Collective Thomson Scattering Diagnostics Research Articles

Commissioning and first results of the 174 GHz collective Thomson scattering diagnostic at Wendelstein 7-X

Collective Thomson Scattering (CTS) diagnostics measure the scattering spectrum of monochromatic incident radiation off collective fluctuations in the plasma. In this contribution, we present the first results from the upgraded CTS diagnostic at Wendelstein 7-X (W7-X) operating in the frequency range between 172 and 176 GHz. This frequency range allows for minimization of noise originating from the electron cyclotron emission in the plasma. Consequently, the good signal-to-noise ratio allows measurements of fast ions or bulk plasma parameters with higher temporal resolution compared with the previously used 140 GHz system.

RAMI analysis of the collective Thomson scattering system front-end – Part1 – Failure modes effects and criticality analysis

ITER is a tokamak nuclear fusion reactor intended to demonstrate the feasibility of nuclear fusion as a carbon-free energy source. In order to control potential technical risks, ITER subsystems are assessed through Reliability, Availability, Maintainability and Inspectability (RAMI) analysis, which includes Failure Modes Effects and Criticality Analysis (FMECA).This paper deals with the FMECA of the front-end components of the Collective Thomson Scattering (CTS) diagnostic, which involves a top-down functional breakdown of the system, identification of critical components and potential failure modes, their effects and consequences for the system. Furthermore, the FMECA methodology allows for the analysis of possible mitigation actions, which may reduce failure mode risk levels.The FMECA was performed in two stages, one concerning the failure modes of the CTS itself, and one concerning the failure modes, which may interrupt ITER operations. Results indicate that there are no failure modes that pose a significant risk to ITER operations. However, mitigation actions were required in order to reduce the risk levels of failure modes, which may compromise CTS diagnostics availability.This article covers the first half of the RAMI analysis of the CTS diagnostic, the FMECA.

Collective Thomson scattering with 77, 154, and 300 GHz sources in LHD

Collective Thomson scattering (CTS) is one of attractive diagnostics for measuring locally and directly the fuel temperature and the velocity distribution of fast ions in fusion plasmas. A mega-watt class source of millimeter or sub-millimeter waves is required to detect a weak scattered radiation superimposed on background radiation owing to electron cyclotron emissions (ECEs) from plasmas. Based on electron cyclotron resonance heating (ECRH) system with the frequencies of 77 GHz and 154 GHz in the Large Helical Device (LHD), the CTS diagnostic system has been developed to measure bulk ion temperatures from a few keV to ∼10 keV and fast ions originated from 180 keV-neutral beam injection in the LHD. The measured CTS spectra and their time evolutions are analyzed with the electrostatic scattering theory. The bulk ion temperatures obtained from CTS spectra increase with the neutral beam injections and decrease with the heating terminated. The velocity map of simulated fast ions explains that the bumps on tail of measured CTS spectra are caused by the co- and counter- fast ions. A new prescription for anisotropic velocity distribution function is proposed. As for 154 GHz bands, the CTS spectrum broadenings for D and H plasmas are distinguished reasonably at the same temperature, and its ion temperatures are comparable to those of the charge exchange recombination spectroscopy. As reactor-relevant diagnostics, a 300 GHz gyrotron and a corresponding receiver system have been implemented in LHD to access high density plasmas with low background ECEs. The recent progress for CTS diagnostics and their spectrum analysis with the probe frequencies of 77 GHz, 154 GHz, and 300 GHz in the LHD experiments is described.

Oscillation Characteristics of a High Power 300 GHz Band Pulsed Gyrotron for Use in Collective Thomson Scattering Diagnostics

Oscillation Characteristics of a High Power 300 GHz Band Pulsed Gyrotron for Use in Collective Thomson Scattering Diagnostics

New receiving line for the remote-steering antenna of the 140 GHz CTS diagnostics in the FTU Tokamak

A new receiving antenna for collecting signals of the Collective Thomson Scattering (CTS) diagnostics in FTU Tokamak has been recently installed. The squared corrugated section and the precisely defined length make it possible to receive from different directions by remotely steering the receiving mirrors. This type of Remote-Steering (RS) antennas, being studied on FTU for the DEMO Electron Cyclotron Heating (ECH) system launch, is already installed on the W7- X stellarator and will be tested in the next campaign. The transmission of the signal from the antenna in the tokamak hall to the CTS diagnostics hall will be mainly realized by means of oversized circular corrugated waveguides carrying the hybrid HE11 (quasi-gaussian) waveguide mode, with inclusion of a special smooth-waveguide section and a short run of reduced-size square-corrugated waveguide through the tokamak bio-shield. The coupling between different waveguide types is made with ellipsoidal focusing mirrors, using quasi-optical matching formulas between the gaussian-shaped beams in input and output to the waveguides. In this work, after a complete study of feasibility of the overall line, a design for the receiving line will be proposed, in order to realize an executive layout to be used as a guideline for the commissioning phase.

Development of 300 GHz Band Gyrotron for Collective Thomson Scattering Diagnostics in the Large Helical Device

Development of 300 GHz Band Gyrotron for Collective Thomson Scattering Diagnostics in the Large Helical Device

First operations with the new Collective Thomson Scattering diagnostic on the Frascati Tokamak Upgrade device

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

The upgraded Collective Thomson Scattering diagnostics of FTU

The 140GHz Collective Thomson Scattering (CTS) diagnostics installed on the Frascati Tokamak Upgrade (FTU) has been upgraded. The new system now is ready both to detect the thermal CTS radiation (for the first time with the probe frequency below the 1st harmonic electron cyclotron resonance) and to study the impact of possible parametric decay instability (PDI) processes on the received signals. The EC front-steering antenna and transmission system have been complemented with a receiving line that matches a quasi-optical line feeding the homodyne multi-channel radiometer. The scattering volume can be placed in a wide range of locations by means of fast poloidal and toroidal rotations of the two plasma-facing mirrors that have an up–down symmetry with respect to the equatorial plane of the torus. The data acquisition system has been improved adding a new digitizer, with a bandwidth of 5GHz and a maximum sampling rate of 12.5GS/s. The possibility of directly sampling and Fourier transforming the down-converted signals greatly improves the suitability of the new diagnostics to carry out thermal ion temperature measurements and to study the competing PDI processes whenever present.

High-power pulsed gyrotron for 300 GHz-band collective Thomson scattering diagnostics in the Large Helical Device

A high-power pulse gyrotron was developed to generate a probe wave for 300 GHz-band collective Thomson scattering (CTS) diagnostics in the Large Helical Device. In this frequency range, avoiding mode competition is critical to realizing high-power and stable oscillation with a narrow frequency bandwidth. A moderately over-moded cavity was investigated to ensure sufficient isolation of a desired mode from neighbouring modes, and to achieve high power output simultaneously. A cavity with the TE14,2 operation mode, a triode electron gun with an intense laminar electron beam, and an internal mode convertor were designed to construct a prototype tube. It was experimentally observed that oscillation of the TE14,2 mode was strong enough for mode competition, and provided high power with sufficient stability. The oscillation characteristics associated with the electron beam properties were compared with the numerical characteristics to find an optimum operating condition. As a result, single-mode operation with maximum output power of 246 kW was demonstrated at 294 GHz with 65 kV/14 A electron beam, yielding efficiency of ∼27%. The radiation pattern was confirmed to be highly Gaussian. The duration of the 130 kW pulse, which is presently limited by the power supply, was extended up to 30 µs. The experimental results validate our design concept and indicate the potential for realizing a gyrotron with higher power and longer pulse toward practical use in 300 GHz CTS diagnostics.

Development of Collective Thomson Scattering Diagnostic for Bulk and Fast Ions in the Large Helical Device

The collective Thomson scattering (CTS) technique has been utilized with the backscattering configuration to diagnose the bulk and fast ions in the Large Helical Device (LHD). The spectrogram of the scattered radiation presents the transient phenomena related to fast ion slowing down in auxiliary neutral beam heated plasmas. This is obtained by the broad band receiver resolving with 32 channels, while the probing beam from 77 GHz gyrotron is modulated with 50 Hz and is injected into plasmas. In the same shot, especially in low temperature plasma (Te, Ti < 1 keV), a wave excitation is also observed in a narrow frequency band. The peaks in the CTS spectrum exist between 0.5 and 1.0 GHz and are observed in both frequency sides asymmetry. For the forthcoming experiments, the feasibility of CTS diagnostics at a new port location is assessed to extend the diagnostic capability for measuring the ion velocity components. Expected spectra for scattered radiation are the same signal level as the present location from a numerical calculation.

Electron Cyclotron Resonance Heating on TEXTOR

TEXTOR is equipped with two gyrotrons at 110 and 140 GHz, respectively. Both share a single power supply and a confocal quasi-optical transmission line. They cannot be operated simultaneously. The 110-GHz gyrotron with limited power and pulse length (300 kW; 200 ms) has been used in a first series of experiments on electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) and for collective Thomson scattering (CTS) diagnostics of energetic ions. In the future the 110-GHz gyrotron will be operated exclusively for CTS diagnostics, while for ECRH and ECCD, the newly installed 140-GHz, high-power (800-kW), long-pulse (>3-s) gyrotron is now available. The highlights of first ECRH experiments with the 110-GHz gyrotron are reported. These include observations of internal transport barriers with ECRH on various target plasmas: in the current plateau phase of both ohmic and radiation improved mode (RI-mode) discharges. In addition, sawtooth control by localized ECRH is demonstrated. First results on CTS include the observation of the slowing down of energetic ions and of the redistribution of energetic ions in sawtooth crashes.