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The effects of exposure in water or water steam on optical properties of mirror samples fabricated of different metals were studied. Materials of samples were: tungsten, beryllium, molybdenum, stainless steel, nickel, copper, rhodium, and amorphous alloys of two compositions. The material of the mirror was found to play a decisive role in the degree of impact of water or water vapor on mirror degradation. The most strongly degraded Mo and Cu mirror samples, and the lowest degradation was observed for SS and Rh/SS mirror samples. The reflectance can be restored to practically initial level when after water impact procedure the mirrors are exposed to ions of Ar plasma with energy ≥100 eV up to ion fluence >1019 ion/cm2.

Features of the recent Uragan-2M campaign are reviewed together with some theoretical advances. They include experiments with B4C limiter, studies of various 1…20 kHz oscillations, development of a new in-situ diagnostics for wall conditions, i.e. the thermal desorption probe, the improved numerical model of RF plasma production in stellarators in the ion cyclotron and electron-cyclotron frequency ranges, a new positive-definite form of time-harmonic Maxwell’s equations and plasma start-up studies.

Samples of two kinds of bulk metallic glasses (BMG) with different compositions, Zr48Cu36Al8Ag8 and Zr57Cu15.4Al10Ni12.6Nb5, were polished to mirror-like quality and, after measuring the initial reflectance at normal light incidence, were subjected to impact of ions of deuterium or argon plasma with fixed energy. The main attention was paid to phenomenon on deuterium absorption that reveals as the weight gain after every exposure of the samples in deuterium plasma. For a fixed ion energy (in the range

Optical properties of zirconium-based amorphous alloys that were subjected to deuterium plasma treatment have been investigated by the multiple-angle-of-incidence single-wavelength and spectral ellipsometry in a spectral range of 0.5–3.5 eV. After bombardment by ions of deuterium plasma of the samples the increase in the intensity of absorption within the spectra of optical conductivity in the indicated spectral range was found. Such behavior of the optical properties can be explained by deuterium plasma ions sputtering of a subsurface layer, and by modification of the electronic properties of a near-surface layer due to deuterium implantation. The optical anisotropy of the samples both in the initial and after treatment in the deuterium plasma was not observed being inherent for amorphous surface layer structures. It has been found that the treatment of mirror-like surfaces by the deuterium plasma increases the roughness of the surface of Zr57Cu15.4Al10Ni12.6Nb5 alloy mirror sample investigated.

Because tungsten (W) is used in present fusion devices and it is a reference material for ITER divertor and possible plasma-facing material for DEMO, we strive to understand the response of different W grades to ion bombardment. In this study, we investigated the behavior of mirrors made of four polycrystalline W grades under long-term ion sputtering. Argon (Ar) and deuterium (D) ions extracted from a plasma were used to investigate the effect of projectile mass on surface modification. Depending on the ion fluence, the reflectance measured at normal incidence was very different for different W grades. The lowest degradation rate of the reflectance was measured for the mirror made of recrystallized W. The highest degradation rate was found for one of the ITER-grade W samples. Pre-irradiation of a mirror with 20-MeV W 6+ ions, as simulation of neutron irradiation in ITER, had no noticeable influence on reflectance degradation under sputtering with either Ar or D ions.

The simultaneous effect of sputtering and neutron irradiation on the optical properties of tungsten mirrors has been considered, which was simulated by bombardment by 20-MeV W+6 ions. The action of charge-exchange atoms was imitated using Ar+ ions with an energy of 600 eV. The dependence of the structure of the surface and optical properties of tungsten on the fluence of Ar ions has been studied using optical microscopy, interferometry, reflectometry, and ellipsometry. It has been shown that irradiation with neutrons should introduce a significant additional contribution to the processes at the tungsten surface that occur under the effect of charge-exchange atoms. An analysis of experimental data obtained by using reflectometry and ellipsometry made it possible to suggest a realistic model of the process of surface modification for samples of ITER-grade tungsten (that were preliminarily irradiated by tungsten ions) using prolonged sputtering by Ar+ ions.

Experiments have been performed to determine the effect of deuterium plasma exposure on mirrors fabricated from Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 alloy (Vitraloy 1, V1) in both the amorphous and crystallized state. It was found that for deuterium ion fluences above ∼2.6 × 10 24 ions/m 2 , cracks and chips appeared on the surface of the crystallized mirrors. At a factor of two higher fluence, 5.2 × 10 24 ions/m 2 of 60 eV ions, one specimen disintegrated into many fragments. The mirror specimens kept in the amorphous state, however, were not observed to have experienced any visual changes even after exposure to ion fluences as high as 1.5 × 10 25 ions/m 2 . The difference in behaviour is explained as being due to hydrogen embrittlement, and differences in hydrogen transport between the two alloy states.

The surface topography and optical properties of recrystallized tungsten exposed to a low-energy (38 eV/D), high flux (10 22 D/m 2 s) deuterium plasma with an ion fluence of 10 26 D/m 2 at various temperatures was investigated. It was found that the surface morphology weakly depends on the exposure temperature in the range 320–695 K with the exception of the narrow temperature region around 535 K, where large changes to all optical characteristics occurs. After plasma exposure at this temperature, the surface topography of the W sample is characterized by active blistering as has already been indicated in previous publications. The reflectance found in direct measurements at normal incidence drops in the wavelength interval 220–650 nm, whereas the estimations of reflectance using the ellipsometry data demonstrate some increase.

The reflectance of Be mirrors due to impact by ions from a deuterium plasma has been studied under several bombardment conditions. Analysis of the resulting surface films has been performed using various diagnostic techniques, with the conclusion that the primary factor leading to the decrease in reflectance following bombardment with energetic ions is the conversion of the surface oxide layer, composed of BeO, to the hydroxide, Be(OD)2, with a corresponding increase in the optical extinction coefficient. The increase in the thickness of the layer is also important. Modifications to the surface layer are thought to involve a balance between the ion-induced diffusion of Be atoms to the surface where they may react with incident D and O atoms, and physical and chemical sputtering processes. For incident ion energies less than � 50 eV, chemical reactions leading to disoxidation of the oxide–hydroxide film dominate, while keV-range ions (primarily D, but with some O impurities) lead to the formation of hydroxide, and an increase in the surface layer thickness.

The ion-induced modification of aluminum alloy mirrors, under bombardment by deuterium plasma ions has been investigated as a simulation of the environment effects on in-vessel mirrors in ITER. Ellipsometry and reflectrometry have been used to characterize the mirror surface, along with several surface diagnostic techniques (XPS, Auger, SIMS). The results of multiangular- and spectro-ellipsometry were analyzed using both a bare surface model, and effective medium model; the medium was composed of Al, Al 2 O 3 (Al(OD) 3 or AlOOD), and voids. It was found that the reflectance decreases following exposure to keV-range ions, but can be restored by subsequent exposing the mirror to low-energy ions (∼60 eV). Chemical processes related to an increased oxide layer are thought to be responsible for the decrease in reflectance, while the reduction of the oxide layer following low-energy D + exposure may lead to the return of high reflectance. By comparing the measurements with the results of modeling, a mechanism is suggested to explain the experimental data. The mechanism is based on: (1) chemical processes in a surface layer and (2) сhanges in the thickness and roughness of the surface layer.

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 th...

In this paper the results of post-mortem analysis of mirror samples (molybdenum, stainless steel and copper) exposed for the 1 year experimental campaign inside the Tore Supra tokamak are presented. The mirrors were not protected during the experiment and therefore were exposed both to glow discharge conditioning of the vessel walls and to working discharges. After exposure, all samples were found to be eroded and at the same time were coated with a thin contaminating deposit. We pay attention to the fact, that the observed erosion cannot be described using the published sputtering yields. In particular, the difference between the erosion of stainless steel and copper exceeded a factor of 10 and the difference between the erosion of stainless steel and molybdenum was less than a factor of 2 in comparison with the corresponding factors of approximately 2.5 and apprximately 10 known from the literature. An attempt is made to correlate these observations with the different sticking coefficients of carbon on ...

In-vessel mirrors are necessary for optical and laser diagnostics of plasmas in next-step fusion devices; however obtaining mirror materials that will maintain their performance in the harsh fusion environment remains problematic. At present, mirrors are fabricated from polycrystals or monocrystals, and metal films on metallic substrate mirrors are also being studied. In this paper, we report on a new family of bulk amorphous alloys that are being investigated in the search for new materials for the fusion environment. Their properties are very different from properties of well-known materials and are insufficiently investigated for scientific and technological applications. We present the results of investigation of modification of the surface and optical properties of amorphous mirror fabricated from Zr(41.2%)Ti(13.8%)Cu(12.5%)Ni(10%)Be(22.5%) alloy after bombardment by ions of deuterium plasma with different fluence and energy.

During the third experimental campaign, three stainless steel mirrors were exposed inside the Large Helical Device (LHD) vacuum vessel and, in order to measure the change in their spectral reflectance R(λ), the effect of this exposure was measured at normal incidence (λ=200–700 nm). The mirrors located in the divertor region and in the diagnostic port became coated with carbon-based films; however, the mirror placed closest to the plasma confinement volume became cleaner than it had initially been. The characteristics of the films were obtained by different techniques. The deposited films were slowly removed by repeatedly exposing the mirrors to low-energy ions of deuterium plasma, and R(λ) was measured after every exposure. In the present paper we compare the reflectance behaviour during this cleaning procedure and the reflectance calculated in the framework of a model assuming that a half-transparent film is formed on stainless steel substrate. The n(λ) and k(λ) data for carbon-based films were taken fr...

Experiments have been performed to evaluate the changes in surface structure and the resulting effects on the optical properties of stainless steel due to hydrogen ion irradiation. Stainless steel (SS) is a standard material used for in-vessel components, including the first mirrors (FMs), in some current generation fusion devices. Optical microscopy, interferometry, scanning electron microscopy (SEM) and ellipsometry were used to characterize the surfaces. Results are presented for the bombardment of SS mirrors with H 3 + ions having various fixed energies (0.3, 0.65, and 1.5 keV/H+), with ion flux densities of (0.5–2) × 1020 H+/m2s and fluences of ∼2.2 × 1024 H+/m2. Variation of the ion energy at a constant fluence had a considerable effect on the damage structure that forms on the SS mirror surfaces. Possible mechanisms for the ion energy effect on the surface are discussed and a model of the damaged surface layer is proposed.

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 ...

Effects of D ion bombardment on Be mirror reflectance, R(λ), in λ=220–650 nm were studied as modelling the impact of charge exchange atoms in a fusion reactor. Without any mass change of mirror, a sharp drop of R(λ) was observed for different kinds of beryllium at high ion energies (600–1350 eV) after ion fluence ∼1018 ions/cm2. It is hypothesized that under deuterium ions the BeO surface film was transformed into the Be(OD)2 film accompanying by changing the film optical properties. Effects of ion energy and ion fluence variation on R(λ) of Be mirror are discussed.

In the l = 3/m = 9 Uragan-3M (U-3M) torsatron (R0 = 1 m, abar; ≈ 12 m, B = 0.7 T, ι(abar)/2π ≈ 0.4), an open helical divertor has been realized. Recently, under RF plasma production and heating conditions, a strong up–down asymmetry of diverted plasma flow has been observed as a result of measurements of distributions of this flow in two symmetric poloidal cross-sections of the U-3M torus. In many aspects, this asymmetry is similar to that observed in the l = 2 Heliotron E (H-E) heliotron/torsatron under neutral beam injection and electron cyclotron heating conditions. The main feature of the asymmetry is a predominant outflow of the diverted plasma in the ion toroidal drift direction. On this basis, the asymmetry can be related to non-uniformity of the distribution of direct charged particle losses in the minor azimuth. In the work reported, the magnitude of diverted plasma flow in U-3M and the degree of its vertical asymmetry are studied as functions of the heating parameter , P being the RF power absorbed in the plasma, and are juxtaposed with corresponding P-related changes in the density, , and suprathermal ion content in the plasma. As the heating power increases, both the temperature of the main ion group and the relative content of suprathermal ions increase. At the same time, a decrease in plasma density is observed, evidencing a rise of particle loss. The rise of particle loss with heating could result from both a shift of diffusion regime towards a lower collisionality and a rise of direct (non-diffusive) loss of high-energy particles. Outside the confinement volume, the total flow of diverted plasma increases together with an increase of vertical flow asymmetry towards the ion toroidal drift side. Such a mutual accordance between the processes in the confinement volume and in the divertor region validates the hypothesis on a dominating role of fast particle loss in the formation of vertical asymmetry of divertor flow in U-3M. In conclusion, the results obtained on U-3M are compared with those from similar research on H-E.

The results of local measurements of RF discharge plasma parameters in the process of internal transport barriers (ITB) formation in the vicinity of rational magnetic surfaces in the Uragan-3M torsatron are presented. The following phenomena were observed in the process of ITB formation: widening of the radial density distribution, formation of plateaus on radial density and electron temperature distributions, formation of regions with high shear of poloidal plasma rotation velocity and radial electric field in the vicinity of stochastic layers of magnetic field lines, decrease of density fluctuations and their radial correlation length, decorrelation of density fluctuations, and increase of the bootstrap current. After the ITB formation, the transition to the improved plasma confinement regime takes place. The transition moves to the beginning of the discharge with the increase of heating power. The possible mechanism of ITB formation near rational surfaces is discussed.

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 ...

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.

The current state of investigations of the problem of providing first mirrors (FMs) for diagnostic systems in a reactor-grade fusion device is summarized. Results obtained in simulation experiments that have been conducted during recent years in several laboratories are presented. Attention is concentrated on two processes that can have an opposite effect but both can lead to degradation of mirror optical properties, namely: sputtering by charge exchange atoms which leads to erosion, and deposition which leads to surface contamination. It is shown in the analysis that when sputtering dominates, mirrors of monocrystalline refractory metals (Mo, W) can have a sufficiently long lifetime even for FMs that have to be located close to the first wall. Similarly, films of low sputtering yield metals on high thermal conductivity substrates (e.g., Rh on Cu) can be used for FMs in locations where the charge exchange flux is reduced to about a tenth of that at the first wall. However, deposition poses a serious threa...

The processes of sputtering of austenitic stainless steel (similar to 316SS) under long-term bombardment with ions of deuterium plasma (energy 0.1–1.5 keV) as well as effects of erosion on optical properties of steel surface are investigated. It was found that for this kind of material there is no selective etching of crystallographic defects that come out to the surface; therefore the rate of reflectance degradation decreases. Investigation of the optical properties of the surface subjected to ion bombardment requires taking into account not only the development of surface microrelief but also radiation damage created in the near-surface layer.

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...

The optical effects of radiation damage on metal mirrors of MeV- and keV-range ions have been examined. Al, Cu and Cr ion beams of 1–3 MeV energy were used to bombard Al, Cu and stainless steel mirrors respectively with doses spanning 10–25 dpa. The effects following long-term sputtering with keV-range ions of hydrogen plasma (or deuterium) on mirror reflectivity R in the UV and visible spectral regions were investigated. The modifications of two stainless steel mirrors' optical properties induced by the deposition of different thicknesses of B layers have also been studied. The ex situ ellipsometric data for multiple angles of incidence (within the interval from 64 to 80°) were recorded at a wavelength of λ =632.8 nm, before both ion irradiation and boronization and after the treatment. It was found that even short keV-range ion bombardment of Al mirrors drastically changed their reflectivity at any given wavelength, in contrast to a stainless steel mirror. The boronization resulted in noticeable changes of the optical constants in the UV. The optical constants of the boron film on the stainless steel substrate ( n =2.53, k ≈0), with thicknesses d ( d =7.3 and 16.6 nm), were calculated from the ellipsometric data. The IR ellipsometric measurements within the frequency range 400–5000 cm −1 were performed for stainless steel mirrors pre-irradiated by Cr ions and then coated by B to clarify the damaged layer formation mechanism. The ellipsometric changes have been attributed to both modification of the metallic mirrors' surface mirror relief and the formation of a skin layer with better conductivity after ion treatment.

We present the results of simulation experiments on the effect of the fusion reactor environment on the optical properties of the first mirrors for spectroscopy and laser diagnostics. We found the greatest effect on the degradation of mirror optical properties was due to charge exchange atoms. These atoms can affect mirror quality in two ways: by sputtering and by redeposition of material sputtered from other inner components of the vacuum chamber. The degradation rates of mirrors made of different metals and subjected to long-term bombardment by ions from hydrogen or deuterium plasmas are compared. From analysis of all data, we concluded that special experiments will have to be conducted in order to make a correct choice of a first mirror material. These tests should include the following: (1) study the role of swelling on mirror surface modification; (2) find the multimillion shot laser-induced-damage threshold of metal mirrors; and (3) develop and test methods to protect the first mirrors from redeposition. In addition, a model should be developed for the charge exchange atom flux and energy distribution that represents real data from operating tokamaks.

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.

The cathodic sputtering of copper mirrors in a hydrogen plasma was realized by means of alternating negative voltage. The time behavior of voltage was chosen to provide the broad ion energy spectrum, in qualitative agreement with the results of measurements of energy spectrum of the charge exchange atoms in fusion devices. The comparison of the degradation rates of mirror reflectivity due to the long-term sputtering was made in a multi-step way in the wavelength range λ =253–650 nm. After several steps of sputtering, the microtopography of the mirror surface was analyzed by means of a scanning electron microscope. It was shown that sputtering with energy-distributed ions leads to different degradation rates of mirror reflectivity compared to the case of monoenergetic ions. This difference is surmised to be linked with peculiarities of defect creation in the near surface layer under conditions of mirror bombardment with variable ion energy in the vicinity of a displacement-induced threshold energy.

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.

Numerical and experimental results of Alfven wave heating of plasmas in the frequency range below the ion cyclotron frequency ( ω ω ci ) are presented. Two different types of antenna were used for plasma production and heating: a frame type antenna (FTA) conventionally used in the URAGAN-3M device and a three-half-turn antenna (THTA) proposed recently to avoid the deleterious effects of conversion of fast wave to slow wave in the plasma periphery and to perform plasma core heating more effectively. Numerical modeling of electromagnetic field excitation in the URAGAN-3M plasma by the FTA and THTA was performed using a one-dimensional code. The results of calculations showed better performance of the compact THTA compared with the FTA for the case of a high density plasma (approximately 10 13 cm −3 ). When using the THTA, the experiments performed showed the possibility of dense plasma production (more than 2 × 10 13 cm −3 ) and heating, which had not been obtained earlier in the URAGAN-3M. Shifting the power deposition profile deeper inside the plasma body with the THTA resulted in modification of the plasma density profile and an improvement in plasma confinement.

The results on modelling the behaviour of metal mirrors under neutron irradiation in a fusion reactor are presented. Mirror-like specimens were irradiated with beams of ions: Cr + (stainless steel), Cu + (copper) and Ne + and/or He + (beryllium). The energy of ions was in the range of a few keV (He + ) to 3 MeV (Cr + and Cu + ). The reflectivity as a function of irradiation dose was measured in the interval of spectrum 0.2–1.0 μm. There was no strong degradation of the mirror reflectivity up to doses ∼ 10 dpa in a near surface layer. In contrast, all used methods showed definite change of surface characteristics.

Fast degradation of in-vessel optics is one of the most serious problems for all optical diagnostics in ITER. To provide the resistance to mechanical and thermal stresses along with a high stability of optical characteristics under deposition-dominated conditions we suggest using high-reflective metallic (Ag or Al) film mirrors coated on silicon substrate and protected with thin oxide film in the divertor Thomson Scattering (TS) diagnostics. The mirrors coated with Al2O3 and ZrO2 films were tested under irradiation by deuterium ions. The experimental results on the oxide films sputtering are discussed in the context of their applicability for the first mirror protection in ITER.

The structure and surface morphology of aluminum and silver mirrors covered with protective dielectric oxide layer were studied by means of TEM and SEM. The presence of needle-like pores throughout the thickness of the ZrO(2) film and bubble-like pores in Al(2)O(3) was observed. The test for resistivity to deuterium ion bombardment shows that the exposition to a fluence of similar to 2 x 10(20) ions/cm(2) with the ion energy of 40-50 eV results in appearance of blisters on the surface of mirrors covered wit h Al(2)O(3). For the mirrors protected with ZrO(2) no noticeable changes in surface morphology and reflectivity were found even after order of magnitude higher ion fluence. The effect of different porous structures on blistering phenomena is discussed. (C) 2010 Elsevier B.V. All rights reserved.

Results are presented from spectroscopic and collector probe measurements in the Uragan-3 torsatron. Impurity influx from the RF antennas is from sputtering by ions following the divertor field lines. Erosion of the helical winding casings is mainly due to background plasma interaction caused by oscillations in plasma potential. The deposition rate of metallic impurities in U-3 is one or two orders of magnitude less than in a conventional stellarator, due to the lack of a vacuum chamber in close proximity to the confinement region.

The authors study the mechanisms by which light (oxygen, carbon) and heavy (metal atoms) impurities are released from the walls in the Uragan-3 torsatron, in which radiofrequency (RF) heating is the only way of heating (and producing) the plasma. Having investigated the relationship between the quantity of impurities in the peripheral plasma, as well as the mean density, the confining magnetic field and the power input, they show that metal impurities appear as a result of sputtering from metal surfaces. Carbon, too, like the metal atoms, enters the plasma as a result of physical sputtering of carbon bearing materials. However, oxygen production occurs in a different way ? through chemical interaction between hydrogen and metal oxides on the surface. By comparing impurity concentrations in the edge plasma and the fluxes of charge-exchange atoms as functions of n?e, power input and magnetic field, the authors show that these fluxes do not play a decisive role, either in surface sputtering of the antenna and the magnet casing or in the chemical interaction responsible for the release of oxygen from the surfaces. Sputtering from the surface of the magnet casing that faces the confinement volume is shown to be governed mainly by the high frequency current associated with RF potential oscillations of the peripheral plasma. Sputtering from the surface of the RF antenna is due to its interaction with the quasi-constant ion current, with an energy of ? 100 eV produced during RF heating. The formation of water molecules on the surface is due to interaction of surface oxides with hydrogen atoms formed during the dissociation of H2 molecules.

Mechanisms of light (oxygen, carbon) and heavy (metal atoms) impurity production have been studied in the URAGAN-3 torsatron, where the RF method is the only one to produce and heat the plasma. The study of the impurity amount in the edge plasma versus the average density n e and the input RF power has shown that the metal and carbon impurities arise due to physical sputtering of metal and carbon-containing materials. Yet, the mechanism of oxygen release is different, and originates from a chemical hydrogen-metal oxide interaction on the surface. The comparison of the impurity concentration in the edge plasma and the charge-exchange atom flux as functions of n e and the magnetic field demonstrates that the charge exchange atoms are not determinative either in surface sputtering of the antenna and the helical winding casing or in the chemical processes of oxygen atom production. The sputtering of the casing surface facing the confinement region is shown to be mainly due to an oscillating charge particle current caused by the RF oscillation of the edge plasma potential. The surface sputtering of the antenna and the casing areas crossed by the divertor flux is caused by a quasisteady flux of ≥100 eV ions that appear during RF heating. The H 2 O molecule formation on the surfaces is attributed to the hydrogen atom-surface interaction, where the H-atoms are produced due to H 2 molecule dissociation.

Two plasma diagnostics in ITER will use cube-corner reflectors (CCR): poloidal polarimetry and toroidal interferometry/polarimetry. The multichannel poloidal polarimetry system is planned to operate at a wavelength of 118.8μm. The multichannel toroidal interferometry/polarimetry system is based on a CO2 laser operating at wavelengths of 10.6 and 9.27μm. The long term sputtering by charge exchange atoms and/or deposition of carbon-based (or beryllium-based) contaminant layers can affect the optical properties of the CCR. The role of both these potentially deleterious effects on the CCR operation is analyzed in this article, taking into account the probing beam wavelength and the CCR locations. The conclusion is that for the intended use of a CCR in the poloidal polarimetry at 118μm neither erosion nor deposition should pose a problem. On the other hand, in the toroidal interferometry/polarimetry system operating at 10μm, care must be taken to reduce the charge exchange atom flux and it is likely that depos...

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.

Recent results of the experimental program on the stellarator-type device Uragan-3M at the IPP in Kharkov are presented. Efforts were focused mainly on optimization of the operation of the frame-type radiofrequency antenna to produce a target plasma for the three-half-turn antenna. Different regimes of the Uragan-3M operation, which are characterized by different temporal behavior of the average plasma density, electron cyclotron emission radiation intensity and particle confinement time, are considered. Elementary atomic processes responsible for plasma creation are studied. The particle confinement time for the Uragan-3M plasmas is estimated. Measurements of energy spectra of charge exchange atoms are carried out. The principal possibility of realizing a 'stellarator–magnetic mirror' scheme as a prototype of a stellarator-mirror fusion–fission hybrid is shown for Uragan-2M. Future plans are discussed.

It is known from experiments on several tokamaks (PLT, ASDEX, JFT-IIM) that energy distribution of charge exchange atoms (CXA) is very wide, from tens eV up to tens keV. Calculation for ITER gave qualitatively similar long-tail distribution of CXA. Evidently, that consequences of such energy-distributed atoms bombardment will differ from those for the case when surface is bombarded with ions having some fixed energy, as it is usually realized in simulation experiments.

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.

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.

In the IPP-Kharkiv there are two torsatrons (stellarators) in operation, and in both of them Alfvén resonance heating under high-k ∥ conditions is used. This method of heating is advantageous for small-size devices, since in contrast to the minority and second-harmonic heating it can be realized at lower plasma densities. A series of experiments has been performed at the Uragan-3M torsatron with an aim to investigate the features of the discharge with a three-half-turn antenna. Electron temperatures in the range are achieved at plasma densities . The plasma energy content has increased by a factor of 2 with respect to the plasma produced with the frame antenna. A new four-strap shielded antenna has been manufactured and installed in the Uragan-2M. A high-frequency discharge for wall conditioning is introduced in the Uragan-2M torsatron. The discharge is sustained by a specially designed small frame antenna, and efficient hydrogen dissociation is achieved. A self-consistent model has been developed for simulation of plasma production in ICRF. The model includes a set of particle and energy-balance equations for the electrons, and the boundary problem for the Maxwell equations. The first calculation results on RF plasma production in the Uragan-2M stellarator with the frame-type antenna are presented.

Possible mechanisms responsible for metal impurity release during ICRF production and heating of the plasma are studied with the URAGAN-3 torsatron. It is shown that the edge plasma potential oscillations at the pumping frequency and its harmonics produce an impurity influx from the surface of the helical winding casings (Fe, Cr). The impurity release from the antenna surface (Ti) is caused by a quasisteady ~ 100 eV ion flux arising in the divertor magnetic flux region during the RF pulse.

Results obtained in the study of some properties of vacuum plasma deposited coatings are reported. Hydrogen permeability, sputtering yields of coatings bombarded with He+ and Ar+ ions, blister formation under irradiation with 80 keV He+ ions, and arcing under the plasma action have been measured. As a result, the TiN coating has been found to be essentially more resistant (as compared with stainless steel) to erosion caused by plasma action. Results obtained from experiments in the Uragan-3 torsatron using stainless steel antennas with and without a TiN coating are compared. The employment of TiN-coated antennas ensured an essential reduction (more than by an order of magnitude) of the flux of metal atoms arriving to the plasma during an RF pulse.