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 Trec=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 TECE=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.

Critical issues highlighted by collective Thomson scattering below electron cyclotron resonance in FTU

“…A diagnostic experiment of collective Thomson scattering (CTS) on thermal density fluctuations designed to provide spatially-and time-resolved measurements of the ion temperature is since long being carried out in FTU (B T0 = 4-8 T, R = 0.93 m, a = 0.3 m, I p = 0.4-1.2 MA) using a high-power gyrotron at f gyr = 140 GHz as the probing source [1]. The sensitivity and the detection bandwidth are similar to those of a previous experiment on W7-AS, where the feasibility of millimetre-wave CTS was first demonstrated 5 Present address: General Atomics, P.O.…”

Recent progress in fast-ion diagnostics for magnetically confined plasmas

“…The continuum is due to Compton interactions. An energy resolution of 3.1% at the 662 keV line from 137 Cs is assumed ∼30% full-energy peak efficiency has now become the reference choice also in view of ITER (Nocente et al 2017;Chugunov et al 2011). When analysis of the fine peak shape is the aim of the measurement, a detector which offers a virtually zero line instrumental broadening is desired.…”

“…Cylindrical detectors with a diameter and length of a few inches as those described so far can also be used for -ray profile measurements. This is the approach envisaged for ITER (Nocente et al 2017). However, in existing tokamaks, for example, at JET, -ray detectors were developed at a later stage than those for -rays born in the 9 Be( 4 He,n) 12 C reaction between fast 4 He ions and 9 Be impurities injected with overnight evaporation are seen.…”

Prospects for a dominantly microwave-diagnosed magnetically confined fusion reactor

“…The convolution is easier to separate if the species of interest have dramatically different ion velocity distributions, due for example to very different masses and/or energies. This implies that CTS can measure the slowdown of α's due to collisions, to radiations and to other effects [106,107], the concentration of high Z impurities, and Z ef f [108,109]. In principle, for sufficiently high precision (requiring an intense source and a low-noise radiometer), CTS can also measure the ion ratio n T /n D or n T /n D /n H , and core density of He ashes [107,109].…”