ADVANCED TECHNIQUES FOR MICROWAVE REFLECTOMETRY

Microwave reflectometry has been applied during the last years as a pl asma diagnostic of increasing interest, mainly due to its simplicity, no need for large access ports and low radiation damage of exposed com ponents. Those characteristics make reflectometry an attractive diagno stic for the next generation devices. Systems used either for density profile or density fluctuations have also shown great development, fro m the original single channel heterodyne to the multichannel homodyne receivers. In the present work we discuss three different advanced ref lectometer systems developed by CIEMAT members in collaboration with d ifferent institutions. The first one is the broadband heterodyne refle ctometer installed on W7AS for density fluctuations measurements. The decoupling of the phase and amplitude of the reflected beam allows for quantitative analysis of the fluctuations. Recent results showing the behavior of the density turbulence during the L-H transition on W7AS are shown. The second system shows how the effect of the turbulence ca n be used for density profile measurements by reflectometry in situati ons where the complicated geometry of the waveguides cannot avoid many parasitic reflections. Experiments from the TJ-I tokamak will be show n. Finally, a reflectometer system based on the Amplitude Modulation ( AM) technique for density profile measurements is discussed and experi mental results from the TJ-I tokamak are shown. The AM system offers t he advantage of being almost insensitive to the effect of fluctuations . It is able to take a direct measurement of the time delay of the mic rowave pulse which propagates to the reflecting layer and is reflected back. In order to achieve fast reconstruction for real time monitorin g of the density profile application of Neural Networks algorithms wil l be presented the method can reduce the computing times by about thre e orders of magnitude.