A. N. Shapoval

Modification of optical characteristics of metallic amorphous mirrors under ion bombardment

In-vessel mirrors are necessary for optical diagnostics of plasmas in next-step fusion devices. These mirrors will be under the influence of the harsh fusion environment, and in these conditions the mirror material should perform its functions. This article describes experiments that have been carried out to evaluate the prospect of amorphous mirrors retaining their optical characteristics under the impact of deuterium or argon plasma ions of different energy. The experiments were undertaken with the use of mirror samples prepared from amorphous alloys Vitreloy-1 and Vitreloy-4. The data reported demonstrate the principal ability of mirrors made of amorphous materials to preserve the initial optical quality in the process of long-term sputtering, and should be considered as proof of the possibility of using amorphous metal mirrors in the erosion-dominated zone of a fusion reactor.

On the choice of materials for the first mirrors of plasma diagnostics in a fusion reactor

Abstract The reflectivity coefficient, R , of mirrors fabricated from different metals (Al, Cu, Mo, Ta, W) in the wavelength range 0.25–0.65 μm was investigated under long-term sputtering with ions of ECR plasma of hydrogen or deuterium, and R values versus thickness of sputtered layer were measured. For copper mirrors the influence of ion energy and ion energy spectrum on optical reflectivity was investigated in detail. Because a strong effect was found, the comparative test of mirrors made of different metals was carried out using ions with a wide energy distribution (0.1–1.5 keV) to have a more adequate simulation of fusion reactor environment conditions. The results obtained show that, with the exception of Al, the decrease of reflectivity coefficient with thickness of sputtered layer has approximately similar rate for metal tested. Thus, mirrors made of materials having lower sputtering coefficient, Y , will withstand a higher charge exchange atom fluence, in qualitative agreement with R / Y criterion suggested by the authors earlier.

Imitation of effects of a fusion reactor environment on optical properties of metallic mirrors

Abstract Results of imitation experiments on effects of charge exchange atoms bombardment on optical properties of metallic mirrors (in the spectrum range of 200–800 nm) are presented. The long-term sputtering of mirror samples made of stainless steel, copper and hot pressed beryllium have been carried out with plasma ions (H + , D + , He + , or Ar + ) of keV energy range. It was shown that in the case of a stainless steel mirror, the sputtering of a layer of ∼2 μm thick did not lead to significant change in reflectivity. As to a copper mirror, a noticeable change of reflectivity was observed after sputtering of ∼0.7 μm layer using He + ions. The highest rate of degradation, especially in the UV region, was found in the case of beryllium mirrors.

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.

On the influence of fusion reactor conditions on optical properties of metallic plasma-viewing mirrors

Abstract This paper presents the results of imitation experiments concerning the effects of fusion reactor conditions on the properties of mirrors Made of stainless steel, copper and beryllium. The neutron irradiation was imitated using MeV energy range ions. To imitate the effects of charge exchange atoms (CXA) bombardment, keV energy range D+ and He+ ions were used. From the data obtained it was concluded that not only the reflectivity but also the resistance to CXA sputtering have to be taken into account when choosing the materials for the first mirrors of a fusion reactor.

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.

On the choice of material for the first mirrors of fusion reactor plasma diagnostics

Abstract The rate of the reflectivity degradation for the nearest UV and visible regions was investigated depending on thickness of a sputtered layer of mirrors made of aluminum, copper, stainless steel (ss), tantalum and tungsten. The thickness of a sputtered layer was measured by the mass loss of mirror samples after every exposure to plasma with a multistep repetition of this procedure and with simultaneous measurements of refiectivity R. The ECR discharge in the H-D mixture or the reflex discharge in D2 was used similar to ion sources. In the ECR discharge the effective energy spectrum of ions bombarding the mirror surface was spread between 0.1 and 1.0 keV or between 0.1 and 1.5 keV with the ion current density ∼ 1 mA. In the reflex discharge the mean ion energy was between 0.65 and 0.8 keV with a half-width ∼0.12keV and approximately the same current density. The measurement results have shown: (1) The rate of a tested mirror degradation depends on the mirror material and on the energy spectrum of p...

Change of Optical Properties of Beryllium Mirrors Under Deuterium Ion Bombardment

In this paper the results of experimental investigations of beryllium mirrors behaviour under bombardment by ions of deuterium plasma are presented. In the experiments small Be mirror samples were exposed to D + ions of fixed energy in the range from 0.05 to 1.35 v keV at deuterium pressure of (3-5) 2 10 m 2 v Pa, and step by step the normal incident reflectivity of the samples was measured after every exposure. The observed dependence of the reflectivity on the thickness of the sputtered layer agrees with an assumption that under D + ion bombardment there occurs the transformation of the BeO surface layer into the layer of hydroxide, Be(OD) 2 . The mass loss at early stage of ion bombardment, when such a transformation takes place, is negligible, and only after much longer exposures the reflectivity is slowly diminishing due to changing the surface layer morphology. The exposed mirrors heating up to >300°C during one hour leads to restoration of their reflectance, probably due to the backward process of BeO layer restoration.

Imitation of Fusion Reactor Environment Effects on the Inner Elements of Spectroscopical, MM and Sub-MM Diagnostics

This paper consists of two parts and concerns the influence of fusion reactor conditions on inner elements of some diagnostic systems. The first part is devoted to results of imitation experiments on the long-term operation of plasma-viewing mirrors, and in the second part there are discussed the results on a possibility to use carbon-graphite materials for fabricating the active elements of mm and sub-mm diagnostics (radiating and receiving antennae, reflectors). It is evident that results of the second part can be useful only in the case that carbon-based materials but not a beryllium will be chosen as the first wall protection.

On the problem of material for the in-vessel mirrors of plasma diagnostics in a fusion reactor

In the paper the problem on the choice of materials for the in-vessel mirrors for plasma diagnostics in the reactor-grade fusion device is discussed. The radiation environment in locations of mirrors is analyzed having for an object to find the correct criteria for the candidate materials. Two groups of mirrors are considered: 1st mirrors that directly view the plasma and 2nd mirrors located behind the radiation shield. The results of experiments simulating the impact of different kinds of radiation on optical properties are compared for mirrors fabricated of polycrystalline and monocrystalline metals, film mirrors on metallic and dielectric substrates.