Side emissions during EC injection for PDI studies in FTU tokamak

Abstract

The evidence of Parametric Decay Instabilities (PDI) excited by the ECH power injected in O-Mode has been explored in FTU Tokamak, using the Collective Thomson Scattering (CTS) diagnostic. The experiments show evidences to support the hypothesis of low-threshold excitation of waves generated by PDI mechanisms, formerly proposed in the case of 2nd harmonic X-mode injection in TEXTOR and ASDEX-U. Theoretical analysis predicts low-threshold parametric decay also for O-mode pump-wave injection, which can be injected in FTU at frequencies close to the first Harmonic EC resonance. Experiments were made at different magnetic fields, injecting the 140 GHz probe and observing the emission from the second antenna of the EC launcher in poloidally symmetric and asymmetric configurations, in presence of MHD islands. The signal is detected by the CTS radiometers, with a fast digitizer allowing the spectral reconstruction at very fine time and frequency scales. Different types of emissions are studied in detail, comparing them with the magnetic island rotation frequency in different plasma conditions. In order to locate the plasma volume originating the emissions, a new antenna and receiving line has been installed.

Microwave diagnostics damage by parametric decay instabilities during electron cyclotron resonance heating in ASDEX Upgrade

“…Second, theoretical predictions of strong PDI-related microwave signals near 1.5 times the ECRH frequency [29] should be investigated experimentally and their potential consequences for ITER assessed. Third, it would be of interest to carry out investigations similar to the ones found in the present paper for PDIs involving an O-mode pump wave decaying to a single trapped UH wave and a low-frequency daughter wave near the UHR when the UHR occurs near a local maximum of the electron density [62][63][64][65][66][67][68][69]. Such PDIs are predicted to occur in connection with MHD modes for 170 GHz fundamental O-mode ECRH of the full field (on-axis toroidal magnetic field strength of 5.3 T) scenarios and 104 GHz (or 110 GHz) fundamental O-mode ECRH of half field scenarios at ITER [62][63][64].…”

Integrated software for the analysis of high-resolution scattering signals during mm-wave beam injection

“…A frequency between 700 and 900 MHz is expected at the point on the UH layer reached by the reflected X wave. Compatible anomalous signals, collected by the receiving system of the CTS diagnostic for that discharge, are shown in figure 5b (more details about of the anomalous emissions of CTS experiments at FTU are reported in [27]). This is an example of the features offered by the CTS data analysis code, which will be exploited for the detailed analysis of the data collected during all the discharges of the past experimental campaigns.…”

Parametric Decay Instabilities during Electron Cyclotron Resonance Heating of Fusion Plasmas, Problems and Possibilities

“…Apart from the PDIs driven by X-mode ECRH, PDIs driven by O-mode ECRH, relevant for the full-field ITER scenarios [12], have also been investigated. Particularly, decay of an O-mode ECRH wave to a trapped UH wave and a low-frequency lower hybrid wave has been investigated theoretically [53][54][55][56], in a lowtemperature plasma filament [3], and at the FTU tokamak [57][58][59]. Additionally, it has recently been suggested that trapped low-frequency waves existing as a consequence of the strong gradients in the ITER and ASDEX Upgrade pedestals could lead to PDIs near the plasma edge during O-mode ECRH [60].…”