Collection optics of JT-60SA edge Thomson scattering diagnostic.

The mission of the JT-60SA project is to complement ITER's capabilities by addressing the fundamental physics and engineering challenges necessary to develop a practical and reliable fusion power plant. Diagnostics play a pivotal role in achieving this mission, especially the Thomson Scattering (TS) diagnostic systems developed by a collaborative Japan-EU team. The edge Thomson scattering of JT-60SA is tailored to measure the low field side outer region of the plasma, in particular, to resolve the electron temperature Te and density ne.

Progress and challenges in the design of ITER's polarimetric Thomson scattering diagnostic system.

Polarimetric Thomson scattering (PTS) is a technique that allows for accurate measurements of electron temperature (Te) in very hot plasmas (Te > 10 keV, a condition expected to be regularly achieved in ITER). Under such conditions, the spectral region spanned by the TS spectrum is large and extends to low wavelengths, where the transmission of the collection optics decreases, available detectors are less efficient, and the high level of plasma background light perturbs the measurements. This work presents the recent developments in the design of a PTS system for ITER, along with the challenges posed by the complex machine design.

Polarimetric Thomson scattering measurements in Joint European Torus high temperature plasmas.

Thomson scattered light is polarized in the same orientation as the incident laser beam at low electron temperatures (T). At high T, part of the spectrum begins to become randomly polarized due to relativistic reasons. First measurements of the depolarized Thomson scattering spectrum were obtained from Joint European Torus (JET) pulses in 2016.

On combining the beam path of similar wavelength lasers for dual-laser Thomson scattering.

A novel method for the path combination of two lasers with very similar wavelengths has been implemented to enable the evaluation of a dual-laser calibration Thomson scattering (TS) technique. The first experimental test of this TS technique has been performed in a RFX-mod plasma device, where, due to experimental constraints, the combination of a Nd:YAG (λ = 1064 nm) and a Nd:YLF (λ = 1053 nm) laser system was the only viable choice available. The method requires that the beam path of both lasers be combined into a single path with the same polarization.

Design of JT-60SA core Thomson scattering diagnostic system.

An incoherent Thomson scattering diagnostic will be installed in the JT-60SA tokamak to measure electron temperature and electron density profiles. The target radial spatial resolution is 25 mm with 46 spatial channels. The accuracy in electron temperature and density is a few percent at n = 7.

Characterization of signals for a Divertor Tokamak Test facility interferometer/polarimeter system.

In magnetically confined fusion experiments, laser interferometer/polarimeter systems allow one to determine plasma density, give valuable information on the internal magnetic fields, and contribute to the evaluation of the plasma magnetic equilibrium and to the real-time estimation of the q profile to allow feedback configuration control. This work presents an analysis of the interferometric and polarimetric signals of a multi-chord far-infrared interferometer/polarimeter for the divertor tokamak test facility, the new tokamak device currently under construction in Italy. The polarimetric signals are calculated both with approximate formulas and by solving the equation describing the evolution of the laser beam polarization inside the plasma using the Mueller formalism.

A feasibility study of a NBI photoneutralizer based on nonlinear gating laser recirculation.

The neutralization efficiency of negative ion neutral beam injectors is a major issue for future fusion reactors. Photon neutralization might be a valid alternative to present gas neutralizers, but still with several challenges for a valid implementation. Some concepts have been presented so far but none has been validated yet.

Dual-angle, self-calibrating Thomson scattering measurements in RFX-MOD.

In the multipoint Thomson scattering (TS) system of the RFX-MOD experiment the signals from a few spatial positions can be observed simultaneously under two different scattering angles. In addition the detection system uses optical multiplexing by signal delays in fiber optic cables of different length so that the two sets of TS signals can be observed by the same polychromator. Owing to the dependence of the TS spectrum on the scattering angle, it was then possible to implement self-calibrating TS measurements in which the electron temperature Te, the electron density ne and the relative calibration coefficients of spectral channels sensitivity Ci were simultaneously determined by a suitable analysis of the two sets of TS data collected at the two angles.

Impurities removal by laser blow-off from in-vacuum optical surfaces on RFX-mod experiment.

An in situ window cleaning system by laser blow-off through optical fiber has been developed on the basis of a feasibility study previously presented. The beam generated by a Q-switched Nd:YAG laser is launched in a vacuum box into a high damage threshold optical fiber through a lens. The fiber output is focused on the impurities-coated surface of a vacuum window exposed to the plasma of the RFX-mod experiment, and it is remotely controlled with an xy motion system to scan the entire surface.

Systematic comparison between line integrated densities measured with interferometry and polarimetry at JET.

A systematic comparison between the line integrated electron density derived from interferometry and polarimetry at JET has been carried out. For the first time the reliability of the measurements of the Cotton-Mouton effect has been analyzed for a wide range of main plasma parameters and the possibility to evaluate the electron density directly from polarimetric data has been studied. The purpose of this work is to recover the interferometric data with the density derived from the measured Cotton-Mouton effect, when the fringe jump phenomena occur.

Enhancement of the JET edge LIDAR Thomson scattering diagnostic with ultrafast detectors.

The edge light detection and ranging (LIDAR) Thomson scattering diagnostic at the Joint European Torus fusion experiment uses a 3 J ruby laser to measure the electron density and temperature profile at the plasma edge. The original system used a 1 GHz digitizer and detectors with response times of approximately 650 ps and effective quantum efficiencies <7%. This system has recently been enhanced with the installation of a new 8 GHz digitizer and four new ultrafast GaAsP microchannel plate photomultiplier tube detectors with response times of <300 ps and effective quantum efficiencies in the range of approximately 13%-20% (averaged over lambda=500-700 nm).

In situ window cleaning by laser blowoff through optical fiber.

The feasibility of a window cleaning system based on the laser blowoff technique is investigated to remove the impurity deposition on vacuum windows of the modified reversed field experiment fusion device. The laser pulse is sent to the window through a fused silica fiber optic (phi=1 mm), then focused on its internal surface, single shot ablating up to approximately 5 mm(2) of the impurity layer; the focused pulse is scanned across the window to clean its entire surface. The composition of the deposited layer is studied through the secondary ion mass spectrometry and profilometry techniques.

ITER LIDAR performance analysis.

The core LIDAR Thomson scattering for ITER is specified for core profile measurements with a spatial resolution of 7 cm (a/30) for the range of 500 eV3x10(19) m(-3) at an accuracy of <10% for T(e). These specifications are verified using a full profile Monte Carlo simulation code. In the simulations it is assumed that the input transmission is 50% and the collection transmission is 10% for lambda=300-1200 nm and F/#=6-17.

Data analysis for a rotating quarter-wave, far-infrared Stokes polarimeter.

Data analysis techniques are reviewed and extended for the measurement of the Stokes vector of partially or completely polarized radiation by the rotating quarter-wave method. It is shown that the conventional technique, based on the Fourier analysis of the recorded signal, can be efficiently replaced by a weighted least-squares best fit, so that the different accuracy of the measured data can be taken into account to calculate the measurement errors of the Stokes vector elements. Measurement errors for the polarization index P and for the azimuth and ellipticity angles psi and chi of the radiation are also calculated by propagation error theory.

Nonlinear optical effects in Raman calibrations of a Thomson scattering system.

We present an investigation of the occurrence of stimulated Raman scattering and other nonlinear optical effects during Raman calibrations of Thomson scattering diagnostic systems for magnetic fusion plasmas. When these effects take place, the intensity of the Raman lines is unpredictable, and the calibrations are impossible. In this research Raman scattering from H(2) and D(2) at filling pressures up to 1 atm has been experimentally investigated using the Thomson scattering system of the ETA-BETA II reversed field pinch device.

Complete calibration of a Thomson scattering spectrometer system by rotational Raman scattering in H(2).

We describe the complete calibration of a ruby laser-based multipoint Thomson scattering system, performed only by rotational Raman scattering from H(2) and the measurement of the absolute Raman cross section of some rotational transitions suitable for such calibrations. The absolute calibration factors (for n(e) measurements by Thomson scattering) have been measured by observing the Stokes J = 1 ? 3 rotational transition at Deltanu = -587 cm(-1) (lambda = 723.8 nm), with a congruent with4.