Aluminum oxide films are used as low refractive index layers in multilayer dielectric mirrors, which are expected to be used in ITER plasma diagnostic optical systems. In such mirrors, a film with a low refractive index is the outer layer. In addition to the normal operating modes of ITER, a number of emergency modes are assumed, one of which is the destruction of the water cooling system of the first wall, divertor or blanket. In this case, the vacuum chamber is filled with steam, the parameters of which depend on the location of the destruction and on the stage of operation of the reactor at which the accident occurs. The maximum steam parameters are set by a special system that limits the pressure to 150 kPa, and the maximum temperature of 250°C can be the case when an accident occurs during vacuum training of the chamber. The work examines the interaction of steam with amorphous films of aluminum oxide deposited on glass of the K-9 grades by reactive magnetron sputtering. It has been shown that exposure of a film 300 nm thick at a temperature of 250°C and a steam pressure of 150 kPa for 2 hours is accompanied by intense hydroxylation of the film and the transformation of the entire oxide film into hydroxides. This leads to severe degradation of light transmission, which may cause a change in the optical properties of the dielectric mirror. As a next step, similar steam exposures are planned of the mirrors in which with a low refractive index layers will alternate with layers of oxides with a high refractive index, such as hafnium, tantalum, and zirconium oxides.

Leucosapphire (c-LS) structure and transmission in visible after surface treatment in 90%H2-10%N2 RF discharge are studied. According to AFM, the number of scratches of the mechanically polished surface decreased significantly after removal of about a 300 nm layer (exposure time of 12 h) under unchanged rms of roughness. According to TEM, a two-layer structure formed in the near-surface region consists of an outer 10 nm amorphous layer followed by a crystalline layer of 40-50 nm with a high defect density. The c-LS transmission in the angle of 400-1000 nm either slightly increased or remained unchanged. The demonstrated transmission stability during exposure in 90%H2-10%N2 RF discharge allows us to consider the plasma sputtering as a promising technique for cleaning contaminated windows protecting first mirror of divertor Thomson scattering being developed for ITER divertor Keywords: Leucosapphire, RF discharge, hydrogen, nitrogen, AFM, TEM, surface layer structure, visible transmission.

Migration of plasma erosion products in plasma facilities is studied experimentally and numerically within the framework of modeling transport of plasma-facing materials in the diagnostic ducts of fusion devices. Material transport simulation is discussed for two cases of low and high background neutral gas pressures. Monte Carlo software KITe was used to simulate transport at a neutral gas background pressure 0.1–0.5 Pa—typical during steady-state tokamak operation and during pressure pulses caused by edge localized modes (ELMs). The simulation approach was implemented to describe experiments at the MAGNUM-PSI facility. Fluid dynamic code FLUENT is used to simulate transport during pressure surges as high as 1000 Pa, which can occur in the case of severe disruptions in tokamak plasma discharges, such as vertical displacement events (VDE) or accidental events. The hydrodynamic approach was verified in simulation of target sputtering in the QSPA plasma gun facility.

The challenges of a large-scale mirror design for the in-vessel collection system of ITER divertor Thomson scattering (DTS) are under consideration. These are the mirrors located out-of-line vision of fusion plasma (so called ‘second mirrors’) with a high-reflective Ag-based coating. The paper outlines: approaches providing optical surface shape and angular position stability; Ag coating applicability for ITER in-vessel environment, including resistance to accidental steam ingress. The proposed solutions may also appear to be of some use in other ITER diagnostics and in diagnostics of other fusion devices.

Studies of contaminants obtained by spraying ITER-grade Be on a quasi-stationary plasma gun facility QSPA-Be are presented. Contaminating films, consisting mainly of Be and O in approximately equal proportions, were deposited on substrates of quartz, sapphire, single crystalline silicon (SC-Si) and NaCl crystal. Characterization of the deposits was performed using SEM, XPS, EBS, AFM, SE, TEM&SAED and micro-interferometry showing polycrystalline BeO films similar as found in JET-ILW. Films on SC-Si and NaCl were used to characterize their composition and morphology. The cleaning rates in the 81.36 MHz RF discharges in He or D2 at 2 Pa were measured on the SC-Si target. The measured etching rate of the deposited films was several times higher than the rate calculated from the theoretical value of beryllium oxide sputtering yield. Test cleaning of these contaminants was carried out in a capacitively coupled RF discharge (CCRF) from the surface of sapphire and quartz plates, which were considered as a mock-ups of the protective window of the first mirror unit (FMU) designed for ITER divertor Thomson scattering diagnostic system (DTS).

The effect of a high-frequency discharge on the change in the topography of the surface of KU-1 optical quartz and transmission of visible light (400–1000 nm) is studied. The working gases of the discharge are D2 and a D2/N2 mixture, in which the fraction of N2 is 25 mol %. The addition of nitrogen increases the rate of sputtering from 60 to 300 nm/h without changing the stoichiometry of the surface layers. After the exposure in plasma, the root-mean-square roughness of the surface decreases from 1.3 to 0.6 nm. The transparence of quartz remains unchanged. The analysis of the roughness and calculation of the diffuse light scattering with a wavelength of 400 nm after passing through quartz are executed using the power spectral density functions.

In plasma devices, in which plasma–wall interaction and material migration are significant, the diagnostic-window transmission may decrease due to the contamination of plasma-cleaning components by sputtered materials. We discuss the cleaning of KU-1 fused silica, which simulates the diagnostic window, from aluminum films in RF plasma generated in pure H2(D2) and Ne and in H2(D2)–0.23Ne mixtures. Aluminum is used as a chemical analogue of Be, which is the main material of the International Thermonuclear Experimental Reactor (ITER) first wall. The morphology of the plasma-treated surface is investigated by atomic force microscopy, the chemical composition is analyzed by X-ray photoelectron spectroscopy, and the transmission spectra in the range 400−1000 nm shows that plasma cleaning is accompanied by the slight reduction of quartz to suboxides and a simultaneous decrease in roughness Rq from 1.3 up to 1.0 nm. After plasma treatment of the quartz surface, a decrease in light transmission by 1.5–2% in the wavelength range 400–750 nm is observed. Further sputtering of the purified quartz surface with the removal of a layer with a thickness of more than 300 nm is accompanied by gradual smoothing of the surface and a decrease in Rq to 1 nm, but with the retention of reduced light transmission. All investigated gases, hydrogen isotopes, neon and D2(H2)–Ne mixtures, are suitable for removing Al films from the quartz surface at a RF power of several W/cm2 and temperatures of 20–100°C.

The intensity of light emitted by plasma and passing through diagnostic quartz windows in tokamaks and other fusion power devices may decrease due to first-wall materials (Be in the International Thermonuclear Experimental Reactor (ITER)) sputtered and re-deposited onto the window surface facing the plasma. In the experiments, aluminum films (analog of Be) are removed from the surface of fused silica KU-1 in plasma of a RF discharge in a H2–N2 mixture prototyping one of the in-situ cleaning techniques. The admixture of nitrogen to hydrogen plasma increases the sputtering rate of aluminum films. Using X-ray photoelectron spectroscopy (XPS) and atomic-force microscopy (AFM), we demonstrate that cleaning is accompanied by the weak reduction of quartz, nitriding with the addition of nitrogen, and the formation of a “honeycomb” structure with increased roughness. These processes are more pronounced with the repeated deposition of Al and subsequent cleaning. Light transmittance in the range 400–1000 nm does not change within 1% in the case of a partial change in the stoichiometry of the surface layers and an increase in the roughness from 1.3 to 4.5 nm. Numerical estimates of the forward and backscattering of light by a rough quartz surface show that the reduction in transmission due to scattering does not exceed a few tenths of a percent, which is in agreement with the measurement data. Thus, the demonstrated stability of the quartz transmittance during the deposition of aluminum and subsequent long exposure in the plasma of a RF discharge makes it possible to consider the cleaning technique under consideration promising for use in ITER.

The effect of cleaning with D2–(2–4) mol % N2 glow discharge plasma on the reflectivity of Mo(111) single-crystal mirrors is studied. The surface of each mirror is prepared by mechanical polishing with diamond pastes or diamond powder. Polishing of the mirrors is accompanied by the formation of a layer of material up to 3 microns thick, different in structure from the bulk metal. The peculiarity of this layer is a gradual improvement in its structure further away from the surface of the mirror and transition from an amorphous layer to a layer with nanoscale crystallites, and then a gradual transformation into the structure of an unbroken single crystal. When polishing, the diamond abrasive is pressed into the surface layer of the mirror. During the plasma-cleaning process, carbon particles pressed into the surface layer of the mirror initiate the development of negative roughness (pits or depressions). At the same time, a 5–10-nm layer of molybdenum nitride is formed. In disordered layers of the mirrors, the probability of the origination and growth of blisters is reduced. Aluminum is chosen as the metal that changes the total reflection coefficient of Mo. The use of a D2–N2 mixture to increase the rate of Al sputtering in the last stage of cleaning is replaced by plasma exposure in deuterium with $${\text{D}}_{3}^{ + }$$ ions, which initiates the dissociation of molybdenum nitride and the removal of nitrogen from the surface layer of the mirror. As a result of prolonged plasma exposure, the reflectivity of the mirrors polished with diamond paste increases to a steady-state value close to the reflection of the reference mirror.

Scientific and technological program of ITER requires measurement of plasma parameters using variety of diagnostics, ~ 20 of which are the optical ones [1]. The plasma emitted light and/or laser beams will travel by in-vessel optical labyrinths between torus and analyzing equipment. The closest to plasma optical elements, so-called first mirrors (FMs) and windows (FWs), will operate in harsh environmental conditions such as high temperature, particle fluxes and deposition of eroded first wall materials (Be, W etc.) [3]. During periods of Mirror Conditioning States (MCS), the optical performance of FMs and FWs has to be restored by an appropriate cleaning system. The system based on the radio frequency capacitively coupled (RFCC) discharge with a mirror or window used as a RF electrode is currently considered as the main approach [4]. However, most research of the plasma cleaning were performed using Be proxy materials (Al, Au etc.) due to its high toxicity. The study of sputtering Be deposits are of high importance to prove the plasma cleaning efficiency. The report presents results of pilot experiments on deposition and in-situ sputtering of Be deposits in the QSPA-Be facility located in Bochvar institute. QSPA-Be facility was designed to simulate plasma heat loads related to ITER ELMs / disruptions [5]. The pure Be target evaporation by one pulse of QSPA-Be provided deposition of ~7 nm contaminant layer with ~70% of beryllium. The beryllium films were characterized using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The in-situ plasma cleaning was realized by exposing in 81 MHz helium discharge at pressure of 2 Pa. The cleaning process was controlled observing beryllium emission line in plasma column spectrum. The FW mock-up characterization was performed after 2 hours of the plasma exposure. Cyclic removal of Be-depositions in QSPA facility is planned to be done in 2020.

The effect of D2–N2 glow discharge plasma on the reflectivity of single-crystal Mo(111) mirrors is studied. The surface of each mirror is processed by mechanical polishing with diamond pastes to a roughness of hq = 5 nm. During polishing, abrasive particles are embedded in the surface layer of the mirror. Exposure to an ion fluence of 1.4 × 1024 m–2 leads to an increase in hq = 7 nm. After irradiation and the removal of a 30 nm-thick Mo layer the total reflection coefficient Rt in a 400–1000 nm region increases by 5–10% to a steady-state value (56–58)%. The increase in Rt may be caused by the preferential removal of carbon during the ion-sputtering process.

The evolution of indestructible blistering in molybdenum foils with the Mo {100} texture is investigated in dc glow discharge in a D 2 –N 2 mixture with a nitrogen molar fraction in the mixture varying from zero to unity at 100 V potential negative with respect to plasma, a total pressure of 15 Pa, and temperatures of 30–60°C. After the addition of 0.01N 2 to the deuterium discharge, the surface area occupied by the blisters increases from 2 to 5% and reaches its maximum of 11% upon exposure to D 2 −0.04N 2 mixture discharge (the fluence is 4 × 10 19 cm –2 ). Afterward, the area decreases, and blistering is absent in the pure N 2 discharge. The amount of deuterium desorbed from the samples upon heating also increases with the addition of nitrogen. In accordance with X-ray photoelectron spectroscopy data, a nitride layer about 5 nm thick is formed if small amounts of N 2 are added to D 2 . This layer is assumed to slow both the recombination rate of atomic deuterium coming from the material bulk to the surface and the transfer of D 2 molecules into the gas phase. At the same time, the nitride layer increases the diffusion flux of D atoms into the foil bulk, promoting blister growth.

To simulate thermal loads in ITER several tungsten (W) water cooled mock-ups were treated by a scanning electron beam (Tsefey-M) at a heat flux up to 1.68 GW m−2 for a duration of 18 μs and a frequency about 30 kHz. Surface morphology, chemical composition, structure, and microhardness of formed W layers were analyzed by SEM, EPMA, XRD, OM, and Vickers hardness tester. The irradiation treatment resulted in W melting to a depth of 0.3 mm, increasing sub-surface grain sizes, grain ordering in the metal bulk, deep cracking, and decreasing of microhardness to a depth of 2–4 mm. Installation of the pre-damaged W samples in a tokamak Globus-M divertor did not change practically discharge conditions.

Divertor targets composed of pre-damaged tungsten (W) tiles were prepared and installed at an outer strike point region of a Globus-M lower divertor.An electron beam was used to irradiate the W tiles close to influence of 1000-2000 disruption events in ITER.Providing adequacy of the e-beam thermal load to the real ITER conditions, the pre-damage treatment was simulated by ANSYS Multiphysics software.The W e-beam treatment results in melting of surface layers, shrinkage and fatigue intergranular cracking of the metal, void formation, grain ordering, and decreasing of microhardness.The first test of the pre-damaged W targets in the Globus-M divertor showed pronounced non-uniform heating caused by plasma disruption events.Temperature field formation may be explained by non-uniform thermal conductivity of the damaged layer or interface between the layer and undamaged bulk structure.One of the most important effects of the pre-damaged W installation is no significant changes in Globus-M discharge behavior.

A technique for tungsten-film deposition on different substrates in asymmetrical high-frequency (1.76 MHz) capacitive discharge in a D2−6.5 mol % O2 mixture under a total pressure of 15 Pa and at 60–130°C is considered. A circular W strip near the upper inner edge of a cylindrical hollow cathode with a radius of 4.2 cm and a height of 10 cm is the source of W particles. The smooth transition from sputtering of the inner surface to deposition occurs at a distance of about 4 cm from the upper boundary of the open part of the cathode. W, Mo, ZrO2, Si, and Cu substrates are placed in the lower closed end (bottom) and on the inner lateral cathode surface. At the upper cathode edge the sputtering yield is (4–5) × 10−2 at/ion. The mass rate of W deposition on the cathode bottom does not depend on the substrate type and is 40 μg/(cm2 h). The peculiarities of the composition, morphology, and structure of W films obtained on the lateral surface and bottom of the hollow cathode are discussed.

The development of the first mirror cleaning and recovery system is one of the challenges for all optical diagnostics in ITER. This study is focused on capacitively coupled radio frequency (CCRF) discharge as a promising method for removal of metal deposits. The physical aspects of the RF discharge application are discussed with a focus on implementation under ITER conditions. The effective sputtering rates of Be, W and Mo were calculated as a function of applied frequency and absorbed power for noble gases-He, Ne, Ar, Kr, Xe, taking into account complex shape of the ion energy distribution function in the electrode sheaths. Helium is shown to be a good candidate for cleaning Mo mirrors from Be deposits in the frequency range similar to 80-100 MHz and pressure of a few Pa.

Sputtering of Mo and Al (as Be proxy) in mixed D2/N2 DC glow discharge was studied in view of the first mirror performance. The composition of the working gas was varied from 100% D2 to 100% N2, while keeping a total pressure of 15Pa. The ion energies striking the sample surface were defined by its 100V biasing negative to a floating potential. It has been obtained that the sputtering yield of Mo and Al increases gradually with N2 concentration up to 4‒16mol% and decreases with further N2 addition. In contrast, the sputtering yield of Be remains unchanged up to 10mol% of N2. Adding 16mol% leads to three-fold decrease in the sputtering rate. The sputtering behavior is discussed in context of surface data analysis and mass spectroscopy of the discharge gas exhaust. Variation in reflectivity of a single crystalline Mo due to plasma exposure under similar conditions is also presented and discussed.

Tritium retention inside the vacuum vessel is a potentially serious constraint in the operation of large-scale fusion machines like ITER. An in situ diagnostics for first wall H/D/T retention by laser induced desorption spectroscopy (LIDS) is proposed for use between plasma discharges. The technique is based on local baking of the first wall by laser irradiation and subsequent analysis of the in-vessel gas by optical emission spectroscopy of plasma radiation. The local heating implementation, kinetics of H/D/T thermal extraction and the accuracy of optical emission spectroscopy measurements are analysed. To resolve the H/D/T lines spectroscopically, their thermal broadening should be minimized to prevent overlapping of the line shapes. A comparative performance analysis of several types of plasma sources with relatively cold ions is made including the following types of discharges: Penning, RF multipactor, laser torch and ECR. All these radiation sources require rather low power and could be used for remote in situ measurements of relative densities of the thermally extracted hydrogen isotopes.

According to recent experiments in plasma fusion machines, the contamination of optical elements under deposition-dominated conditions in ITER will result in fast degradation of their optical characteristics. The development of deposition-mitigation techniques is an important part of R&D program of ITER optical diagnostics. The paper describes the approaches to be used for the recovery of first optical elements of the Divertor Thomson scattering (DTS) diagnostics involving plasma cleaning and laser ablation. The long-term laser cleaning efficiency under continuous deposition of Al and W was studied for fused silica and alumina windows. The laser-induced damage threshold was expected to decrease by about the factor of three in highly contaminated areas and up to six times in the case of contamination with dust particles as compared to clean windows. The implementation of the laser cleaning technique for DTS diagnostics in ITER is discussed. Another technique, shown to be efficient for removal of metal and metal oxide deposits, is plasma treatment. The physical aspects of plasma cleaning based on capacitively coupled (CC) RF discharge are considered with the focus on the ITER-specific requirements to the system. Ion energy distribution and flux density in capacitively coupled RF discharge were measured as function of RF frequency and power. The grounded sheath voltage was found to be nearly constant at different discharge frequencies and absorbed power. The frequency dependence of effective sputtering yield of Be and Mo is presented for Ne and He plasma discharges.