A. V. Rogov

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.

Angular distribution of atoms during the magnetron sputtering of polycrystalline targets

The angular distributions of atoms are measured during the dc magnetron sputtering of Mg, Al, Cu, Ag, Ta, Pt, Au, Ti, Cr, Zn, Zr, and Nb polycrystals by Ar ions with an energy up to 0.5 keV. These angular distributions are phenomenologically approximated, and adjustable parameters are found for each element. Computer simulation of sputtering based on the pair collision approximation is used to determine the coefficient of proportionality between the magnetron discharge voltage and the average sputtering ion energy and to find the interatomic interaction potentials that provide the most accurate description of the experimental data. Both the angular distribution width and the sputtering coefficient are shown to have a periodic dependence on the atomic number of the target Z2, and materials with the maximum sputtering coefficients have the narrowest distribution of sputtered atoms.

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.

Factors determining the efficiency of magnetron sputtering. Optimization criteria

We report on the results of experimental study of the dependence of sputtering energy efficiency Kw in a dc planar magnetron sputtering setup on the discharge power, working gas pressure, magnetic field, cathode erosion depth, and the structure of the gas puffing system and anode. We propose that this parameter be used for comparing the degree of perfection of the magnetron design irrespective of the magnetron size and structural features. The results of measurements of Kw in sputtering of Al, Ti, Cr, Cu, Zn, Zr, Nb, Mo, Ag, In, Sn, Ta, W, Pt, and Au are considered. The optimization criterion is worked out for the magnetic system of the magnetron, which ensures the minimal working pressure and the maximal sputtering rate for the cathode. The results are analyzed theoretically.

Magnetron Deposition of Molybdenum Mirrors and Micrometer-Thick Mirror Foil with an Ordered Columnar Nanocrystalline Structure

The formation of a molybdenum reflecting coating with a columnar nanocrystalline structure is studied under the condition when molybdenum is deposited by dc magnetron sputtering on single-crystalline molybdenum mirrors (substrates). The coating is found to peel off together with a polishing-induced imperfect amorphous surface layer of the substrate. This effect aids in obtaining a free mirror-like molybdenum foil. When the amorphous imperfect layer is removed from the substrate before deposition, the reflecting mirror coating has a good adhesion to the substrate.

Experimental Study of First Mirror Degradation Under Influence of Iter Relevant Conditions

Being placed in a straight view of burning plasma the First Mirror (FM) is one of most critical elements of ITER diagnostics. The sputtering by charge exchange atoms (CXA) and the deposition of contaminating films and dust on a mirror surface will be the main reasons of FM reflectivity degradation under ITER conditions, and for laser diagnostics the thermomechanical stresses occurring at every laser shot may accelerate the mirror destruction [1]. The materials traditionally used for optical mirrors are not suitable for building-up the ITER diagnostic mirrors. The use of mirror with reflective coating from metals with low sputtering yield and sufficiently high reflectivity (e.g., Rh, Mo, W, Ag) on metallic substrate is one of the possible variant of FM choice for a survival in extreme ITER environments [2]. The production practice of mirrors with Rh and Mo coatings on SS and Mo substrates (Rh/SS and Mo/Mo, correspondingly) by means of sputter deposition of reflective coating on polished substrates is elaborated using a dc planar magnetron [3]. The influence of main damage factors on FM prototypes and experiments on a remote cleaning of mirrors from carbon films by pulsed laser radiation are considered in the paper.

Improvement in the homogeneity of molybdenium coatings deposited onto polished polycrystalline molybdenium substrates

The surface morphology of optical sputter-resistant nanocrystal molybdenum coatings formed by magnetron sputtering is investigated. The coatings deposited onto polished polycrystalline molybdenum mirrors have defects of two types: large crystalline inclusions protruding above the coating surface and areas with sharp boundaries corresponding to those of grains on the polycrystalline substrate surface. Preliminary treatment of the substrate surface by sputtering with the simultaneous deposition of substrate material removed defects of both types. A theoretical model of coating formation explaining the mechanism of improvement in its structural uniformity during surface preparation by the proposed method is presented.

Approximation of the erosion zone profile in planar magnetrons with a disk cathode

We have proposed a universal approximation of the normalized erosion zone profile of planar magnetrons with a disk cathode by a composite function that includes the probability density function for the minimal distribution of extremal values for the region from the center of the disk cathode to the maximum of the erosion zone and the survival Weibull distribution function (from the maximum to the outer boundary of the sputtering zone). The accuracy of the approximation has been verified for six magnetrons differing in the cathode size or in the design of the magnetic systems. In all cases, good agreement has been observed between the approximation and experimentally measured values. The results reported here can be used to analyze processes that occur on the cathode during sputtering and to refine the calculations of coating profiles.

Formation of gold nanoparticles and their aggregates in a liquid at magnetron deposition

The influence of magnetron deposition conditions on the size of Au nanoparticles and their aggregates obtained by condensation in a neutral liquid is studied experimentally. A model is suggested in which the nanoparticles and aggregates form in a thin subsurface layer, which becomes oversaturated by atoms and resulting nanoparticles when the liquid flows through a localized deposition zone. The process stops when the products leave this zone because of stirring. The size of nanoparticles and aggregates depends on the particle flux density and exposure time in the deposition zone. The final size of nanoparticles depends on the exposure time only slightly, while that of aggregates significantly depends on the exposure time. This allows one to prepare a concentrated solution of almost monodisperse nanoparticles with a low degree of aggregation by properly selecting deposition conditions and multiply passing the liquid through the deposition zone.

Analysis of the Mo/Mo Nanocrystalline Optical Coating Obtained with the Aid of Simultaneous Deposition and Ion Sputtering

Structure, optical properties, and resistance to sputtering are studied for a reflecting Mo-coating that is fabricated using magnetron deposition with simultaneous low-energy ion sputtering at the deposition rate that is higher than the etching rate. A Mo-polycrystalline mirror is used as a substrate. It is shown that the coating exhibits textured nanocrystalline structure with a relatively low spread of crystallite sizes and high resistance to sputtering. It is also demonstrated that the spectral reflection coefficient of such a Mo-coating differs from the spectral reflection coefficient of polycrystalline and single-crystalline Mo and the difference results from the effect of the structure of coating on its optical properties. A theoretical model of the coating formation is proposed.