Progress And Prospects Of The Ftu Collective Thomson Scattering Diagnostics

Abstract

During the last years anomalous signals have been found in the spectra analyzed with the Collective Thomson Scattering (CTS) diagnostics in tokamak devices like TEXTOR, ASDEX and FTU. The signal frequency, down-shifted with respect to the probing one, suggested the explanation of their origin in terms of Parametric Decay Instability (PDI) processes correlated with the presence of magnetic islands and taking place even for values of the pumping wave power below the threshold predicted by conventional models. A threshold lower or comparable with the power levels routinely used could pose limitations to the use of the ECRH in fusion devices. An accurate characterization of the conditions for the occurrence of this phenomenon and of its consequences is thus of primary importance. The CTS diagnostics of FTU has been recently improved for this aim with a new fast data acquisition, allowing spectra reconstruction by direct FFT of the intermediate frequency signal, added in parallel to the pre-existing multichannel spectrum analyzer. A section with polarizing mirrors has been introduced in the quasi-optical transmission line to control the polarization of the received signals. Exploiting the front-steering configuration available with the real-time launcher of FTU, the setup allows studying the anomalous emissions in conditions of density and wave injection similar to ITER. Given the possibility of the CTS in FTU to operate in both O-and X-mode, a second radiometer is being installed, to detect simultaneously signals scattered in both polarizations. The upgrades mentioned above are presented in this work as well as the first data obtained in 2014, during the experimental activity carried out under an EUROfusion Enabling Research project. 1st EPS conference on Plasma Diagnostics