I.I. Arkhipov

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.

Overview of results obtained at the Globus-M spherical tokamak

Experiments and simulations to achieve high values of plasma parameters at the Globus-M spherical tokamak (ST) at moderate absolute auxiliary heating power (up to 0.8 MW) and high specific heating power (up to 2–3 MW m−3) are described. Important distinguishing features are the low edge safety factor range, which is unusual for STs, 2.7 < q < 5 and small plasma–outer wall space (3–5 cm). High ion heating efficiency with neutral beam injection (NBI) was demonstrated. Results of numerical simulation of fast ion trajectories are described and fast ion generation during the NBI and ion cyclotron resonance heating is discussed. Investigations on their confinement and slowing down are also presented. Reasons for achievement of high IC heating efficiency are outlined. Reliable H-mode regime achievement is described. Transport ASTRA modelling demonstrated that during NB heated H-mode ion heat diffusivity remains neoclassical and the particle diffusion coefficient inside transport barrier decreases significantly. Analysis was performed of divertor tile and special probe surfaces after irradiation by plasma during a large number of shots (3000–10 000 shots). Mixed layer composition is measured and deuterium retention in different tokamak first wall areas is estimated. Plasma jet injection experiments with upgraded plasma jet are described. Jet penetration to the plasma centre with immediate increase of density and temperature drop is proved and analogy with pellet injection is outlined.

Laboratory study of the transport and condensation of hydrocarbon radicals and its consequences for mitigating the tritium inventory in the ITER-FEAT divertor

Abstract The surface loss probability β of CH 3 radicals at a-C:H surfaces has been determined by analysis of the carbon deposition profiles along a tube flow reactor directly coupled with a methane RF discharge. β is equal to (1.0±0.2)×10 −3 for a methyl to atomic hydrogen flux ratio of 10:1. β remains the same in the temperature range 300–800 K and decreases slightly between 800 and 1200 K. The deposition rate drops drastically in the range 400–800 K. Above 800 K the carbon deposition is regained. From separate experiments on magnetron sputtering of graphite by D ions, it is inferred that at 300 and 900 kinetic CH x species react at the C:D surface with the same probability. In contrast, the deposition of thermal C x H y radicals decreases by a factor of 25 with a temperature rise from 320 to 400 K, remaining at this low level up to 1000 K. The implications of these results for co-deposition in the ITER-FEAT divertor are discussed.

Transport of and deposition from hydrocarbon radicals in a flow tube downstream from a CH4 RF discharge

Abstract The stream technique is used for the measurement of the surface loss probability ( β ) and sticking coefficient ( s ) of a hydrocarbon radical. A CH 4 RF inductive coupled discharge is used as source of CH i radicals and H atoms. These species lead to film formation in the downstream region of the tube. The thickness profiles along tube of the deposited carbon films are measured. The areal density and the structure of the carbon deposits on Si wafers, which are mounted inside tube, are studied by EPMA and RHEED. At 300 K the deposition profile of the a-C:H films can be approximated by exp(− x / L ), where x is the position in the tube and L is the transport length. By measuring L , β coefficients can be determined. On the basis of a comparison with literature, one can conclude that methyl radicals contribute predominantly to a-C:H deposition.

Bulk retention of deuterium in graphites exposed to deuterium plasma at high temperature

Abstract A highly ionized deuterium plasma with a low residual gas pressure and a high intensity D2+-ion beam were used for the study of deuterium retention in RG-Ti-91 and POCO AXF-5Q graphites. Deuterium retention in the samples was estimated by TDS during heating to 2000 K. Mechanical removal of a surface layer 100 μm thick was used to distinguish bulk and surface fractions of retained deuterium. The samples of RG-Ti and POCO graphites were exposed to a plasma with an ion flux of 3 × 1017 D/cm2 · s in the ‘Lenta’ plasma device for 10 to 104 s at residual deuterium pressure of 0.04 Pa at 1400 K. Under plasma exposure deuterium capture in RG-Ti graphite reached the saturation level at a fluence of 4 × 1020 D/cm2 while the bulk inventory was negligible. As for POCO graphite, deuterium retention increased with fluence and was equal to 18 appm in the bulk for a fluence of 7 × 1021 D/cm2. The same amount of deuterium in the bulk was obtained after gas exposure of POCO at an effective pressure of 0.8 Pa (1400 K, 6 h). With this result, the tritium concentration in the plasma-facing graphite materials can reach 1500 appm or 380 grams of tritium per ton of graphite. To understand the role of ion flux in generation of effective pressure, POCO was irradiated with 16 keV D2+-ions at 1400 K for 4 h to 8 × 1020 D/cm2 (ion flux was 6 × 1016 D/cm2 · s, residual deuterium pressure was 0.004 Pa). The results are discussed on the basis of structural differences for POCO and RG-Ti graphites.

Experimental Modeling of Transport and Deposition of Hydrocarbon Radicals on ITER-FEAT Cold Trap

Abstract The surface loss probability, β , of methyl (CH 3 ) radicals and atomic (H) hydrogen on a growing film surface was measured at lowered temperatures. The loss of reactive species was investigated by analysis of the deposition profiles along a quartz tube reactor directly coupled with a methane RF discharge (so-called ‘stream’ technique). The experiments were performed using special external containers with dry ice (∼200 K) and liquid nitrogen (77 K). It has been found that β (CH 3 ) on the growing film surfaces increases from 1.6×10 −4 at 300 K to 6.2×10 −4 at 200 K. Based on the data analysis, β for H can be deduced. The β (H) values were 3×10 −4 , 1×10 −3 and at least 10 −2 at 300 K, ∼200 K and 77 K, respectively. Film deposition from hydrocarbon radicals generated in a magnetron discharge is studied at temperatures from 300 K down to 120 K. Transition from formation of film-like to drop-like deposit is observed with cooling the substrate from 220 to 120 K. The application of the results to the understanding of transport and deposition of neutral reactive species on ITER-FEAT cold trap have been given.

Erosion mechanisms and products in graphite targets under simulated disruption conditions

Plasma/material interaction was studied in disruption simulation experiment at plasma gun facility MK-200 UG. Graphite was exposed to intense plasma stream under heat fluxes typical to ITER hard disruption. Plasma-induced surface damage and carbon erosion products were analysed.

LABORATORY EXPERIMENTS ON MODELING OF TRANSFER AND DEPOSITION OF HYDROCARBONS IN TOKAMAK MAIN CHAMBER AREAS SHADOWED FROM PLASMA

Одной из приоритетных научных и инженерных задач в современных токамаках и строящемся ИТЭР является поиск возможности подавления или контролируемого осаждения углеводородных радикалов и молекул в диверторе и откачивающем тракте с целью минимизации накопления трития в машине и устойчивой работы реактора. С помощью струевой техники выполнен сравнительный анализ закономерностей осаждения аморфных углеводородных (а-С:Н)-плёнок при температуре 300—1000 К из метильных радикалов, переносимых газом-носителем СН4/С2Нy/Н2 (y = 2, 4, 6), в кварцевой трубе с цилиндрическими вставками из Cu, Ni, Fe, W и нержавеющей стали (SS) как исходной, так и покрытой тонкими плёнками Pd или Rh. В нагретой части трубы с температурой от 300 до 1000 К процесс осаждения метила полностью подавлялся в секции трубы с температурой 380— 800 К для всех перечисленных вставок. Интервал температур 400—800 К, в котором осаждение углеводородов из метильных радикалов отсутствовало, был назван «температурным транспортным окном» (ТТО). Наиболее эффективным катализатором реакции гидрирования оказалась SS. Радикалы и ненасыщенные углеводороды, способные к полимеризации при 300—400 К, полностью выводились из потока газа-носителя (СН4/С2Нy/Н2) за несколько сотен соударений с поверхностью SS с температурой 420—470 К. Показана возможность создания рекомбинационного фильтра из SS для углеводородных радикалов (осуществления реакции гидрирования радикалов), переносимых ламинарным потоком СН4/С2Нy/Н2. Полученные данные могут быть использованы при выборе температурных режимов конструкционных элементов дивертора ИТЭР, изготовленных из вольфрама и нержавеющей стали.

Structural-phase transformations at annealing of highly reflective mirrors based on aluminum and silver

Multilayered prototypes of Al and Ag total reflection mirrors with a protective coating of amorphous ZrO2 oxide were analyzed using transmission electron (cross-sectional method), optical, and scanning (with electron probe microanalysis) microscopes. The prototypes’ reflectivity was measured by the recording of reflection spectra in the wavelength range 200–1100 nm. The structure of the mirrors was studied by X-ray diffraction reflection. High temperature annealing (8 h) did not change the stoichiometric composition of the protective coating. The reflectivity of the Al mirrors in the wavelength range 700–1100 nm and the Ag mirrors in the wavelength range of 500–1100 nm remained unchanged in spite of recrystallization of the polycrystalline metal films and the incipient process of amorphous ZrO2 crystallization. The mass of the crystalline oxide after annealing was several percent of the total amorphous oxide mass. At the same time annealing led to the occurrence of local voids on the aluminum/upper oxide interface. Such defects were not observed on the Ag mirrors. As a whole, the Ag mirrors turned out to be more stable to heating as compared to those made of Al.