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[en] Accounting for the lattice discreteness and the sheath electric field nonlinearity in dusty plasma crystals, it is demonstrated that highly localized structures (discrete breathers) involving vertical (transverse, off-plane) oscillations of charged dust grains may exist in a dust lattice. These structures correspond to either extremely localized bright breather excitations (pulses) or dark excitations composed of dips/voids. Explicit criteria for selecting different breather modes are presented
[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 . • 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] It is shown that there are two low-frequency modes associated with vertical oscillations in the Coulomb crystal of dust grains arranged in two horizontal chains in a sheath region of a low-temperature gas discharge plasma. The dispersion relations and characteristic frequencies of the modes are found. copyright 1998 American Institute of Physics
[en] Power exhaust is one of the big challenges for future fusion reactors. The power load at the divertor targets, the primary plasma-wall interaction zone, would exceed material limits and, thus, must be reduced. Therefore, 90% of the exhaust power needs to be dissipated and the divertor is anticipated to be in the detached regime, where the interaction of the plasma with the wall is significantly reduced. Radiation is the dominant dissipation process and is increased by impurity seeding. The radiation distribution can be tailored by using different seed impurities (N for radiation outside, Ne and Ar for radiation at the edge of and Kr for radiation inside the confined region). The tailoring of the radiation profile is required in order to maximize the radiated power and at the same time minimize the impact on the energy confinement. Recent experiments with intense impurity seeding at the ASDEX Upgrade tokamak demonstrate operation at highest heat fluxes and detached divertor targets at radiated power fractions of up to 90%. In these scenarios the radiation originates predominantly from the confined region and leads to an unexpectedly small confinement reduction.
[en] We demonstrate how the observation of dynamic Mach cones in dusty plasma experiments allows one to follow the evolution of the dust acoustic wave velocity as charges on dust particles change, or as their mass changes. In experiments in which only the charge is changing due to, e.g., the application of a thin coating on UV-illuminated dust particles, the coating rate may be inferred through the analysis of the Mach cone pattern. In other experiments where the dust sizes are changing, also leading to some change in charge, the growth rate can be followed
[en] The concept of a cold radiative plasma boundary is presented as a possible solution of the energy exhaust problem in a fusion reactor. The most relevant processes which determine level and location of the radiation from low-Z impurities are discussed in detail. Experimental results are used to demonstrate the general feasibility of generating a stable and quasi-stationary plasma with a cold radiating layer on a high power level. Furthermore, the limitations of the concept are briefly analyzed addressing feed-back control of the impurity level, thermal stability, particle transport and impurity contamination
[en] It is shown that oscillating dipoles of elongated grains (dipole oscillons) can create oscillatory wake potentials which can focus ions and attract nonspherical grains having the same polarity. This may provide the possibility for the formation of a linear ordered structure, which has been observed in association with ultra-low low-frequency oscillations in a liquid plasma crystal [V. Molotkov et al., JETP Lett. 71, 102 (2000); Rahman et al., Phys. Scr. T 89, 186 (2001)]
[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] An experiment has been performed to study the behavior of dust acoustic waves driven at high frequencies (f>100 Hz), extending the range of previous work. In this study, two previously unreported phenomena are observed--interference effects between naturally excited dust acoustic waves and driven dust acoustic waves, and the observation of finite dust temperature effects on the dispersion relation
[en] A three-component fluid model for a dusty plasma-sheath in an oblique magnetic field is presented. The study is carried out for the conditions when the thermophoretic force associated with the electron temperature gradient is one of the most important forces affecting dust grains in the sheath. It is shown that the sheath properties (the sheath size, the electron, ion and dust particle densities and velocities, the electric field potential, and the forces affecting the dust particles) are functions of the neutral gas pressure and ion temperature, the dust size, the dust material density, and the electron temperature gradient. Effects of plasma-dust collisions on the sheath structure are studied. It is shown that an increase in the forces pushing dust particles to the wall is accompanied by a decrease in the sheath width. The results of this work are particularly relevant to low-temperature plasma-enabled technologies, where effective control of nano- and microsized particles near solid or liquid surfaces is required.