K.Yu. Vukolov

Progress in research and development of mirrors for ITER diagnostics

Metallic mirrors will be used as plasma-viewing elements in all optical and laser diagnostic systems in ITER. In the harsh environment of ITER, the performance of mirrors will decrease mainly because of the erosion of their surfaces and deposition of impurities. The deterioration of the optical properties of diagnostic mirrors will directly affect the entire performance of the respective ITER diagnostics, possibly leading to their shutdown. Therefore, R&D on mirrors is of crucial importance for ITER diagnostics. There is a coordinated worldwide R&D programme supervised by the Specialists Working Group on first mirrors of the International Tokamak Physics Activity, Topical Group on Diagnostics. This paper provides an overview of new results in the field of first mirrors, covering the manufacturing of ITER mirror prototypes, investigations of mitigation of deposition and mirror cleaning and the predictive modelling of the mirror performance in ITER. The current status of research on beryllium deposition—a new critical area of mirror research—is given along with an outlook for future activities.

First mirrors in ITER: material choice and deposition prevention/cleaning techniques

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.

Results of irradiation tests of KU-1 and KS-4V silica glasses as ITER candidate window materials

Abstract Previous irradiation tests have shown that KU-1 silica glass may be used for ITER diagnostic windows in the spectral region of 0.35–2.5 μm. To help clarify the possibility of increasing this spectral range to the UV region irradiation tests of KS-4V high purity silica in Co60 gamma source and nuclear reactor were undertaken. KS-4V has an order of magnitude lower induced absorption in UV range compared with KU-1 after gamma irradiation up to 3 MGy, but there is no marked difference in transparency of KU-1 and KS-4V silica glasses after reactor irradiation up to F>0.1≅3×1016 n/cm2 (Dγ≅1 MGy) although induced absorption of KS-4V is slightly lower. In situ tests of transparency degradation under neutron and gamma irradiation of KU-1 and KS-4V silica glasses in UV range will be necessary for final clarification and ITER database preparing.

First mirrors for diagnostic systems of an experimental fusion reactor I. Simulation mirror tests under neutron and ion bombardment

Among the diagnostic systems planned for use in the International Thermonuclear Experimental Reactor to control the reactor operation, a large number of these systems have to use the mirrors to input or output the electromagnetic radiation to or from the burning plasma in different parts of the spectrum. The mirrors placed inside the vacuum vessel will be subjected to the impact of several factors, resulting in degradation in their optical characteristics. The most critical factors are erosion under the bombardment with a flux of high-energy particles, deuterons and tritons, and the deposition of the products of erosion of the in-vessel components. The first part of this review presents the results of the simulation experiments studying the effect of sputtering and deposition of contaminants on the optical properties of mirrors fabricated from different materials. In the second part of the review, the results of mirror testing on the operating large-scale fusion devices are considered.

Exposure of stainless steel mirrors in T-10 tokamak

Stainless steel (SS) mirrors were exposed in the upper port of T-10 in front of the graphite limiter during the experimental campaign. Three mirrors were located at different distances from the plasma (22 cm, 45 cm, 66 cm), and three more were placed at 22 cm from the plasma inside a box that was opened at working discharges only. Deposits were found on all the mirrors as a result of the exposure. The deposited films not only reduced the intensity of the reflected radiation, but also strongly changed its spectra. The deposits had a layered structure and consisted of complex CnDm compounds with a D/C ratio of about 0.3. The film thickness was about 12 µm on the mirror located without protection 22 cm from the plasma – that is, about 100 times larger than that on the samples placed inside the box. The deposition rate for the protected mirrors was about 0.2 nm/s. The ablation of deposits under pulsed excimer laser radiation was demonstrated by a complete recovery of the initial reflectance of the mirrors.

Diagnostic First Mirrors for Burning Plasma Experiments

The lifetime of refractive components exposed to reactor grade plasmas will be very short and so all diagnostics which use UV (λ > 5nm), Visible and IR radiation (λ up to ∼100μm) have to view the plasma via a mirror. The diagnostic first mirrors (FM) must survive in extremely hostile conditions and maintain a good optical performance for the duration of reactor operation. In ITER-FEAT the FMs will receive intense UV and X-ray radiation, neutron and gamma fluxes, and particle fluxes (due to charge exchange atoms (CXA)). In addition, they will be subjected to the deposition of material eroded from the divertor and first wall. Of the different kinds of radiation and fluxes only CXA impact will result in direct surface modification of metallic FMs that can lead to degradation of optical properties. The fluxes of all radiation components to the mirror surface depend strongly on the mirror location. For example, the FMs of a wide-angle observation system (endoscope with open architecture) in ITER will be bombarded by CXA fluxes of about the same magnitude as the first wall. On the other hand, the FMs in the LIDAR system are located in a long duct (∼2 m in length) and will receive CXA fluxes ∼ 10−2 of the first wall flux. This corresponds to about the lowest flux received by a FM in ITER.

Quartz KU-1 optical density measurements after irradiation in the nuclear reactor IR-8

Abstract Optical characteristics of quartz glass KU-1 were investigated after irradiation in a water-pull nuclear reactor IR-8 up lo the fluence of 6 × 1019n/cm2. Quartz samples - cylinders with diameters 16 mm and heights 8 mm - were placed in two aluminum containers active zone in one of the central channel of the reactor IR-8 where the neutron flux was 8× 1013 n/cm2 s. Quartz glass KU-1 optical characteristics were investigated after irradiation at the dose rate 4kGy(Si)/s. Made from one melt, samples (though even this does not guarantee their complete identity) were irradiated up to the neutron fluence for the first group of 6 × 1019n/cm (gamma dose of 2.5GGy) and for the second group of 6× 1018n/cm2 (gamma dose of 270 MGy) at the temperature of 55°C. Transparency spectra of unirradiated and irradiated samples, in the wavelength range from 0.2 to 2.5 m, were measured with the aid of a Hitachi Recording Spectrometer 20 days after irradiation. As it was observed at a lower fluence an essential degradati...

The test of Mo and W mirrors under the long-term bombardment by ions of deuterium plasma

The refractory metals are mostly suitable material for fabricating first mirrors for plasma diagnostics in a fusion reactor. Their low sputtering yields under the impact of charge exchange atoms can guarantee long-term mirror operation. An important factor influencing the rate of mirror degradation and reproducibility of results is the structure of its material: monocrystal W and Mo mirrors fabricated of the densest face (110) are shown to have the best characteristic. After mechanical treatment (in the process of monocrystal mirror fabrication), Mo (111) degrades noticeably faster than a perfect monocrystal. The lowest resistance was shown by mirrors fabricated of rolled sheet TZM alloy. The difference in the dependences of reflectance behavior on sputtered depth is explained by microrelief development due to sputtering erosion. In simulation experiments, the sputtering yield was not the only factor influencing the rate of reflectance degradation under bombardment by ions of deuterium plasma. For mirror test data to be reproducible, the material has to be certified; and special precautions should be taken when fabricating mirrors.

Testing of molybdenum film mirrors under bombardment by deuterium plasma ions

Results are presented on the behaviour of molybdenum coatings on substrates from TZM alloy and single-crystal molybdenum when bombarded by deuterium plasma ions with a wide energy distribution. Blisters were found to appear in the near-surface layer of the substrate in the case of TZM alloy and on the film–substrate interface in the case of single-crystal molybdenum. By improving the vacuum conditions during film deposition, we managed to suppress the blistering process and to produce films which are able to retain their optical properties for the whole time of operation of the International Thermonuclear Experimental Reactor, provided that the flux of charge exchange atoms to their surface is weakened by not less than an order of magnitude in comparison with the flux to the first wall.

First mirrors for diagnostic systems of an experimental fusion reactor II. The mirror tests on the large fusion devices under operation

In part II of this review, the results of experiments on the exposure of mirrors in the operating fusion devices are described (we restricted the results to those that had been obtained up to the end of 2004). It is shown that in the locations that are suitable for the positions of the mirrors (at the ports and divertor), as well as near the carbon graphite limiters, the appearance of deposits predominates. However, there are also regions in the vacuum vessel where the main effect is mirror sputtering. In this connection, two methods of mirror cleaning that can possibly be used in situ are described: by a gas-discharge low-temperature plasma and by repeated laser shots. Also, the effect of multiple-shot laser illumination on the survivability of metallic mirrors of laser diagnostics is analysed.