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

Abstract

The capability of the CTS diagnostic to detect emissions at frequencies close to the injected EC power can be used to study the occurrence of parametric decay instabilities (PDI) excited by the ECH power. 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]. In this paper the observations obtained with new experiments are compared to the previous ones. The design of the new receiving line equipped with a RS antenna and the status of the work are presented. Experiment preparationOn the diagnostic side an effort is ongoing to exclude any parasitic effect of gyrotron spectrum, of highfrequency front-end components, of local breakdown in the antenna and other disturbances. The gyrotron frequency spectrum, previously obtained from a lossy pickup line 50 m long used to measure the gyrotron main line frequency vs time (without using the notch filter), was now obtained with a direct pick-up from the stray radiation in the launcher region while shooting the gyrotron on the local dummy load. In this way a higher signal is collected and a sensitive spectrum of the gyrotron radiation is obtained, using the notch filter to suppress the main line. The resulting spectra from the gyrotron often include spurious lines that appear also in some CTS spectra from plasma, as shown in Fig. 1. The lines of this type have always the same behavior, starting from the beginning of the gyrotron pulse and lasting tens or hundreds of ms with a frequency decreasing with oscillations around an average speed of 2 GHz/s (Fig. 1 top right).Whether these are due to gyrotron spurious mode or to a signal treatment in the HF diagnostic section is under evaluation. The level of the spurious line, not observed in the 2015 shots, is estimated at least 50 dB below the main gyrotron line.The calibration of spectra received from the plasma is now performed comparing the spectra with the ones obtained during Ohmic pulses with the EC resonance inside the plasma (where the emission is only thermal ECE) and subtracting the noise spectra obtained during the same shot with the aid of a chopper/modulator working at around 10 kHz. The spectra obtained during the two chopping phases (on-off) are evaluated by proper data analysis, summed separately and then the noise part is subtracted....