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

The broadband microwave reflectometry system on ASDEX Upgrade has been rece ntly upgraded with new channels. It has now 12 channels that probe simultan eously the high and low fields plasma, with ultrafast sweeping (20-100 mu s ). 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 X 10(19) and 1.5 X 10(19) m(-3). A fast sweeping heterodyne system was developed for the highest freq uency channels (50-110 GHz) to cope with higher losses and lower incident p ower. Fixed frequency operation is employed for fluctuation measurements. A dedicated channel (33-55 GHz) operating in fixed frequency provides a sign al to continuously monitor the level of density fluctuations (e. g., L-H tr ansition), 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. Exper imental 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 r otating magnetic islands illustrates the potentialities of the reflectometr y to estimate the location of the rational q surfaces. (C) 1999 American In stitute of Physics. [S0034-6748(99)69801-X].