Characterization and preliminary results of the collective Thomson scattering system on FTU tokamak

Abstract

A collective Thomson scattering (CTS) experiment is now installed on FTU aimed to measure the plasma ion temperature and test the theory of CTS. A high power 140 GHz gyrotron beam is focalized into the plasma center by means of a circular corrugated transmission line and an in-vessel optical system to a radius of 0.025 m. The scattered radiation is collected at 90° scattering angle by a symmetric in-vessel optical system and a low-power quasioptical transmission line which collects the scattered radiation on a superheterodyne receiver. Both in-vessel optical systems are placed in a vertical port. The receiver is based on 32 channels, 1.2 GHz global bandwidth, with a measured noise temperature T rec =1 eV and minimum integration time of 1 ms. The antenna decoupling measurement resulted about 60 dB, while the attenuation introduced by the collecting system is evaluated in 5 dB. The angle of the scattering wavevector and the magnetic field directions is 25°. The experiment is run at a magnetic field B=7.4–7.7 T , i.e., the gyrotron frequency is well below the first electron cyclotron emission (ECE) harmonic, so the ECE plasma background temperature is low and it is measured T ECE =70–50 eV , respectively. The detected polarization is the ordinary mode. In this experiment the calculated signal is of the order of the ECE noise level. Indeed in a series of dedicated FTU runs, CTS signals with signal-to-noise consistent with calculation (made using standard formulas) were found. Some features of the detected scattered spectra are reported, as well as plasma radiation measurements in various plasma conditions including lower hybrid rf heating.