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.

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Thomson scattering (TS) is a proven diagnostic technique that will be implemented in ITER in three independent systems. The Edge TS will measure electron temperature T e and electron density n e profiles at high resolution in the region with r/a>0.8 (with a the minor radius). The Core TS will cover the region r/a<0.85 and shall be able to measure electron temperatures up to 40 keV. The Divertor TS will observe a segment of the divertor plasma more than 700 mm long and is designed to detect T e as low as 0.3 eV.The Edge and Core systems are primary contributors to T e and n e profiles. Both are installed in equatorial port 10 and very close together with the toroidal distance between the two laser beams of less than 600 mm at the first wall (∼ 6 • toroidal separation), a characteristic that should allow to reliably match the two profiles in the region 0.8 < r/a < 0.85.Today almost every existing fusion machine has one or more TS systems installed, therefore substantial experience has been accumulated worldwide on practical methods for the optimization of the technique. However the ITER environment is imposing specific loads (e.g. gamma and neutron radiation, temperatures, disruption-induced stresses) and also access and reliability constraints that require new designs for many of the sub-systems.The challenges and the proposed solutions for all three TS systems are presented. K : Plasma diagnostics -interferometry, spectroscopy and imaging; Plasma diagnosticscharged-particle spectroscopy 1Corresponding author.

This paper describes the challenges of Thomson Scattering implementation in the ITER divertor and evaluates the capability to satisfy project requirements related to the range of the measured electron temperature and density. A number of aspects of data interpretation are also discussed. Although this assessment and the proposed solutions are considered in terms of ITER compatibility, they may also be of some use in currently operating magnetic confinement devices.

i) The problem: to measure T e as high as 40 keV using Thomson Scattering in the reactor core both for Maxwellian and non-Maxwellian case of electron velocity distribution function especially in the case of unknown system spectral responsivity. (ii) The suggested solutions:to use IR probing laser 1320 nm additionally to convenient NIR laser 1064 nm to improve measurement accuracy for T e~ 40keV;to use specific algorithm for TS data processing in case of non-Maxwellian eVDF; to use multi-laser approach, that suggests plasma probing with 3 lasers -946 nm/1064 nm/1320 nm simultaneously in the case of unknown system spectral sensitivity.(iii) Next steps -test multi-laser approach and designed data procession technique in real experiment on existing fusion device.

Almost all optical diagnostic systems in ITER will require the implementation of mirror recovery and protection systems. Plasma cleaning is considered to be the most promising technique for the removal of metal deposits from optical surfaces. The engineering and physical aspects of RF discharge application for continuous or periodic plasma treatment are discussed with a focus on implementation under ITER conditions. The ion flux parameters obtained in capacitively coupled (CC) RF discharge were measured in the mock-up of a plasma cleaning system. The uniformity of sputtering in CC RF discharge with and without a magnetic field was studied experimentally for the cylindrical discharge reactor geometry and compared with numerical simulations. The sharp increase in the sputtering rate resulting from the non-uniform radial distribution of the ion flux was observed near the electrode edges. The longitudinal magnetic field improves sputtering uniformity. It was demonstrated that Al/Al 2 O 3 deposits can be removed in the Ne and D 2 plasma of CC RF discharge but longterm exposition results in the degradation of the polycrystalline molybdenum mirror surface. The efficiency of Al sputtering in the atmosphere containing O 2 and N 2 fractions was studied in the D 2 /O 2 and D 2 /N 2 plasma of glow discharge. The addition of 2% of oxygen or nitrogen increases the sputtering yield by 3-4 times as compared with that in a nominally pure D 2 discharge. The impact of metal deposits on the performance of diagnostic mirrors is discussed. It was shown that an ultrathin metallic film with a thickness as low as a few nm may cause a significant degradation of diagnostic mirrors with a transparent coating.

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