Upgrade of the X-mode Reflectometry Diagnostic for Radial Correlation Measurements in JET

The recent upgrades and the potential of the X-mode reflectometry diagnostic on JET, dedicated to measurement of the turbulence radial correlation length, are presented in this paper. The reflectometer system now consists of four independent heterodyne instruments working at different probing frequencies, which in principle enables analysis of the density fluctuations at four distinct radial regions in the plasma. A VME based computer connected to the JET database computer system is used to control all the diagnostic, in particular frequency of the tuneable sources and frequency of the fast acquisition boards. The diagnostic is fully operated by remote control. A study of the effects of type I ELMs observed at JET illustrates its potential. The main limitation comes from the low quality of the transmission lines, which leads to a strong attenuation of the signal and makes the measurements difficult especially in the plasma core. To reach higher performances, new low attenuation corrugated circular waveguides (with an expected gain of 20 dB) will be installed next summer 2004 during the JET shutdown.
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