S.A. Grashin

Characterization of a-B/C:H films deposited from different boron containing precursors

This paper describes the characteristics of a-B/C:H films deposited by PCVD processes with a new harmless precursor-carborane (C2B10H12) in the model device as well as in the T-11M tokamak. Deposition conditions, density, thickness, structure, chemical bonds, content and depth distribution of elements, and some other properties of the films, were investigated in detail. The structure and properties of the films were compared with those for films deposited by other boronization techniques with different precursors (diborane and trimethylborane) and also with a-C:H films. Application of carborane for boronization leads us to the increase of B/C ratio in the films up to a value of 3–4 which is optimal for the erosion process. The structure of the nearest order became icosahedral instead of a mixture of diamond-like and icosahedral as in the case of deposition with diborane. All amorphous a-B/C:H films are homogeneous, and uniform in depth. The hydrogen content is the same as for other B containing films.

Boronization of Russian tokamaks from carborane precursors

Abstract A new and cheap boronization technique using the nontoxic and nonexplosive solid substance carborane has been developed and succesfully applied to the Russian tokamaks T-11M, T-3M, T-10 and TUMAN-3. The glow discharge in a mixture of He and carborane vapor produced the amorphous B C coating with the B C ratio varied from 2.0–3.7. The deposition rate was about 150 nm/h. The primary effect of boronization was a significant reduction of the impurity influx and the plasma impurity contamination, a sharp decrease of the plasma radiated power, and a decrease of the effective charge. Boronization strongly suppressed the impurity influx caused by additional plasma heating. ECR- and ICR-heating as well as ECR current drive were more effective in boronized vessels. Boronization resulted in a signficant extension of the Ne- and q-region of stable tokamak operation. The density limit rose strongly. In Ohmic H-mode energy confinement time increased significantly (by a factor of 2) after boronization. It rose linearly with plasma current Ip and was 10 times higher than Neo-Alcator time at maximum current.

Experimental study of plasma fluxes in the shadow of a scoop limiter on T-10

Abstract Investigation of plasma fluxes measured by Langmuir probes on both sides of the T-10 scoop limiter shows the existence of large asymmetry, depending on discharge parameters. Two mechanisms can be responsible for such asymmetry. At low density, rotation of the plasma periphery in the direction of the plasma current causes the asymmetry. At high density it arises due to the preferential escape to the external side of the torus and its further spread along the magnetic field lines. The dependence of the neutral gas pressure under the limiter on the plasma influx to the channels was not linear. A pressure up to 30 mTorr has been observed.

Properties of a-C/B : H films relevant to plasma—surface interactions

Abstract Thin boron doped carbonaceous films have been deposited on metal alloys (stainless steel, Inconel), graphite and silicon single crystals during the boronization procedures in TEXTOR.The deposits were investigated by X-ray- and electron-diffraction (XRD, RHEED), transmission electron microscopy (TEM), nuclear reaction and backscattering techniques (NRA, ERD, RBS), electron microprobe (EPMA) and surface analysis techniques combined with ion sputtering (AES, XPS, SNMS). The films show a very good adhesion to all the substrates investigated, they are hard, semitransparent, amorphous and homogeneous down to about 3 nm. Colour-thickness correlation implies a refractive index n of about 2.4. The density of the film is ≈ 1.2 g cm−3, the average atomic distance d ≈ 0.23 nm (C-C ≈ 0.25 nm, C-B ≈ 0.15 nm, B-B ≈ 0.31 nm). The energy shift of the B(ls) core level in XPS indicates a covalent character of the B-C bond in the deposit. No indication of B-O bond has been found.

Radial distributions and poloidal asymmetries of T-10 SOL parameters and turbulence

Abstract This work presents the results of T-10 SOL investigations with multipin Langmuir probes and correlation reflectometry. A special regime with a rail limiter deeply introduced from the bottom of the machine at SOL temperatures of up to 50 eV has been studied. Five types of plasma small scale turbulence were observed in the SOL, including the three previously found types of core turbulence. It was shown that the main SOL parameters are poloidally uniform. A high poloidal asymmetry of the fluctuations and the turbulent flux of particles, was found.

Thermal load of a scoop limiter with changeable geometry

Abstract The thermal load of a T-10 scoop limiter with changeable geometry has been investigated by infrared thermography. The radial dependence of the energy flux was found to be non-exponential. The limiter load decreases sharply if the mean electron density exceeds a critical value of n c ∼ 4.5 × 10 19 m −3 . It shows maxima for q(a) near 2 and 3.

On the distribution of metals deposited onto the limiter and the liner of tokamaks after long-term operation

Abstract Three inspections of the inner parts of the discharge vessels of T-10 and TM1-MH after long-term operation revealed that metals originating from the various construction materials are distributed inhomogeneously over the first wall of these tokamaks. This partially allows one to identify local metal sources and to indicate anisotropies of the transport. Different materials from inner structures, even if they were only used in earlier experiments, are observed at all limiter surfaces and as components of the debris consisting of macroparticles of different size, shape and elemental composition. There are metallic deposits of the form of structured films or of solidified droplets.

A simple boronization technique for T-3M and T-11M tokamak chambers

Abstract A new technique for chamber boronization has been tested on tokanaks T-11M ( R = 0.7 m, a = 0.2 m, B t = 1 T, J p = 80 kA) and T-3M ( R = 1.06 m, a = 0.26 m, B t = 1 T, J p = 120 kA). This technique is not subject to drawbacks of conventional boronization, which makes use of the highly toxic and explosive gas diborane. The boron-containing gas was produced in the course of simple decomposition of ecologically safe and explosion proof solid substance in helium glow discharge for 2–3 h with the chamber wall at room temperature. In T-3M the graphite limiter and stainless steel wall and in T-11M the molybdenum wall and diaphragms were boronized. Despite the differences in wall and diaphragm structural materials the results produced by boronization were found to be similar. The radiation losses and effective ionic charge in the plasma were reduced. The critical density limit was increased by a factor of at least 3, and the loop voltage was decreased to 1 V. The hydrogen recycling was reduced after heat treatment or glow discharge conditioning. This simple, ecologically clean, hazard free and inexpensive boronization technique is recommended for extensive application in tokamaks.

Electron structure investigations of homogeneous hydrocarbon films formed in plasma conditions of T-10 tokamak

Electron structure investigations of smooth hydrocarbon thick films with a high atomic D/C ratio, redeposited from deuterium plasma discharge onto the vacuum vessel regions of the T-10 tokamak, are continued. For the first time, the investigations of both film sides and in a valence band region were performed by X-ray photoemission spectroscopy, combined with “conventional” measurements of survey spectra, C1s, O1s core levels, and by using Auger X-ray electron spectroscopy for sp2/sp3 ratio determination. The distinctions of electron structure of both sides of flakes which were noticed earlier using another techniques, were found, differing from the structure of thin films deposited in cleaning discharges of a low-temperature plasma. It is shown how these differences are connected with different processes of film deposition in tokamaks.

Interaction of hydrogen with the material of the discharge chamber of Tokamak TM-4

Abstract A new stage in the study of the interaction of hydrogen with the materials of fusion devices has been developed in recent years. At present real thermonuclear devices are used for this purpose because the model experiments on reemission, diffusion, accumulation, trapping, etc. cannot take into consideration the simultaneous action on the metal of various types of radiation: ion, electron, X-ray, light, neutron, etc. Upto the present time a number of experiments on isotope exchange have been performed on tokamaks DITE [1], TFR [2], ISX-B [3]. The results provide the possibility to evaluate the reemission kinetics of hydrogen. The present paper reports the data from experiments on recycling and accumulation of hydrogen in the wall of the tokamak TM-4 (I.V. Kurchatov Institute of Atomic Energy) during a series of shots.