V.N. Bondarenko

Diagnostic first mirrors for burning plasma experiments (invited)

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...

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.

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.

Effect of exposure inside the Large Helical Device vessel on the optical properties of stainless steel mirrors

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 from experiments and the literature.

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.

Plasma heating effects on divertor flow vertical asymmetries in the Uragan-3M torsatron

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.

Interpretation of Tore Supra in-vessel mirror experiments

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 to different substrates.

The Influence of Stochastic Layers of Magnetic Field Lines on Transport Barrier Formation in a Stellarator System

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.

A study of three-half-turn and frame antennae for ion cyclotron range of frequency plasma heating in the URAGAN-3M torsatron

Abstract 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.

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.