. Automatic profile inversion was recently implemented using data validation and rejection algorithms. Experimental results are presented to illustrate the profile measurements with O mode and with O and X mode combined operation. We present density profiles measured in a wide range of plasma regimes. Profile modifications due to rotating magnetic islands illustrates the potentialities of the reflectometry to estimate the location of the rational q surfaces.