Side emissions during EC injection for PDI studies in FTU tokamak

¹ Istituto di Fisica del Plasma, Consiglio Nazionale delle Ricerche , via R. Cozzi 53 20125 Milano Italy
² ENEA Fusion and Nuclear Safety Department, C.R.Frascati via E. Fermi 45 00044 Frascati (Roma) Italy
³ Swiss Plasma Center, Ecole Polytéchnique Fédérale de Lausanne , Lausanne Switzerland
⁴ Institute of Applied Physics 46 Ulyanov st Nizhny Novgorod 603950 Russia
⁵ Consorzio CREATE, Università degli Studi di Napoli , Napoli Italy

^* Corresponding author: bruschi@ifp.cnr.it

Published online: 25 March 2019

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.