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.
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.
The lifetime of optical components unprotected from reactor grade plasmas may be very short due to contamination with carbon and beryllium-based materials eroded by plasma from beryllium walls and carbon tiles. Deposits result in a significant reduction of optical transmission. In addition, even rather thin and transparent deposits can dramatically change the shape of reflectance spectra owing to interference of reflected beams, especially for mirrors with rather low reflectivity, like W or Mo. Development of optics-cleaning and deposition-mitigating techniques is a key factor in the construction and operation of optical diagnostics in ITER. The most severe problem faces optical elements positioned in the divertor region. The latest achievements in protection of in-vessel optics are presented by example of deposition prevention/cleaning techniques for inmachine components of a Thomson scattering system in divertor. Careful consideration of well-known and novel protection approaches shows that neither of them provides guaranteed survivability of the first in-vessel optics in divertor. Only a set of mutually complementing prevention/cleaning techniques, that include special materials for mirrors and inhibition additives for plasma, is able to manage the challenging task. The essential issue, which needs to be addressed in the nearest future, is an extensive development of introduced techniques under experimental conditions (exposure time and contamination fluxes) similar to those expected in ITER.
Nucl. Fusion 59 (2019) 066029 (10pp) baffled cassette with mirrors was exposed at the main wall of JET for 23,6 plasma hours. No significant degradation of reflectivity was measured on mirrors located in the ducts.Predictive modeling was further advanced. A model for the particle transport, deposition and erosion at the port-plug was used in selecting an optical layout of several ITER diagnostics. These achievements contributed to the focusing of the first mirror research thus accelerating the diagnostic development. Modeling requires more efforts. Remaining crucial issues will be in a focus of the future work of the FM SWG.
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.
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