Observation of short time-scale spectral emissions at millimeter wavelengths with the new CTS diagnostic on the FTU tokamak

Abstract

The Collective Thomson Scattering (CTS) diagnostic on FTU tokamak was renewed for investigations on the excitation of Parametric Decay Instabilities (PDI) by Electron Cyclotron (EC) beams in presence of magnetic islands and their effects on the EC absorption.

Fast events detection with the CTS diagnostic on FTU and plans for improvement

“…One of the features reported in [4] was studied in more detail, showing correlation with the appearance of a growing m:n = 2:1 magnetic island at 4.7 T central magnetic field, in asymmetric configuration (see [4]) at line-integrated density of around 1 × 10 20 m -3 . In Fig.…”

“…In this way, from the detection of back-scattered power in the case of 2nd harmonic X-mode injection [1,2], the sources of the signals were well explained by waves generated by PDI mechanisms [3]. On FTU, during the work with the CTS diagnostics focused on studying the effects of possible PDI occurrence on EC beams (under Enabling Research project ER-ENEA-06), other still unexplained fast signals were detected in the spectra, in experiments with injection of EC power in O-mode at the fundamental EC harmonic [4]. The possibility to analyze data with a high time resolution is crucial for the study of these fast events, and the use of a fast rate (12 GS/s max) digitizer and of the FFT of the sampled intermediate frequency signal (taken at the output of the 140 GHz heterodyne radiometer front-end) over different time-scales is mandatory [5].…”

Anomalous absorption in ECRH experiments due to the parametric low-threshold excitation of localized UH waves

“…We focus on the case of the one-step down-conversion leading to the low-threshold excitation of the IB and radially trapped UH waves. We derive a set of equations describing the cascade of decays and then solve it numerically under the conditions similar to the on-axis O1-mode ECRH experiments at FTU [6] in which the nonmonotonic density profile arises due to the density pump-out effect from the ECR layer.…”

“…The parametric decay instabilities (PDIs), which can accompany the ECRH experiments, are believed to be deeply suppressed by the huge energy loss of daughter waves from the decay region, according to the predictions of theory developed in 80 th [1]. However, during the last decade numerous ECRH experiments performed at TEXTOR, TCV, TJ-II, ASDEX-UG, LHD, T-10 and FTU [2][3][4][5][6][7][8][9][10][11][12][13] have demonstrated excitation of the anomalous phenomena. The clearest evidence of the nonlinear phenomena excitation was obtained at TEXTOR [2,3] where the strong backscattering signals down-shifted in frequency and amplitude modulated at the frequency of magnetic island were observed.…”

“…means of the WKB approximation in the same way as in equations(6) and(7). The choice 1y, 0 z q = , 1y 0 q′ = allows us to minimize the energy losses of UH plasmons from the decay region.…”