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. The system performance is assessed for an updated geometry (with respect to previous publication), showing that, with a scattering angle θscat = 167°, the expected signal is strongly reduced. Potential alternatives are analyzed: (1) a system employing a different laser injection position, allowing for a more favorable scattering angle and (2) a recently proposed dual-polarization laser pulse technique. The latter is evaluated for the possible ITER geometry, again showing that a more favorable scattering angle is needed for a robust performance.

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. The collection optics of the edge TS system have a critical role in meeting the required field of view and spatial resolution despite the limited space. This work presents a comprehensive optomechanical design of the optics assembly, whose main features are telecentricity and compactness, highlighting its capabilities. The tests undertaken to verify its performance: focal plane identification, thermal cycle, and magnification, are described.

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 (Te). At high Te, 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. This paper builds upon these initial measurements with the data obtained during 2021. These new measurements improve upon first results, in particular, by obtaining spectral measurements of the depolarized spectrum. The recent JET campaign was well suited to these measurements with long and hot plasmas. The resulting data are averaged over many plasmas and laser pulses to obtain a measurement of the amount of "p" and "s" scattered light as a function of Te. This experimentally obtained d(p/s)/dTe is then fitted and found to show reasonable agreement with the theoretically predicted depolarized fraction. Error estimates on the measured "p/s" have been obtained and show that the measurements are meaningful. This is good news for ITER for which the intention is to use this measurement as a check on Te determined by the core plasma Thomson scattering diagnostic by using conventional spectral measurement techniques.

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. This presents a unique challenge due to the small difference between the two laser wavelengths. In this paper, we describe two methods for beam combination: first via a prism, eventually dismissed as unpractical, and second via a polarizing beam splitter in reverse with a dual-wavelength waveplate. We detail the optical setup, waveplate design, and successful implementation of this second method.

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. The latter method correctly accounts for crosstalk between Faraday rotation and the Cotton-Mouton effect. The impact of the plasma birefringence on the interferometric phase shift is also studied, and it is found that a perturbation of the interferometric phase shift is present also in the case of an initial fixed linear polarization of the probe laser beam.

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 ne = 7.5 × 1019 m-3, which is the expected value in the plasma core. This paper presents the designs of collection optics, fibers with their alignment system, and polychromators. The collection optics overcomes unique issues for superconducting fusion devices, i.e., limited design space, high-temperature measurements, and harsh radiation condition. When in several years the more performing plasma will generate intense nuclear radiation, the lens materials of the optics can be replaced by radiation resistant glasses without major changes in the lens holder. It will prevent transmission degradation and keep stable measurement accuracy.

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. A novel photoneutralization concept is discussed here, based on an annular cavity and a duplicated frequency laser beam (recirculation injection by nonlinear gating). The choice of lithium triborate as the material for the second harmonic extractor is discussed and a possible cooling method via crystal slicing is presented; laser intensity enhancement within the cavity is evaluated in order to quantify the achievable neutralization rate. Mockups of the critical components are proposed as intermediate steps toward system realization.

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. The analysis has shown that, in spite of the small difference in the spectra obtained at the two angles, reliable values of the relative calibration coefficients can be determined by the analysis of good S/N dual­angle spectra recorded in a few tens of plasma shots. This analysis suggests that in RFX-MOD the calibration of the entire set of TS polychromators by means of the similar, dual-laser (Nd:YAG/Nd:YLF) TS technique, should be feasible.

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. We first investigate the energy density threshold necessary to ablate the deposited impurity substrate on removed dirty windows: above threshold, a single laser pulse recovers ∼95% of the window transmission before its exposure to the plasma, while below it the efficiency of the cleaning process is too poor. The system so conceived was then used to clean the three collection windows of the Main Thomson scattering diagnostic on RFX-mod. We also present results obtained applying the same technique to the SiO-protected Al mirror used for the Z(eff) diagnostic: an energy threshold for efficient impurity removal without mirror damage is first identified, then ablation tests are executed and analyzed in terms of recovered reflectivity. The SIMS technique is used both with windows and mirror to study the composition of surfaces before and after the ablation.

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. The results show that the difference between the line integrated electron density from interferometry and polarimetry is with one fringe (1.143 x 10(19) m(-2)) for more than 90% of the cases. It is possible to consider polarimetry as a satisfactory alternative method to interferometry to measure the electron density and it could be used to recover interferometric signal when a fringe jumps occurs, preventing difficulties for the real-time control of many experiments at the JET machine.

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). This upgrade has enabled the spatial resolution to be reduced to approximately 6.3 cm along the laser line of sight for a laser pulse of 300 ps full width at half maximum, which is close to the requirements for the ITER core LIDAR. Performance analysis shows that the new system will have an effective spatial resolution of up to 1 cm in the magnetic midplane via magnetic flux surface mapping.

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. A crucial design decision lies on the choice of laser and detector combination. It is evaluated that the system can meet its spatial and accuracy specifications for higher temperatures of T(e)>5 keV with a combination of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (lambda(0)=1064 nm, Delta lambda<1 nm, 5 J, and Delta t(FWHM)=250 ps, 5-10 Hz) and S20, GaAs, and GaAsP microchannel plate photomultipliers (Delta t(FWHM)<300 ps, effective quantum efficiency, EQE=3%-4%, and D=18 mm). In order to reach the required T(e) of 500 eV with Nd:YAG first harmonic, this choice requires a development of fast near infrared detectors.

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. Effectiveness of cleaning is analyzed in terms of quality of the cleaned spot, its dimension, repetition rate of the laser, and its wavelength. The energy damage threshold of the fiber optic is also investigated. Three different lasers (microjoule Nd:YAG, Nd:YLF, and ruby) are first tested directly on the window; then only the ruby laser beam is propagated through an optical fiber and tested.

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 with4.5% accuracy. The relative calibration (for T(e) measurements) has also been performed by Raman scattering, observing the same line in all the spectral channels. The results agree to within congruent with10% with those obtained by a tungsten lamp and a lock-in detection technique. From a comparison of the results of the two methods, systematic errors on the measured T(e) and n(e) of less than ~10% (for T(e) up to 200 eV) are inferred. The absolute cross section of the rotational Raman transitions at Deltaupsilon = -587 cm(-1) (lambda = 723.8 nm), Deltanu = -354 cm(-1) (lambda = 711.8 nm), and Deltanu = 354 cm(-1) (lambda = 677.6 nm) has also been measured, by comparison with the intensity of the Rayleigh line. Significant deviations from thermal equilibrium population in Raman levels, expected from the possible occurrence of stimulated Raman emission, are not observed. From the measured data the anisotropic polarizability of H(2) at lambda = 694.3 nm was estimated to be gamma = 2.93 x 10(-25) cm(3) +/- 3%. The absolute cross section of the strongest rotational lines of H(2) is also given.

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. Stimulated Raman Stokes light has been observed at filling pressures above 230 and 500 mbars for H(2) and D(2), respectively, for input laser pulses of 8 J and 30 ns (FWHM) duration. Evidence has been found that the stimulated Raman light does not originate from the observed scattering volume but is detected as light diffused into the vacuum chamber. To explain these results, the Raman gain and the intensity of the stimulated Raman light are calculated, taking into account the multimode structure of the laser beam. We find that significant power conversion from the input laser beam to the Stokes wave takes place near the output window of the vacuum chamber. Part of this radiation is diffused back into the machine, and this part is detected as superimposed on the spontaneous Raman signal. Finally we discuss the Raman calibrations in RFX, a larger plasma device in which the Raman medium will be N(2) at a temperature up to 350°C, and show that a filling pressure of 100 mbar gives a sufficient calibration signal, avoiding any nonlinear effect.