Abstract
Anomalous emissions were found over the last few years in spectra of Collective Thomson Scattering (CTS) diagnostics in tokamak devices such as TEXTOR, ASDEX and FTU, in addition to real CTS signals. The signal frequency, down-shifted with respect to the probing one, suggested a possible origin in Parametric Decay Instability (PDI) processes correlated with the presence of magnetic islands and occurring for pumping wave power levels well below the threshold predicted by conventional models. A threshold below or close to the Electron Cyclotron Resonance Heating (ECRH) power levels could limit, under certain circumstances, 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. Exploiting the front-steering configuration available with the real-time launcher, the implementation of a new CTS setup now allows studying these anomalous emission phenomena in FTU under conditions of density and wave injection geometry that are more similar to those envisaged for CTS in ITER. The upgrades of the diagnostic are presented as well as a few preliminary spectra detected with the new system during the very first operations in 2014. The present work has been carried out under an EUROfusion Enabling Research project.A shorter version of this contribution is due to be published in PoS at: 1st EPS conference on Plasma Diagnostics
Highlights
Anomalous emissions were found over the last few years in spectra of Collective Thomson Scattering (CTS) diagnostics in tokamak devices such as TEXTOR, ASDEX and FTU, in addition to real CTS signals
The signal frequency, down-shifted with respect to the probing one, suggested a possible origin in Parametric Decay Instability (PDI) processes correlated with the presence of magnetic islands and occurring for pumping wave power levels well below the threshold predicted by conventional models
The CTS diagnostic of FTU has been recently improved with a new fast data acquisition, implemented for the first time in ASDEX [19], to allow studying the rapidly modulated emissions correlated with rotating islands
Summary
2.1 Diagnostic layout In the new CTS layout, the lower of the two lines of the real-time front-steering ECRH launcher of FTU [10,11,12] is used to launch the probe beam and the upper one to collect the signals. The upper line is designed either to inject ECH power or to collect radiation at 140 GHz for studies of reverse OXB mode coupling scheme [14, 15] or for the radiometric system during CTS experiments. The transmission line used for delivering CTS signals from the torus hall to the radiometer has been recently installed on FTU. It consists of a mix of quasi-optical mirrors and a section of low loss HE11 overmoded corrugated waveguide with 88.9 mm diameter. A numerical code, developed on purpose, computes the polarization changes after the 20 reflections made in the CTS line, so allowing to define the correct configuration of the polarization section, which is checked with low power measurements before starting the measurements
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
