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We developed high energy Q-switched 1064 nm and 946 nm Nd:YAG lasers for ITER Divertor Thomson Scattering (DTS). The 1064 nm Nd:YAG laser is designed to be used as the main DTS probing laser and features output energy of 2 J at 3 ns pulse duration. MOPA configuration with phase conjugation mirror provides divergence of 1.3 times the diffraction limit and high stability of the output beam parameters with repetition rate varied from 1 to 50 Hz. The 946 nm Nd:YAG laser designed for calibration by an auxiliary Thomson spectrum at the neighboring wavelength. The 946 nm laser consists of MO based on cavity dumping, high gain regenerative preamplifier and zero diffraction length ring cavity regenerative power amplifier with image-rotation providing 1 J × 50 Hz at 3 ns pulse duration with divergence of 1.5 the diffraction limit. As far as we know this is the highest value of the output energy obtained at 946 nm with nanosecond pulse duration until now. Both lasers use unified pump modules providing high maintainability of the system. The paper describes laser design approaches, state of the art and outlook of further developments.

Laser-induced fluorescence (LIF) on ITER will be used for local measurement of helium density nHe and ion temperature Ti in divertor. The LIF diagnostic is combined with the divertor Thomson scattering (DTS) via common laser injection and signal collection optics. Collisional-radiative model developed for helium ions is used to estimate laser parameters required for robust measurement of laser-induced signals against plasma background radiation. The estimations demonstrate feasibility of measuring nHeII with laser parameters provided by available optical parametric oscillator pumped by Nd:YAG. New approach for measuring Ti is proposed and the expected errors are estimated using numerical simulations.

Laser-induced fluorescence diagnostics on ITER will be used for local measurements of helium density (nHeI) and ion temperature (Ti) in the divertor area. The diagnostics is combined with divertor Thomson scattering via common laser injection and signal collection optics. Multichannel filter polychromators with design similar to those developed for Thomson scattering will be used to perform spectral analysis of the fluorescent signals. Both the fluorescent signals and the background radiation are estimated using the collisional-radiative models developed for He I and He II. The estimations demonstrate the feasibility of measuring nHeI with the temporal resolution 20 ms and accuracy ΔnHeI

The ITER divertor Thomson scattering (TS) system has been significantly upgraded in the last few years in terms of equipment and experimental setup design. In this paper, we are focusing on lasers, digital filter polychromator and piezo actuators, which could be also useful for other diagnostics. Capabilities of the probing and calibration lasers are specified and compared against ITER requirements. Key features of the digital polychromator design for TS applications are presented and obtained signal-to-noise ratios are discussed. The main reason to use piezo actuators for in-vessel remotely controlled devices is high radiation and magnetic resistance of piezo elements. Piezo actuators of inertial and ultrasonic types were produced and their promising performance for fusion devices was demonstrated.

This note is devoted to a filter polychromator combining built-in optical, data acquisition and processing subsystems designed for electron temperature and density measurements in fusion plasma by Thomson scattering (TS). Special attention is paid to assessing the accuracy of TS signals measured in the polychromator spectral channels. The potential for estimating TS error caused by background emission is demonstrated using high-pass signal. The recent polychromator tests have shown APD gain stabilization better than ± 0.5% over the ambient temperature range from 17.5 °C to 35 °C

The paper describes the digital filter polychromator developed for Thomson scattering (TS) diagnostics. The device having ultra-low noise, high-speed detection system and up to 7 spectral channels is equipped with digitizer and single board computer for system control and data processing. The data acquisition system is capable to detect very short laser pulses of few ns width with low magnitude. This feature opens up new possibilities for the TS diagnostics design in modern fusion devices, which includes: operation with high background illumination, measurement of very low electron temperature and density at the plasma edge and scrape-off layer (SOL), separation of the TS signal and the stray light coming with a delay of several ns. The analysis of the TS signal waveform also gives a possibility to distinguish the TS and Mie scattering from the dust particles that is important for measurements in SOL. Owing to compactness and ultra-low energy consumption both optical and data processing units interfaced with optical Gigabit Ethernet were combined in a stand-alone device. Such all-in-one design with optical fiber input and optical digital output provides enhanced resistance to electromagnetic disturbances and simplify operation. A set of such polychromators can be mounted in a standard 19” rack and can be integrated with outer CODAC (Control, Data Access and Communication) system. Testing of the polychromators at the Globus-M2 tokamak has shown excellent performance of the developed serial devices.

Erosion and re-deposition of plasma-facing components in tokamaks require the development of in-vessel optics protection and cleaning techniques. Front-end diagnostic optics in large-scale fusion m...

The metal mirrors for deflecting laser radiation in the divertor Thomson scattering system of ITER are found to be inapplicable due to high density of laser radiation on a mirror. This is caused by the short distance from the laser focus to the mirror surface. We report on investigations of the possibility to use dielectric mirrors and protective sapphire plates in a rotary unit. We study the laser damage thresholds of optical elements, which are supposed to be used in the laser input channel. These optical elements are the dielectric mirrors deposited on different substrates (quartz, sapphire, and single-crystal molybdenum) and the protecting sapphire plate. A number of sapphire samples were irradiated by a neutron flux up to 1019 n/cm2 (E > 100 keV) and annealed. Laser damage thresholds of the elements were measured at room temperature and at the ITER operating temperature of 150°C.A YAG:Nd laser operating with 10-Hz repetition rate was used in the experiments. The laser pulse parameters ...

ITER design has highlighted the fundamental need to monitor and deepen understanding of divertor operations. The designed Thomson scattering (TS) for the divertor leg and SOL near the X-point, has gained a lot from present State-of-the-Art in the diagnostics. However, hostile environment, very restricted access, and required spatial resolution, all impose severe constraints on the diagnostic design for ITER. Signal transmission from diagnostics introduces the further considerations of relative movements of the machine components. Despite the constraints it is feasible using of crossed probe and viewing beams to make measurements of the electron temperature and density profiles in outer divertor leg at the extent from the strike-point to X-point vicinity, and the LIDAR TS technique to make the electron temperature and density scan along the major radial line-of-sight at ∼0.5 m above the X-point. The interfaces for each of the subsystems are presented and discussed in the sections below.

A wide bandpass double-grating polychromator with high rejection and high transmission has been designed and manufactured for laser-aided plasma diagnostics. The special mount utilizes subtractive dispersion in the second stage of the double polychromator such that the larger dispersion of the second stage is reduced by that of the first stage. This affects the intensity of the stray light background at the laser wavelength. The background at the edge of the laser line was measured at 10−5 of the light incident on the input slit. At the short end of the 200 nm bandpass, the stray light relative intensity approached 10−7.

Fast degradation of in-vessel optics is one of the most serious problems for all optical diagnostics in ITER. To provide the resistance to mechanical and thermal stresses along with a high stability of optical characteristics under deposition-dominated conditions we suggest using high-reflective metallic (Ag or Al) film mirrors coated on silicon substrate and protected with thin oxide film in the divertor Thomson Scattering (TS) diagnostics. The mirrors coated with Al2O3 and ZrO2 films were tested under irradiation by deuterium ions. The experimental results on the oxide films sputtering are discussed in the context of their applicability for the first mirror protection in ITER.

The structure and surface morphology of aluminum and silver mirrors covered with protective dielectric oxide layer were studied by means of TEM and SEM. The presence of needle-like pores throughout the thickness of the ZrO(2) film and bubble-like pores in Al(2)O(3) was observed. The test for resistivity to deuterium ion bombardment shows that the exposition to a fluence of similar to 2 x 10(20) ions/cm(2) with the ion energy of 40-50 eV results in appearance of blisters on the surface of mirrors covered wit h Al(2)O(3). For the mirrors protected with ZrO(2) no noticeable changes in surface morphology and reflectivity were found even after order of magnitude higher ion fluence. The effect of different porous structures on blistering phenomena is discussed. (C) 2010 Elsevier B.V. All rights reserved.

A new kind of large dispersion lower order blaze grating spectrometer is built. The design is based on the use of uncustomary large angles of incidence and diffraction approaching 80°. The double diffraction optical scheme is employed to fit the design conception. A set of compact size grating spectrometers of various focal lengths and grating ruled widths has been manufactured and explored. The measured grating efficiency related to different blaze angles shows the evidence for the instrument gainful operation. The spectrometer imaging properties are much promising to enable a fine spectral resolution to 0.005 nm under a short focal length ∼100 mm. The estimation of flux luminosity is made in comparison with Fabry–Perot interferometer.

The electron component is the main channel for anomalous power loss and the main indicator of transient processes in the tokamak plasma. The electron temperature and density profiles mainly determine the operational mode of the machine. This imposes demanding requirements on the precision and on the spatial and temporal resolution of the Thomson scattering (TS) measurements. Measurements of such high electron temperature with good accuracy in a large fusion device such as ITER using TS encounter a number of physical problems. The 40 keV TS spectrum has a significant blue shift. Due to the transmission functions of the fibres and to their darkening that can occur under a strong neutron irradiation, the operational wavelength range is bounded on the blue side. For example, high temperature measurements become impossible with the 1064 nm probing wavelength since the TS signal within the boundaries of the operational window weakly depends on Te. The second problem is connected with the TS calibration. The TS system for a large fusion machine like ITER will have a set of optical components inaccessible for maintenance, and their spectral characteristics may change with time. Since the present concept of the TS system for ITER relies on the classical approach to measuring the shape of the scattered spectra using wide spectral channels, the diagnostic will be very sensitive to the changes in the optical transmission. The third complication is connected with the deviation of the electron velocity distribution function from a Maxwellian that can happen under a strong ECRH/ECCD, and it may additionally hamper the measurements. This paper analyses the advantages of a 'multi-laser approach' implementation for the current design of the core TS system. Such an approach assumes simultaneous plasma probing with different wavelengths that allows the measurement accuracy to be improved significantly and to perform the spectral calibration of the TS system. Comparative analysis of the conservative and advanced approaches is given.

A technique based on laser-induced breakdown spectroscopy (LIBS) is proposed for detecting in-situ dust on the plasma-exposed surfaces and in the grooves of plasmafacing components in the next-generation of fusion devices (e.g., ITER-FEAT). It is based on laser-induced ablation of wall material and spectral analysis of the laser spark flash-light collected by imaging optics and transmitted to the detection system. This could give space- and time-resolved information on the presence of dust, or loosely bound films, their characteristic deposition patterns, elemental composition, and possibly their hydrogen content, without the necessity of breaking the machine vacuum. We have performed some simple proof-of-principle experiments to demonstrate the suitability of this technique, which might provide an effective non-intrusive in-situ surface analysis method for surveying in-vessel dust accumulation in future fusion devices.

A divertor Thomson scattering (TS) system being developed for ITER has incorporated proven solutions from currently available TS systems. On the other hand any ITER diagnostic has to operate in a hostile environment and very restricted access geometry. Therefore the operation in an environment of intensive stray light, plasma background radiation, the necessity meet the requirement using only a 20 mm gap between divertor cassettes for plasma diagnosis as well as to measure plasma temperatures as low as 1 eV severely constrain the divertor TS diagnostic design. The challenging solutions of this novel diagnostic system which has to ensure its steady performance and also the operability and maintenance are the focus of this report. One of the most demanding parts of the in-vessel diagnostic equipment development is the design assessment using different engineering analyses. The task definition and first results of thermal, e/m and seismic analyses are provided. The process of further improving of the design involves identification of susceptible areas and multiple iterations of the design, as needed. One of the key points for all Thomson scattering diagnostics are the laser capabilities. A high-performance and high-power laser system using a steady-state and high-repetitive mode Nd:YAG laser (2J, 50–100Hz, 3ns) has been developed. The reduced laser pulse duration matched with high-speed low-noise APD detector can be very important under high background light level. For diagnostics such as Thomson scattering and Raman spectroscopy, a high-degree of discrimination against stray light at the laser wavelength is required for successful detection of wavelength-shifted light from the laser-plasma interaction region. For this case of high stray light level, a triple grating polychromator characterized by high rejection and high transmission has been designed and developed. The novel polychromator design minimizes stray light while still maintaining a relatively high transmission.

We present here our recent results on the development and testing of the first mirrors for the divertor Thomson scattering diagnostics in ITER. The Thomson scattering system is based on several large-scale (tens of centimetres) mirrors that will be located in an area with extremely high (3–10%) concentration of contaminants (mainly hydrocarbons) and our main concern is to prevent deposition-induced loss of mirror reflectivity in the spectral range 1000–1064 nm. The suggested design of the mirrors—a high-reflective metal layer on a Si substrate with an oxide coating—combines highly stable optical characteristics under deposition-dominated conditions with excellent mechanical properties. For the mirror layer materials we consider Ag and Al allowing the possibility of sharing the Thomson scattering mirror collecting system with a laser-induced fluorescence system operating in the visible range. Neutron tests of the mirrors of this design are presented along with numerical simulation of radiation damage and transmutation of mirror materials. To provide active protection of the large-scale mirrors we use a number of deposition-mitigating techniques simultaneously. Two main techniques among them, plasma treatment and blowing-out, are considered in detail. The plasma conditions appropriate for mirror cleaning are determined from experiments using plasma-induced erosion/deposition in a CH4/H2 gas mixture. We also report data on the numerical simulation of plasma parameters of a capacitively-coupled discharge calculated using a commercial CFD-ACE code. A comparison of these data with the results for mirror testing under deuterium ion bombardment illustrates the possibility of using the capacitively-coupled discharge for in situ non-destructive deposition mitigation/cleaning.

A multipass/multipulse Thomson scattering system has been implemented on CDX-U in collaboration with the Ioffe Institute, St. Petersburg. The system consists of a low energy (1.5– 2.5 J), passively Q-switched ruby laser, and a multipass optical cavity enclosing the plasma. Multiple reflections of the beam within the cavity increase by about an order of magnitude the number of scattered photons, allowing temperature density to be measured with good accuracy even at very low plasma density. By feeding the returned beam back into the laser, the system can deliver several pulses over a 1 ms period. However, the experiments on CDX-U show that a mechanical shock wave reaching the multipass system affects the feedback and laser output per pulse drops significantly. Therefore we operate the system by fine tuning the laser cavity, so that output is practically independent of feedback from multipass cavity. Also, by optimizing the transmission of the passive Q-switch and the pumping power, we obtain that laser outp...