[en] Full text of publication follows: The explanation of formation of macroscopic particles agglomerates in rarefied media is an intriguing problem that appears in the context of laboratory and space physics observations. The fast formation of agglomerates in low pressure conditions should be unlikely because of the low density of precursor species, and it is even more puzzling to understand the production of solid-state dust particles in a chemically active plasma. In this work we present evidence of formation of C dust grains in an unusual low pressure plasma confined in a magnetic cusp trap and interpret the major pathways of nucleation of C particles aggregates in a plasma environment of interest both for technological applications and for nuclear fusion research. Optical emission spectroscopy has been used to monitor the methane fragmentation in plasma phase and mass spectrometry has been used to monitor the residual gas during the particles formation in plasma discharge for various Ar/CH₄ gas mixture. The formation of spherical nano-size particles was detected ex-situ by means of AFM (Atomic Force Microscopy) and SEM (Scanning Electron Microscopy) analysis of a collecting Si substrate and in-situ by means of a novel technique based on the induced electrostatic fluctuation spectrum. The chemical nature of the particles was studied ex-situ by means of FT-IR spectroscopy. The production of acetylene compounds (C₂H_x) seems to be a key mechanism for the dust formation in methane plasma. (authors)

[en] Highlights: • Volumetric plasma recombination and impurity radiation cause the “rollover”. • Plasma-neutral friction is not directly involved in reduction of plasma flux. • Increase of impurity radiation results in a smooth transition to detachment. • Detachment onset in a specific flux tube happens when $\frac{P_{\text{up}}}{q_{\text{recycl}}}>{\left(\frac{P_{\text{up}}}{q_{\text{recycl}}}\right)}_{\text{crit}}$. • The latter holds for both pure deuterium and impurity-seeded plasmas. - Abstract: Divertor plasma detachment is analyzed from the viewpoint of energy and particle balance in the edge plasma. It is shown that volumetric recombination and impurity radiation losses are responsible for the transition to the detached plasma regime, whereas “momentum removal” plays although important, but auxiliary role providing conditions necessary for the first two to become efficient. A criterion of the local (on an isolated flux tube) detachment onset is studied for both pure and impurity-seeded plasmas.

[en] In this work, it is shown that RF voltages are induced along the sides of the Faraday shields of ICRF (ion cyclotron range of frequency) antennas and that these voltages cause modifications of plasma parameters and impurity production in the edge regions of fusion research plasmas. Specifically, in-air measurements with a capacitive probe on a model antenna and a Phaedrus-T two-strap antenna showed that RF voltages were induced along the sides of the Faraday shield when the sides were slotted. RF B field measurements with a B dot probe suggested that these voltages were induced primarily by the currents on the radial feeds of the antennas. RF-induced modifications of edge plasma parameters were found along the field lines that mapped to the Faraday shield in both the Phaedrus-B tandem mirror and the Phaedrus-T tokamak. In Phaedrus-T, when capacitive coupling between the edge plasma and the induced voltage was prevented by the attachment of boron nitride (an electric insulator) plates to the sides of the Faraday shield, both the RF-induced modifications of the edge plasma and the impurity production during ICRF were found to be greatly reduced

[en] Most divertor impurity modelling gives the result that negligible leakage occurs of impurities produced at the plates, to the confined plasma. This is not, however, found experimentally. A Monte Carlo impurity code has been employed in exploratory studies aimed at identifying possible leakage pathways to the main plasma. It was found that for substantial range of divertor plasma densities and temperatures, the friction force dominated all other forces on the impurities, including the temperature gradient forces which are generally directed away from the plates. Thus, if it was assumed that the deuterium plasma flow was everywhere directed toward the plates, negligible leakage to the confined plasma occurred. The possibility of deuterium flow reversal was also considered where it was assumed that over some radial fraction of the SOL, the deuterium plasma flow was directed away from the plates, starting at a distance from the plates equal to the deuterium average ionization distance. The spatial distribution of impurity (carbon) physically sputtered neutrals was modelled and it was found that a fraction of the impurity neutrals were ionized in the deuterium flow reversal zone. When these impurity particles were then tracked through further ionization, and their parallel and cross-field transport was followed, a substantial impurity density was found in the confined plasma with Z_eff values of 3 or more. It was thus concluded that flow reversal of the deuterium plasma is a promising possible explanation of the leakage of divertor plate impurities to the confined plasma

[en] The 35-channel H_α (λ = 656.3 nm) line intensity and impurities behaviour of a 9-channel C III, 7-channel bremsstrahlung emission, single channel O II (λ = 441.5 nm) and O V (λ = 278.1 nm) diagnostics were investigated in ac discharges on the HT-7 tokamak. The data analysis identified that the intensity of multi-channel H_α emission and impurities emission of C III (λ = 464.7 nm), O II and O V for the negative current plasma are much lower than for the positive current plasma. Lower hybrid wave plays a different role for the co-driven and anti-driven directions during current transition in ac discharges. Therefore, the emission of C III, O II and H_α behaviours is very different when the current direction transits from positive to negative, and vice versa.

[en] The pumped divertor experiment, in demonstrating before full deuterium-tritium operation an effective method of impurity control, aims to provide essential design data for a Next Step tokamak fusion device. (orig.)

[en] Through direct visualization at the discrete level, the microexcitations in cold 2+1D dusty plasma liquids formed by negatively charged dusts suspended in low pressure gaseous discharges were experimentally investigated, in which the downward ion flow wake field induces strong vertical coupling and chain bundle structure. It is found that the horizontal structure and motion are similar to those of the two-dimensional liquid. Different types of basic cooperative chain excitations: straight vertical chains with small amplitude jittering, chain tilting-restraightening, bundle twisting-restraightening, and chain breaking-reconnection, are observed. The region with good (poor) horizontal structural order prefers the straight (tilted or broken) chains with little (large) titling and tilting rate.

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[en] A brief summary is presented of recent developments in laboratory studies of complex (dusty) plasmas, both in terrestrial laboratories and under microgravity conditions, together with a short discussion of possible future developments